![Chapter 1
Techno-Economic Analysis
of Different Energy Storage Technologies
Hussein Ibrahim and Adrian Ilinca
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/52220
1. Introduction
Overall structure of electrical power system is in the process of changing. For incremental
growth, it is moving away from fossil fuels - major source of energy in the world today - to
renewable energy resources that are more environmentally friendly and sustainable [1].
Factors forcing these considerations are (a) the increasing demand for electric power by both
developed and developing countries, (b) many developing countries lacking the resources to
build power plants and distribution networks, (c) some industrialized countries facing
insufficient power generation and (d) greenhouse gas emission and climate change
concerns. Renewable energy sources such as wind turbines, photovoltaic solar systems,
solar-thermo power, biomass power plants, fuel cells, gas micro-turbines, hydropower
turbines, combined heat and power (CHP) micro-turbines and hybrid power systems will be
part of future power generation systems [2-8].
Nevertheless, exploitation of renewable energy sources (RESs), even when there is a good
potential resource, may be problematic due to their variable and intermittent nature. In
addition, wind fluctuations, lightning strikes, sudden change of a load, or the occurrence of
a line fault can cause sudden momentary dips in system voltage [4]. Earlier studies have
indicated that energy storage can compensate for the stochastic nature and sudden
deficiencies of RESs for short periods without suffering loss of load events and without the
need to start more generating plants [4], [9], [10]. Another issue is the integration of RESs
into grids at remote points, where the grid is weak, that may generate unacceptable voltage
variations due to power fluctuations. Upgrading the power transmission line to mitigate this
problem is often uneconomic. Instead, the inclusion of energy storage for power smoothing
and voltage regulation at the remote point of connection would allow utilization of the
power and could offer an economic alternative to upgrading the transmission line.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-12-320.jpg)
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The current status shows that several drivers are emerging and will spur growth in the
demand for energy storage systems [11]. These include: the growth of stochastic generation
from renewables; an increasingly strained transmission infrastructure as new lines lag
behind demand; the emergence of micro-grids as part of distributed grid architecture; and
the increased need for reliability and security in electricity supply [12]. However, a lot of
issues regarding the optimal active integration (operational, technical and market) of these
emerging energy storage technologies into the electric grid are still not developed and need
to be studied, tested and standardized. The integration of energy storage systems (ESSs) and
further development of energy converting units (ECUs) including renewable energies in the
industrial nations must be based on the existing electric supply system infrastructure. Due
to that, a multi-dimensional integration task regarding the optimal integration of energy
storage systems will result.
The history of the stationary Electrical Energy Storage (EES) dates back to the turn of the 20th
century, when power stations were often shut down overnight, with lead-acid accumulators
supplying the residual loads on the direct current networks [13–15]. Utility companies
eventually recognised the importance of the flexibility that energy storage provides in
networks and the first central station for energy storage, a Pumped Hydroelectric Storage
(PHS), was put to use in 1929 [13,16,17]. The subsequent development of the electricity
supply industry, with the pursuit of economy of scale, at large central generating stations,
with their complementary and extensive transmission and distribution networks, essentially
consigned interest in storage systems up until relatively recent years. Up to 2005, more than
200 PHS systems were in use all over the world providing a total of more than 100 GW of
generation capacity [16–18]. However, pressures from deregulation and environmental
concerns lead to investment in major PHS facilities falling off, and interest in the practical
application of EES systems is currently enjoying somewhat of a renaissance, for a variety of
reasons including changes in the worldwide utility regulatory environment, an ever-
increasing reliance on electricity in industry, commerce and the home, power
quality/quality-of-supply issues, the growth of renewable as a major new source of
electricity supply, and all combined with ever more stringent environmental requirements
[14,19-20]. These factors, combined with the rapidly accelerating rate of technological
development in many of the emerging EESs, with anticipated unit cost reductions, now
make their practical applications look very attractive on future timescales of only a few
years.
This document aims to review the state-of-the-art development of EES technologies
including PHS [18,21], Compressed Air Energy Storage system (CAES) [22–26], Battery [27–
31], Flow Battery [14-15,20,32], Fuel Cell [33-34], Solar Fuel [15,35], Superconducting
Magnetic Energy Storage system (SMES) [36–38], Flywheel [32,39-41], Capacitor and
Supercapacitor [15,39], and Thermal Energy Storage system (TES) [42–50]. Some of them are
currently available and some are still under development. The applications, classification,
technical characteristics, research and development (R&D) progress and deployment status
of these EES technologies will be discussed in the following sections.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-13-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 3
2. Electrical energy storage
2.1. Definition of electrical energy storage
Electrical Energy Storage (EES) refers to a process of converting electrical energy from a
power network into a form that can be stored for converting back to electrical energy when
needed [13–14,51]. Such a process enables electricity to be produced at times of either low
demand, low generation cost or from intermittent energy sources and to be used at times of
high demand, high generation cost or when no other generation means is available [13–
15,19,51] (Figure 1). EES has numerous applications including portable devices, transport
vehicles and stationary energy resources [13-15], [19-20], [51-54]. This document will
concentrate on EES systems for stationary applications such as power generation,
distribution and transition network, distributed energy resource, renewable energy and
local industrial and commercial customers.
Figure 1. Fundamental idea of the energy storage [55]
2.2. Role of energy storage systems
Breakthroughs that dramatically reduce the costs of electricity storage systems could drive
revolutionary changes in the design and operation of the electric power system [52]. Peak load
problems could be reduced, electrical stability could be improved, and power quality
disturbances could be eliminated. Indeed, the energy storage plays a flexible and
multifunctional role in the grid of electric power supply, by assuring more efficient
management of available power. The combination with the power generation systems by the
conversion of renewable energy, the Energy Storage System (ESS) provide, in real time, the
balance between production and consumption and improve the management and the
reliability of the grid [56]. Furthermore, the ESS makes easier the integration of the renewable](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-14-320.jpg)
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resources in the energy system, increases their penetration rate of energy and the quality of the
supplied energy by better controlling frequency and voltage. Storage can be applied at the
power plant, in support of the transmission system, at various points in the distribution
system and on particular appliances and equipments on the customer’s side of the meter [52].
Figure 2. New electricity value chain with energy storage as the sixth dimension [11]
The ESS can be used to reduce the peak load and eliminate the extra thermal power plant
operating only during the peak periods, enabling better utilization of the plant functioning
permanently and outstanding reduction of emission of greenhouse gases (GHG) [57].
Energy storage systems in combination with advanced power electronics (power electronics
are often the interface between energy storage systems and the electrical grid) have a great
technical role and lead to many financial benefits. Some of these are summarized in the
following sections. Figure 2 shows how the new electricity value chain is changing
supported by the integration of energy storage systems (ESS). More details about the
different applications of energy storage systems will be presented in the section 4.
3. Energy storage components
Before discussing the technologies, a brief explanation of the components within an energy
storage device are discussed. Every energy storage facility is comprised of three primary
components [58]:
Storage Medium
Power Conversion System (PCS)
Balance of Plant (BOP)
3.1. Storage medium
The storage medium is the ‘energy reservoir’ that retains the potential energy within a
storage device. It ranges from mechanical (Pumped Heat Electricity Storage – PHES),](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-15-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 5
chemical (Battery Energy Storage - BES) and electrical (Superconductor Magnetic Energy
Storage – SMES) potential energy [58].
3.2. Power Conversion System (PCS)
It is necessary to convert from Alternating Current (AC) to Direct Current (DC) and vice
versa, for all storage devices except mechanical storage devices e.g. PHES and CAES
(Compressed Air Energy Storage) [59]. Consequently, a PCS is required that acts as a
rectifier while the energy device is charged (AC to DC) and as an inverter when the device is
discharged (DC to AC). The PCS also conditions the power during conversion to ensure that
no damage is done to the storage device.
The customization of the PCS for individual storage systems has been identified as one of
the primary sources of improvement for energy storage facilities, as each storage device
operates differently during charging, standing and discharging [59]. The PCS usually costs
from 33% to 50% of the entire storage facility. Development of PCSs has been slow due to
the limited growth in distributed energy resources e.g. small scale power generation
technologies ranging from 3 to 10,000 kW [60].
3.3. Balance-of-Plant (BOP)
These are all the devices that [58]:
Are used to house the equipment
Control the environment of the storage facility
Provide the electrical connection between the PCS and the power grid
It is the most variable cost component within an energy storage device due to the various
requirements for each facility. The BOP typically includes electrical interconnections, surge
protection devices, a support rack for the storage medium, the facility shelter and
environmental control systems [59].
The balance‐of‐plant includes structural and mechanical equipment such as protective
enclosure, Heating/Ventilation/Air Conditioning (HVAC), and maintenance/auxiliary
devices. Other BOP features include the foundation, structure (if needed), electrical
protection and safety equipment, metering equipment, data monitoring equipment, and
communications and control equipment. Other cost such as the facility site, permits, project
management and training may also be considered here [61].
4. Applications and technical benefits of energy storage systems
The traditional electricity value chain has been considered to consist of five links:
fuel/energy source, generation, transmission, distribution and customer-side energy service
as shown in Figure 3. By supplying power when and where needed, ESS is on the brink of
becoming the ‘‘sixth link” by integrating the existing segments and creating a more
responsive market [62]. Stored energy integration into the generation-grid system is](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-16-320.jpg)
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illustrated in Figure 4 [32]. It can be seen that potential applications of EES are numerous
and various and could cover the full spectrum ranging from larger scale, generation and
transmission-related systems, to those primarily related to the distribution network and
even ‘beyond the meter’, into the customer/end-user site [13]. Some important applications
have been summarised in [13–15], [32], [52], [62–66]:
Figure 3. Benefits of ESS along the electricity value chain [62].
Figure 4. Energy storage applications into grid [32].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-17-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 7
4.1. Generation
Commodity Storage: Storing bulk energy generated at night for use during peak demand
periods during the day. This allows for arbitrating the production price of the two
periods and a more uniform load factor for the generation, transmission, and
distribution systems [62].
Contingency Service: Contingency reserve is power capacity capable of providing power
to serve customer demand should a power facility fall off-line. Spinning reserves are
ready instantaneously, with non-spinning and long-term reserves ready in 10 minutes
or longer. Spinning Reserve is defined as the amount of generation capacity that can be
used to produce active power over a given period of time which has not yet been
committed to the production of energy during this period [67].
Area Control: Prevent unplanned transfer of power between one utility and another.
Grid Frequency Support: Grid Frequency Support means real power provided to the
electrical distribution grid to reduce any sudden large load/generation imbalance and
maintain a state of frequency equilibrium for the system’s 60Hz (cycles per second)
during regular and irregular grid conditions. Large and rapid changes in the electrical
load of a system can damage the generator and customers’ electrical equipment [62].
Black-Start: This refers to units with the capability to start-up on their own in order to
energize the transmission system and assist other facilities to start-up and synchronize
to the grid.
4.2. Transmission and distribution
System Stability: The ability to maintain all system components on a transmission line in
synchronous operation with each other to prevent a system collapse [62].
Grid Angular Stability: Grid Angular Stability means reducing power oscillations (due to
rapid events) by injection and absorption of real power.
Grid Voltage Support: Grid Voltage Support means power provided to the electrical
distribution grid to maintain voltages within the acceptable range between each end of
all power lines. This involves a trade-off between the amount of “real” energy produced
by generators and the amount of “reactive” power produced [68].
Asset Deferral: Defer the need for additional transmission facilities by supplementing
and existing transmission facilities—saving capital that otherwise goes underutilized
for years [69].
4.3. Energy service
Energy Management (Load Levelling / Peak Shaving): Load Levelling is rescheduling
certain loads to cut electrical power demand, or the production of energy during off-
peak periods for storage and use during peak demand periods. Whilst Peak Shaving is
reducing electric usage during peak periods or moving usage from the time of peak
demand to off-peak periods. This strategy allows to customers to peak shave by shifting
energy demand from one time of the day to another. This is primarily used to reduce
their time-of-use (demand) charges [62].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-18-320.jpg)
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Unbalanced Load Compensation: This can be done in combination with four-wire inverters
and also by injecting and absorbing power individually at each phase to supply
unbalanced loads.
Power Quality improvement: Power Quality is basically related to the changes in magnitude
and shape of voltage and current. This result in different issues including: Harmonics,
Power Factor, Transients, Flicker, Sag and Swell, Spikes, etc. Distributed energy storage
systems (DESS) can mitigate these problems and provide electrical service to the customer
without any secondary oscillations or disruptions to the electricity "waveform" [67].
Power Reliability: Can be presented as the percentage/ratio of interruption in delivery of
electric power (may include exceeding the threshold and not only complete loss of power)
versus total uptime. DESS can help provide reliable electric service to consumers (UPS) to
‘ride-through’ a power disruption. Coupled with energy management storage, this allows
remote power operation [68].
4.4. Supporting the integration of intermittent renewable energy sources
The development and use of renewable energy has experienced rapid growth over the past
few years. In the next 20–30 years all sustainable energy systems will have to be based on
the rational use of traditional resources and greater use of renewable energy.
Decentralized electrical production from renewable energy sources yields a more assured
supply for consumers with fewer environmental hazards. However, the unpredictable
character of these sources requires that network provisioning and usage regulations be
established for optimal system operation.
Figure 5. Integration of extrapolated (x6) wind power using energy storage on the Irish electricity grid [58]
However, renewable energy resources have two problems. First, many of the potential
power generation sites are located far from load centers. Although wind energy generation
facilities can be constructed in less than one year, new transmission facilities must be](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-19-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 9
constructed to bring this new power source to market. Since it can take upwards of 7 years
to build these transmission assets, long, lag-time periods can emerge where wind generation
is "constrained-off" the system [62]. For many sites this may preclude them from delivering
power to existing customers, but it opens the door to powering off-grid markets—an
important and growing market.
The second problem is that the renewable resources fluctuate independently from demand.
Therefore, the most of the power accessible to the grids is generated when there is low
demand for it. By storing the power from renewable sources from off-peak and releasing it
during on-peak, energy storage can transform this low value, unscheduled power into
schedulable, high-value product (see Figure 5). Beyond energy sales, with the assured
capability of dispatching power into the market, a renewable energy source could also sell
capacity into the market through contingency services.
This capability will make the development of renewable resources far more cost-effective —
by increasing the value of renewables it may reduce the level of subsidy down to where it is
equal to the environmental value of the renewable, at which point it is no longer a subsidy
but an environmental credit [62].
Frequency and synchronous spinning reserve support: In grids with a significant share of
wind generation, intermittency and variability in wind generation output due to
sudden shifts in wind patterns can lead to significant imbalances between generation
and load that in turn result in shifts in grid frequency [68]. Such imbalances are usually
handled by spinning reserve at the transmission level, but energy storage can provide
prompt response to such imbalances without the emissions related to most
conventional solutions.
Transmission Curtailment Reduction: Wind power generation is often located in remote
areas that are poorly served by transmission and distribution systems. As a result,
sometimes wind operators are asked to curtail their production, which results in lost
energy production opportunity, or system operators are required to invest in expanding
the transmission capability. An EES unit located close to the wind generation can allow
the excess energy to be stored and then delivered at times when the transmission
system is not congested [68].
Time Shifting: Wind turbines are considered as non-dispatchable resources. EES can be
used to store energy generated during periods of low demand and deliver it during
periods of high demand (Figure 5). When applied to wind generation, this application is
sometimes called “firming and shaping” because it changes the power profile of the
wind to allow greater control over dispatch [68].
Forecast Hedge: Mitigation of errors (shortfalls) in wind energy bids into the market
prior to required delivery, thus reducing volatility of spot prices and mitigating risk
exposure of consumers to this volatility [69].
Fluctuation suppression: Wind farm generation frequency can be stabilised by
suppressing fluctuations (absorbing and discharging energy during short duration
variations in output) [69].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-20-320.jpg)
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5. Financial benefits of energy storage systems
In [70] detailed analysis of energy storage benefits is done including market analysis, the
following are some highlights:
1. Cost Reduction or Revenue Increase of Bulk Energy Arbitrage: Arbitrage involves purchase
of inexpensive electricity available during low demand periods to charge the storage
plant, so that the low priced energy can be used or sold at a later time when the price
for electricity is high [11].
2. Cost Avoid or Revenue Increase of Central Generation Capacity: For areas where the supply
of electric generation capacity is tight, energy storage could be used to offset the need
to: a) purchase and install new generation and/or b) “rent” generation capacity in the
wholesale electricity marketplace.
3. Cost Avoid or Revenue Increase of Ancillary Services: It is well known that energy storage
can provide several types of ancillary services. In short, these are what might be called
support services used to keep the regional grid operating. Two more familiar ones are
spinning reserve and load following [11].
4. Cost Avoid or Revenue Increase for Transmission Access/Congestion: It is possible that use of
energy storage could improve the performance of the Transmission and Distribution
(T&D) system by giving the utilities the ability to increase energy transfer and stabilize
voltage levels. Further, transmission access/congestion charges can be avoided because
the energy storage is used.
5. Reduced Demand Charges: Reduced demand charges are possible when energy storage is
used to reduce an electricity end-user’s use of the electric grid during times grid is high
(i.e., during peak electric demand periods) [11].
6. Reduced Reliability-related Financial Losses: Storage reduces financial losses associated
with power outages. This benefit is very end-user-specific and applies to commercial
and industrial (C&I) customers, primarily those for which power outages cause
moderate to significant losses.
7. Reduced Power Quality-related Financial Losses: Energy storage reduces financial losses
associated with power quality anomalies. Power quality anomalies of interest are those
that cause loads to go off-line and/or that damage electricity-using equipment and
whose negative effects can be avoided if storage is used [11].
8. Increased Revenue from Renewable Energy Sources: Storage could be used to time-shift
electric energy generated by renewables. Energy is stored when demand and price for
power are low, so the energy can be used when a) demand and price for power is high
and b) output from the intermittent renewable generation is low.
The previous listed functionalities point out that those energy storages in combination with
power electronics will have a huge impact in future electrical supply systems. This is why
any planning and implementation strategy should be related to the real-time control and
operational functionalities of the ESS in combination with Distributed Energy Resources
(DER) in order to get rapid integration process.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-21-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 11
6. Techno-economic characteristics of energy storage systems
The main characteristics of storage systems on which the selection criteria are based are the
following [73]:
6.1. Storage capacity
This is the quantity of available energy in the storage system after charging. Discharge is
often incomplete. For this reason, it is defined on the basis of total energy stored, which is
superior to that actually retrieved (operational). The usable energy, limited by the depth of
discharge, represents the limit of discharge depth (minimum-charge state). In conditions of
quick charge or discharge, the efficiency deteriorates and the retrievable energy can be
much lower than storage capacity (Figure 6). On the other hand, self-discharge is the
attenuating factor under very slow regime.
Figure 6. Variation of energy capacity, self-discharge and internal resistance of a nickel-metal-hydride
battery with the number of cycles [71]
6.2. Storage System Power
This parameter determines the constitution and size of the motor-generator in the stored
energy conversion chain. A storage system’s power rating is assumed to be the system’s
nameplate power rating under normal operating conditions [73]. Furthermore, that rating is
assumed to represent the storage system’s maximum power output under normal operating
conditions. In this document, the normal discharge rate used is commonly referred to as the
system’s ‘design’ or ‘nominal’ (power) rating.
6.3. Storage ‘Emergency’ Power Capability
Some types of storage systems can discharge at a relatively high rate (e.g., 1.5 to 2 times their
nominal rating) for relatively short periods of time (e.g., several minutes to as much as 30
minutes). One example is storage systems involving a Na/S battery, which is capable of
producing two times its rated (normal) output for relatively short durations [72].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-22-320.jpg)
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That feature – often referred to as the equipment’s ‘emergency’ rating – is valuable if there
are circumstances that occur infrequently that involve an urgent need for relatively high
power output, for relatively short durations.
Importantly, while discharging at the higher rate, storage efficiency is reduced (relative to
efficiency during discharge at the nominal discharge rate), and storage equipment damage
increases (compared to damage incurred at the normal discharge rate).
So, in simple terms, storage with emergency power capability could be used to provide the
nominal amount of power required to serve a regularly occurring need (e.g., peak demand
reduction) while the same storage could provide additional power for urgent needs that
occur infrequently and that last for a few to several minutes at a time [72].
6.4. Autonomy
Autonomy or discharge duration autonomy is the amount of time that storage can discharge
at its rated output (power) without recharging. Discharge duration is an important criterion
affecting the technical viability of a given storage system for a given application and storage
plant cost [73]. This parameter depends on the depth of discharge and operational
conditions of the system, constant power or not. It is a characteristic of system adequacy for
certain applications. For small systems in an isolated area relying on intermittent renewable
energy, autonomy is a crucial criterion. The difficulty in separating the power and energy
dimensions of the system makes it difficult to choose an optimum time constant for most
storage technologies [74].
6.5. Energy and power density
Power density is the amount of power that can be delivered from a storage system with a
given volume or mass. Similarly, energy density is the amount of energy that can be stored
in a storage device that has a given volume or mass. These criteria are important in
situations for which space is valuable or limited and/or if weight is important (especially for
mass density of energy in portable applications, but less so for permanent applications).
6.6. Space requirements for energy storage
Closely related to energy and power density are footprint and space requirements for
energy storage. Depending on the storage technology, floor area and/or space constraints
may indeed be a challenge, especially in heavily urbanized areas.
6.7. Efficiency
All energy transfer and conversion processes have losses. Energy storage is no different.
Storage system round-trip efficiency (efficiency) reflects the amount of energy that comes
out of storage relative to the amount put into the storage. This definition is often
oversimplified because it is based on a single operation point [75]. The definition of](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-23-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 13
efficiency must therefore be based on one or more realistic cycles for a specific application.
Instantaneous power is a defining factor of efficiency (Figure 7). This means that, for
optimum operation, the power-transfer chain must have limited losses in terms of energy
transfer and self-discharge. This energy conservation measure is an essential element for
daily network load-levelling applications.
Typical values for efficiency include the following: 60% to 75% for conventional
electrochemical batteries; 75% to 85% for advanced electrochemical batteries; 73% to 80% for
CAES; 75% to 78% for pumped hydro; 80% to 90% for flywheel storage; and 95% for
capacitors and SMES [72], [76].
Figure 7. Power efficiency of a 48V-310Ah (15 kWh/10 h discharge) lead accumulator [77]
6.8. Storage operating cost
Storage total operating cost (as distinct from plant capital cost or plant financial carrying
charges) consists of two key components: 1) energy-related costs and 2) operating costs not
related to energy. Non-energy operating costs include at least four elements: 1) labor
associated with plant operation, 2) plant maintenance, 3) equipment wear leading to loss-of-
life, and 4) decommissioning and disposal cost [73].
1. Charging Energy-Related Costs: The energy cost for storage consists of all costs incurred
to purchase energy used to charge the storage, including the cost to purchase energy
needed to make up for (round trip) energy losses [73]. For a storage system with 75%
efficiency, if the unit price for energy used for charging is 4¢/kWh, then the plant
energy cost is 5.33¢/kWh.
2. Labor for Plant Operation: In some cases, labor may be required for storage plant
operation. Fixed labor costs are the same magnitude irrespective of how much the
storage is used. Variable labor costs are proportional to the frequency and duration of
storage use [73]. In many cases, labor is required to operate larger storage facilities
and/or ‘blocks’ of aggregated storage capacity whereas little or no labor may be needed
for smaller/distributed systems that tend to be designed for autonomous operation. No
explicit value is ascribed to this criterion, due in part to the wide range of labor costs
that are possible given the spectrum of storage types and storage system sizes [73].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-24-320.jpg)
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Figure 8. Storage total variable operation cost for 75% storage efficiency [73]
3. Plant Maintenance: Plant maintenance costs are incurred to undertake normal,
scheduled, and unplanned repairs and replacements for equipment, buildings, grounds,
and infrastructure. Fixed maintenance costs are the same magnitude irrespective of how
much the storage is used [73]. Variable maintenance costs are proportional to the
frequency and duration of storage use.
4. Replacement Cost: If specific equipment or subsystems within a storage system are
expected to wear out during the expected life of the system, then a ‘replacement cost’
will be incurred. In such circumstances, a ‘sinking fund’ is needed to accumulate funds
to pay for replacements when needed [73]. That replacement cost is treated as a variable
cost (i.e., the total cost is spread out over each unit of energy output from the storage
plant).
5. Variable Operating Cost: A storage system’s total variable operating cost consists of
applicable non-energy-related variable operating costs plus plant energy cost, possibly
including charging energy, labor for plant operation, variable maintenance, and
replacement costs. Variable operating cost is a key factor affecting the cost-effectiveness
of storage [73]. It is especially important for ‘high-use’ value propositions involving
many charge-discharge cycles.
Ideally, storage for high-use applications should have relatively high or very high efficiency
and relatively low variable operating cost. Otherwise, the total cost to charge then discharge
the storage is somewhat-to-very likely to be higher than the benefit. That can be a significant
challenge for some storage types and value propositions.
Consider the example illustrated in Figure 8, which involves a 75% efficient storage system
with a non-energy-related variable operating cost of 4¢/kWhout. If that storage system is](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-25-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 15
charged with energy costing 4¢/kWhin, then the total variable operating cost – for energy
output – is about 9.33¢/kWhout [73].
6.9. Durability
Lifetime or durability refers to the number of times the storage unit can release the energy
level it was designed for after each recharge, expressed as the maximum number of cycles
N (one cycle corresponds to one charge and one discharge) [81]. All storage systems
degrade with use because they are subject to fatigue or wear by usage use (i.e., during each
charge-discharge cycle). This is usually the principal cause of aging, ahead of thermal
degradation. The rate of degradation depends on the type of storage technology, operating
conditions, and other variables. This is especially important for electrochemical batteries
[73].
For some storage technologies – especially batteries – the extent to which the system is
emptied (discharged) also affects the storage media’s useful life. Discharging a small portion
of stored energy is a ‘shallow’ discharge and discharging most or all of the stored energy is a
‘deep’ discharge. For these technologies, a shallow discharge is less damaging to the storage
medium than a deep discharge [73].
To the extent that the storage medium degrades and must be replaced during the expected
useful life of the storage system, the cost for that replacement must be added to the variable
operating cost of the storage system.
Figure 9. Evolution of cycling capacity as a function of depth of discharge for a lead-acid battery [79]
The design of a storage system that considers the endurance of the unit in terms of cycles
should be a primary importance when choosing a system. However, real fatigue processes
are often complex and the cycling capacity is not always well defined. In all cases, it is
strongly linked to the amplitude of the cycles (Figure 9) and/or the average state of charge
[78]. As well, the cycles generally vary greatly, meaning that the quantification of N is
delicate and the values given represent orders of magnitude [74].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-26-320.jpg)
![Energy Storage – Technologies and Applications
16
6.10. Reliability
Like power rating and discharge duration, storage system reliability requirements are
circumstance-specific. Little guidance is possible. Storage-system reliability is always an
important factor because it is a guarantee of on-demand service [81]. The project design
engineer is responsible for designing a plant that provides enough power and that is as
reliable as necessary to serve the specific application.
6.11. Response time
Storage response time is the amount of time required to go from no discharge to full
discharge. At one extreme, under almost all conditions, storage has to respond quite rapidly
if used to provide capacity on the margin in lieu of transmission and distribution (T&D)
capacity. That is because the output from T&D equipment (i.e., wires and transformers)
changes nearly instantaneously in response to demand [73].
In contrast, consider storage used in lieu of generation capacity. That storage does not need
to respond as quickly because generation tends to respond relatively slowly to demand
changes. Specifically, some types of generation – such as engines and combustion turbines
– take several seconds to many minutes before generating at full output. For other
generation types, such as those fueled by coal and nuclear energy, the response time may
be hours [73].
Most types of storage have a response time of several seconds or less. CAES and pumped
hydroelectric storage tend to have a slower response, though they still respond quickly
enough to serve several important applications.
6.12. Ramp rate
An important storage system characteristic for some applications is the ramp rate – the rate
at which power output can change. Generally, storage ramp rates are rapid (i.e., output can
change quite rapidly); pumped hydro is the exception. Power devices with a slow response
time tend also to have a slow ramp rate [73].
6.13. Charge rate
Charge rate – the rate at which storage can be charged – is an important criterion
because, often, modular energy storage (MES) must be recharged so it can serve load
during the next day [58]. If storage cannot recharge quickly enough, then it will not have
enough energy to provide the necessary service. In most cases, storage charges at a rate
that is similar to the rate at which it discharges [73]. In some cases, storage may charge
more rapidly or more slowly, depending on the capacity of the power conditioning
equipment and the condition and/or chemistry and/or physics of the energy storage
medium.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-27-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 17
6.14. Self-discharge and energy retention
Energy retention time is the amount of time that storage retains its charge. The concept of
energy retention is important because of the tendency for some types of storage to self-
discharge or to otherwise dissipate energy while the storage is not in use. In general terms,
energy losses could be referred to as standby losses [74].
Storage that depends on chemical media is prone to self-discharge. This self-discharge is due
to chemical reactions that occur while the energy is stored. Each type of chemistry is
different, both in terms of the chemical reactions involved and the rate of self-discharge.
Storage that uses mechanical means to store energy tends to be prone to energy dissipation.
For example, energy stored using pumped hydroelectric storage may be lost to evaporation.
CAES may lose energy due to air escaping from the reservoir [73].
To the extent that storage is prone to self-discharge or energy dissipation, retention time is
reduced. This characteristic tends to be less important for storage that is used frequently. For
storage that is used infrequently (i.e., is in standby mode for a significant amount of time
between uses), this criterion may be very important [72].
6.15. Transportability
Transportability can be an especially valuable feature of storage systems for at least two
reasons. First, transportable storage can be (re)located where it is needed most and/or where
benefits are most significant [58]. Second, some locational benefits only last for one or two
years. Given those considerations, transportability may significantly enhance the prospects
that lifecycle benefits will exceed lifecycle cost.
6.16. Power conditioning
To one extent or another, most storage types require some type of power conditioning (i.e.,
conversion) subsystem. Equipment used for power conditioning – the power conditioning
unit (PCU) – modifies electricity so that the electricity has the necessary voltage and the
necessary form; either alternating current (AC) or direct current (DC). The PCU, in concert
with an included control system, must also synchronize storage output with the oscillations
of AC power from the grid [73].
Output from storage with relatively low-voltage DC output must be converted to AC with
higher voltage before being discharged into the grid and/or before being used by most load
types. In most cases, conversion from DC to AC is accomplished using a device known as an
inverter [73].
For storage requiring DC input, the electricity used for charging must be converted from the
form available from the grid (i.e., AC at relatively high voltage) to the form needed by the
storage system (e.g., DC at lower voltage). That is often accomplished via a PCU that can
function as a DC ‘power supply’ [73].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-28-320.jpg)
![Energy Storage – Technologies and Applications
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6.17. Power quality
Although requirements for applications vary, the following storage characteristics may or
may not be important. To one extent or another, they are affected by the PCU used and/or
they drive the specifications for the PCU. In general, higher quality power (output) costs
more.
6. Power Factor: Although detailed coverage of the concept of power factor is beyond the
scope of this report, it is important to be aware of the importance of this criterion. At a
minimum, the power output from storage should have an acceptable power factor,
where acceptable is somewhat circumstance variable power factor.
7. Voltage Stability: In most cases, it is important for storage output voltage to remain
somewhat-to-very constant. Depending on the circumstances, voltage can vary; though,
it should probably remain within about 5% to 8% of the rated value.
8. Waveform: Assuming that storage output is AC, in most cases, the waveform should be
as close as possible to that of a sine wave. In general, higher quality PCUs tend to have
waveforms that are quite close to that of a sine wave whereas output from lower quality
PCUs tends to have a waveform that is somewhat square.
9. Harmonics: Harmonic currents in distribution equipment can pose a significant
challenge. Harmonic currents are components of a periodic wave whose frequency is an
integral multiple of the fundamental frequency [73]. In this case, the fundamental
frequency is the utility power line frequency of 60 Hz.
6.18. Modularity
One attractive feature of modular energy storage is the flexibility that system ‘building
blocks’ provide. Modularity allows for more optimal levels and types of capacity and/or
discharge duration because modular resources allow utilities to increase or decrease storage
capacity, when and where needed, in response to changing conditions [72-73]. Among other
attractive effects, modular capacity provides attractive means for utilities to address
uncertainty and to manage risk associated with large, ‘lumpy’ utility T&D investments.
6.19. Storage system reactive power capability
One application (Voltage Support) and one incidental benefit (Power Factor Correction)
described in this guide involve storage whose capabilities include absorbing and injecting
reactive power (expressed in units of volt-Amperes reactive or VARs) [58], [72-73]. This
feature is commonly referred as VAR support. In most cases, storage systems by themselves
do not have reactive power capability. For a relatively modest incremental cost, however,
reactive power capability can be added to most storage system types.
6.20. Feasibility and adaptation to the generating source
To be highly efficient, a storage system needs to be closely adapted to the type of application
(low to mid power in isolated areas, network connection, etc.) and to the type of production](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-29-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 19
(permanent, portable, renewable, etc.) (Figure 10) it is meant to support. It needs to be
harmonized with the network.
Figure 10. Fields of application of the different storage techniques according to stored energy and
power output [80]
6.21. Monitoring, control and communications equipments
This equipment, on both the quality and safety of storage levels, has repercussions on the
accessibility and availability of the stored energy [74]. Indeed, storage used for most
applications addressed in this report must receive and respond to appropriate control
signals. In some cases, storage may have to respond to a dispatch control signal. In other
cases, the signal may be driven by a price or prices [73]. Storage response to a control signal
may be a simple ramp up or ramp down of power output in proportion to the control signal.
A more sophisticated response, requiring one or more control algorithms, may be needed.
6.22. Interconnection
If storage will be charged with energy from the grid or will inject energy into the grid, it must
meet applicable interconnection requirements. At the distribution level, an important point of
reference is the Institute of Electronics and Electrical Engineers (IEEE) Standard 1547 [82].
Some countries and utilities have more specific interconnection rules and requirements.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-30-320.jpg)
![Energy Storage – Technologies and Applications
20
6.23. Operational constraints
Especially related to safety (explosions, waste, bursting of a flywheel, etc.) or other
operational conditions (temperature, pressure, etc.), they can influence the choice of a
storage technology as a function of energy needs [74].
6.24. Environmental aspect
While this parameter is not a criterium of storage-system capacity, the environmental aspect
of the product (recyclable materials) is a strong sales pitch. For example, in Nordic countries
(Sweden, Norway), a definite margin of the population prefers to pay more for energy than
to continue polluting the country [83]. This is a dimension that must not, therefore, be
overlooked.
6.25. Decommissioning and disposal needs and cost
In most cases there will be non-trivial decommissioning costs associated with almost any
storage system [73]. For example, eventually batteries must be dismantled and the chemicals
must be removed. Ideally, dismantled batteries and their chemicals can be recycled, as is the
case for the materials in lead-acid batteries.
Ultimately, decommissioning-related costs should be included in the total cost to own and
to operate storage.
6.26. Other characteristics
The ease of maintenance, simple design, operational flexibility (this is an important
characteristic for the utility), fast response time for the release of stored energy, etc.
Finally, it is important to note that these characteristics apply to the overall storage system:
storage units and power converters alike [74].
7. Classification of energy storage systems
There are two criteria to categorise the various ESSs: function and form. In terms of the
function, ESS technologies can be categorised into those that are intended firstly for high
power ratings with a relatively small energy content making them suitable for power
quality or UPS [69]; and those designed for energy management, as shown in Figure 11.
PHS, CAES, TES, large-scale batteries, flow batteries, fuel cells, solar fuel and TES fall into
the category of energy management, whereas capacitors/super-capacitors, SMES, flywheels
and batteries are in the category of power quality and reliability. This simple classification
glosses over the wide range of technical parameters of energy storage devices.
Although electricity is not easy to be directly stored cheaply, it can be easily stored in other
forms and converted back to electricity when needed. Storage technologies for electricity can
also be classified by the form of storage into the following [69]:](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-31-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 21
Figure 11. Energy storage classification with respect to function [69].
1. Electrical energy storage: (i) Electrostatic energy storage including capacitors and super-
capacitors; (ii) Magnetic/current energy storage including SMES.
2. Mechanical energy storage: (i) Kinetic energy storage (flywheels); (ii) Potential energy
storage (PHES and CAES).
3. Chemical energy storage: (i) electrochemical energy storage (conventional batteries such
as lead-acid, nickel metal hydride, lithium ion and flow-cell batteries such as zinc
bromine and vanadium redox); (ii) chemical energy storage (fuel cells, Molten-
Carbonate Fuel Cells – MCFCs and Metal-Air batteries); (iii) thermochemical energy
storage (solar hydrogen, solar metal, solar ammonia dissociation–recombination and
solar methane dissociation–recombination).
4. Thermal energy storage: (i) Low temperature energy storage (Aquiferous cold energy
storage, cryogenic energy storage); (ii) High temperature energy storage (sensible heat
systems such as steam or hot water accumulators, graphite, hot rocks and concrete,
latent heat systems such as phase change materials).
8. Description of energy storage technologies
8.1. Pumped hydro storage (PHS)
In pumping hydro storage, a body of water at a relatively high elevation represents a
potential or stored energy. During peak hours the water in the upper reservoir is lead
through a pipe downhill into a hydroelectric generator and stored in the lower reservoir.
Along off-peak periods the water is pumped back up to recharge the upper reservoir and
the power plant acts like a load in power system [72], [84].
Pumping hydro energy storage system (figure 12) consists in two large water reservoirs,
electric machine (motor/generator) and reversible pump-turbine group or pump and turbine
separated. This system can be started-up in few minutes and its autonomy depends on the
volume of stored water.
Restrictions to pumping hydro energy storage are related with geographical constraints and
weather conditions. In periods of much rain, pumping hydro capacity can be reduced.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-32-320.jpg)
![Energy Storage – Technologies and Applications
22
Pumped hydroelectric systems have conversion efficiency, from the point of view of a
power network, of about 65–80%, depending on equipment characteristics [72]. Considering
the cycle efficiency, 4 kWh are needed to generate three. The storage capacity depends on
two parameters: the height of the waterfall and the volume of water. A mass of 1 ton falling
100 m generates 0.272 kWh.
Figure 12. Wind-Pumped hydro energy storage hybrid system [69].
8.2. Batteries energy storage
Batteries store energy in electrochemical form creating electrically charged ions. When the
battery charges, a direct current is converted in chemical energy, when discharges, the
chemical energy is converted back into a flow of electrons in direct current form [75].
Electrochemical batteries use electrodes both as part of the electron transfer process and
store the products or reactants via electrode solid-state reactions [85]. Batteries are the most
popular energy storage devices. However, the term battery comprises a sort of several
technologies applying different operation principals and materials. There is a wide range of
technologies used in the fabrication of electrochemical accumulators (lead–acid (Figure 13),
nickel–cadmium, nickel–metal hydride, nickel–iron, zinc–air, iron–air, sodium–sulphur,
lithium–ion, lithium–polymer, etc.) and their main assets are their energy densities (up to
150 and 2000 Wh/kg for lithium) and technological maturity. Their main inconvenient
however is their relatively low durability for large-amplitude cycling (a few 100 to a few
1000 cycles). They are often used in portable systems, but also in permanent applications
(emergency network back-up, renewable-energy storage in isolated areas, etc.) [83].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-33-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 23
The minimum discharge period of the electrochemical accumulators rarely reaches below 15
minutes. However, for some applications, power up to 100 W/kg, even a few kW/kg, can be
reached within a few seconds or minutes. As opposed to capacitors, their voltage remains
stable as a function of charge level. Nevertheless, between a high-power recharging
operation at near-maximum charge level and its opposite, that is to say a power discharge
nearing full discharge, voltage can easily vary by a ratio of two [74].
Figure 13. Structure of a lead‐acid battery [86]
8.3. Flow batteries energy storage (FBES)
Flow batteries are a two-electrolyte system in which the chemical compounds used for
energy storage are in liquid state, in solution with the electrolyte. They overcome the
limitations of standard electrochemical accumulators (lead-acid or nickel-cadmium for
example) in which the electrochemical reactions create solid compounds that are stored
directly on the electrodes on which they form. This is therefore a limited-mass system,
which obviously limits the capacity of standard batteries.
Various types of electrolyte have been developed using bromine as a central element: with zinc
(ZnBr), sodium (NaBr) (Figure 14), vanadium (VBr) and, more recently, sodium polysulfide.
The electrochemical reaction through a membrane in the cell can be reversed (charge-
discharge). By using large reservoirs and coupling a large number of cells, large quantities of
energy can be stored and then released by pumping electrolyte into the reservoirs.
The main advantages of the technology include the following [87]: 1) high power and
energy capacity; 2) fast recharge by replacing exhaust electrolyte; 3) long life enabled by](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-34-320.jpg)
![Energy Storage – Technologies and Applications
24
easy electrolyte replacement; 4) full discharge capability; 5) use of nontoxic materials; and 6)
low-temperature operation. The main disadvantage of the system is the need for moving
mechanical parts such as pumping systems that make system miniaturization difficult.
Therefore, the commercial uptake to date has been limited. The best example of flow battery
was developed in 2003 by Regenesys Technologies, England, with a storage capacity of 15
MW-120 MWh. It has since been upgraded to an electrochemical system based entirely on
vanadium. The overall electricity storage efficiency is about 75 % [88].
Figure 14. Illustration of a flow-battery
8.4. Flywheel energy storage (FES)
Flywheel energy accumulators are comprised of a massive or composite flywheel coupled
with a motor-generator and special brackets (often magnetic), set inside a housing at very
low pressure to reduce self-discharge losses (Figure 15) [9]. They have a great cycling
capacity (a few 10,000 to a few 100,000 cycles) determined by fatigue design.
To store energy in an electrical power system, high-capacity flywheels are needed. Friction
losses of a 200 tons flywheel are estimated at about 200 kW. Using this hypothesis and
instantaneous efficiency of 85 %, the overall efficiency would drop to 78 % after 5 hours, and
45 % after one day. Long-term storage with this type of apparatus is therefore not foreseeable.
From a practical point of view, electromechanical batteries are more useful for the
production of energy in isolated areas. Kinetic energy storage could also be used for the
distribution of electricity in urban areas through large capacity buffer batteries, comparable
to water reservoirs, aiming to maximize the efficiency of the production units. For example,
large installations made up of forty 25kW-25kWh systems are capable of storing 1 MW that
can be released within one hour.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-35-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 25
Figure 15. Flywheel energy accumulators [89]
8.5. Supercapacitors energy storage (SES)
Supercapacitors are the latest innovational devices in the field of electrical energy storage. In
comparison with a battery or a traditional capacitor, the supercapacitor allows a much
powerful power and energy density [15]. Supercapacitors are electrochemical double layer
capacitors that store energy as electric charge between two plates, metal or conductive,
separated by a dielectric, when a voltage differential is applied across the plates. As like
battery systems, capacitors work in direct current.
The energy/volume obtained is superior to that of capacitors (5 Wh/kg or even 15 Wh/kg), at
very high cost but with better discharge time constancy due to the slow displacement of ions
in the electrolyte (power of 800–2000 W/kg). Super-capacitors generally are very durable,
that is to say 8–10 years, 95% efficiency and 5% per day self-discharge, which means that the
stored energy must be used quickly.
Supercapacitors find their place in many applications where energy storage is needed, like
uninterruptible power supplies, or can help in smoothing strong and short-time power
solicitations of weak power networks. Their main advantages are the long life cycle and the
short charge/discharge time [2], [19].
8.6. Superconducting magnetic energy storage (SMES)
An emerging technology, systems store energy in the magnetic field created by the flow of
direct current in a coil of cryogenically cooled, superconducting material. Due to their
construction, they have a high operating cost and are therefore best suited to provide
constant, deep discharges and constant activity. The fast response time (under 100 ms) of
these systems makes them ideal for regulating network stability (load levelling). Power is
available almost instantaneously and very high power output can be provided for a brief
period of time [20-21]. These facilities currently range in size up to 3 MW units and are](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-36-320.jpg)
![Energy Storage – Technologies and Applications
26
generally used to provide grid stability in a distribution system and power quality at
manufacturing facilities requiring ultra-clean power such a chip fabrication facility.
One advantage of this storage system is its great instantaneous efficiency, near 95 % for a
charge-discharge cycle [90]. Moreover, these systems are capable of discharging the near
totality of the stored energy, as opposed to batteries. They are very useful for
applications requiring continuous operation with a great number of complete charge-
discharge cycles.
8.7. Fuel cells-Hydrogen energy storage (HES)
Fuel cells are a means of restoring spent energy to produce hydrogen through water
electrolysis. The storage system proposed includes three key components: electrolysis which
consumes off-peak electricity to produce hydrogen, the fuel cell which uses that hydrogen
and oxygen from air to generate peak-hour electricity, and a hydrogen buffer tank to ensure
adequate resources in periods of need.
Fuel cells can be used in decentralized production (particularly low-power stations –
residential, emergency...), spontaneous supply related or not to the network, mid-power
cogeneration (a few hundred kW), and centralized electricity production without heat
upgrading. They can also represent a solution for isolated areas where the installation of
power lines is too difficult or expensive (mountain locations, etc.). There are several
hydrogen storage modes, such as: compressed, liquefied, metal hydride, etc. For station
applications, pressurized tanks with a volume anywhere between 10-2 m3 and 10,000 m3 are
the simplest solution to date. Currently available commercial cylinders can stand pressures
up to 350 bars.
Combining an electrolyser and a fuel cell for electrical energy storage is a low-efficiency
solution (at best 70 % for the electrolyser and 50 % for the fuel cell, and 35 % for the
combination). As well, the investment costs are prohibitive and life expectancy is very
limited, especially for power network applications [74].
8.8. Thermal energy storage (TES)
Thermal energy storage (TES) already exists in a wide spectrum of applications. It uses
materials that can be kept at high/low temperatures in insulated containments. Heat/cold
recovered can then be applied for electricity generation using heat engine cycles.
Energy input can, in principle, be provided by electrical resistance heating or
refrigeration/cryogenic procedures, hence the overall round trip efficiency of TES is low
(30–60%) although the heat cycle efficiency could be high (70–90%), but it is benign to the
environment and may have particular advantages for renewable and commercial
buildings.
TES systems can be classified into low-temperature TES and high-temperature TES
depending on whether the operating temperature of the energy storage material is higher](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-37-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 27
than the room temperature. More precisely, TES can be categorised into industrial cooling
(below -18 C), building cooling (at 0-12 C), building heating (at 25-50 C) and industrial
heat storage (higher than 175 C).
8.9. Compressed Air Energy Storage (CAES)
This method consist to use off-peak power to pressurize air into an underground reservoir
(salt cavern, abandoned hard rock mine or aquifer) which is then released during peak
daytime hours to power a turbine/generator for power production. CAES (Figure 16) is the
only other commercially available technology (besides pumped-hydro) able to provide the
very-large system energy storage deliverability (above 100 MW in single unit sizes) to use
for commodity storage or other large-scale setting [74].
Figure 16. Illustration of compressed-air energy storage
The energy density for this type of system is in the order of 12 kWh/m3 [91], while the
estimated efficiency is around 70 % [92]. Let us note that to release 1 kWh into the network,
0.7–0.8 kWh of electricity needs to be absorbed during off-peak hours to compress the air, as
well as 1.22 kWh of natural gas during peak hours (retrieval). Two plants currently exist,
with several more under development. The first operating unit is a 290 MW unit built in
Huntorf, Germany in 1978. The second plant is a 110 MW unit built in McIntosh, Alabama in
1991. Small-scale compressed air energy storage (SSCAES), compressed air storage under
high pressure in cylinders (up to 300 bars with carbon fiber structures) are still developing
and seem to be a good solution for small- and medium-scale applications.
9. Assessment and comparison of the energy storage technologies
Following, some figures are presented that compare different aspects of storage
technologies. These aspects cover topics such as: technical maturity, range of applications,
efficiencies, lifetime, costs, mass and volume densities, etc.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-38-320.jpg)
![Energy Storage – Technologies and Applications
28
9.1. Technical maturity
The technical maturity of the EES systems is shown in Figure 17. The EES technologies can
be classified into three categories in terms of their maturity [69]:
1. Mature technologies: PHS and lead-acid battery are mature and have been used for
over 100 years.
2. Developed technologies: CAES, NiCd, NaS, ZEBRA Li-ion, Flow Batteries, SMES,
flywheel, capacitor, supercapacitor, Al-TES (Aquiferous low- temperature – Thermal
energy storage) and HT-TES (High temperature – Thermal energy storage) are developed
technologies. All these EES systems are technically developed and commercially available;
however, the actual applications, especially for large-scale utility, are still not widespread.
Their competitiveness and reliability still need more trials by the electricity industry and
the market.
3. Developing technologies: Fuel cell, Meta-Air battery, Solar Fuel and CES (Cryogenic
Energy Storage) are still under development. They are not commercially mature
although technically possible and have been investigated by various institutions. On the
other hand, these developing technologies have great potential for industrial take up in
the near future. Energy costs and environmental concerns are the main drivers.
Figure 17. Technical maturity of EES systems [69]
9.2. Power rating and discharge time
The power ratings of various EESs are compared in Table 1. Broadly, the EESs fall into three
types according to their applications [69], [74]:
1. Energy management: PHS, CAES and CES are suitable for applications in scales above
100 MW with hourly to daily output durations. They can be used for energy
management for large-scale generations such as load leveling, ramping/load following,
and spinning reserve. Large-scale batteries, flow batteries, fuel cells, CES and TES are
suitable for medium-scale energy management with a capacity of 10–100 MW [55], [69].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-39-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 29
2. Power quality: Flywheel, batteries, SMES, capacitor and supercapacitor have a fast
response (milliseconds) and therefore can be utilized for power quality such as the
instantaneous voltage drop, flicker mitigation and short duration UPS. The typical
power rating for this kind of application is lower than 1 MW [55].
3. Bridging power: Batteries, flow batteries, fuel cells and Metal-Air cells not only have a
relatively fast response (<1 s) but also have relatively long discharge time (hours),
therefore they are more suitable for bridging power. The typical power rating for these
types of applications is about 100 kW–10 MW [55], [74].
Table 1. Comparison of technical characteristics of EES systems [69]
9.3. Storage duration
Table 1 also illustrates the self-discharge (energy dissipation) per day for EES systems. One
can see that PHS, CAES, Fuel Cells, Metal-Air Cells, solar fuels and flow batteries have a
very small self-discharge ratio so are suitable for a long storage period. Lead-Acid, NiCd, Li-
ion, TESs and CES have a medium self-discharge ratio and are suitable for a storage period
not longer than tens of days [69].
NaS, ZEBRA, SMES, capacitor and supercapacitor have a very high self-charge ratio of 10–
40% per day. They can only be implemented for short cyclic periods of a maximum of
several hours. The high self-discharge ratios of NaS and ZEBRA are from the high working
temperature which needs to be self-heating to maintain the use of the storage energy [69].
Flywheels will discharge 100% of the stored energy if the storage period is longer than about
1 day. The proper storage period should be within tens of minutes.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-40-320.jpg)
![Energy Storage – Technologies and Applications
30
9.4. Capital cost
Capital cost is one of the most important factors for the industrial take-up of the EES. They
are expressed in the forms shown in Table 2, cost per kWh, per kW and per kWh per cycle.
All the costs per unit energy shown in the table have been divided by the storage efficiency
to obtain the cost per output (useful) energy [69]. The per cycle cost is defined as the cost per
unit energy divided by the cycle life which is one of the best ways to evaluate the cost of
energy storage in a frequent charge/discharge application, such as load levelling. For
example, while the capital cost of lead-acid batteries is relatively low, they may not
necessarily be the least expensive option for energy management (load levelling) due to
their relatively short life for this type of application. The costs of operation and maintenance,
disposal, replacement and other ownership expenses are not considered, because they are
not available for some emerging technologies [55], [69].
Table 2. Comparison of technical characteristics of EES systems [69]
CAES, Metal-Air battery, PHS, TESs and CES are in the low range in terms of the capital cost
per kWh. The Metal-Air batteries may appear to be the best choice based on their high
energy density and low cost, but they have a very limited life cycle and are still under
development. Among the developed techniques, CAES has the lowest capital cost compared
to all the other systems. The capital cost of batteries and flow batteries is slightly higher than
the break even cost against the PHS although the gap is gradually closing. The SMES,
flywheel, capacitor and supercapacitor are suitable for high power and short duration](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-41-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 31
applications, since they are cheap on the output power basis but expensive in terms of the
storage energy capacity [55], [69].
The costs per cycle kWh of PHS and CAES are among the lowest among all the EES
technologies, the per cycle cost of batteries and flow batteries are still much higher than PHS
and CAES although a great decrease has occurred in recent years. CES is also a promising
technology for low cycle cost. However, there are currently no commercial products
available. Fuel cells have the highest per cycle cost and it will take a long time for them to be
economically competitive. No data have been found for the solar fuels as they are in the
early stage of development [55], [69].
It should also be noted that the capital cost of energy storage systems can be significantly
different from the estimations given here due to, for example, breakthroughs in
technologies, time of construction, location of plants, and size of the system. The
information summarised here should only be regarded as being preliminary.
9.5. Cycle efficiency
The cycle efficiency of EES systems during one charge-discharge cycle is illustrated in Figure
18. The cycle efficiency is the ‘‘round-trip” efficiency defined as ratio between output energy
and input energy. The self-discharge loss during the storage is not considered. One can see
that the EES systems can be broadly divided into three groups:
1. Very high efficiency: SMES, flywheel, supercapacity and Li-ion battery have a very high
cycle efficiency of > 90%.
2. High efficiency: PHS, CAES, batteries (except for Li-ion), flow batteries and conventional
capacitor have a cycle efficiency of 60–90%. It can also be seen that storing electricity by
compression and expansion of air using the CAES is usually less efficient than pumping
and discharging water with PHSs, since rapid compression heats up a gas, increasing its
pressure thus making further compression more energy consuming [55], [69].
3. Low efficiency: Hydrogen, DMFC, Metal-Air, solar fuel, TESs and CES have an efficiency
lower than 60% mainly due to large losses during the conversion from the commercial
AC side to the storage system side. For example, hydrogen storage of electricity has
relatively low round-trip energy efficiency (20–50%) due to the combination of
electrolyser efficiency and the efficiency of re-conversion back to electricity [55], [69].
It must be noted that there is a trade-off between the capital cost and round-trip efficiency,
at least to some extent. For example, a storage technology with a low capital cost but a low
round-trip efficiency may well be competitive with a high cost, high round-trip efficiency
technology.
9.6. Energy and power density
The power density (W/kg or W/litre) is the rated output power divided by the volume of the
storage device. The energy density is calculated as a stored energy divided by the volume.
The volume of the storage device is the volume of the whole energy storage system](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-42-320.jpg)
![Energy Storage – Technologies and Applications
32
Figure 18. Cycle efficiency of EES systems. [69]
including the energy storing element, accessories and supporting structures, and the
inverter system. As can be seen from Table 1, the Fuel Cells, Metal-Air battery, and Solar
fuels have an extremely high energy density (typically 1000 Wh/kg), although, as
mentioned above, their cycle efficiencies are very low. Batteries, TESs, CES and CAES have
medium energy density. The energy density of PHS, SMES, Capacitor/supercapacitor and
flywheel are among the lowest below 30 Wh/kg. However, the power densities of SMES,
capacitor/supercapacitor and flywheel are very high which are, again, suitable for
applications for power quality with large discharge currents and fast responses [69], [74].
NaS and Li-ion have a higher energy density than other conventional batteries. The energy
densities of flow batteries are slightly lower than those of conventional batteries. It should
be noted that there are differences in the energy density of the same type of EES made by
different manufacturers [55].
9.7. Life time and cycle life
Also compared in Table 1 are life time and/or cycle life for various EESs. It can be seen that
the cycle lives of EES systems whose principles are largely based on the electrical
technologies are very long normally greater than 20,000. Examples include SMES, capacitor
and supercapacitor. Mechanical and thermal energy storage systems, including PHS, CAES,
flywheel, AL-TES, CES and HT-TES, also have long cycle lives. These technologies are based
on conventional mechanical engineering, and the life time is mainly determined by the life
time of the mechanical components. The cycle abilities of batteries, flow batteries, and fuel
cells are not as high as other systems due to chemical deterioration with the operating time.
Metal-Air battery only has a life of a few hundred cycles and obviously needs to be further
developed [55], [69].](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-43-320.jpg)
![Techno-Economic Analysis of Different Energy Storage Technologies 33
10. Conclusion
The key element of this analysis is the review of the available energy storage techniques
applicable to electrical power systems.
There is obviously a cost associated to storing energy, but we have seen that, in many cases,
storage is already cost effective. More and more application possibilities will emerge as
further research and development is made in the field [91].
Storage is a major issue with the increase of renewable but decentralized energy sources that
penetrate power networks [93]. Not only is it a technical solution for network management,
ensuring real-time load leveling, but it is also a mean of better utilizing renewable resources
by avoiding load shedding in times of overproduction. Coupled with local renewable
energy generation, decentralized storage could also improve power network sturdiness
through a network of energy farms supplying a specific demand zone.
Many solutions are available to increase system security, but they are so different in terms of
specifications that they are difficult to compare. This is why we tried to bring out a group of
technical and economical characteristics which could help improve performance and cost
estimates for storage systems.
Based on the review, the following conclusions could be drawn [55], [69], [74]:
1. Although there are various commercially available EES systems, no single storage
system meets all the requirements for an ideal EES - being mature, having a long
lifetime, low costs, high density and high efficiency, and being environmentally benign.
Each EES system has a suitable application range. PHS, CAES, large-scale batteries,
flow batteries, fuel cells, solar fuels, TES and CES are suitable for energy management
application; flywheels, batteries, capacitors and supercapacitors are more suitable for
power quality and short duration UPS, whereas batteries, flow batteries, fuel cells and
Metal-Air cells are promising for the bridging power.
2. PHS and Lead-Acid battery are technically mature; CAES, NiCd, NaS, ZEBRA Li-ion,
flow battery, SMES, flywheel, capacitor, Supercapacitor, AL-TES and HT-TES are
technically developed and commercially available; Fuel Cell, Meta-Air battery, Solar
Fuel and CES are under development. The capital costs of CAES, Metal-Air battery,
PHS, TES and CES are lower than other EESs. CAES has the lowest capital cost among
the developed technologies. Metal-Air battery has the potential to be the cheapest
among currently known EES systems.
3. The cycle efficiencies of SMES, flywheel, capacitor/supercapacitor, PHS, CAES,
batteries, flow batteries are high with the cycle efficiency above 60%. Fuel Cell, DMFC,
Metal-Air, solar fuel, TES and CES have a low efficiency mainly due to large losses
during the conversion from commercial AC to the storage energy form.
4. The cycle lives of the EES systems based on the electrical technologies, such as SMES,
capacitor and supercapacitor, are high. Mechanical and thermal based EES, including
PHS, CAES, flywheel, ALTES, CES and HT-TES, also have a long cycle life. The cycle
abilities of batteries, flow batteries, and fuel cells are not as high as other systems due to](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-44-320.jpg)
![Energy Storage – Technologies and Applications
34
chemical deterioration with the operating time. Metal-Air battery has the lowest life
time at least currently.
5. PHS, CAES, batteries, flow batteries, fuel cells and SMES are considered to have some
negative effects on the environment due to one or more of the following: fossil
combustion, strong magnetic field, landscape damage, and toxic remains. Solar fuels
and CES are more environmentally friendly. However, a full life-cycle analysis should
be done before a firm conclusion can be drawn.
Based on the contents of this study and carefully measuring the stakes, we find that:
1. The development of storage techniques requires the improvement and optimization of
power electronics, often used in the transformation of electricity into storable energy,
and vice versa.
2. The rate of penetration of renewable energy will require studies on the influence of the
different storage options, especially those decentralized, on network sturdiness and
overall infrastructure and energy production costs.
3. The study of complete systems (storage, associated transformation of electicity, power
electronics, control systems...) will lead to the optimization of the techniques in terms of
cost, efficiency, reliability, maintenance, social and environmental impacts, etc.
4. It is important to assess the national interest for compressed gas storage techniques.
5. Investment in research and development on the possibility of combining several storage
methods with a renewable energy source will lead to the optimization of the overall
efficiency of the system and the reduction of greenhouse gases created by conventional
gas-burning power plants.
6. Assessing the interest for high-temperature thermal storage systems, which have a huge
advantage in terms of power delivery, will lead to the ability of safely establish them
near power consumption areas,
7. The development of supercapacitors will lead to their integration into the different
types of usage.
8. The development of low-cost, long-life flywheel storage systems will lead to increased
potential, particularly for decentralized applications.
9. To increase the rate of penetration and use of hydrogen-electrolysor fuel-cell storage
systems, a concerted R&D effort will have to be made in this field.
Author details
Hussein Ibrahim
TechnoCentre éolien, Gaspé, QC, Canada
Adrian Ilinca
Université du Québec à Rimouski, Rimouski, QC, Canada
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[81] Bonneville JM. Stockage cinétique. Institut de Recherche de l’Hydro-Québec, Varennes,
Québec, Canada. Rapport interne; février 1975.
[82] IEEE 1547 Standard for Interconnecting Distributed Resources with Electric Power
Systems. Approved by the IEEE Standards Board in June 2003. Approved as an
American National Standard in October 2003.
[83] Faure F. Suspension magnétique pour volant d’inertie. Thèse de doctorat. Institut
National Polytechnique de Grenoble,France;Juin 2003.
[84] Alaa Mohd, Egon Ortjohann, Andreas Schmelter, Nedzad Hamsic, Danny Morton.
Challenges in integrating distributed Energy storage systems into future smart grid.
IEEE International Symposium on Industrial Electronics, June 30 - July 2, 2008.
[85] Rudell A. Storage and Fuel Cells. EPSRC SuperGen Workshop : Future Technologies for
a Sustainable Electricity System. Univ. of Cambridge, 7 november2003.
[86] Lead/acid batteries, University of Cambridge, 15th October 2007,
http://www.doitpoms.ac.uk/tlplib/batteries/batteries_lead_acid.php
[87] Sergio Vazquez, Srdjan M. Lukic, Eduardo Galvan, Leopoldo G. Franquelo, Juan M.
Carrasco, Energy Storage Systems for Transport and Grid Applications, IEEE
TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 12, DECEMBER
2010, Pages. 3881-3895.
[88] www.cea.fr
[89] http://www.power-thru.com/flywheel_detail.html
[90] Anzano JP, Jaud P, Madet D. Stockage de l'électricité dans le système de production
électrique. Techniques de l'ingénieur, traité de Génie Électrique D4030; 1989.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-50-320.jpg)
![Energy Storage – Technologies and Applications
40
[91] Multon B, Ruer J. Stocker l’électricité : Oui, c’est indispensable, et c’est possible !
pourquoi, où, comment?. Publication ECRIN en contribution au débat national sur
l’énergie; Avril 2003.
[92] Robyns B. Contribution du stockage de l’énergie électrique à la participation au services
système des éoliennes. Séminaire SRBE – SEE – L2EP « Éolien et réseaux : enjeux », 22
mars 2005.
[93] Multon B. L’énergie électrique : analyse des ressources et de la production. Journées
section électrotechnique du club EEA. Paris, France, 28-29 janvier 1999, p. 8.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-51-320.jpg)
![Energy Storage – Technologies and Applications
42
wind speed data of that location for detailed analysis. Figure 1 shows the daily load profile
(summer: January 01, 2009 and winter: July 01 2009) of Capricornia region of Rockhampton,
a regional city in Australia. Ergon Energy [1] is the utility operator in that area. However
load demand of the residential load in that area depends on the work time patter which is
different than the load profile of Figure 1. Overall electricity demand is very high in the
evening and also in the morning for the residential load however PV generates electricity
mostly during the day time therefore residents need to purchase costly electricity during
peak demand in the evening. Similarly wind energy also unable to follow the residential
load profile. Therefore properly estimated storage needs to be integrated to overcome this
situation.
Figure 1. Daily load profile of Capricornia region in 2009 (Summer & Winter)
This chapter explores the need of storage systems to maximize the use of RE, furthermore
estimates the required capacity of storage to meet the daily need which will gradually
eliminate the dependency on conventional energy sources. Estimation of storage sizing is
explained in section 3. This chapter also conducts the feasibility assessment of storage in
terms of economic and environmental perspective which is explained in section 4.
2. Background
Solar and Wind are the two major sources of RE. Australia is one of best places for these
sources. In regional areas of Australia, roof top Solar PV is installed in many residential
houses either in off-grid or grid connected configurations and most residential wind turbine
are for specific applications in off-grid configuration. In grid connected solar PV systems
where storage is not integrated, the energy output from this system does not satisfy to the
desired level. Currently installed most of the residential PV systems are designed in an](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-53-320.jpg)
![Estimation of Energy Storage and Its Feasibility Analysis 43
unplanned way that even with battery integrated system is not able to support the load in
reliable way. Figure 2 illustrates a typical situation when whole system in jeopardize as the
estimation of storage system was not done correctly.
Figure 2. Typical condition of failure in storage integrated RE system
The adoption of storage with the PV system certainly incurs additional cost to the system but
the benefits of adding storage has not been clearly assessed. Therefore this chapter aims to
achieve two objectives. One is to estimate the required storage for the grid connected PV
system or grid connected wind turbine or combination of grid connected PV and wind turbine
system to achieve the maximum daily use of RE. Second objective is to identify the effects of
storage on the designed system in terms of environment and economic by comparing the same
system with and without storage. The feasibility of the designed system is expressed as, the
Cost of Energy (COE) is closer to the present system while providing environmental benefits
by reducing Greenhouse Gas (GHG) emission and improving the Renewable Fraction (RF).
Data was collected for the Capricornia region of Rockhampton city in Queensland,
Australia. Load data was collected for a 3 bed room house by estimating all the electrical
appliances demand and average usage period considering its ratings. Daily load profile
drawn from hourly load data and total daily load was estimated by calculating the area
under the daily load profile curve using trapezoidal method. Weather data was collected for
the year 2009 from [2] for this location and calculated the energy output from PV array and](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-54-320.jpg)
![danaro, com'era solito in simili frangenti di fare), ma l'esarco
Romano non gliel voleva permettere: del che si duol egli forte
coll'arcivescovo suddetto. E tanto più, perchè essendo stato
rinforzato Ariolfo dalle soldatesche di due altri condottieri di armi,
Autari e Nordolfo, difficilmente volea più dar orecchio a trattati di
pace. Pertanto il prega che se ha luogo di parlar di tali affari con sì
strambo ministro, cerchi di condurlo alla pace, con ricordargli
specialmente che s'era levato di Roma il nerbo maggiore delle
milizie, per sostenere l'occupata Perugia, come egli deplora altrove
[Gregorius M., lib. 5, ep. 40.], nè vi era restata altra guarnigione che il
reggimento teodosiano, così appellato da Teodosio Augusto, figliuolo
di Maurizio imperadore, il quale ancora, per essere privo delle sue
paghe, stentava ad accomodarsi alla guardia delle mura. Aggiugne
che anche Arichi, ossia Arigiso duca di Benevento, il quale era
succeduto a Zottone primo duca di quella contrada, instigato da
Ariolfo, rotte le capitolazioni precedenti, avea mosse le sue armi
contra de' Napoletani, e minacciava quella città.
Non si doveano credere i Longobardi obbligati ad alcun trattato
precedente, da che l'esarco sotto la buona fede aveva occupato ad
essi Perugia con altre città. Paolo Diacono [Paulus Diaconus, lib. 4, cap.
19.] parla della morte di Zottone suddetto dopo venti anni di ducato,
con dire che in suo luogo succedette Arigiso, mandato colà dal re
Agilolfo, e per conseguente o in questo o nel precedente anno, con
intendersi da ciò che il ducato beneventano dovette aver principio
circa l'anno 571, come pensò il padre Antonio Caracciolo. Era Arigiso
nato nel Friuli, avea servito d'ajo a' figliuoli di Gisolfo duca del Friuli,
ed era parente del medesimo Gisolfo. Risulta poi dalla suddetta
lettera di san Gregorio all'arcivescovo di Ravenna, che la città di Fano
era posseduta allora dai Longobardi, e vi si trovavano molti fatti
schiavi, per la liberazion de' quali aveva il caritativo papa voluto
inviare nel precedente anno una persona con danaro; ma questa
non si era arrischiata di passare pel ducato di Spoleti, che divideva
Roma da quella città ed era sotto il dominio de' Longobardi. Tuttavia
non lasciò Fortunato, vescovo d'essa città, di riscattarli, con
aggravarsi di molti debiti per questa santa azione [Greg. Magnus, lib. 7,](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-56-320.jpg)
![epist. 13.]; e san Gregorio gli concedette dipoi che potesse vendere i
vasi sacri delle chiese per pagare i creditori. Quel Severo vescovo
scismatico, la cui città era stata bruciata, e per cui l'arcivescovo di
Ravenna chiedeva delle limosine a san Gregorio, vien creduto
vescovo di Aquileja dal cardinal Baronio [Baron., Annal. Eccl.] e dal
padre Mabillone [Mabill., in Annal. Bened., lib. 8, cap. 37.]. Io il tengo per
Severo vescovo d'Ancona, nominato altrove da san Gregorio, giacchè
egli dice: Juxta quippe est civitas Fanum: il che non conviene nè a
Grado nè ad Aquileja. Nell'edizione di san Gregorio fatta da' padri
Benedettini, la lettera sedicesima del libro nono [Greg. M., lib. 9, ep. 16,
edition. Bened.] è ad Serenum anconitanum episcopum. Si ha da
leggere ad Severum, apparendo ciò dalla susseguente lettera
ottantesima nona [Idem, ibid. epist. 89.]. Dovea questo vescovo,
addottrinato dalle disgrazie della sua città, avere abbandonato lo
scisma e meritata la grazia di san Gregorio.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-57-320.jpg)
![Anno di
Cristo DXCIII. Indizione XI.
Gregorio I papa 4.
Maurizio imperadore 12.
Agilolfo re 3.
L'anno X dopo il consolato di Maurizio Augusto.
Ci fa sapere Paolo Diacono, che irritato forte il re Agilolfo per la
perdita di Perugia e dell'altre suddette città, si mosse
immediatamente da Pavia con un possente esercito per riacquistare
quella città. E però potrebbe essere che appartenesse al precedente
anno questo suo sforzo. Ma non parlando punto san Gregorio di
Agilolfo nelle lettere scritte in quell'anno, nè essendo molto esatto
nell'ordine dei tempi lo storico suddetto, chieggo licenza di poter
riferire al presente anno l'avvenimento suddetto. Venne dunque il
bellicoso re con grandi forze all'assedio di Perugia, e con tal vigore
sollecitò quell'impresa, che tornò alle sue mani essa città, e Maurizio
preso pagò colla sua testa il tradimento fatto. Come poi e quando
Perugia tornasse in poter dei Romani, nol so. Certo è che vi tornò.
Par ben credibile che Agilolfo ricuperasse ancora l'altre città a lui
tolte dall'esarco. Nè questo gli bastò. Volle anche tentare Roma
stessa: al che non fece mente Paolo Diacono, allorchè scrisse, che
dopo la presa di Perugia Agilolfo se ne tornò a Pavia. Racconta il
santo pontefice [Gregor. M., Praefat. lib. 2, in Ezechi.] ch'egli era dietro a](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-58-320.jpg)
![spiegare al popolo il capitolo quarantesimo di Ezechiello, allorchè
s'intese jam Agilulphum Longobardorum regem, ad obsidionem
nostram summopere festinantem, Padum transisse. E che seguissero
dipoi dei gran travagli e danni al popolo romano, si raccoglie da
quanto seguita appresso a dire il medesimo san Gregorio [Paulus
Diaconus, lib. 4, cap. 8.]: Ubique luctus aspicimus. Ubique gemitus
audivimus; destructae urbes, eversa sunt castra, depopulati sunt
agri, in solitudinem terra redacta est. Alios in captivitatem duci, alios
detruncari, alios interfici videmus. Aggiugne più sotto [Greg. M., Homil.
6, lib. 2.]: Nemo autem me reprehendat, si post hanc locutionem
cessavero, quia, sicut omnes cernitis, nostrae tribulationes
excreverunt. Undique gladio circumfusi sumus, undique imminens
mortis periculum timemus. Alti detruncatis ad nos manibus redeunt;
alii captivi, alii interemti ad nos nuntiantur. Jam cogor linguam ab
expositione retinere. E queste parole son quelle che fecero dire a
Paolo Diacono [Idem, lib. 2, Homil. ultim.], il qual sembra discorde da sè
medesimo, essere rimasto sì atterrito il beato Gregorio papa
dall'arrivo del re Agilolfo, che cessò dal proseguire la spiegazion del
testo di Ezechiello. Crede il cardinal Baronio che questi guai di Roma
succedessero nell'anno 595, quando tutte le apparenze sono che
molto prima arrivasse un sì atroce flagello addosso a quella città. Ed
è fuor di dubbio che Roma, tuttochè guernita d'un debolissimo
presidio, valorosamente si difese in quelle strettezze, di modo che il
re Agilolfo, scorgendo la difficoltà dell'impresa, fors'anche
segretamente commosso dalle preghiere e dai regali, che a tempo
opportuno soleva impiegare per bene del suo popolo il generoso
papa Gregorio, si ritirò da quei contorni, e dopo tanti danni inferiti
lasciò in pace i Romani. Mancò di vita in quest'anno uno dei re
franchi, cioè Guntranno re della Borgogna, principe per la pietà e per
altre virtù assai commendato. Perchè in questi tempi non si durava
gran fatica a canonizzare gli uomini, e specialmente i principi
dabbene per santi, però anche a lui toccò d'essere messo in quel
ruolo. Morì senza figliuoli, e lasciò tutti i suoi stati al re di Austrasia
Childeberto, la cui potenza con una sì gran giunta divenne
formidabile. E buon pei Longobardi che neppur egli sopravvivesse di
molto a questo suo zio.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-59-320.jpg)
![Anno di
Cristo DXCIV. Indizione XII.
Gregorio I papa 5.
Maurizio imperadore 13.
Agilolfo re 4.
L'anno XI dopo il consolato di Maurizio Augusto.
Credesi che nell'anno precedente san Gregorio papa prendesse a
scrivere i suoi Dialoghi; ma c'è anche motivo di giudicare che ciò
succedesse nell'anno presente, scrivendo egli [Gregor. Magnus, Dialog.,
lib. 3, cap. 19.] che cinque anni prima era seguita la fiera innondazione
del Tevere. Manteneva intanto il santo pontefice buona
corrispondenza con Teodelinda regina dei Longobardi, principessa
piissima e bene attaccata alla religione cattolica: il che giovò non
poco per rendere il re Agilolfo suo consorte, benchè ariano, ben
affetto e favorevole ai Cattolici stessi, e servì in fine, siccome diremo,
ad abbracciare la stessa fede cattolica, se pur sussiste ciò che ne
lasciò scritto Paolo Diacono. Era stato eletto arcivescovo di Milano
Costanzo; e perchè si sparse voce ch'egli avesse condannati i tre
capitoli del concilio calcedonense, ed accettato il concilio quinto, tre
vescovi suoi suffraganei, fra' quali specialmente quello di Brescia,
non solamente si separarono dalla di lui comunione, ma eziandio
indussero la regina a fare lo stesso. Restano due lettere scritte da
san Gregorio [Idem, lib. 4, ep. 4, et 38.] alla medesima regina, nelle quali](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-61-320.jpg)
![si duole ch'ella si sia lasciata sedurre, quasi la dottrina del concilio
calcedonense, principalmente sostenuta dalla Chiesa romana, avesse
patito alcun detrimento per le persone condannate dipoi nel quinto
concilio generale. Da altre lettere del medesimo papa pare che si
raccolga essersi Teodelinda umilmente accomodata alle di lui
esortazioni. Ma veggasi all'anno 604. Abbiamo anche da Paolo
Diacono [Paulus Diaconus, lib. 4, cap. 5.] che a questa buona principessa
san Gregorio, non si sa quando, inviò in dono i Dialoghi suddetti.
Una delle maggiori premure, che circa questi tempi nudriva
l'infaticabil pontefice, era quella di stabilir la pace coi Longobardi. A
così lodevol pensiero chi s'opponesse lo vedremo nell'anno seguente,
contuttochè io non lasci di sospettare che possa tal pace
appartenere all'anno presente, non essendo noi certi che tutte le
lettere di san Gregorio papa sieno disposte con ordine esattissimo di
tempo. Comunque sia, in una lettera scritta da esso papa sotto
l'indizione duodecima, cioè sotto quest'anno, al sopra citato Costanzo
arcivescovo di Milano, si vede che il ringrazia delle nuove dategli del
re Agone (così ancora veniva chiamato, siccome già accennai, il re
Agilolfo) e dei re de' Franchi, e desidera d'essere informato di tutto
altro che possa accadere. Dice in fine una particolarità degna
d'attenzione nelle seguenti parole, cioè: Se vedrete che Agone re de'
Longobardi non possa accordarsi col patrizio (ossia con Romano
esarco), fategli sapere che si prometta meglio di me, perchè son
pronto a spendere, s'egli vorrà consentire in qualche partito
vantaggioso al romano imperio. Desiderava Gregorio che seguisse la
pace generale, e perchè ciò venisse effettuato, si esibiva a pagare; e
quando poi non si potesse concludere questa general pace,
proponeva di farla almeno col ducato romano, per non vedere più
esposto alle miserie della guerra il popolo, ch'egli più degli altri era
tenuto ad amare. Sono di parere i padri Benedettini, nella edizione di
san Gregorio, che a quest'anno appartenga una lettera del
medesimo santo papa [Gregor. Magnus, lib. 4, ep. 47.] scritta a Sabiniano
suo apocrisario, ossia nunzio alla corte di Costantinopoli, con
ordinargli di dire ai serenissimi nostri padroni, che se Gregorio lor
servo si fosse voluto mischiare nella morte dei Longobardi, oggidì la
nazione longobarda non avrebbe nè re, nè duchi, nè conti, e si](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-62-320.jpg)
![Anno di
Cristo DXCV. Indizione XIII.
Gregorio I papa 6.
Maurizio imperadore 14.
Agilolfo re 5.
L'anno XII dopo il consolato di Maurizio Augusto.
Non cessava il santo pontefice Gregorio di far delle premure
perchè si venisse ad una pace fra l'imperio e i Longobardi, sì perchè
avea troppo in orrore gl'infiniti disordini prodotti dalla guerra, e sì
perchè toccava con mano la debolezza dell'imperio stesso, che non
poteva se non perdere continuando la discordia. Ora egli a tal fine
scrisse in questo anno a Severo, scolastico (cioè consultore)
dell'esarco [Gregor. Magnus, lib. 5, ep. 36.], con fargli sapere che Agilolfo
re de' Longobardi non ricusava di fare una pace generale, purchè
l'esarco volesse emendare i danni a lui dati, prima che fosse venuta
l'ultima rottura, esibendosi anch'egli pronto a fare lo stesso, se i suoi
nel tempo della pace aveano danneggiato le terre dell'imperio. Però
il prega di adoperarsi, acciocchè l'esarco acconsenta alla pace; che
per altro Agilolfo si mostrava anche disposto a stabilirla coi soli
Romani. Oltre a ciò, avvertisce l'esarco che varii luoghi ed isole erano
in pericolo manifesto di perdersi; e però s'affrettasse ad abbracciar la
proposta concordia, per poter avere un po' di quiete, e mettersi
intanto in forze da poter meglio resistere. Ma l'esarco Romano era](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-64-320.jpg)
![della razza di coloro che antepongono il proprio vantaggio a quello
del pubblico. Se la guerra recava immensi mali alla misera Italia,
fruttava ben di molti guadagni alla borsa sua. E perciò non
solamente abborriva la pace, ma giunse infino a caricar di calunnie il
santo pontefice alla corte, in maniera che circa il mese di giugno
Maurizio Augusto scrivendo ad esso papa e ad altri delle lettere, il
trattò da uomo semplice e poco accorto, quasichè si lasciasse burlare
da Ariolfo duca di Spoleti con varie lusinghe di pace, ed avesse
rappresentato alla corte o all'esarco delle cose insussistenti. Chi
legge la lettera scritta in questo proposito dall'incomparabil
pontefice, non può di meno di non ammirare e benedire la singolar
sua umiltà e la destrezza, con cui seppe sostenere il suo decoro, e
nello stesso tempo non mancar di rispetto a chi era principe
temporale di Roma. Duolsi egli, fra l'altre cose, che sia stata rotta
dagli uffiziali cesarei la pace da lui stabilita coi Longobardi della
Toscana, mercè dell'occupazion di Perugia: poscia dopo la rottura,
che sieno stati levati di Roma i soldati ivi soliti a stare di presidio, per
guernire Narni e Perugia, lasciando in tal guisa abbandonata ed
esposta a pericoli di perdersi quell'augusta città. Aggiugne essere
stata la piaga maggiore l'arrivo di Agilolfo, perchè si videro tanti
miseri Romani legati con funi al collo a guisa di cani, e condotti a
vendere in Francia, dove dovea praticarsi un gran mercato di schiavi,
benchè cristiani. Tali parole fecero credere al Sigonio [Sigon., de Regn.
Ital., lib. 1.] che l'assedio di Roma fatto da Agilolfo s'abbia da riferire
all'anno precedente 594, e non è dispregevole la di lui conghiettura,
quantunque a me sembri più probabile che quel fatto succedesse
prima. Si lagna ancora il buon papa che dopo essere i Romani
scampati da quel fiero turbine, si voglia ancora crederli colpevoli per
la scarsezza del frumento, in cui si trovava allora la città, quando
s'era già rappresentato alla corte che non si potea lungo tempo
conservare in Roma una gran provvisione di grano. E sofferiva bene
esso papa con pazienza tante contrarietà; ma non sapeva già
digerire che gli Augusti padroni fossero in collera contra di Gregorio
prefetto di Roma, e di Castorio generale delle milizie, che pure
aveano fatto de' miracoli nella difesa della città.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-65-320.jpg)
![Di questo passo andavano allora gli affari d'Italia con un principe
che vendeva le cariche, che credeva più ai cattivi che ai buoni
consiglieri, e sceglieva ministri malvagi, i quali venivano in Italia, non
per far del bene ai popoli, ma per ismugnere il loro sangue. Di
questo ne abbiam la testimonianza dello stesso san Gregorio in una
lettera scritta a Costantina Augusta moglie dell'imperadore Maurizio
[Greg. Magnus, lib. 5, ep. 41.], dove le significa d'aver convertito alla fede
molti gentili che erano nell'isola di Sardegna, e scoperto in tal
congiuntura che costoro pagavano dianzi un tanto al governatore per
aver licenza di sagrificare agl'idoli; e che anche dopo la lor
conversione seguitava il governatore a voler che pagassero. Ripreso
dal vescovo per tale avania, avea risposto d'aver promesso alla corte
tanto danaro per ottener quella carica, e che neppur questo bastava
per soddisfare al suo impegno. Nella Corsica poi tante erano le
gravezze, che gli abitanti per pagarle erano costretti fino a vendere i
proprii figliuoli, di maniera che moltissimi, i quali possedevano beni
in quell'isola, erano forzati a ricoverarsi sotto il dominio della
nefandissima nazion dei Longobardi, la quale dovea trattar meglio i
sudditi suoi, e superava nel buon governo i Greci. Così in Sicilia eravi
un esattore imperiale per nome Stefano, che senza processo
confiscava a più non posso i beni di que' possidenti. Peggio
nondimeno che gli altri operava Romano patrizio, esarco di Ravenna.
Con tutta la sua umiltà e pazienza il santo pontefice Gregorio non
potè di meno di non accennare a Sebastiano vescovo del Sirmio
[Greg. Magnus, ep. 42.], amico d'esso esarco, le oppressioni che Roma
pativa per l'iniquità di costui. Breviter dico (sono sue parole) quia
ejus in nos malitia gladios Longobardorum vicit, ita ut benigniores
videantur hostes, qui nos interimunt, quam reipublicae judices, qui
nos malitia sua, rapinis atque fallaciis in cogitatione consumunt.
Eppure i soli Longobardi erano trattati da nefandissimi. Venne a
morte in quest'anno Giovanni arcivescovo di Ravenna, e in suo luogo
fu eletto Mariniano, a cui papa Gregorio concedette il pallio.
Rapporta eziandio Girolamo Rossi [Rubeus, Hist. Ravenn., lib. 4.] una
bolla di papa Gregorio, confirmatoria de' privilegii della chiesa
ravennate; ma che contien troppe difficultà per crederla vera. Il
cardinal Baronio [Baronal. An. Eccl.] ne ha mostrata la falsità. Passò](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-66-320.jpg)
![ancora a miglior vita san Gregorio vescovo Turonense, insigne
storico delle Gallie. Circa questi tempi fu creato duca di Baviera
Tassilone da Childeberto re dell'Austrasia. Egli è chiamato re della
Baviera da Paolo Diacono [Paulus Diaconus, lib. 4, cap. 7.] e da Sigeberto
[Sigebertus, in Chron.] copiatore d'esso Paolo. Ma niun d'essi e niuna
delle memorie antiche ci fa sapere cosa divenisse di Garibaldo duca
o re d'essa Baviera, padre, siccome dicemmo, di Teodelinda regina
de' Longobardi. Credesi che egli terminasse il corso de' suoi giorni,
oppure che Childeberto sovrano della Baviera, a cagion dell'alleanza
da lui contratta per via del matrimonio suddetto coi re longobardi, e
da lui mal veduta, gli movesse guerra e il deponesse. Si sa ch'egli
ebbe un figliuolo per nome Gundoaldo, che venne in Italia colla
sorella Teodelinda, e questi, per attestato di Fredegario [Fredegar., in
Chron., cap. 34.], si accasò con una donna nobile di nazion longobarda,
e n'ebbe de' figliuoli. Avremo occasione di parlare di questi principi
più abbasso. Nè vo' lasciar di dire che in questi tempi l'umile
pontefice romano ebbe da combattere colla superbia di Giovanni il
Digiunatore, patriarca di Costantinopoli, il quale voleva attribuirsi il
titolo di vescovo ecumenico ossia universale. A questa usurpazione
egli si oppose con tutta forza e mansuetudine. Ne scrisse a lui
[Gregor. Magnus, lib. 5, epist. 21.], all'imperadore, e a Costantina
imperadrice, dolendosi specialmente con quest'ultima, perchè si
permettesse che fosse maltrattata la Chiesa romana, capo di tutte.
Dice, fra le altre cose, in essa lettera, essere già ventisett'anni che i
Romani viveano fra le spade dei Longobardi (prendendo le afflizioni
dell'Italia dall'anno 568, in cui i Longobardi vi entrarono), e che la
Chiesa romana avea fatto e faceva di grandi spese della propria
borsa per regalare essi Longobardi, e salvare con tal mezzo il suo
popolo: di modo che siccome l'imperadore teneva in Ravenna il suo
tesoriere e spenditore per pagare l'esercito, così esso papa era
divenuto spenditore in Roma, con impiegar nello stesso tempo le sue
rendite in mantenimento del clero, de' monisteri e de' poveri, e in
placare essi Longobardi. Contuttociò si vedeva questa deformità, che
la Chiesa romana era astretta a sofferir tali strapazzi dall'ambizion
del vescovo di Costantinopoli. Ma Giovanni digiunatore finì in
quest'anno medesimo la lite col fine della sua vita: uomo per altro](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-67-320.jpg)
![Anno di
Cristo DXCVI. Indizione XIV.
Gregorio I papa 7.
Maurizio imperadore 15.
Agilolfo re 6.
L'anno XIII dopo il consolato di Maurizio Augusto.
Si andava tuttavia maneggiando l'affare della pace tra il re
Agilolfo e l'esarco di Ravenna. Ma perciocchè non mancavano
persone che per privati riguardi attraversavano il pubblico bene, s.
Gregorio [Gregor. Magnus, lib. 6, ep. 30 et 31.] diede incumbenza a
Castorio suo notaio residente in Ravenna di sollecitar questo
aggiustamento, senza il quale soprastavano dei gravi pericoli a Roma
stessa e a diverse isole. Ma in Ravenna da gente maligna fu di notte
attaccato alle colonne un cartello in discredito, non solo del suddetto
Castorio, ma del medesimo papa, quasichè per fini storti amendue
promovessero l'affare di essa pace. S. Gregorio ne scrisse a
Mariniano arcivescovo, al clero, ai nobili, ai soldati e al popolo di
quella città, con ordinare che pubblicassero la scomunica contra gli
autori d'esso cartello. Nella Campania dovette esser guerra in questo
anno, ed in essa furono presi molti Napoletani dai Longobardi. Non
fu pigro il pietoso cuore del pontefice romano a scrivere tosto ad
Antemio suddiacono, suo agente in Napoli [Idem, ib., ep. 35.], con
inviargli una buona somma di danaro per riscattare chiunque non](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-69-320.jpg)
![avea tanto da potere ricuperare la libertà. In quest'anno ancora
l'infaticabil papa prese la gloriosa risoluzione di spedire in Inghilterra
s. Agostino monaco del monistero di s. Andrea di Roma, con altri
compagni, a fin di convertire alla fede di Cristo gli Anglo-Sassoni,
Barbari che da gran tempo aveano occupata la maggior parte della
Bretagna maggiore. Questa memorabil impresa è una di quelle, per
le quali il santo pontefice specialmente si acquistò il titolo di grande,
e quello ancora di apostolo dell'Inghilterra, titolo parimente dato al
medesimo Agostino, che fu creato primo arcivescovo di Cantuaria, e
fece delle maraviglie per ridurre que' popoli alla greggia di Cristo.
Riferisce Beda [Beda, Hist. Angl., lib. I, cap 23.] una lettera di s. Gregorio
papa, rapportata anche da Gotselino [Gotselinus, in Vita S. August.
Cantuar. n. 7 et 8.] nella vita del suddetto s. Agostino, e scritta die X
kalendas augusti, imperante D. N. Mauricio Tiberio piissimo Augusto,
anno XIV post consulatum ejusdem domini nostri anno XIII,
Indictione XIV. Leggonsi le medesime note cronologiche in un'altra
lettera del medesimo papa ad Eterio vescovo, oppure a Virgilio
vescovo, o ad altri (il che poco importa), riferita dal medesimo
Gotselino. Ora queste indicano precisamente il presente anno,
perchè nel dì 25 luglio dell'anno 596 correva tuttavia l'anno
quattordicesimo dell'imperio di Maurizio, e l'indizione
quattordicesima. E perciocchè in questo tempo concorre l'anno
decimoterzo dopo il consolato di esso Augusto, si viene a conoscere
aver io fondatamente messo il consolato di Maurizio nell'anno 583,
contro il parere del padre Pagi. Seguì nell'anno presente la morte
ben frettolosa di Childeberto II, potentissimo re dell'Austrasia e della
Borgogna, che avea recato tanti fastidii ai Longobardi e tanti danni
alla Italia. Non avea più di venticinque o ventisei anni d'età; ed
essendo pur morta nello stesso giorno, o poco dopo, la regina
Faileuba sua moglie, fu creduto che amendue fossero portati via dal
veleno; ed alcuni scrittori moderni ne han fatto cadere il sospetto
sopra la regina Brunechilde sua madre, principessa che nulla
trascurò per regnare. Ma nulla di ciò dicendone gli antichi, niun
fondamento v'ha di questa diceria. Lasciò due figliuoli piccioli,
Teodeberto re dell'Austrasia, e Teoderico re della Borgogna. Abbiamo
da Paolo Diacono [Paulus Diaconus, lib. 4, cap. 11 et 14.] che il re Agilolfo](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-70-320.jpg)
![mandò, non si sa in qual anno, ambasciatori ad esso re Teoderico, o,
per dir meglio, alla suddetta regina Brunechilde, che come tutrice de'
nipoti governava gli stati, e stabilì una pace perpetua con esso.
Racconta il medesimo storico che circa questi tempi si videro per la
prima volta in Italia de' cavalli selvatici e de' bufali, che erano
riguardati per maraviglia dagl'Italiani. E perciocchè Romano esarco
era pertinace in non voler la pace, apprendiamo da una lettera di san
Gregorio [Gregor. Magnus, lib. 4, ep. 60.] ad Eulogio patriarca
d'Alessandria, che i Romani pagavano la pena dell'iniquità di costui,
scrivendo egli con sommo dolore, che non passava giorno senza
qualche saccheggio, o morti, o ferite di quel popolo a cagion della
guerra coi Longobardi. Da un'altra lettera del medesimo santo
pontefice, scritta a Teottista patrizia [Idem, lib. 7, ep. 26.], ricaviamo
che in questo anno essi Longobardi condotti o spediti da Arichi, ossia
da Arigiso duca di Benevento, presero la città di Crotone, oggidì
Cotrone nella Calabria ulteriore, e condussero via schiavi molti
uomini e donne, pel riscatto dei quali si affaticò la non mai stanca
carità di questo inclito papa. Non apparisce che i Longobardi si
mantenessero in quella città, troppo esposta alle forze marittime de'
Greci.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-71-320.jpg)
![Gennadio patrizio ed esarco dell'Africa [Gregor. Magnus, lib. 4, ep. 3.], gli
raccomandasse in quest'anno di vegliare alla sicurezza dell'isola di
Corsica, sottoposta al governatore dell'Africa, perchè temeva di uno
sbarco dei Longobardi in quell'isola e nella vicina Sardegna, come in
fatti da lì a non molto accadde. Abbiamo poi da Teofilatto [Theophilact.
l. 8, cap. 11.] che verisimilmente nell'anno presente caduto infermo
Maurizio Augusto, fece testamento, in cui lasciò l'imperio d'Oriente a
Teodosio Augusto, il maggiore de' suoi figliuoli, e l'Italia colle isole
adiacenti a Tiberio suo figliuolo minore. Egli poi si riebbe da quel
malore. Quanto meglio avrebbe egli operato se avesse inviato in
Italia questo suo secondogenito! Sarebbe stata in salvo la di lui vita:
e forse la presenza di questo principe avrebbe rimesso in migliore
stato gli affari d'Italia. Non so dire se intorno a questi tempi
terminasse i suoi giorni in Ravenna Romano patrizio ed esarco, uomo
nemico della pace, e che pescava meglio nel torbido. Pare che si
possa ricavare da un'epistola di s. Gregorio [Greg. Magnus, lib. 7, ep.
29.], che venisse in quest'anno a Ravenna Callinico suo successore,
personaggio di massime più diritte e più riverente verso il santo
pontefice Gregorio. Certo è solamente che esso esarco si trova in
Ravenna nell'anno 599. Negli Atti de' santi [Acta Sanctorum Bolland. ad
diem 13 junii.], raccolti ed illustrati dal padre Bollando e da' suoi
successori della Compagnia di Gesù, abbiamo la vita di s. Ceteo
vescovo di Amiterno, città florida una volta, ed oggidì distrutta, dalle
cui rovine nacque la moderna città dell'Aquila, distante cinque miglia
di là. Ivi è detto ch'egli era vescovo di quella città ai tempi di s.
Gregorio il grande e di Faroaldo duca di Spoleti, nel cui ducato era
compreso Amiterno. Furono deputati al governo di essa terra due
Longobardi ariani, come erano i più di questa nazione, chiamati Alais
ed Umbolo. Per la lor crudeltà Ceteo vescovo se ne fuggì a Roma, e
fu a trovare il santo papa Gregorio. Richiamato dal popolo alla sua
residenza, godeva egli quiete e pace, quando Alais inviperito contro
del compagno, mandò segretamente a Veriliano conte d'Orta, città
che doveva essere allora in poter dei Greci, acciocchè venisse una
notte alla distruzion di Amiterno. Andarono gli Ortani; ma scoperto a
tempo il lor tentativo, furono ripulsati. Alais restò convinto del
tradimento, e perchè il vescovo Ceteo volle salvargli la vita, fu](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-73-320.jpg)
![Anno di
Cristo DXCVIII. Indizione I.
Gregorio I papa 9.
Maurizio imperadore 17.
Agilolfo re 8.
L'anno XV dopo il consolato di Maurizio Augusto.
Da una lettera [Greg. Magnus, lib. 8, ep. 18.] scritta in questo anno da
s. Gregorio ad Agnello vescovo di Terracina, si ricava, che tuttavia
restavano in quella città delle reliquie del paganesimo, le quali il
santo papa procurò di schiantare. A questo fine si raccomandò
ancora a Mauro visconte d'essa città, acciocchè assistesse col braccio
secolare alle diligenze del vescovo. Ordinò nello stesso tempo che
niuno fosse esentato dal far le guardie alla città: al che ne' bisogni
erano tenuti anche gli ecclesiastici; e che neppure i monaci
godessero esenzione da questo peso, si raccoglie da un'altra lettera
dello stesso pontefice [Idem, lib. 9, ep. 73.]. Questo ci fa vedere che
continuasse la guerra, e fin dove arrivassero in questi tempi le
scorrerie dei Longobardi. Riconosce egli dipoi [Idem, lib. 8, ep. 22.]
l'essersi da tanto tempo preservata essa città dal cadere in mano de'
nemici suddetti dalla protezion del principe degli apostoli s. Pietro,
giacchè quella città si trovava allora senza gran popolo e senza
guarnigione, almen sufficiente, di soldati. Il nome di visconte, che
abbiam veduto poco fa, vuol che io ricordi qui come in questi secoli](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-75-320.jpg)
![era in uso, e questo durò molti secoli dipoi, che i governatori d'una
città erano appellati comites, conti. Aveano questi il loro
luogotenente, chiamato perciò vicecomes, che nella lingua volgare
italiana passò in viceconte, e finalmente in visconte. Dalle parole di
s. Gregorio sovraccitate si raccoglie che nelle città tuttavia soggette
all'imperio vi doveva essere il visconte, e per conseguenza il conte.
Lo stesso si praticava in Francia. Veramente i Longobardi soleano
chiamar giudici i governatori delle loro città, come consta dalle lor
leggi. Contuttociò talvolta ancora questi giudici portano il nome di
conte. L'ordinario poi significato del titolo di duca competeva a quei
solamente che comandavano a qualche provincia, ed avevano sotto
di sè più conti. Trovansi nondimeno duchi d'una sola città. Ma di
queste cose ho io abbastanza trattato nelle Antichità estensi [Antichità
Estensi, cap. 1, part. 1.] e nelle Antichità italiane [Antiq. Italic., Dissert. VIII.].
Quello ancora ch'è da notare, non era per anche nato in questi tempi
il titolo di marchese; e però la bolla che il Rossi, per quanto accennai
di sopra, riferisce data da s. Gregorio a Mariniano arcivescovo in
Ravenna, si scuopre falsa al vedere fatta ivi menzione dei marchesi,
nome nato circa due secoli dipoi. Penso io che al presente anno
appartenga la notizia di uno sbarco fatto dai Longobardi nell'isola di
Sardegna, di cui siam debitori ad una lettera di san Gregorio [Greg.
Magnus, lib. 9, ep. 4.], scritta ne' primi mesi della Indizione seconda,
cominciata nel settembre di quest'anno. L'aveva già preveduto il
buon pontefice, senza lasciare di portarne per tempo colà l'avviso,
acciocchè si facesse buona guardia, ma non gli fu creduto nè
ubbidito. Ora colla presente lettera, scritta a Gennaro vescovo di
Cagliari, significa che finalmente era riuscito all'abbate Probo, inviato
da esso papa al re Agilolfo, d'intavolar la pace. Ma perchè ci voleva
del tempo, prima che ne fossero sottoscritte le capitolazioni da tutte
e due le parti, perciò lo esorta ad ordinar una miglior guardia delle
mura e ne' siti pericolosi, affinchè non venga voglia ai nemici di
tornare in questo mentre a visitarli. Convien poi credere che
nascesse qualche difficoltà, per cui paresse intorbidata la speranza
d'essa pace; perciocchè da lì a poco (se pure non v'ha sbaglio
nell'ordine e nella distribuzion delle lettere di s. Gregorio) torna egli
a scriver al medesimo vescovo [Gregor. Magnus, lib. 9, ep. 6.], che finita](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-76-320.jpg)
![Anno di
Cristo DXCIX. Indizione II.
Gregorio I papa 10.
Maurizio imperadore 18.
Agilolfo re 9.
L'anno XVI dopo il consolato di Maurizio Augusto.
Finalmente in quest'anno fu conchiusa la pace fra il re Agilolfo e
Callinico, esarco di Ravenna. Ne fa menzione Paolo Diacono [Paulus
Diaconus, lib. 4, cap. 13.], e l'anno si ricava dalle lettere scritte sotto la
presente indizione seconda da san Gregorio papa [Greg. Magnus, lib. 9,
ep. 42 et 43.], non solo alla cattolica regina Teodelinda, ma anco ad
esso re Agilolfo, forse tuttavia ariano; non apparendo ch'egli avesse
peranche abbracciata la religion cattolica. Ringrazia dunque Agilolfo
della pace fatta, il prega di ordinare ai suoi duchi che la osservino e
non cerchino dei pretesti per guastarla. Il saluta ancora con paterna
carità: parole che paiono indirizzate ad un re cattolico, ma che
sembrano poi non accordarsi coll'altre che egli soggiugne alla regina.
Perciocchè dopo averla ringraziata dell'efficace mano che ella aveva
avuta per condurre alla pace il regal consorte, l'esorta, ut apud
excellentissimum conjugem vestrum ita agatis, quatenus christianae
reipublicae societatem non rejiciat. Nam sicut ei vos scire credimus,
multis modis est utile, si se ad ejus amicitias conferre voluerit.
Queste parole paiono significare, desiderarsi dal papa una lega dei](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-78-320.jpg)
![Longobardi coll'imperadore; ma può anche sospettarsi desiderio nel
pontefice che la regina s'ingegni di tirare il marito al cattolicismo: il
che per molte cagioni gli sarebbe riuscito di profitto, perchè certo
tanti Cattolici suoi sudditi non miravano di buon occhio un principe
ariano, e molto meno i Cattolici non suoi sudditi. Anche secondo
l'umana politica sarebbe tornato il conto ad Agilolfo l'unirsi colla
Chiesa cattolica; e questo punto l'intese bene Clodoveo il grande re
de' Franchi e Recaredo re dei Visigoti, principi che abbracciarono la
fede cattolica romana, e meglio con ciò si stabilirono nei loro regni. E
che così facesse anche il re Agilolfo l'abbiamo da Paolo Diacono
[Paulus Diaconus, lib. 4, cap. 6.], là dove scrive ch'egli mosso dalle
salutevoli preghiere della regina Teodelinda, catholicam fidem tenuit,
et multas possessiones Ecclesiae Christi largitus est, atque
episcopos, qui in depressione et abjectione erant, ad dignitatis
solitae honorem reduxit. Ma ciò dovette seguire più tardi, siccome
vedremo più abbasso. Intanto certa cosa è che il re Agilolfo, cattolico
o ariano che si fosse in questi tempi, non inquietava punto per conto
della religione i Cattolici, e lasciava tutta la convenevole libertà ai
vescovi di esercitare il sacro lor ministero, di comunicare colla santa
sede, e di passare, occorrendo bisogni ecclesiastici, a Roma e a
Ravenna, tuttochè città nemiche. In somma s'egli non avea per
anche abjurato l'arianismo, almeno per le premure di Teodelinda
piissima e cattolica regina, amorevolmente trattava i professori del
cattolicismo. Non so io poi intendere come san Gregorio dopo avere
scritte le lettere suddette, in una altra indirizzata ad Eulogio patriarca
[Greg. Magnus, lib. 9, ep. 78.], sotto la stessa Indizione II, gli dica di
trovarsi oppresso dai dolori della podagra e dalle spade dei
Longobardi. Se la pace era fatta, come poi lagnarsi della guerra che
suppone fatta dai Longobardi ai Romani? Ciò mi fa dubitare se a
questa lettera sia stato assegnato il suo convenevol sito. Ma è ben
degna di attenzione un'altra lettera scritta da questo glorioso
pontefice a Teodoro curator di Ravenna [Idem, ibid., ep. 98.], ministro
che cooperato avea non poco alla conclusion della pace. Gli fa
dunque sapere che Ariolfo duca di Spoleti non avea voluto
sottoscrivere la pace puramente, come il re Agilolfo avea fatto, con
avervi apposto due condizioni, cioè ch'egli l'accettava, purchè dalla](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-79-320.jpg)
![parte dei Romani non si commettesse in avvenire eccesso alcuno
contra de' Longobardi, nè potessero i Romani far guerra ad Arichi,
ossia Arigiso duca di Benevento, confinante col ducato di Spoleti e
collegato di esso Ariolfo. Nell'edizione di san Gregorio è scritto
Arogis, ma si ha da scrivere Arigis.
Questa maniera di giurar la pace con tali riserve comparve a san
Gregorio insidiosa e furbesca, affinchè restasse aperto l'adito a
nuove rotture, non mancando mai pretesti per far guerra a chi ha in
odio la pace. E tanto più trovava egli delle magagne in questo
aggiustamento, perchè Varnilfrida (forse moglie d'esso Ariolfo, non
parendo questo un nome di maschio, che sarebbe stato Varnilfrido)
non l'avea voluto sottoscrivere. Aggiunge che gli uomini mandati dal
re Agilolfo a Roma esigevano che dal medesimo papa fossero
sottoscritti i capitoli della suddetta pace: segno della considerazione
e stima che quel re avea del romano pontefice, oppure che, non
fidandosi dei Romani, esigesse per sigurtà lo stesso pontefice. Ma
san Gregorio abborriva di farlo, sì perchè gli erano state riferite da
Basilio, uomo chiarissimo, delle parole ingiuriose proferite da esso re
contra della sede apostolica, e dello stesso papa Gregorio, benchè
Agilolfo negasse a spada tratta di averle dette; e sì ancora, perchè
se mai si fosse mancato da lì innanzi contro i patti, egli non voleva
averne da render conto, premendogli di non disgustare un principe,
di cui avea troppo bisogno pel governo di tante chiese poste sotto il
di lui dominio. Però si raccomanda affin d'essere esentato da quella
sottoscrizione. Stendeva in addietro il vescovo di Torino la sua
giurisdizione nella valle di Morienna e di Susa. Furono occupati
questi paesi da Guntranno re di Borgogna, allorchè i Longobardi
fecero le irruzioni nelle Gallie, come raccontammo di sopra, ed uniti
al suo regno della Borgogna. Ciò fatto, non piacendo ad esso re che
que' popoli neppure pel governo spirituale fossero sottoposti al
vescovo di Torino, cioè di una città sottoposta ai Longobardi, fece
creare un nuovo vescovo della Morienna. Se ne dolse Ursicino
vescovo di Torino con san Gregorio, il quale sopra ciò scrisse due
lettere [Gregor. Magnus, lib. 9, ep. 95 et 96.], l'una a Siagrio vescovo
d'Autun, e l'altra a Teoderico e Teodeberto re de' Franchi, con](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-80-320.jpg)
![pregarli che non fosse recato pregiudizio ai diritti del vescovo
torinese. Ma egli cantò a gente sorda; il vescovato di Morienna
sussistè, e tuttavia sussiste. E da una d'esse lettere apparisce che il
vescovo di Torino avea patito dei saccheggi nelle sue parrocchie, e
che il popolo era stato condotto (certamente dai Franchi) in
ischiavitù negli anni addietro. Rapporta l'Ughelli [Ughellius Italia Sacr.,
tom. 4, in Episcop. Bobiens.] una carta d'oblazione fatta da san
Colombano abate del monistero di Bobio a san Gregorio papa anno
pontificatus domni Gregorii summi pontificis et universalis papae IV,
Indictione III sub die III mensis novembris. L'indizione terza
cominciata nel settembre mostra appartener quella carta all'anno
presente. Ma il lettore osservando che non correva in quest'anno
l'anno quarto di san Gregorio, e che non fu in uso di que' tempi il
chiamare il romano pontefice, benchè capo della Chiesa di Dio, papa
universale: (titolo che lo stesso san Gregorio impugnò cotanto nel
patriarca di Costantinopoli); e che questa carta discorda dall'altre
antiche memorie che fanno, siccome diremo più abbasso, fondato
molto più tardi il monistero di Bobio; e che non si fa menzione degli
anni dell'imperadore, come era il costume, benchè la carta si
supponga scritta in Roma: non saprà, dissi, il lettore prestar fede ad
un sì fatto documento.](https://image.slidesharecdn.com/2032120-250526102458-7649db4c/85/Energy-Storage-Technologies-And-Applications-Ahmed-Faheem-Zobaa-81-320.jpg)
Energy Storage Technologies And Applications Ahmed Faheem Zobaa
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- 5. APPLICATIONS ENERGY STORAGE TECHNOLOGIES AND Edited by Ahmed Faheem Zobaa
- 6. ENERGY STORAGE – TECHNOLOGIES AND APPLICATIONS Edited by Ahmed Faheem Zobaa
- 7. Energy Storage – Technologies and Applications http://dx.doi.org/10.5772/2550 Edited by Ahmed Faheem Zobaa Contributors Hussein Ibrahim, Adrian Ilinca, Mohammad Taufiqul Arif, Amanullah M. T. Oo, A. B. M. Shawkat Ali, Petr Krivik, Petr Baca, Haisheng Chen, Xinjing Zhang, Jinchao Liu, Chunqing Tan, Luca Petricca, Per Ohlckers, Xuyuan Chen, Yong Xiao, Xiaoyu Ge, Zhe Zheng, Masatoshi Uno, Yu Zhang, Jinliang Liu, George Cristian Lazaroiu, Sonia Leva, Ying Zhu, Wenhua H. Zhu, Bruce J. Tatarchuk, Ruddy Blonbou, Stéphanie Monjoly, Jean-Louis Bernard, Antonio Ernesto Sarasua, Marcelo Gustavo Molina, Pedro Enrique Mercado Published by InTech Janeza Trdine 9, 51000 Rijeka, Croatia Copyright © 2013 InTech All chapters are Open Access distributed under the Creative Commons Attribution 3.0 license, which allows users to download, copy and build upon published articles even for commercial purposes, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. After this work has been published by InTech, authors have the right to republish it, in whole or part, in any publication of which they are the author, and to make other personal use of the work. Any republication, referencing or personal use of the work must explicitly identify the original source. Notice Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those of the editors or publisher. No responsibility is accepted for the accuracy of information contained in the published chapters. The publisher assumes no responsibility for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained in the book. Publishing Process Manager Sandra Bakic Typesetting InTech Prepress, Novi Sad Cover InTech Design Team First published January, 2013 Printed in Croatia A free online edition of this book is available at www.intechopen.com Additional hard copies can be obtained from orders@intechopen.com Energy Storage – Technologies and Applications, Edited by Ahmed Faheem Zobaa p. cm. ISBN 978-953-51-0951-8
- 9. Contents Chapter 1 Techno-Economic Analysis of Different Energy Storage Technologies 1 Hussein Ibrahim and Adrian Ilinca Chapter 2 Estimation of Energy Storage and Its Feasibility Analysis 41 Mohammad Taufiqul Arif, Amanullah M. T. Oo and A. B. M. Shawkat Ali Chapter 3 Electrochemical Energy Storage 79 Petr Krivik and Petr Baca Chapter 4 Compressed Air Energy Storage 101 Haisheng Chen, Xinjing Zhang, Jinchao Liu and Chunqing Tan Chapter 5 The Future of Energy Storage Systems 113 Luca Petricca, Per Ohlckers and Xuyuan Chen Chapter 6 Analysis and Control of Flywheel Energy Storage Systems 131 Yong Xiao, Xiaoyu Ge and Zhe Zheng Chapter 7 Single- and Double-Switch Cell Voltage Equalizers for Series-Connected Lithium-Ion Cells and Supercapacitors 149 Masatoshi Uno Chapter 8 Hybrid Energy Storage and Applications Based on High Power Pulse Transformer Charging 177 Yu Zhang and Jinliang Liu Chapter 9 Low Voltage DC System with Storage and Distributed Generation Interfaced Systems 219 George Cristian Lazaroiu and Sonia Leva Chapter 10 In-Situ Dynamic Characterization of Energy Storage and Conversion Systems 239 Ying Zhu, Wenhua H. Zhu and Bruce J. Tatarchuk
- 10. VI Contents Chapter 11 Dynamic Energy Storage Management for Dependable Renewable Electricity Generation 271 Ruddy Blonbou, Stéphanie Monjoly and Jean-Louis Bernard Chapter 12 Dynamic Modelling of Advanced Battery Energy Storage System for Grid-Tied AC Microgrid Applications 295 Antonio Ernesto Sarasua, Marcelo Gustavo Molina and Pedro Enrique Mercado
- 12. Chapter 1 Techno-Economic Analysis of Different Energy Storage Technologies Hussein Ibrahim and Adrian Ilinca Additional information is available at the end of the chapter http://dx.doi.org/10.5772/52220 1. Introduction Overall structure of electrical power system is in the process of changing. For incremental growth, it is moving away from fossil fuels - major source of energy in the world today - to renewable energy resources that are more environmentally friendly and sustainable [1]. Factors forcing these considerations are (a) the increasing demand for electric power by both developed and developing countries, (b) many developing countries lacking the resources to build power plants and distribution networks, (c) some industrialized countries facing insufficient power generation and (d) greenhouse gas emission and climate change concerns. Renewable energy sources such as wind turbines, photovoltaic solar systems, solar-thermo power, biomass power plants, fuel cells, gas micro-turbines, hydropower turbines, combined heat and power (CHP) micro-turbines and hybrid power systems will be part of future power generation systems [2-8]. Nevertheless, exploitation of renewable energy sources (RESs), even when there is a good potential resource, may be problematic due to their variable and intermittent nature. In addition, wind fluctuations, lightning strikes, sudden change of a load, or the occurrence of a line fault can cause sudden momentary dips in system voltage [4]. Earlier studies have indicated that energy storage can compensate for the stochastic nature and sudden deficiencies of RESs for short periods without suffering loss of load events and without the need to start more generating plants [4], [9], [10]. Another issue is the integration of RESs into grids at remote points, where the grid is weak, that may generate unacceptable voltage variations due to power fluctuations. Upgrading the power transmission line to mitigate this problem is often uneconomic. Instead, the inclusion of energy storage for power smoothing and voltage regulation at the remote point of connection would allow utilization of the power and could offer an economic alternative to upgrading the transmission line.
- 13. Energy Storage – Technologies and Applications 2 The current status shows that several drivers are emerging and will spur growth in the demand for energy storage systems [11]. These include: the growth of stochastic generation from renewables; an increasingly strained transmission infrastructure as new lines lag behind demand; the emergence of micro-grids as part of distributed grid architecture; and the increased need for reliability and security in electricity supply [12]. However, a lot of issues regarding the optimal active integration (operational, technical and market) of these emerging energy storage technologies into the electric grid are still not developed and need to be studied, tested and standardized. The integration of energy storage systems (ESSs) and further development of energy converting units (ECUs) including renewable energies in the industrial nations must be based on the existing electric supply system infrastructure. Due to that, a multi-dimensional integration task regarding the optimal integration of energy storage systems will result. The history of the stationary Electrical Energy Storage (EES) dates back to the turn of the 20th century, when power stations were often shut down overnight, with lead-acid accumulators supplying the residual loads on the direct current networks [13–15]. Utility companies eventually recognised the importance of the flexibility that energy storage provides in networks and the first central station for energy storage, a Pumped Hydroelectric Storage (PHS), was put to use in 1929 [13,16,17]. The subsequent development of the electricity supply industry, with the pursuit of economy of scale, at large central generating stations, with their complementary and extensive transmission and distribution networks, essentially consigned interest in storage systems up until relatively recent years. Up to 2005, more than 200 PHS systems were in use all over the world providing a total of more than 100 GW of generation capacity [16–18]. However, pressures from deregulation and environmental concerns lead to investment in major PHS facilities falling off, and interest in the practical application of EES systems is currently enjoying somewhat of a renaissance, for a variety of reasons including changes in the worldwide utility regulatory environment, an ever- increasing reliance on electricity in industry, commerce and the home, power quality/quality-of-supply issues, the growth of renewable as a major new source of electricity supply, and all combined with ever more stringent environmental requirements [14,19-20]. These factors, combined with the rapidly accelerating rate of technological development in many of the emerging EESs, with anticipated unit cost reductions, now make their practical applications look very attractive on future timescales of only a few years. This document aims to review the state-of-the-art development of EES technologies including PHS [18,21], Compressed Air Energy Storage system (CAES) [22–26], Battery [27– 31], Flow Battery [14-15,20,32], Fuel Cell [33-34], Solar Fuel [15,35], Superconducting Magnetic Energy Storage system (SMES) [36–38], Flywheel [32,39-41], Capacitor and Supercapacitor [15,39], and Thermal Energy Storage system (TES) [42–50]. Some of them are currently available and some are still under development. The applications, classification, technical characteristics, research and development (R&D) progress and deployment status of these EES technologies will be discussed in the following sections.
- 14. Techno-Economic Analysis of Different Energy Storage Technologies 3 2. Electrical energy storage 2.1. Definition of electrical energy storage Electrical Energy Storage (EES) refers to a process of converting electrical energy from a power network into a form that can be stored for converting back to electrical energy when needed [13–14,51]. Such a process enables electricity to be produced at times of either low demand, low generation cost or from intermittent energy sources and to be used at times of high demand, high generation cost or when no other generation means is available [13– 15,19,51] (Figure 1). EES has numerous applications including portable devices, transport vehicles and stationary energy resources [13-15], [19-20], [51-54]. This document will concentrate on EES systems for stationary applications such as power generation, distribution and transition network, distributed energy resource, renewable energy and local industrial and commercial customers. Figure 1. Fundamental idea of the energy storage [55] 2.2. Role of energy storage systems Breakthroughs that dramatically reduce the costs of electricity storage systems could drive revolutionary changes in the design and operation of the electric power system [52]. Peak load problems could be reduced, electrical stability could be improved, and power quality disturbances could be eliminated. Indeed, the energy storage plays a flexible and multifunctional role in the grid of electric power supply, by assuring more efficient management of available power. The combination with the power generation systems by the conversion of renewable energy, the Energy Storage System (ESS) provide, in real time, the balance between production and consumption and improve the management and the reliability of the grid [56]. Furthermore, the ESS makes easier the integration of the renewable
- 15. Energy Storage – Technologies and Applications 4 resources in the energy system, increases their penetration rate of energy and the quality of the supplied energy by better controlling frequency and voltage. Storage can be applied at the power plant, in support of the transmission system, at various points in the distribution system and on particular appliances and equipments on the customer’s side of the meter [52]. Figure 2. New electricity value chain with energy storage as the sixth dimension [11] The ESS can be used to reduce the peak load and eliminate the extra thermal power plant operating only during the peak periods, enabling better utilization of the plant functioning permanently and outstanding reduction of emission of greenhouse gases (GHG) [57]. Energy storage systems in combination with advanced power electronics (power electronics are often the interface between energy storage systems and the electrical grid) have a great technical role and lead to many financial benefits. Some of these are summarized in the following sections. Figure 2 shows how the new electricity value chain is changing supported by the integration of energy storage systems (ESS). More details about the different applications of energy storage systems will be presented in the section 4. 3. Energy storage components Before discussing the technologies, a brief explanation of the components within an energy storage device are discussed. Every energy storage facility is comprised of three primary components [58]: Storage Medium Power Conversion System (PCS) Balance of Plant (BOP) 3.1. Storage medium The storage medium is the ‘energy reservoir’ that retains the potential energy within a storage device. It ranges from mechanical (Pumped Heat Electricity Storage – PHES),
- 16. Techno-Economic Analysis of Different Energy Storage Technologies 5 chemical (Battery Energy Storage - BES) and electrical (Superconductor Magnetic Energy Storage – SMES) potential energy [58]. 3.2. Power Conversion System (PCS) It is necessary to convert from Alternating Current (AC) to Direct Current (DC) and vice versa, for all storage devices except mechanical storage devices e.g. PHES and CAES (Compressed Air Energy Storage) [59]. Consequently, a PCS is required that acts as a rectifier while the energy device is charged (AC to DC) and as an inverter when the device is discharged (DC to AC). The PCS also conditions the power during conversion to ensure that no damage is done to the storage device. The customization of the PCS for individual storage systems has been identified as one of the primary sources of improvement for energy storage facilities, as each storage device operates differently during charging, standing and discharging [59]. The PCS usually costs from 33% to 50% of the entire storage facility. Development of PCSs has been slow due to the limited growth in distributed energy resources e.g. small scale power generation technologies ranging from 3 to 10,000 kW [60]. 3.3. Balance-of-Plant (BOP) These are all the devices that [58]: Are used to house the equipment Control the environment of the storage facility Provide the electrical connection between the PCS and the power grid It is the most variable cost component within an energy storage device due to the various requirements for each facility. The BOP typically includes electrical interconnections, surge protection devices, a support rack for the storage medium, the facility shelter and environmental control systems [59]. The balance‐of‐plant includes structural and mechanical equipment such as protective enclosure, Heating/Ventilation/Air Conditioning (HVAC), and maintenance/auxiliary devices. Other BOP features include the foundation, structure (if needed), electrical protection and safety equipment, metering equipment, data monitoring equipment, and communications and control equipment. Other cost such as the facility site, permits, project management and training may also be considered here [61]. 4. Applications and technical benefits of energy storage systems The traditional electricity value chain has been considered to consist of five links: fuel/energy source, generation, transmission, distribution and customer-side energy service as shown in Figure 3. By supplying power when and where needed, ESS is on the brink of becoming the ‘‘sixth link” by integrating the existing segments and creating a more responsive market [62]. Stored energy integration into the generation-grid system is
- 17. Energy Storage – Technologies and Applications 6 illustrated in Figure 4 [32]. It can be seen that potential applications of EES are numerous and various and could cover the full spectrum ranging from larger scale, generation and transmission-related systems, to those primarily related to the distribution network and even ‘beyond the meter’, into the customer/end-user site [13]. Some important applications have been summarised in [13–15], [32], [52], [62–66]: Figure 3. Benefits of ESS along the electricity value chain [62]. Figure 4. Energy storage applications into grid [32].
- 18. Techno-Economic Analysis of Different Energy Storage Technologies 7 4.1. Generation Commodity Storage: Storing bulk energy generated at night for use during peak demand periods during the day. This allows for arbitrating the production price of the two periods and a more uniform load factor for the generation, transmission, and distribution systems [62]. Contingency Service: Contingency reserve is power capacity capable of providing power to serve customer demand should a power facility fall off-line. Spinning reserves are ready instantaneously, with non-spinning and long-term reserves ready in 10 minutes or longer. Spinning Reserve is defined as the amount of generation capacity that can be used to produce active power over a given period of time which has not yet been committed to the production of energy during this period [67]. Area Control: Prevent unplanned transfer of power between one utility and another. Grid Frequency Support: Grid Frequency Support means real power provided to the electrical distribution grid to reduce any sudden large load/generation imbalance and maintain a state of frequency equilibrium for the system’s 60Hz (cycles per second) during regular and irregular grid conditions. Large and rapid changes in the electrical load of a system can damage the generator and customers’ electrical equipment [62]. Black-Start: This refers to units with the capability to start-up on their own in order to energize the transmission system and assist other facilities to start-up and synchronize to the grid. 4.2. Transmission and distribution System Stability: The ability to maintain all system components on a transmission line in synchronous operation with each other to prevent a system collapse [62]. Grid Angular Stability: Grid Angular Stability means reducing power oscillations (due to rapid events) by injection and absorption of real power. Grid Voltage Support: Grid Voltage Support means power provided to the electrical distribution grid to maintain voltages within the acceptable range between each end of all power lines. This involves a trade-off between the amount of “real” energy produced by generators and the amount of “reactive” power produced [68]. Asset Deferral: Defer the need for additional transmission facilities by supplementing and existing transmission facilities—saving capital that otherwise goes underutilized for years [69]. 4.3. Energy service Energy Management (Load Levelling / Peak Shaving): Load Levelling is rescheduling certain loads to cut electrical power demand, or the production of energy during off- peak periods for storage and use during peak demand periods. Whilst Peak Shaving is reducing electric usage during peak periods or moving usage from the time of peak demand to off-peak periods. This strategy allows to customers to peak shave by shifting energy demand from one time of the day to another. This is primarily used to reduce their time-of-use (demand) charges [62].
- 19. Energy Storage – Technologies and Applications 8 Unbalanced Load Compensation: This can be done in combination with four-wire inverters and also by injecting and absorbing power individually at each phase to supply unbalanced loads. Power Quality improvement: Power Quality is basically related to the changes in magnitude and shape of voltage and current. This result in different issues including: Harmonics, Power Factor, Transients, Flicker, Sag and Swell, Spikes, etc. Distributed energy storage systems (DESS) can mitigate these problems and provide electrical service to the customer without any secondary oscillations or disruptions to the electricity "waveform" [67]. Power Reliability: Can be presented as the percentage/ratio of interruption in delivery of electric power (may include exceeding the threshold and not only complete loss of power) versus total uptime. DESS can help provide reliable electric service to consumers (UPS) to ‘ride-through’ a power disruption. Coupled with energy management storage, this allows remote power operation [68]. 4.4. Supporting the integration of intermittent renewable energy sources The development and use of renewable energy has experienced rapid growth over the past few years. In the next 20–30 years all sustainable energy systems will have to be based on the rational use of traditional resources and greater use of renewable energy. Decentralized electrical production from renewable energy sources yields a more assured supply for consumers with fewer environmental hazards. However, the unpredictable character of these sources requires that network provisioning and usage regulations be established for optimal system operation. Figure 5. Integration of extrapolated (x6) wind power using energy storage on the Irish electricity grid [58] However, renewable energy resources have two problems. First, many of the potential power generation sites are located far from load centers. Although wind energy generation facilities can be constructed in less than one year, new transmission facilities must be
- 20. Techno-Economic Analysis of Different Energy Storage Technologies 9 constructed to bring this new power source to market. Since it can take upwards of 7 years to build these transmission assets, long, lag-time periods can emerge where wind generation is "constrained-off" the system [62]. For many sites this may preclude them from delivering power to existing customers, but it opens the door to powering off-grid markets—an important and growing market. The second problem is that the renewable resources fluctuate independently from demand. Therefore, the most of the power accessible to the grids is generated when there is low demand for it. By storing the power from renewable sources from off-peak and releasing it during on-peak, energy storage can transform this low value, unscheduled power into schedulable, high-value product (see Figure 5). Beyond energy sales, with the assured capability of dispatching power into the market, a renewable energy source could also sell capacity into the market through contingency services. This capability will make the development of renewable resources far more cost-effective — by increasing the value of renewables it may reduce the level of subsidy down to where it is equal to the environmental value of the renewable, at which point it is no longer a subsidy but an environmental credit [62]. Frequency and synchronous spinning reserve support: In grids with a significant share of wind generation, intermittency and variability in wind generation output due to sudden shifts in wind patterns can lead to significant imbalances between generation and load that in turn result in shifts in grid frequency [68]. Such imbalances are usually handled by spinning reserve at the transmission level, but energy storage can provide prompt response to such imbalances without the emissions related to most conventional solutions. Transmission Curtailment Reduction: Wind power generation is often located in remote areas that are poorly served by transmission and distribution systems. As a result, sometimes wind operators are asked to curtail their production, which results in lost energy production opportunity, or system operators are required to invest in expanding the transmission capability. An EES unit located close to the wind generation can allow the excess energy to be stored and then delivered at times when the transmission system is not congested [68]. Time Shifting: Wind turbines are considered as non-dispatchable resources. EES can be used to store energy generated during periods of low demand and deliver it during periods of high demand (Figure 5). When applied to wind generation, this application is sometimes called “firming and shaping” because it changes the power profile of the wind to allow greater control over dispatch [68]. Forecast Hedge: Mitigation of errors (shortfalls) in wind energy bids into the market prior to required delivery, thus reducing volatility of spot prices and mitigating risk exposure of consumers to this volatility [69]. Fluctuation suppression: Wind farm generation frequency can be stabilised by suppressing fluctuations (absorbing and discharging energy during short duration variations in output) [69].
- 21. Energy Storage – Technologies and Applications 10 5. Financial benefits of energy storage systems In [70] detailed analysis of energy storage benefits is done including market analysis, the following are some highlights: 1. Cost Reduction or Revenue Increase of Bulk Energy Arbitrage: Arbitrage involves purchase of inexpensive electricity available during low demand periods to charge the storage plant, so that the low priced energy can be used or sold at a later time when the price for electricity is high [11]. 2. Cost Avoid or Revenue Increase of Central Generation Capacity: For areas where the supply of electric generation capacity is tight, energy storage could be used to offset the need to: a) purchase and install new generation and/or b) “rent” generation capacity in the wholesale electricity marketplace. 3. Cost Avoid or Revenue Increase of Ancillary Services: It is well known that energy storage can provide several types of ancillary services. In short, these are what might be called support services used to keep the regional grid operating. Two more familiar ones are spinning reserve and load following [11]. 4. Cost Avoid or Revenue Increase for Transmission Access/Congestion: It is possible that use of energy storage could improve the performance of the Transmission and Distribution (T&D) system by giving the utilities the ability to increase energy transfer and stabilize voltage levels. Further, transmission access/congestion charges can be avoided because the energy storage is used. 5. Reduced Demand Charges: Reduced demand charges are possible when energy storage is used to reduce an electricity end-user’s use of the electric grid during times grid is high (i.e., during peak electric demand periods) [11]. 6. Reduced Reliability-related Financial Losses: Storage reduces financial losses associated with power outages. This benefit is very end-user-specific and applies to commercial and industrial (C&I) customers, primarily those for which power outages cause moderate to significant losses. 7. Reduced Power Quality-related Financial Losses: Energy storage reduces financial losses associated with power quality anomalies. Power quality anomalies of interest are those that cause loads to go off-line and/or that damage electricity-using equipment and whose negative effects can be avoided if storage is used [11]. 8. Increased Revenue from Renewable Energy Sources: Storage could be used to time-shift electric energy generated by renewables. Energy is stored when demand and price for power are low, so the energy can be used when a) demand and price for power is high and b) output from the intermittent renewable generation is low. The previous listed functionalities point out that those energy storages in combination with power electronics will have a huge impact in future electrical supply systems. This is why any planning and implementation strategy should be related to the real-time control and operational functionalities of the ESS in combination with Distributed Energy Resources (DER) in order to get rapid integration process.
- 22. Techno-Economic Analysis of Different Energy Storage Technologies 11 6. Techno-economic characteristics of energy storage systems The main characteristics of storage systems on which the selection criteria are based are the following [73]: 6.1. Storage capacity This is the quantity of available energy in the storage system after charging. Discharge is often incomplete. For this reason, it is defined on the basis of total energy stored, which is superior to that actually retrieved (operational). The usable energy, limited by the depth of discharge, represents the limit of discharge depth (minimum-charge state). In conditions of quick charge or discharge, the efficiency deteriorates and the retrievable energy can be much lower than storage capacity (Figure 6). On the other hand, self-discharge is the attenuating factor under very slow regime. Figure 6. Variation of energy capacity, self-discharge and internal resistance of a nickel-metal-hydride battery with the number of cycles [71] 6.2. Storage System Power This parameter determines the constitution and size of the motor-generator in the stored energy conversion chain. A storage system’s power rating is assumed to be the system’s nameplate power rating under normal operating conditions [73]. Furthermore, that rating is assumed to represent the storage system’s maximum power output under normal operating conditions. In this document, the normal discharge rate used is commonly referred to as the system’s ‘design’ or ‘nominal’ (power) rating. 6.3. Storage ‘Emergency’ Power Capability Some types of storage systems can discharge at a relatively high rate (e.g., 1.5 to 2 times their nominal rating) for relatively short periods of time (e.g., several minutes to as much as 30 minutes). One example is storage systems involving a Na/S battery, which is capable of producing two times its rated (normal) output for relatively short durations [72].
- 23. Energy Storage – Technologies and Applications 12 That feature – often referred to as the equipment’s ‘emergency’ rating – is valuable if there are circumstances that occur infrequently that involve an urgent need for relatively high power output, for relatively short durations. Importantly, while discharging at the higher rate, storage efficiency is reduced (relative to efficiency during discharge at the nominal discharge rate), and storage equipment damage increases (compared to damage incurred at the normal discharge rate). So, in simple terms, storage with emergency power capability could be used to provide the nominal amount of power required to serve a regularly occurring need (e.g., peak demand reduction) while the same storage could provide additional power for urgent needs that occur infrequently and that last for a few to several minutes at a time [72]. 6.4. Autonomy Autonomy or discharge duration autonomy is the amount of time that storage can discharge at its rated output (power) without recharging. Discharge duration is an important criterion affecting the technical viability of a given storage system for a given application and storage plant cost [73]. This parameter depends on the depth of discharge and operational conditions of the system, constant power or not. It is a characteristic of system adequacy for certain applications. For small systems in an isolated area relying on intermittent renewable energy, autonomy is a crucial criterion. The difficulty in separating the power and energy dimensions of the system makes it difficult to choose an optimum time constant for most storage technologies [74]. 6.5. Energy and power density Power density is the amount of power that can be delivered from a storage system with a given volume or mass. Similarly, energy density is the amount of energy that can be stored in a storage device that has a given volume or mass. These criteria are important in situations for which space is valuable or limited and/or if weight is important (especially for mass density of energy in portable applications, but less so for permanent applications). 6.6. Space requirements for energy storage Closely related to energy and power density are footprint and space requirements for energy storage. Depending on the storage technology, floor area and/or space constraints may indeed be a challenge, especially in heavily urbanized areas. 6.7. Efficiency All energy transfer and conversion processes have losses. Energy storage is no different. Storage system round-trip efficiency (efficiency) reflects the amount of energy that comes out of storage relative to the amount put into the storage. This definition is often oversimplified because it is based on a single operation point [75]. The definition of
- 24. Techno-Economic Analysis of Different Energy Storage Technologies 13 efficiency must therefore be based on one or more realistic cycles for a specific application. Instantaneous power is a defining factor of efficiency (Figure 7). This means that, for optimum operation, the power-transfer chain must have limited losses in terms of energy transfer and self-discharge. This energy conservation measure is an essential element for daily network load-levelling applications. Typical values for efficiency include the following: 60% to 75% for conventional electrochemical batteries; 75% to 85% for advanced electrochemical batteries; 73% to 80% for CAES; 75% to 78% for pumped hydro; 80% to 90% for flywheel storage; and 95% for capacitors and SMES [72], [76]. Figure 7. Power efficiency of a 48V-310Ah (15 kWh/10 h discharge) lead accumulator [77] 6.8. Storage operating cost Storage total operating cost (as distinct from plant capital cost or plant financial carrying charges) consists of two key components: 1) energy-related costs and 2) operating costs not related to energy. Non-energy operating costs include at least four elements: 1) labor associated with plant operation, 2) plant maintenance, 3) equipment wear leading to loss-of- life, and 4) decommissioning and disposal cost [73]. 1. Charging Energy-Related Costs: The energy cost for storage consists of all costs incurred to purchase energy used to charge the storage, including the cost to purchase energy needed to make up for (round trip) energy losses [73]. For a storage system with 75% efficiency, if the unit price for energy used for charging is 4¢/kWh, then the plant energy cost is 5.33¢/kWh. 2. Labor for Plant Operation: In some cases, labor may be required for storage plant operation. Fixed labor costs are the same magnitude irrespective of how much the storage is used. Variable labor costs are proportional to the frequency and duration of storage use [73]. In many cases, labor is required to operate larger storage facilities and/or ‘blocks’ of aggregated storage capacity whereas little or no labor may be needed for smaller/distributed systems that tend to be designed for autonomous operation. No explicit value is ascribed to this criterion, due in part to the wide range of labor costs that are possible given the spectrum of storage types and storage system sizes [73].
- 25. Energy Storage – Technologies and Applications 14 Figure 8. Storage total variable operation cost for 75% storage efficiency [73] 3. Plant Maintenance: Plant maintenance costs are incurred to undertake normal, scheduled, and unplanned repairs and replacements for equipment, buildings, grounds, and infrastructure. Fixed maintenance costs are the same magnitude irrespective of how much the storage is used [73]. Variable maintenance costs are proportional to the frequency and duration of storage use. 4. Replacement Cost: If specific equipment or subsystems within a storage system are expected to wear out during the expected life of the system, then a ‘replacement cost’ will be incurred. In such circumstances, a ‘sinking fund’ is needed to accumulate funds to pay for replacements when needed [73]. That replacement cost is treated as a variable cost (i.e., the total cost is spread out over each unit of energy output from the storage plant). 5. Variable Operating Cost: A storage system’s total variable operating cost consists of applicable non-energy-related variable operating costs plus plant energy cost, possibly including charging energy, labor for plant operation, variable maintenance, and replacement costs. Variable operating cost is a key factor affecting the cost-effectiveness of storage [73]. It is especially important for ‘high-use’ value propositions involving many charge-discharge cycles. Ideally, storage for high-use applications should have relatively high or very high efficiency and relatively low variable operating cost. Otherwise, the total cost to charge then discharge the storage is somewhat-to-very likely to be higher than the benefit. That can be a significant challenge for some storage types and value propositions. Consider the example illustrated in Figure 8, which involves a 75% efficient storage system with a non-energy-related variable operating cost of 4¢/kWhout. If that storage system is
- 26. Techno-Economic Analysis of Different Energy Storage Technologies 15 charged with energy costing 4¢/kWhin, then the total variable operating cost – for energy output – is about 9.33¢/kWhout [73]. 6.9. Durability Lifetime or durability refers to the number of times the storage unit can release the energy level it was designed for after each recharge, expressed as the maximum number of cycles N (one cycle corresponds to one charge and one discharge) [81]. All storage systems degrade with use because they are subject to fatigue or wear by usage use (i.e., during each charge-discharge cycle). This is usually the principal cause of aging, ahead of thermal degradation. The rate of degradation depends on the type of storage technology, operating conditions, and other variables. This is especially important for electrochemical batteries [73]. For some storage technologies – especially batteries – the extent to which the system is emptied (discharged) also affects the storage media’s useful life. Discharging a small portion of stored energy is a ‘shallow’ discharge and discharging most or all of the stored energy is a ‘deep’ discharge. For these technologies, a shallow discharge is less damaging to the storage medium than a deep discharge [73]. To the extent that the storage medium degrades and must be replaced during the expected useful life of the storage system, the cost for that replacement must be added to the variable operating cost of the storage system. Figure 9. Evolution of cycling capacity as a function of depth of discharge for a lead-acid battery [79] The design of a storage system that considers the endurance of the unit in terms of cycles should be a primary importance when choosing a system. However, real fatigue processes are often complex and the cycling capacity is not always well defined. In all cases, it is strongly linked to the amplitude of the cycles (Figure 9) and/or the average state of charge [78]. As well, the cycles generally vary greatly, meaning that the quantification of N is delicate and the values given represent orders of magnitude [74].
- 27. Energy Storage – Technologies and Applications 16 6.10. Reliability Like power rating and discharge duration, storage system reliability requirements are circumstance-specific. Little guidance is possible. Storage-system reliability is always an important factor because it is a guarantee of on-demand service [81]. The project design engineer is responsible for designing a plant that provides enough power and that is as reliable as necessary to serve the specific application. 6.11. Response time Storage response time is the amount of time required to go from no discharge to full discharge. At one extreme, under almost all conditions, storage has to respond quite rapidly if used to provide capacity on the margin in lieu of transmission and distribution (T&D) capacity. That is because the output from T&D equipment (i.e., wires and transformers) changes nearly instantaneously in response to demand [73]. In contrast, consider storage used in lieu of generation capacity. That storage does not need to respond as quickly because generation tends to respond relatively slowly to demand changes. Specifically, some types of generation – such as engines and combustion turbines – take several seconds to many minutes before generating at full output. For other generation types, such as those fueled by coal and nuclear energy, the response time may be hours [73]. Most types of storage have a response time of several seconds or less. CAES and pumped hydroelectric storage tend to have a slower response, though they still respond quickly enough to serve several important applications. 6.12. Ramp rate An important storage system characteristic for some applications is the ramp rate – the rate at which power output can change. Generally, storage ramp rates are rapid (i.e., output can change quite rapidly); pumped hydro is the exception. Power devices with a slow response time tend also to have a slow ramp rate [73]. 6.13. Charge rate Charge rate – the rate at which storage can be charged – is an important criterion because, often, modular energy storage (MES) must be recharged so it can serve load during the next day [58]. If storage cannot recharge quickly enough, then it will not have enough energy to provide the necessary service. In most cases, storage charges at a rate that is similar to the rate at which it discharges [73]. In some cases, storage may charge more rapidly or more slowly, depending on the capacity of the power conditioning equipment and the condition and/or chemistry and/or physics of the energy storage medium.
- 28. Techno-Economic Analysis of Different Energy Storage Technologies 17 6.14. Self-discharge and energy retention Energy retention time is the amount of time that storage retains its charge. The concept of energy retention is important because of the tendency for some types of storage to self- discharge or to otherwise dissipate energy while the storage is not in use. In general terms, energy losses could be referred to as standby losses [74]. Storage that depends on chemical media is prone to self-discharge. This self-discharge is due to chemical reactions that occur while the energy is stored. Each type of chemistry is different, both in terms of the chemical reactions involved and the rate of self-discharge. Storage that uses mechanical means to store energy tends to be prone to energy dissipation. For example, energy stored using pumped hydroelectric storage may be lost to evaporation. CAES may lose energy due to air escaping from the reservoir [73]. To the extent that storage is prone to self-discharge or energy dissipation, retention time is reduced. This characteristic tends to be less important for storage that is used frequently. For storage that is used infrequently (i.e., is in standby mode for a significant amount of time between uses), this criterion may be very important [72]. 6.15. Transportability Transportability can be an especially valuable feature of storage systems for at least two reasons. First, transportable storage can be (re)located where it is needed most and/or where benefits are most significant [58]. Second, some locational benefits only last for one or two years. Given those considerations, transportability may significantly enhance the prospects that lifecycle benefits will exceed lifecycle cost. 6.16. Power conditioning To one extent or another, most storage types require some type of power conditioning (i.e., conversion) subsystem. Equipment used for power conditioning – the power conditioning unit (PCU) – modifies electricity so that the electricity has the necessary voltage and the necessary form; either alternating current (AC) or direct current (DC). The PCU, in concert with an included control system, must also synchronize storage output with the oscillations of AC power from the grid [73]. Output from storage with relatively low-voltage DC output must be converted to AC with higher voltage before being discharged into the grid and/or before being used by most load types. In most cases, conversion from DC to AC is accomplished using a device known as an inverter [73]. For storage requiring DC input, the electricity used for charging must be converted from the form available from the grid (i.e., AC at relatively high voltage) to the form needed by the storage system (e.g., DC at lower voltage). That is often accomplished via a PCU that can function as a DC ‘power supply’ [73].
- 29. Energy Storage – Technologies and Applications 18 6.17. Power quality Although requirements for applications vary, the following storage characteristics may or may not be important. To one extent or another, they are affected by the PCU used and/or they drive the specifications for the PCU. In general, higher quality power (output) costs more. 6. Power Factor: Although detailed coverage of the concept of power factor is beyond the scope of this report, it is important to be aware of the importance of this criterion. At a minimum, the power output from storage should have an acceptable power factor, where acceptable is somewhat circumstance variable power factor. 7. Voltage Stability: In most cases, it is important for storage output voltage to remain somewhat-to-very constant. Depending on the circumstances, voltage can vary; though, it should probably remain within about 5% to 8% of the rated value. 8. Waveform: Assuming that storage output is AC, in most cases, the waveform should be as close as possible to that of a sine wave. In general, higher quality PCUs tend to have waveforms that are quite close to that of a sine wave whereas output from lower quality PCUs tends to have a waveform that is somewhat square. 9. Harmonics: Harmonic currents in distribution equipment can pose a significant challenge. Harmonic currents are components of a periodic wave whose frequency is an integral multiple of the fundamental frequency [73]. In this case, the fundamental frequency is the utility power line frequency of 60 Hz. 6.18. Modularity One attractive feature of modular energy storage is the flexibility that system ‘building blocks’ provide. Modularity allows for more optimal levels and types of capacity and/or discharge duration because modular resources allow utilities to increase or decrease storage capacity, when and where needed, in response to changing conditions [72-73]. Among other attractive effects, modular capacity provides attractive means for utilities to address uncertainty and to manage risk associated with large, ‘lumpy’ utility T&D investments. 6.19. Storage system reactive power capability One application (Voltage Support) and one incidental benefit (Power Factor Correction) described in this guide involve storage whose capabilities include absorbing and injecting reactive power (expressed in units of volt-Amperes reactive or VARs) [58], [72-73]. This feature is commonly referred as VAR support. In most cases, storage systems by themselves do not have reactive power capability. For a relatively modest incremental cost, however, reactive power capability can be added to most storage system types. 6.20. Feasibility and adaptation to the generating source To be highly efficient, a storage system needs to be closely adapted to the type of application (low to mid power in isolated areas, network connection, etc.) and to the type of production
- 30. Techno-Economic Analysis of Different Energy Storage Technologies 19 (permanent, portable, renewable, etc.) (Figure 10) it is meant to support. It needs to be harmonized with the network. Figure 10. Fields of application of the different storage techniques according to stored energy and power output [80] 6.21. Monitoring, control and communications equipments This equipment, on both the quality and safety of storage levels, has repercussions on the accessibility and availability of the stored energy [74]. Indeed, storage used for most applications addressed in this report must receive and respond to appropriate control signals. In some cases, storage may have to respond to a dispatch control signal. In other cases, the signal may be driven by a price or prices [73]. Storage response to a control signal may be a simple ramp up or ramp down of power output in proportion to the control signal. A more sophisticated response, requiring one or more control algorithms, may be needed. 6.22. Interconnection If storage will be charged with energy from the grid or will inject energy into the grid, it must meet applicable interconnection requirements. At the distribution level, an important point of reference is the Institute of Electronics and Electrical Engineers (IEEE) Standard 1547 [82]. Some countries and utilities have more specific interconnection rules and requirements.
- 31. Energy Storage – Technologies and Applications 20 6.23. Operational constraints Especially related to safety (explosions, waste, bursting of a flywheel, etc.) or other operational conditions (temperature, pressure, etc.), they can influence the choice of a storage technology as a function of energy needs [74]. 6.24. Environmental aspect While this parameter is not a criterium of storage-system capacity, the environmental aspect of the product (recyclable materials) is a strong sales pitch. For example, in Nordic countries (Sweden, Norway), a definite margin of the population prefers to pay more for energy than to continue polluting the country [83]. This is a dimension that must not, therefore, be overlooked. 6.25. Decommissioning and disposal needs and cost In most cases there will be non-trivial decommissioning costs associated with almost any storage system [73]. For example, eventually batteries must be dismantled and the chemicals must be removed. Ideally, dismantled batteries and their chemicals can be recycled, as is the case for the materials in lead-acid batteries. Ultimately, decommissioning-related costs should be included in the total cost to own and to operate storage. 6.26. Other characteristics The ease of maintenance, simple design, operational flexibility (this is an important characteristic for the utility), fast response time for the release of stored energy, etc. Finally, it is important to note that these characteristics apply to the overall storage system: storage units and power converters alike [74]. 7. Classification of energy storage systems There are two criteria to categorise the various ESSs: function and form. In terms of the function, ESS technologies can be categorised into those that are intended firstly for high power ratings with a relatively small energy content making them suitable for power quality or UPS [69]; and those designed for energy management, as shown in Figure 11. PHS, CAES, TES, large-scale batteries, flow batteries, fuel cells, solar fuel and TES fall into the category of energy management, whereas capacitors/super-capacitors, SMES, flywheels and batteries are in the category of power quality and reliability. This simple classification glosses over the wide range of technical parameters of energy storage devices. Although electricity is not easy to be directly stored cheaply, it can be easily stored in other forms and converted back to electricity when needed. Storage technologies for electricity can also be classified by the form of storage into the following [69]:
- 32. Techno-Economic Analysis of Different Energy Storage Technologies 21 Figure 11. Energy storage classification with respect to function [69]. 1. Electrical energy storage: (i) Electrostatic energy storage including capacitors and super- capacitors; (ii) Magnetic/current energy storage including SMES. 2. Mechanical energy storage: (i) Kinetic energy storage (flywheels); (ii) Potential energy storage (PHES and CAES). 3. Chemical energy storage: (i) electrochemical energy storage (conventional batteries such as lead-acid, nickel metal hydride, lithium ion and flow-cell batteries such as zinc bromine and vanadium redox); (ii) chemical energy storage (fuel cells, Molten- Carbonate Fuel Cells – MCFCs and Metal-Air batteries); (iii) thermochemical energy storage (solar hydrogen, solar metal, solar ammonia dissociation–recombination and solar methane dissociation–recombination). 4. Thermal energy storage: (i) Low temperature energy storage (Aquiferous cold energy storage, cryogenic energy storage); (ii) High temperature energy storage (sensible heat systems such as steam or hot water accumulators, graphite, hot rocks and concrete, latent heat systems such as phase change materials). 8. Description of energy storage technologies 8.1. Pumped hydro storage (PHS) In pumping hydro storage, a body of water at a relatively high elevation represents a potential or stored energy. During peak hours the water in the upper reservoir is lead through a pipe downhill into a hydroelectric generator and stored in the lower reservoir. Along off-peak periods the water is pumped back up to recharge the upper reservoir and the power plant acts like a load in power system [72], [84]. Pumping hydro energy storage system (figure 12) consists in two large water reservoirs, electric machine (motor/generator) and reversible pump-turbine group or pump and turbine separated. This system can be started-up in few minutes and its autonomy depends on the volume of stored water. Restrictions to pumping hydro energy storage are related with geographical constraints and weather conditions. In periods of much rain, pumping hydro capacity can be reduced.
- 33. Energy Storage – Technologies and Applications 22 Pumped hydroelectric systems have conversion efficiency, from the point of view of a power network, of about 65–80%, depending on equipment characteristics [72]. Considering the cycle efficiency, 4 kWh are needed to generate three. The storage capacity depends on two parameters: the height of the waterfall and the volume of water. A mass of 1 ton falling 100 m generates 0.272 kWh. Figure 12. Wind-Pumped hydro energy storage hybrid system [69]. 8.2. Batteries energy storage Batteries store energy in electrochemical form creating electrically charged ions. When the battery charges, a direct current is converted in chemical energy, when discharges, the chemical energy is converted back into a flow of electrons in direct current form [75]. Electrochemical batteries use electrodes both as part of the electron transfer process and store the products or reactants via electrode solid-state reactions [85]. Batteries are the most popular energy storage devices. However, the term battery comprises a sort of several technologies applying different operation principals and materials. There is a wide range of technologies used in the fabrication of electrochemical accumulators (lead–acid (Figure 13), nickel–cadmium, nickel–metal hydride, nickel–iron, zinc–air, iron–air, sodium–sulphur, lithium–ion, lithium–polymer, etc.) and their main assets are their energy densities (up to 150 and 2000 Wh/kg for lithium) and technological maturity. Their main inconvenient however is their relatively low durability for large-amplitude cycling (a few 100 to a few 1000 cycles). They are often used in portable systems, but also in permanent applications (emergency network back-up, renewable-energy storage in isolated areas, etc.) [83].
- 34. Techno-Economic Analysis of Different Energy Storage Technologies 23 The minimum discharge period of the electrochemical accumulators rarely reaches below 15 minutes. However, for some applications, power up to 100 W/kg, even a few kW/kg, can be reached within a few seconds or minutes. As opposed to capacitors, their voltage remains stable as a function of charge level. Nevertheless, between a high-power recharging operation at near-maximum charge level and its opposite, that is to say a power discharge nearing full discharge, voltage can easily vary by a ratio of two [74]. Figure 13. Structure of a lead‐acid battery [86] 8.3. Flow batteries energy storage (FBES) Flow batteries are a two-electrolyte system in which the chemical compounds used for energy storage are in liquid state, in solution with the electrolyte. They overcome the limitations of standard electrochemical accumulators (lead-acid or nickel-cadmium for example) in which the electrochemical reactions create solid compounds that are stored directly on the electrodes on which they form. This is therefore a limited-mass system, which obviously limits the capacity of standard batteries. Various types of electrolyte have been developed using bromine as a central element: with zinc (ZnBr), sodium (NaBr) (Figure 14), vanadium (VBr) and, more recently, sodium polysulfide. The electrochemical reaction through a membrane in the cell can be reversed (charge- discharge). By using large reservoirs and coupling a large number of cells, large quantities of energy can be stored and then released by pumping electrolyte into the reservoirs. The main advantages of the technology include the following [87]: 1) high power and energy capacity; 2) fast recharge by replacing exhaust electrolyte; 3) long life enabled by
- 35. Energy Storage – Technologies and Applications 24 easy electrolyte replacement; 4) full discharge capability; 5) use of nontoxic materials; and 6) low-temperature operation. The main disadvantage of the system is the need for moving mechanical parts such as pumping systems that make system miniaturization difficult. Therefore, the commercial uptake to date has been limited. The best example of flow battery was developed in 2003 by Regenesys Technologies, England, with a storage capacity of 15 MW-120 MWh. It has since been upgraded to an electrochemical system based entirely on vanadium. The overall electricity storage efficiency is about 75 % [88]. Figure 14. Illustration of a flow-battery 8.4. Flywheel energy storage (FES) Flywheel energy accumulators are comprised of a massive or composite flywheel coupled with a motor-generator and special brackets (often magnetic), set inside a housing at very low pressure to reduce self-discharge losses (Figure 15) [9]. They have a great cycling capacity (a few 10,000 to a few 100,000 cycles) determined by fatigue design. To store energy in an electrical power system, high-capacity flywheels are needed. Friction losses of a 200 tons flywheel are estimated at about 200 kW. Using this hypothesis and instantaneous efficiency of 85 %, the overall efficiency would drop to 78 % after 5 hours, and 45 % after one day. Long-term storage with this type of apparatus is therefore not foreseeable. From a practical point of view, electromechanical batteries are more useful for the production of energy in isolated areas. Kinetic energy storage could also be used for the distribution of electricity in urban areas through large capacity buffer batteries, comparable to water reservoirs, aiming to maximize the efficiency of the production units. For example, large installations made up of forty 25kW-25kWh systems are capable of storing 1 MW that can be released within one hour.
- 36. Techno-Economic Analysis of Different Energy Storage Technologies 25 Figure 15. Flywheel energy accumulators [89] 8.5. Supercapacitors energy storage (SES) Supercapacitors are the latest innovational devices in the field of electrical energy storage. In comparison with a battery or a traditional capacitor, the supercapacitor allows a much powerful power and energy density [15]. Supercapacitors are electrochemical double layer capacitors that store energy as electric charge between two plates, metal or conductive, separated by a dielectric, when a voltage differential is applied across the plates. As like battery systems, capacitors work in direct current. The energy/volume obtained is superior to that of capacitors (5 Wh/kg or even 15 Wh/kg), at very high cost but with better discharge time constancy due to the slow displacement of ions in the electrolyte (power of 800–2000 W/kg). Super-capacitors generally are very durable, that is to say 8–10 years, 95% efficiency and 5% per day self-discharge, which means that the stored energy must be used quickly. Supercapacitors find their place in many applications where energy storage is needed, like uninterruptible power supplies, or can help in smoothing strong and short-time power solicitations of weak power networks. Their main advantages are the long life cycle and the short charge/discharge time [2], [19]. 8.6. Superconducting magnetic energy storage (SMES) An emerging technology, systems store energy in the magnetic field created by the flow of direct current in a coil of cryogenically cooled, superconducting material. Due to their construction, they have a high operating cost and are therefore best suited to provide constant, deep discharges and constant activity. The fast response time (under 100 ms) of these systems makes them ideal for regulating network stability (load levelling). Power is available almost instantaneously and very high power output can be provided for a brief period of time [20-21]. These facilities currently range in size up to 3 MW units and are
- 37. Energy Storage – Technologies and Applications 26 generally used to provide grid stability in a distribution system and power quality at manufacturing facilities requiring ultra-clean power such a chip fabrication facility. One advantage of this storage system is its great instantaneous efficiency, near 95 % for a charge-discharge cycle [90]. Moreover, these systems are capable of discharging the near totality of the stored energy, as opposed to batteries. They are very useful for applications requiring continuous operation with a great number of complete charge- discharge cycles. 8.7. Fuel cells-Hydrogen energy storage (HES) Fuel cells are a means of restoring spent energy to produce hydrogen through water electrolysis. The storage system proposed includes three key components: electrolysis which consumes off-peak electricity to produce hydrogen, the fuel cell which uses that hydrogen and oxygen from air to generate peak-hour electricity, and a hydrogen buffer tank to ensure adequate resources in periods of need. Fuel cells can be used in decentralized production (particularly low-power stations – residential, emergency...), spontaneous supply related or not to the network, mid-power cogeneration (a few hundred kW), and centralized electricity production without heat upgrading. They can also represent a solution for isolated areas where the installation of power lines is too difficult or expensive (mountain locations, etc.). There are several hydrogen storage modes, such as: compressed, liquefied, metal hydride, etc. For station applications, pressurized tanks with a volume anywhere between 10-2 m3 and 10,000 m3 are the simplest solution to date. Currently available commercial cylinders can stand pressures up to 350 bars. Combining an electrolyser and a fuel cell for electrical energy storage is a low-efficiency solution (at best 70 % for the electrolyser and 50 % for the fuel cell, and 35 % for the combination). As well, the investment costs are prohibitive and life expectancy is very limited, especially for power network applications [74]. 8.8. Thermal energy storage (TES) Thermal energy storage (TES) already exists in a wide spectrum of applications. It uses materials that can be kept at high/low temperatures in insulated containments. Heat/cold recovered can then be applied for electricity generation using heat engine cycles. Energy input can, in principle, be provided by electrical resistance heating or refrigeration/cryogenic procedures, hence the overall round trip efficiency of TES is low (30–60%) although the heat cycle efficiency could be high (70–90%), but it is benign to the environment and may have particular advantages for renewable and commercial buildings. TES systems can be classified into low-temperature TES and high-temperature TES depending on whether the operating temperature of the energy storage material is higher
- 38. Techno-Economic Analysis of Different Energy Storage Technologies 27 than the room temperature. More precisely, TES can be categorised into industrial cooling (below -18 C), building cooling (at 0-12 C), building heating (at 25-50 C) and industrial heat storage (higher than 175 C). 8.9. Compressed Air Energy Storage (CAES) This method consist to use off-peak power to pressurize air into an underground reservoir (salt cavern, abandoned hard rock mine or aquifer) which is then released during peak daytime hours to power a turbine/generator for power production. CAES (Figure 16) is the only other commercially available technology (besides pumped-hydro) able to provide the very-large system energy storage deliverability (above 100 MW in single unit sizes) to use for commodity storage or other large-scale setting [74]. Figure 16. Illustration of compressed-air energy storage The energy density for this type of system is in the order of 12 kWh/m3 [91], while the estimated efficiency is around 70 % [92]. Let us note that to release 1 kWh into the network, 0.7–0.8 kWh of electricity needs to be absorbed during off-peak hours to compress the air, as well as 1.22 kWh of natural gas during peak hours (retrieval). Two plants currently exist, with several more under development. The first operating unit is a 290 MW unit built in Huntorf, Germany in 1978. The second plant is a 110 MW unit built in McIntosh, Alabama in 1991. Small-scale compressed air energy storage (SSCAES), compressed air storage under high pressure in cylinders (up to 300 bars with carbon fiber structures) are still developing and seem to be a good solution for small- and medium-scale applications. 9. Assessment and comparison of the energy storage technologies Following, some figures are presented that compare different aspects of storage technologies. These aspects cover topics such as: technical maturity, range of applications, efficiencies, lifetime, costs, mass and volume densities, etc.
- 39. Energy Storage – Technologies and Applications 28 9.1. Technical maturity The technical maturity of the EES systems is shown in Figure 17. The EES technologies can be classified into three categories in terms of their maturity [69]: 1. Mature technologies: PHS and lead-acid battery are mature and have been used for over 100 years. 2. Developed technologies: CAES, NiCd, NaS, ZEBRA Li-ion, Flow Batteries, SMES, flywheel, capacitor, supercapacitor, Al-TES (Aquiferous low- temperature – Thermal energy storage) and HT-TES (High temperature – Thermal energy storage) are developed technologies. All these EES systems are technically developed and commercially available; however, the actual applications, especially for large-scale utility, are still not widespread. Their competitiveness and reliability still need more trials by the electricity industry and the market. 3. Developing technologies: Fuel cell, Meta-Air battery, Solar Fuel and CES (Cryogenic Energy Storage) are still under development. They are not commercially mature although technically possible and have been investigated by various institutions. On the other hand, these developing technologies have great potential for industrial take up in the near future. Energy costs and environmental concerns are the main drivers. Figure 17. Technical maturity of EES systems [69] 9.2. Power rating and discharge time The power ratings of various EESs are compared in Table 1. Broadly, the EESs fall into three types according to their applications [69], [74]: 1. Energy management: PHS, CAES and CES are suitable for applications in scales above 100 MW with hourly to daily output durations. They can be used for energy management for large-scale generations such as load leveling, ramping/load following, and spinning reserve. Large-scale batteries, flow batteries, fuel cells, CES and TES are suitable for medium-scale energy management with a capacity of 10–100 MW [55], [69].
- 40. Techno-Economic Analysis of Different Energy Storage Technologies 29 2. Power quality: Flywheel, batteries, SMES, capacitor and supercapacitor have a fast response (milliseconds) and therefore can be utilized for power quality such as the instantaneous voltage drop, flicker mitigation and short duration UPS. The typical power rating for this kind of application is lower than 1 MW [55]. 3. Bridging power: Batteries, flow batteries, fuel cells and Metal-Air cells not only have a relatively fast response (<1 s) but also have relatively long discharge time (hours), therefore they are more suitable for bridging power. The typical power rating for these types of applications is about 100 kW–10 MW [55], [74]. Table 1. Comparison of technical characteristics of EES systems [69] 9.3. Storage duration Table 1 also illustrates the self-discharge (energy dissipation) per day for EES systems. One can see that PHS, CAES, Fuel Cells, Metal-Air Cells, solar fuels and flow batteries have a very small self-discharge ratio so are suitable for a long storage period. Lead-Acid, NiCd, Li- ion, TESs and CES have a medium self-discharge ratio and are suitable for a storage period not longer than tens of days [69]. NaS, ZEBRA, SMES, capacitor and supercapacitor have a very high self-charge ratio of 10– 40% per day. They can only be implemented for short cyclic periods of a maximum of several hours. The high self-discharge ratios of NaS and ZEBRA are from the high working temperature which needs to be self-heating to maintain the use of the storage energy [69]. Flywheels will discharge 100% of the stored energy if the storage period is longer than about 1 day. The proper storage period should be within tens of minutes.
- 41. Energy Storage – Technologies and Applications 30 9.4. Capital cost Capital cost is one of the most important factors for the industrial take-up of the EES. They are expressed in the forms shown in Table 2, cost per kWh, per kW and per kWh per cycle. All the costs per unit energy shown in the table have been divided by the storage efficiency to obtain the cost per output (useful) energy [69]. The per cycle cost is defined as the cost per unit energy divided by the cycle life which is one of the best ways to evaluate the cost of energy storage in a frequent charge/discharge application, such as load levelling. For example, while the capital cost of lead-acid batteries is relatively low, they may not necessarily be the least expensive option for energy management (load levelling) due to their relatively short life for this type of application. The costs of operation and maintenance, disposal, replacement and other ownership expenses are not considered, because they are not available for some emerging technologies [55], [69]. Table 2. Comparison of technical characteristics of EES systems [69] CAES, Metal-Air battery, PHS, TESs and CES are in the low range in terms of the capital cost per kWh. The Metal-Air batteries may appear to be the best choice based on their high energy density and low cost, but they have a very limited life cycle and are still under development. Among the developed techniques, CAES has the lowest capital cost compared to all the other systems. The capital cost of batteries and flow batteries is slightly higher than the break even cost against the PHS although the gap is gradually closing. The SMES, flywheel, capacitor and supercapacitor are suitable for high power and short duration
- 42. Techno-Economic Analysis of Different Energy Storage Technologies 31 applications, since they are cheap on the output power basis but expensive in terms of the storage energy capacity [55], [69]. The costs per cycle kWh of PHS and CAES are among the lowest among all the EES technologies, the per cycle cost of batteries and flow batteries are still much higher than PHS and CAES although a great decrease has occurred in recent years. CES is also a promising technology for low cycle cost. However, there are currently no commercial products available. Fuel cells have the highest per cycle cost and it will take a long time for them to be economically competitive. No data have been found for the solar fuels as they are in the early stage of development [55], [69]. It should also be noted that the capital cost of energy storage systems can be significantly different from the estimations given here due to, for example, breakthroughs in technologies, time of construction, location of plants, and size of the system. The information summarised here should only be regarded as being preliminary. 9.5. Cycle efficiency The cycle efficiency of EES systems during one charge-discharge cycle is illustrated in Figure 18. The cycle efficiency is the ‘‘round-trip” efficiency defined as ratio between output energy and input energy. The self-discharge loss during the storage is not considered. One can see that the EES systems can be broadly divided into three groups: 1. Very high efficiency: SMES, flywheel, supercapacity and Li-ion battery have a very high cycle efficiency of > 90%. 2. High efficiency: PHS, CAES, batteries (except for Li-ion), flow batteries and conventional capacitor have a cycle efficiency of 60–90%. It can also be seen that storing electricity by compression and expansion of air using the CAES is usually less efficient than pumping and discharging water with PHSs, since rapid compression heats up a gas, increasing its pressure thus making further compression more energy consuming [55], [69]. 3. Low efficiency: Hydrogen, DMFC, Metal-Air, solar fuel, TESs and CES have an efficiency lower than 60% mainly due to large losses during the conversion from the commercial AC side to the storage system side. For example, hydrogen storage of electricity has relatively low round-trip energy efficiency (20–50%) due to the combination of electrolyser efficiency and the efficiency of re-conversion back to electricity [55], [69]. It must be noted that there is a trade-off between the capital cost and round-trip efficiency, at least to some extent. For example, a storage technology with a low capital cost but a low round-trip efficiency may well be competitive with a high cost, high round-trip efficiency technology. 9.6. Energy and power density The power density (W/kg or W/litre) is the rated output power divided by the volume of the storage device. The energy density is calculated as a stored energy divided by the volume. The volume of the storage device is the volume of the whole energy storage system
- 43. Energy Storage – Technologies and Applications 32 Figure 18. Cycle efficiency of EES systems. [69] including the energy storing element, accessories and supporting structures, and the inverter system. As can be seen from Table 1, the Fuel Cells, Metal-Air battery, and Solar fuels have an extremely high energy density (typically 1000 Wh/kg), although, as mentioned above, their cycle efficiencies are very low. Batteries, TESs, CES and CAES have medium energy density. The energy density of PHS, SMES, Capacitor/supercapacitor and flywheel are among the lowest below 30 Wh/kg. However, the power densities of SMES, capacitor/supercapacitor and flywheel are very high which are, again, suitable for applications for power quality with large discharge currents and fast responses [69], [74]. NaS and Li-ion have a higher energy density than other conventional batteries. The energy densities of flow batteries are slightly lower than those of conventional batteries. It should be noted that there are differences in the energy density of the same type of EES made by different manufacturers [55]. 9.7. Life time and cycle life Also compared in Table 1 are life time and/or cycle life for various EESs. It can be seen that the cycle lives of EES systems whose principles are largely based on the electrical technologies are very long normally greater than 20,000. Examples include SMES, capacitor and supercapacitor. Mechanical and thermal energy storage systems, including PHS, CAES, flywheel, AL-TES, CES and HT-TES, also have long cycle lives. These technologies are based on conventional mechanical engineering, and the life time is mainly determined by the life time of the mechanical components. The cycle abilities of batteries, flow batteries, and fuel cells are not as high as other systems due to chemical deterioration with the operating time. Metal-Air battery only has a life of a few hundred cycles and obviously needs to be further developed [55], [69].
- 44. Techno-Economic Analysis of Different Energy Storage Technologies 33 10. Conclusion The key element of this analysis is the review of the available energy storage techniques applicable to electrical power systems. There is obviously a cost associated to storing energy, but we have seen that, in many cases, storage is already cost effective. More and more application possibilities will emerge as further research and development is made in the field [91]. Storage is a major issue with the increase of renewable but decentralized energy sources that penetrate power networks [93]. Not only is it a technical solution for network management, ensuring real-time load leveling, but it is also a mean of better utilizing renewable resources by avoiding load shedding in times of overproduction. Coupled with local renewable energy generation, decentralized storage could also improve power network sturdiness through a network of energy farms supplying a specific demand zone. Many solutions are available to increase system security, but they are so different in terms of specifications that they are difficult to compare. This is why we tried to bring out a group of technical and economical characteristics which could help improve performance and cost estimates for storage systems. Based on the review, the following conclusions could be drawn [55], [69], [74]: 1. Although there are various commercially available EES systems, no single storage system meets all the requirements for an ideal EES - being mature, having a long lifetime, low costs, high density and high efficiency, and being environmentally benign. Each EES system has a suitable application range. PHS, CAES, large-scale batteries, flow batteries, fuel cells, solar fuels, TES and CES are suitable for energy management application; flywheels, batteries, capacitors and supercapacitors are more suitable for power quality and short duration UPS, whereas batteries, flow batteries, fuel cells and Metal-Air cells are promising for the bridging power. 2. PHS and Lead-Acid battery are technically mature; CAES, NiCd, NaS, ZEBRA Li-ion, flow battery, SMES, flywheel, capacitor, Supercapacitor, AL-TES and HT-TES are technically developed and commercially available; Fuel Cell, Meta-Air battery, Solar Fuel and CES are under development. The capital costs of CAES, Metal-Air battery, PHS, TES and CES are lower than other EESs. CAES has the lowest capital cost among the developed technologies. Metal-Air battery has the potential to be the cheapest among currently known EES systems. 3. The cycle efficiencies of SMES, flywheel, capacitor/supercapacitor, PHS, CAES, batteries, flow batteries are high with the cycle efficiency above 60%. Fuel Cell, DMFC, Metal-Air, solar fuel, TES and CES have a low efficiency mainly due to large losses during the conversion from commercial AC to the storage energy form. 4. The cycle lives of the EES systems based on the electrical technologies, such as SMES, capacitor and supercapacitor, are high. Mechanical and thermal based EES, including PHS, CAES, flywheel, ALTES, CES and HT-TES, also have a long cycle life. The cycle abilities of batteries, flow batteries, and fuel cells are not as high as other systems due to
- 45. Energy Storage – Technologies and Applications 34 chemical deterioration with the operating time. Metal-Air battery has the lowest life time at least currently. 5. PHS, CAES, batteries, flow batteries, fuel cells and SMES are considered to have some negative effects on the environment due to one or more of the following: fossil combustion, strong magnetic field, landscape damage, and toxic remains. Solar fuels and CES are more environmentally friendly. However, a full life-cycle analysis should be done before a firm conclusion can be drawn. Based on the contents of this study and carefully measuring the stakes, we find that: 1. The development of storage techniques requires the improvement and optimization of power electronics, often used in the transformation of electricity into storable energy, and vice versa. 2. The rate of penetration of renewable energy will require studies on the influence of the different storage options, especially those decentralized, on network sturdiness and overall infrastructure and energy production costs. 3. The study of complete systems (storage, associated transformation of electicity, power electronics, control systems...) will lead to the optimization of the techniques in terms of cost, efficiency, reliability, maintenance, social and environmental impacts, etc. 4. It is important to assess the national interest for compressed gas storage techniques. 5. Investment in research and development on the possibility of combining several storage methods with a renewable energy source will lead to the optimization of the overall efficiency of the system and the reduction of greenhouse gases created by conventional gas-burning power plants. 6. Assessing the interest for high-temperature thermal storage systems, which have a huge advantage in terms of power delivery, will lead to the ability of safely establish them near power consumption areas, 7. The development of supercapacitors will lead to their integration into the different types of usage. 8. The development of low-cost, long-life flywheel storage systems will lead to increased potential, particularly for decentralized applications. 9. To increase the rate of penetration and use of hydrogen-electrolysor fuel-cell storage systems, a concerted R&D effort will have to be made in this field. Author details Hussein Ibrahim TechnoCentre éolien, Gaspé, QC, Canada Adrian Ilinca Université du Québec à Rimouski, Rimouski, QC, Canada 11. References [1] Smith, S.C.; Sen, P.K.; Kroposki, B., Advancement of Energy Storage Devices and Applications in Electrical Power System. Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 22-24 2008
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- 51. Energy Storage – Technologies and Applications 40 [91] Multon B, Ruer J. Stocker l’électricité : Oui, c’est indispensable, et c’est possible ! pourquoi, où, comment?. Publication ECRIN en contribution au débat national sur l’énergie; Avril 2003. [92] Robyns B. Contribution du stockage de l’énergie électrique à la participation au services système des éoliennes. Séminaire SRBE – SEE – L2EP « Éolien et réseaux : enjeux », 22 mars 2005. [93] Multon B. L’énergie électrique : analyse des ressources et de la production. Journées section électrotechnique du club EEA. Paris, France, 28-29 janvier 1999, p. 8.
- 52. Chapter 2 Estimation of Energy Storage and Its Feasibility Analysis Mohammad Taufiqul Arif, Amanullah M. T. Oo and A. B. M. Shawkat Ali Additional information is available at the end of the chapter http://dx.doi.org/10.5772/52218 1. Introduction Storage significantly adds flexibility in Renewable Energy (RE) and improves energy management. This chapter explains the estimation procedures of required storage with grid connected RE to support for a residential load. It was considered that storage integrated RE will support all the steady state load and grid will support transient high loads. This will maximize the use of RE. Proper sized RE resources with proper sized storage is essential for best utilization of RE in a cost effective way. This chapter also explains the feasibility analysis of storage by comparing the economical and environmental indexes. Most of the presently installed Solar PV or Wind turbines are without storage while connected to the grid. The intermittent nature of solar radiation and wind speed limits the capacity of RE to follow the load demand. The available standards described sizing and requirements of storage in standalone systems. However standards available for distributed energy resources (DER) or distributed resources (DR) to connect to the grid while considering solar photovoltaic (PV), wind turbine and storage as DR. Bearing this limitation, this chapter followed the sizing guidelines for standalone system to estimate the required storage for the grid connected RE applications. Solar PV is unable provide electricity during night and cloudy days; similarly wind energy also unable to follow load demand. Moreover PV and/or wind application is not able to follow the load demand; when these RE generators are just in the stage to start generating energy and when these RE are in highest mode of generating stage while load demand falls to the lowest level. Therefore it can be said that RE is unable to generate energy by following the load demand which is a major limitation in energy management. Storage can play this critical role of proper energy management. Moreover storage helps in reducing the intermittent nature of RE and improve the Power Quality (PQ). This study considers regional Australia as the study area also considered residential load, solar radiation and
- 53. Energy Storage – Technologies and Applications 42 wind speed data of that location for detailed analysis. Figure 1 shows the daily load profile (summer: January 01, 2009 and winter: July 01 2009) of Capricornia region of Rockhampton, a regional city in Australia. Ergon Energy [1] is the utility operator in that area. However load demand of the residential load in that area depends on the work time patter which is different than the load profile of Figure 1. Overall electricity demand is very high in the evening and also in the morning for the residential load however PV generates electricity mostly during the day time therefore residents need to purchase costly electricity during peak demand in the evening. Similarly wind energy also unable to follow the residential load profile. Therefore properly estimated storage needs to be integrated to overcome this situation. Figure 1. Daily load profile of Capricornia region in 2009 (Summer & Winter) This chapter explores the need of storage systems to maximize the use of RE, furthermore estimates the required capacity of storage to meet the daily need which will gradually eliminate the dependency on conventional energy sources. Estimation of storage sizing is explained in section 3. This chapter also conducts the feasibility assessment of storage in terms of economic and environmental perspective which is explained in section 4. 2. Background Solar and Wind are the two major sources of RE. Australia is one of best places for these sources. In regional areas of Australia, roof top Solar PV is installed in many residential houses either in off-grid or grid connected configurations and most residential wind turbine are for specific applications in off-grid configuration. In grid connected solar PV systems where storage is not integrated, the energy output from this system does not satisfy to the desired level. Currently installed most of the residential PV systems are designed in an
- 54. Estimation of Energy Storage and Its Feasibility Analysis 43 unplanned way that even with battery integrated system is not able to support the load in reliable way. Figure 2 illustrates a typical situation when whole system in jeopardize as the estimation of storage system was not done correctly. Figure 2. Typical condition of failure in storage integrated RE system The adoption of storage with the PV system certainly incurs additional cost to the system but the benefits of adding storage has not been clearly assessed. Therefore this chapter aims to achieve two objectives. One is to estimate the required storage for the grid connected PV system or grid connected wind turbine or combination of grid connected PV and wind turbine system to achieve the maximum daily use of RE. Second objective is to identify the effects of storage on the designed system in terms of environment and economic by comparing the same system with and without storage. The feasibility of the designed system is expressed as, the Cost of Energy (COE) is closer to the present system while providing environmental benefits by reducing Greenhouse Gas (GHG) emission and improving the Renewable Fraction (RF). Data was collected for the Capricornia region of Rockhampton city in Queensland, Australia. Load data was collected for a 3 bed room house by estimating all the electrical appliances demand and average usage period considering its ratings. Daily load profile drawn from hourly load data and total daily load was estimated by calculating the area under the daily load profile curve using trapezoidal method. Weather data was collected for the year 2009 from [2] for this location and calculated the energy output from PV array and
- 55. Another Random Scribd Document with Unrelated Content
- 56. danaro, com'era solito in simili frangenti di fare), ma l'esarco Romano non gliel voleva permettere: del che si duol egli forte coll'arcivescovo suddetto. E tanto più, perchè essendo stato rinforzato Ariolfo dalle soldatesche di due altri condottieri di armi, Autari e Nordolfo, difficilmente volea più dar orecchio a trattati di pace. Pertanto il prega che se ha luogo di parlar di tali affari con sì strambo ministro, cerchi di condurlo alla pace, con ricordargli specialmente che s'era levato di Roma il nerbo maggiore delle milizie, per sostenere l'occupata Perugia, come egli deplora altrove [Gregorius M., lib. 5, ep. 40.], nè vi era restata altra guarnigione che il reggimento teodosiano, così appellato da Teodosio Augusto, figliuolo di Maurizio imperadore, il quale ancora, per essere privo delle sue paghe, stentava ad accomodarsi alla guardia delle mura. Aggiugne che anche Arichi, ossia Arigiso duca di Benevento, il quale era succeduto a Zottone primo duca di quella contrada, instigato da Ariolfo, rotte le capitolazioni precedenti, avea mosse le sue armi contra de' Napoletani, e minacciava quella città. Non si doveano credere i Longobardi obbligati ad alcun trattato precedente, da che l'esarco sotto la buona fede aveva occupato ad essi Perugia con altre città. Paolo Diacono [Paulus Diaconus, lib. 4, cap. 19.] parla della morte di Zottone suddetto dopo venti anni di ducato, con dire che in suo luogo succedette Arigiso, mandato colà dal re Agilolfo, e per conseguente o in questo o nel precedente anno, con intendersi da ciò che il ducato beneventano dovette aver principio circa l'anno 571, come pensò il padre Antonio Caracciolo. Era Arigiso nato nel Friuli, avea servito d'ajo a' figliuoli di Gisolfo duca del Friuli, ed era parente del medesimo Gisolfo. Risulta poi dalla suddetta lettera di san Gregorio all'arcivescovo di Ravenna, che la città di Fano era posseduta allora dai Longobardi, e vi si trovavano molti fatti schiavi, per la liberazion de' quali aveva il caritativo papa voluto inviare nel precedente anno una persona con danaro; ma questa non si era arrischiata di passare pel ducato di Spoleti, che divideva Roma da quella città ed era sotto il dominio de' Longobardi. Tuttavia non lasciò Fortunato, vescovo d'essa città, di riscattarli, con aggravarsi di molti debiti per questa santa azione [Greg. Magnus, lib. 7,
- 57. epist. 13.]; e san Gregorio gli concedette dipoi che potesse vendere i vasi sacri delle chiese per pagare i creditori. Quel Severo vescovo scismatico, la cui città era stata bruciata, e per cui l'arcivescovo di Ravenna chiedeva delle limosine a san Gregorio, vien creduto vescovo di Aquileja dal cardinal Baronio [Baron., Annal. Eccl.] e dal padre Mabillone [Mabill., in Annal. Bened., lib. 8, cap. 37.]. Io il tengo per Severo vescovo d'Ancona, nominato altrove da san Gregorio, giacchè egli dice: Juxta quippe est civitas Fanum: il che non conviene nè a Grado nè ad Aquileja. Nell'edizione di san Gregorio fatta da' padri Benedettini, la lettera sedicesima del libro nono [Greg. M., lib. 9, ep. 16, edition. Bened.] è ad Serenum anconitanum episcopum. Si ha da leggere ad Severum, apparendo ciò dalla susseguente lettera ottantesima nona [Idem, ibid. epist. 89.]. Dovea questo vescovo, addottrinato dalle disgrazie della sua città, avere abbandonato lo scisma e meritata la grazia di san Gregorio.
- 58. Anno di Cristo DXCIII. Indizione XI. Gregorio I papa 4. Maurizio imperadore 12. Agilolfo re 3. L'anno X dopo il consolato di Maurizio Augusto. Ci fa sapere Paolo Diacono, che irritato forte il re Agilolfo per la perdita di Perugia e dell'altre suddette città, si mosse immediatamente da Pavia con un possente esercito per riacquistare quella città. E però potrebbe essere che appartenesse al precedente anno questo suo sforzo. Ma non parlando punto san Gregorio di Agilolfo nelle lettere scritte in quell'anno, nè essendo molto esatto nell'ordine dei tempi lo storico suddetto, chieggo licenza di poter riferire al presente anno l'avvenimento suddetto. Venne dunque il bellicoso re con grandi forze all'assedio di Perugia, e con tal vigore sollecitò quell'impresa, che tornò alle sue mani essa città, e Maurizio preso pagò colla sua testa il tradimento fatto. Come poi e quando Perugia tornasse in poter dei Romani, nol so. Certo è che vi tornò. Par ben credibile che Agilolfo ricuperasse ancora l'altre città a lui tolte dall'esarco. Nè questo gli bastò. Volle anche tentare Roma stessa: al che non fece mente Paolo Diacono, allorchè scrisse, che dopo la presa di Perugia Agilolfo se ne tornò a Pavia. Racconta il santo pontefice [Gregor. M., Praefat. lib. 2, in Ezechi.] ch'egli era dietro a
- 59. spiegare al popolo il capitolo quarantesimo di Ezechiello, allorchè s'intese jam Agilulphum Longobardorum regem, ad obsidionem nostram summopere festinantem, Padum transisse. E che seguissero dipoi dei gran travagli e danni al popolo romano, si raccoglie da quanto seguita appresso a dire il medesimo san Gregorio [Paulus Diaconus, lib. 4, cap. 8.]: Ubique luctus aspicimus. Ubique gemitus audivimus; destructae urbes, eversa sunt castra, depopulati sunt agri, in solitudinem terra redacta est. Alios in captivitatem duci, alios detruncari, alios interfici videmus. Aggiugne più sotto [Greg. M., Homil. 6, lib. 2.]: Nemo autem me reprehendat, si post hanc locutionem cessavero, quia, sicut omnes cernitis, nostrae tribulationes excreverunt. Undique gladio circumfusi sumus, undique imminens mortis periculum timemus. Alti detruncatis ad nos manibus redeunt; alii captivi, alii interemti ad nos nuntiantur. Jam cogor linguam ab expositione retinere. E queste parole son quelle che fecero dire a Paolo Diacono [Idem, lib. 2, Homil. ultim.], il qual sembra discorde da sè medesimo, essere rimasto sì atterrito il beato Gregorio papa dall'arrivo del re Agilolfo, che cessò dal proseguire la spiegazion del testo di Ezechiello. Crede il cardinal Baronio che questi guai di Roma succedessero nell'anno 595, quando tutte le apparenze sono che molto prima arrivasse un sì atroce flagello addosso a quella città. Ed è fuor di dubbio che Roma, tuttochè guernita d'un debolissimo presidio, valorosamente si difese in quelle strettezze, di modo che il re Agilolfo, scorgendo la difficoltà dell'impresa, fors'anche segretamente commosso dalle preghiere e dai regali, che a tempo opportuno soleva impiegare per bene del suo popolo il generoso papa Gregorio, si ritirò da quei contorni, e dopo tanti danni inferiti lasciò in pace i Romani. Mancò di vita in quest'anno uno dei re franchi, cioè Guntranno re della Borgogna, principe per la pietà e per altre virtù assai commendato. Perchè in questi tempi non si durava gran fatica a canonizzare gli uomini, e specialmente i principi dabbene per santi, però anche a lui toccò d'essere messo in quel ruolo. Morì senza figliuoli, e lasciò tutti i suoi stati al re di Austrasia Childeberto, la cui potenza con una sì gran giunta divenne formidabile. E buon pei Longobardi che neppur egli sopravvivesse di molto a questo suo zio.
- 61. Anno di Cristo DXCIV. Indizione XII. Gregorio I papa 5. Maurizio imperadore 13. Agilolfo re 4. L'anno XI dopo il consolato di Maurizio Augusto. Credesi che nell'anno precedente san Gregorio papa prendesse a scrivere i suoi Dialoghi; ma c'è anche motivo di giudicare che ciò succedesse nell'anno presente, scrivendo egli [Gregor. Magnus, Dialog., lib. 3, cap. 19.] che cinque anni prima era seguita la fiera innondazione del Tevere. Manteneva intanto il santo pontefice buona corrispondenza con Teodelinda regina dei Longobardi, principessa piissima e bene attaccata alla religione cattolica: il che giovò non poco per rendere il re Agilolfo suo consorte, benchè ariano, ben affetto e favorevole ai Cattolici stessi, e servì in fine, siccome diremo, ad abbracciare la stessa fede cattolica, se pur sussiste ciò che ne lasciò scritto Paolo Diacono. Era stato eletto arcivescovo di Milano Costanzo; e perchè si sparse voce ch'egli avesse condannati i tre capitoli del concilio calcedonense, ed accettato il concilio quinto, tre vescovi suoi suffraganei, fra' quali specialmente quello di Brescia, non solamente si separarono dalla di lui comunione, ma eziandio indussero la regina a fare lo stesso. Restano due lettere scritte da san Gregorio [Idem, lib. 4, ep. 4, et 38.] alla medesima regina, nelle quali
- 62. si duole ch'ella si sia lasciata sedurre, quasi la dottrina del concilio calcedonense, principalmente sostenuta dalla Chiesa romana, avesse patito alcun detrimento per le persone condannate dipoi nel quinto concilio generale. Da altre lettere del medesimo papa pare che si raccolga essersi Teodelinda umilmente accomodata alle di lui esortazioni. Ma veggasi all'anno 604. Abbiamo anche da Paolo Diacono [Paulus Diaconus, lib. 4, cap. 5.] che a questa buona principessa san Gregorio, non si sa quando, inviò in dono i Dialoghi suddetti. Una delle maggiori premure, che circa questi tempi nudriva l'infaticabil pontefice, era quella di stabilir la pace coi Longobardi. A così lodevol pensiero chi s'opponesse lo vedremo nell'anno seguente, contuttochè io non lasci di sospettare che possa tal pace appartenere all'anno presente, non essendo noi certi che tutte le lettere di san Gregorio papa sieno disposte con ordine esattissimo di tempo. Comunque sia, in una lettera scritta da esso papa sotto l'indizione duodecima, cioè sotto quest'anno, al sopra citato Costanzo arcivescovo di Milano, si vede che il ringrazia delle nuove dategli del re Agone (così ancora veniva chiamato, siccome già accennai, il re Agilolfo) e dei re de' Franchi, e desidera d'essere informato di tutto altro che possa accadere. Dice in fine una particolarità degna d'attenzione nelle seguenti parole, cioè: Se vedrete che Agone re de' Longobardi non possa accordarsi col patrizio (ossia con Romano esarco), fategli sapere che si prometta meglio di me, perchè son pronto a spendere, s'egli vorrà consentire in qualche partito vantaggioso al romano imperio. Desiderava Gregorio che seguisse la pace generale, e perchè ciò venisse effettuato, si esibiva a pagare; e quando poi non si potesse concludere questa general pace, proponeva di farla almeno col ducato romano, per non vedere più esposto alle miserie della guerra il popolo, ch'egli più degli altri era tenuto ad amare. Sono di parere i padri Benedettini, nella edizione di san Gregorio, che a quest'anno appartenga una lettera del medesimo santo papa [Gregor. Magnus, lib. 4, ep. 47.] scritta a Sabiniano suo apocrisario, ossia nunzio alla corte di Costantinopoli, con ordinargli di dire ai serenissimi nostri padroni, che se Gregorio lor servo si fosse voluto mischiare nella morte dei Longobardi, oggidì la nazione longobarda non avrebbe nè re, nè duchi, nè conti, e si
- 63. troverebbe in una somma confusione. Ma perchè egli ha timore di Dio, teme di mischiarsi nella morte di chicchessia. Parole degne d'attenzione, per conoscere sempre più la santità di Gregorio, e qual fosse il governo de' Longobardi, del quale parleremo in altro luogo. Era imputato il santo pontefice d'aver fatto morire in carcere Malco vescovo longobardo, oppure di qualche città suggetta ai Longobardi; e però si giustificò colle suddette espressioni.
- 64. Anno di Cristo DXCV. Indizione XIII. Gregorio I papa 6. Maurizio imperadore 14. Agilolfo re 5. L'anno XII dopo il consolato di Maurizio Augusto. Non cessava il santo pontefice Gregorio di far delle premure perchè si venisse ad una pace fra l'imperio e i Longobardi, sì perchè avea troppo in orrore gl'infiniti disordini prodotti dalla guerra, e sì perchè toccava con mano la debolezza dell'imperio stesso, che non poteva se non perdere continuando la discordia. Ora egli a tal fine scrisse in questo anno a Severo, scolastico (cioè consultore) dell'esarco [Gregor. Magnus, lib. 5, ep. 36.], con fargli sapere che Agilolfo re de' Longobardi non ricusava di fare una pace generale, purchè l'esarco volesse emendare i danni a lui dati, prima che fosse venuta l'ultima rottura, esibendosi anch'egli pronto a fare lo stesso, se i suoi nel tempo della pace aveano danneggiato le terre dell'imperio. Però il prega di adoperarsi, acciocchè l'esarco acconsenta alla pace; che per altro Agilolfo si mostrava anche disposto a stabilirla coi soli Romani. Oltre a ciò, avvertisce l'esarco che varii luoghi ed isole erano in pericolo manifesto di perdersi; e però s'affrettasse ad abbracciar la proposta concordia, per poter avere un po' di quiete, e mettersi intanto in forze da poter meglio resistere. Ma l'esarco Romano era
- 65. della razza di coloro che antepongono il proprio vantaggio a quello del pubblico. Se la guerra recava immensi mali alla misera Italia, fruttava ben di molti guadagni alla borsa sua. E perciò non solamente abborriva la pace, ma giunse infino a caricar di calunnie il santo pontefice alla corte, in maniera che circa il mese di giugno Maurizio Augusto scrivendo ad esso papa e ad altri delle lettere, il trattò da uomo semplice e poco accorto, quasichè si lasciasse burlare da Ariolfo duca di Spoleti con varie lusinghe di pace, ed avesse rappresentato alla corte o all'esarco delle cose insussistenti. Chi legge la lettera scritta in questo proposito dall'incomparabil pontefice, non può di meno di non ammirare e benedire la singolar sua umiltà e la destrezza, con cui seppe sostenere il suo decoro, e nello stesso tempo non mancar di rispetto a chi era principe temporale di Roma. Duolsi egli, fra l'altre cose, che sia stata rotta dagli uffiziali cesarei la pace da lui stabilita coi Longobardi della Toscana, mercè dell'occupazion di Perugia: poscia dopo la rottura, che sieno stati levati di Roma i soldati ivi soliti a stare di presidio, per guernire Narni e Perugia, lasciando in tal guisa abbandonata ed esposta a pericoli di perdersi quell'augusta città. Aggiugne essere stata la piaga maggiore l'arrivo di Agilolfo, perchè si videro tanti miseri Romani legati con funi al collo a guisa di cani, e condotti a vendere in Francia, dove dovea praticarsi un gran mercato di schiavi, benchè cristiani. Tali parole fecero credere al Sigonio [Sigon., de Regn. Ital., lib. 1.] che l'assedio di Roma fatto da Agilolfo s'abbia da riferire all'anno precedente 594, e non è dispregevole la di lui conghiettura, quantunque a me sembri più probabile che quel fatto succedesse prima. Si lagna ancora il buon papa che dopo essere i Romani scampati da quel fiero turbine, si voglia ancora crederli colpevoli per la scarsezza del frumento, in cui si trovava allora la città, quando s'era già rappresentato alla corte che non si potea lungo tempo conservare in Roma una gran provvisione di grano. E sofferiva bene esso papa con pazienza tante contrarietà; ma non sapeva già digerire che gli Augusti padroni fossero in collera contra di Gregorio prefetto di Roma, e di Castorio generale delle milizie, che pure aveano fatto de' miracoli nella difesa della città.
- 66. Di questo passo andavano allora gli affari d'Italia con un principe che vendeva le cariche, che credeva più ai cattivi che ai buoni consiglieri, e sceglieva ministri malvagi, i quali venivano in Italia, non per far del bene ai popoli, ma per ismugnere il loro sangue. Di questo ne abbiam la testimonianza dello stesso san Gregorio in una lettera scritta a Costantina Augusta moglie dell'imperadore Maurizio [Greg. Magnus, lib. 5, ep. 41.], dove le significa d'aver convertito alla fede molti gentili che erano nell'isola di Sardegna, e scoperto in tal congiuntura che costoro pagavano dianzi un tanto al governatore per aver licenza di sagrificare agl'idoli; e che anche dopo la lor conversione seguitava il governatore a voler che pagassero. Ripreso dal vescovo per tale avania, avea risposto d'aver promesso alla corte tanto danaro per ottener quella carica, e che neppur questo bastava per soddisfare al suo impegno. Nella Corsica poi tante erano le gravezze, che gli abitanti per pagarle erano costretti fino a vendere i proprii figliuoli, di maniera che moltissimi, i quali possedevano beni in quell'isola, erano forzati a ricoverarsi sotto il dominio della nefandissima nazion dei Longobardi, la quale dovea trattar meglio i sudditi suoi, e superava nel buon governo i Greci. Così in Sicilia eravi un esattore imperiale per nome Stefano, che senza processo confiscava a più non posso i beni di que' possidenti. Peggio nondimeno che gli altri operava Romano patrizio, esarco di Ravenna. Con tutta la sua umiltà e pazienza il santo pontefice Gregorio non potè di meno di non accennare a Sebastiano vescovo del Sirmio [Greg. Magnus, ep. 42.], amico d'esso esarco, le oppressioni che Roma pativa per l'iniquità di costui. Breviter dico (sono sue parole) quia ejus in nos malitia gladios Longobardorum vicit, ita ut benigniores videantur hostes, qui nos interimunt, quam reipublicae judices, qui nos malitia sua, rapinis atque fallaciis in cogitatione consumunt. Eppure i soli Longobardi erano trattati da nefandissimi. Venne a morte in quest'anno Giovanni arcivescovo di Ravenna, e in suo luogo fu eletto Mariniano, a cui papa Gregorio concedette il pallio. Rapporta eziandio Girolamo Rossi [Rubeus, Hist. Ravenn., lib. 4.] una bolla di papa Gregorio, confirmatoria de' privilegii della chiesa ravennate; ma che contien troppe difficultà per crederla vera. Il cardinal Baronio [Baronal. An. Eccl.] ne ha mostrata la falsità. Passò
- 67. ancora a miglior vita san Gregorio vescovo Turonense, insigne storico delle Gallie. Circa questi tempi fu creato duca di Baviera Tassilone da Childeberto re dell'Austrasia. Egli è chiamato re della Baviera da Paolo Diacono [Paulus Diaconus, lib. 4, cap. 7.] e da Sigeberto [Sigebertus, in Chron.] copiatore d'esso Paolo. Ma niun d'essi e niuna delle memorie antiche ci fa sapere cosa divenisse di Garibaldo duca o re d'essa Baviera, padre, siccome dicemmo, di Teodelinda regina de' Longobardi. Credesi che egli terminasse il corso de' suoi giorni, oppure che Childeberto sovrano della Baviera, a cagion dell'alleanza da lui contratta per via del matrimonio suddetto coi re longobardi, e da lui mal veduta, gli movesse guerra e il deponesse. Si sa ch'egli ebbe un figliuolo per nome Gundoaldo, che venne in Italia colla sorella Teodelinda, e questi, per attestato di Fredegario [Fredegar., in Chron., cap. 34.], si accasò con una donna nobile di nazion longobarda, e n'ebbe de' figliuoli. Avremo occasione di parlare di questi principi più abbasso. Nè vo' lasciar di dire che in questi tempi l'umile pontefice romano ebbe da combattere colla superbia di Giovanni il Digiunatore, patriarca di Costantinopoli, il quale voleva attribuirsi il titolo di vescovo ecumenico ossia universale. A questa usurpazione egli si oppose con tutta forza e mansuetudine. Ne scrisse a lui [Gregor. Magnus, lib. 5, epist. 21.], all'imperadore, e a Costantina imperadrice, dolendosi specialmente con quest'ultima, perchè si permettesse che fosse maltrattata la Chiesa romana, capo di tutte. Dice, fra le altre cose, in essa lettera, essere già ventisett'anni che i Romani viveano fra le spade dei Longobardi (prendendo le afflizioni dell'Italia dall'anno 568, in cui i Longobardi vi entrarono), e che la Chiesa romana avea fatto e faceva di grandi spese della propria borsa per regalare essi Longobardi, e salvare con tal mezzo il suo popolo: di modo che siccome l'imperadore teneva in Ravenna il suo tesoriere e spenditore per pagare l'esercito, così esso papa era divenuto spenditore in Roma, con impiegar nello stesso tempo le sue rendite in mantenimento del clero, de' monisteri e de' poveri, e in placare essi Longobardi. Contuttociò si vedeva questa deformità, che la Chiesa romana era astretta a sofferir tali strapazzi dall'ambizion del vescovo di Costantinopoli. Ma Giovanni digiunatore finì in quest'anno medesimo la lite col fine della sua vita: uomo per altro
- 68. dipinto dai Greci per prelato di virtù cospicue, per le quali fu poi da essi messo nel ruolo dei santi.
- 69. Anno di Cristo DXCVI. Indizione XIV. Gregorio I papa 7. Maurizio imperadore 15. Agilolfo re 6. L'anno XIII dopo il consolato di Maurizio Augusto. Si andava tuttavia maneggiando l'affare della pace tra il re Agilolfo e l'esarco di Ravenna. Ma perciocchè non mancavano persone che per privati riguardi attraversavano il pubblico bene, s. Gregorio [Gregor. Magnus, lib. 6, ep. 30 et 31.] diede incumbenza a Castorio suo notaio residente in Ravenna di sollecitar questo aggiustamento, senza il quale soprastavano dei gravi pericoli a Roma stessa e a diverse isole. Ma in Ravenna da gente maligna fu di notte attaccato alle colonne un cartello in discredito, non solo del suddetto Castorio, ma del medesimo papa, quasichè per fini storti amendue promovessero l'affare di essa pace. S. Gregorio ne scrisse a Mariniano arcivescovo, al clero, ai nobili, ai soldati e al popolo di quella città, con ordinare che pubblicassero la scomunica contra gli autori d'esso cartello. Nella Campania dovette esser guerra in questo anno, ed in essa furono presi molti Napoletani dai Longobardi. Non fu pigro il pietoso cuore del pontefice romano a scrivere tosto ad Antemio suddiacono, suo agente in Napoli [Idem, ib., ep. 35.], con inviargli una buona somma di danaro per riscattare chiunque non
- 70. avea tanto da potere ricuperare la libertà. In quest'anno ancora l'infaticabil papa prese la gloriosa risoluzione di spedire in Inghilterra s. Agostino monaco del monistero di s. Andrea di Roma, con altri compagni, a fin di convertire alla fede di Cristo gli Anglo-Sassoni, Barbari che da gran tempo aveano occupata la maggior parte della Bretagna maggiore. Questa memorabil impresa è una di quelle, per le quali il santo pontefice specialmente si acquistò il titolo di grande, e quello ancora di apostolo dell'Inghilterra, titolo parimente dato al medesimo Agostino, che fu creato primo arcivescovo di Cantuaria, e fece delle maraviglie per ridurre que' popoli alla greggia di Cristo. Riferisce Beda [Beda, Hist. Angl., lib. I, cap 23.] una lettera di s. Gregorio papa, rapportata anche da Gotselino [Gotselinus, in Vita S. August. Cantuar. n. 7 et 8.] nella vita del suddetto s. Agostino, e scritta die X kalendas augusti, imperante D. N. Mauricio Tiberio piissimo Augusto, anno XIV post consulatum ejusdem domini nostri anno XIII, Indictione XIV. Leggonsi le medesime note cronologiche in un'altra lettera del medesimo papa ad Eterio vescovo, oppure a Virgilio vescovo, o ad altri (il che poco importa), riferita dal medesimo Gotselino. Ora queste indicano precisamente il presente anno, perchè nel dì 25 luglio dell'anno 596 correva tuttavia l'anno quattordicesimo dell'imperio di Maurizio, e l'indizione quattordicesima. E perciocchè in questo tempo concorre l'anno decimoterzo dopo il consolato di esso Augusto, si viene a conoscere aver io fondatamente messo il consolato di Maurizio nell'anno 583, contro il parere del padre Pagi. Seguì nell'anno presente la morte ben frettolosa di Childeberto II, potentissimo re dell'Austrasia e della Borgogna, che avea recato tanti fastidii ai Longobardi e tanti danni alla Italia. Non avea più di venticinque o ventisei anni d'età; ed essendo pur morta nello stesso giorno, o poco dopo, la regina Faileuba sua moglie, fu creduto che amendue fossero portati via dal veleno; ed alcuni scrittori moderni ne han fatto cadere il sospetto sopra la regina Brunechilde sua madre, principessa che nulla trascurò per regnare. Ma nulla di ciò dicendone gli antichi, niun fondamento v'ha di questa diceria. Lasciò due figliuoli piccioli, Teodeberto re dell'Austrasia, e Teoderico re della Borgogna. Abbiamo da Paolo Diacono [Paulus Diaconus, lib. 4, cap. 11 et 14.] che il re Agilolfo
- 71. mandò, non si sa in qual anno, ambasciatori ad esso re Teoderico, o, per dir meglio, alla suddetta regina Brunechilde, che come tutrice de' nipoti governava gli stati, e stabilì una pace perpetua con esso. Racconta il medesimo storico che circa questi tempi si videro per la prima volta in Italia de' cavalli selvatici e de' bufali, che erano riguardati per maraviglia dagl'Italiani. E perciocchè Romano esarco era pertinace in non voler la pace, apprendiamo da una lettera di san Gregorio [Gregor. Magnus, lib. 4, ep. 60.] ad Eulogio patriarca d'Alessandria, che i Romani pagavano la pena dell'iniquità di costui, scrivendo egli con sommo dolore, che non passava giorno senza qualche saccheggio, o morti, o ferite di quel popolo a cagion della guerra coi Longobardi. Da un'altra lettera del medesimo santo pontefice, scritta a Teottista patrizia [Idem, lib. 7, ep. 26.], ricaviamo che in questo anno essi Longobardi condotti o spediti da Arichi, ossia da Arigiso duca di Benevento, presero la città di Crotone, oggidì Cotrone nella Calabria ulteriore, e condussero via schiavi molti uomini e donne, pel riscatto dei quali si affaticò la non mai stanca carità di questo inclito papa. Non apparisce che i Longobardi si mantenessero in quella città, troppo esposta alle forze marittime de' Greci.
- 72. Anno di Cristo DXCVII. Indizione XV. Gregorio I papa 8. Maurizio imperadore 16. Agilolfo re 7. L'anno XIV dopo il consolato di Maurizio Augusto. Siam qui abbandonati dalla storia, senza sapere qual fatto rilevante accadesse in quest'anno in Italia, a riserva delle azioni di s. Gregorio magno papa nel governo della Chiesa di Dio, che si possono leggere presso il cardinal Baronio e nella vita scrittane dai monaci Benedettini di s. Mauro. Certo durava tuttavia la guerra fra i Longobardi e i sudditi del romano imperio; ed essendo sì confusi i confini delle due diverse giurisdizioni, facile è che succedessero delle ostilità fra le due parti. Avevano i Greci mantenuto fin qui il loro dominio, non solamente nell'esarcato di Ravenna e nel ducato romano, ma ancora in Cremona, in Padova ed in altre città, massimamente marittime, ed anche Mantova era tornata alle loro mani. Non si sa intendere come i Longobardi più poderosi de' Greci non formassero l'assedio o il blocco di tali città che cotanto s'internavano ne' loro stati. Ma forse non istettero colle mani alla cintola, e noi solamente per mancanza di memorie, delle quali era privo anche Paolo Diacono, non abbiam contezza degli avvenimenti d'allora. Si crede nondimeno che san Gregorio papa in inscrivendo a
- 73. Gennadio patrizio ed esarco dell'Africa [Gregor. Magnus, lib. 4, ep. 3.], gli raccomandasse in quest'anno di vegliare alla sicurezza dell'isola di Corsica, sottoposta al governatore dell'Africa, perchè temeva di uno sbarco dei Longobardi in quell'isola e nella vicina Sardegna, come in fatti da lì a non molto accadde. Abbiamo poi da Teofilatto [Theophilact. l. 8, cap. 11.] che verisimilmente nell'anno presente caduto infermo Maurizio Augusto, fece testamento, in cui lasciò l'imperio d'Oriente a Teodosio Augusto, il maggiore de' suoi figliuoli, e l'Italia colle isole adiacenti a Tiberio suo figliuolo minore. Egli poi si riebbe da quel malore. Quanto meglio avrebbe egli operato se avesse inviato in Italia questo suo secondogenito! Sarebbe stata in salvo la di lui vita: e forse la presenza di questo principe avrebbe rimesso in migliore stato gli affari d'Italia. Non so dire se intorno a questi tempi terminasse i suoi giorni in Ravenna Romano patrizio ed esarco, uomo nemico della pace, e che pescava meglio nel torbido. Pare che si possa ricavare da un'epistola di s. Gregorio [Greg. Magnus, lib. 7, ep. 29.], che venisse in quest'anno a Ravenna Callinico suo successore, personaggio di massime più diritte e più riverente verso il santo pontefice Gregorio. Certo è solamente che esso esarco si trova in Ravenna nell'anno 599. Negli Atti de' santi [Acta Sanctorum Bolland. ad diem 13 junii.], raccolti ed illustrati dal padre Bollando e da' suoi successori della Compagnia di Gesù, abbiamo la vita di s. Ceteo vescovo di Amiterno, città florida una volta, ed oggidì distrutta, dalle cui rovine nacque la moderna città dell'Aquila, distante cinque miglia di là. Ivi è detto ch'egli era vescovo di quella città ai tempi di s. Gregorio il grande e di Faroaldo duca di Spoleti, nel cui ducato era compreso Amiterno. Furono deputati al governo di essa terra due Longobardi ariani, come erano i più di questa nazione, chiamati Alais ed Umbolo. Per la lor crudeltà Ceteo vescovo se ne fuggì a Roma, e fu a trovare il santo papa Gregorio. Richiamato dal popolo alla sua residenza, godeva egli quiete e pace, quando Alais inviperito contro del compagno, mandò segretamente a Veriliano conte d'Orta, città che doveva essere allora in poter dei Greci, acciocchè venisse una notte alla distruzion di Amiterno. Andarono gli Ortani; ma scoperto a tempo il lor tentativo, furono ripulsati. Alais restò convinto del tradimento, e perchè il vescovo Ceteo volle salvargli la vita, fu
- 74. preteso complice, e però barbaramente gittato nel fiume Pescara ivi si annegò, e ne fu poi fatto un martire. In quella leggenda v'ha delle frottole: contuttociò non è da disprezzare il racconto suddetto.
- 75. Anno di Cristo DXCVIII. Indizione I. Gregorio I papa 9. Maurizio imperadore 17. Agilolfo re 8. L'anno XV dopo il consolato di Maurizio Augusto. Da una lettera [Greg. Magnus, lib. 8, ep. 18.] scritta in questo anno da s. Gregorio ad Agnello vescovo di Terracina, si ricava, che tuttavia restavano in quella città delle reliquie del paganesimo, le quali il santo papa procurò di schiantare. A questo fine si raccomandò ancora a Mauro visconte d'essa città, acciocchè assistesse col braccio secolare alle diligenze del vescovo. Ordinò nello stesso tempo che niuno fosse esentato dal far le guardie alla città: al che ne' bisogni erano tenuti anche gli ecclesiastici; e che neppure i monaci godessero esenzione da questo peso, si raccoglie da un'altra lettera dello stesso pontefice [Idem, lib. 9, ep. 73.]. Questo ci fa vedere che continuasse la guerra, e fin dove arrivassero in questi tempi le scorrerie dei Longobardi. Riconosce egli dipoi [Idem, lib. 8, ep. 22.] l'essersi da tanto tempo preservata essa città dal cadere in mano de' nemici suddetti dalla protezion del principe degli apostoli s. Pietro, giacchè quella città si trovava allora senza gran popolo e senza guarnigione, almen sufficiente, di soldati. Il nome di visconte, che abbiam veduto poco fa, vuol che io ricordi qui come in questi secoli
- 76. era in uso, e questo durò molti secoli dipoi, che i governatori d'una città erano appellati comites, conti. Aveano questi il loro luogotenente, chiamato perciò vicecomes, che nella lingua volgare italiana passò in viceconte, e finalmente in visconte. Dalle parole di s. Gregorio sovraccitate si raccoglie che nelle città tuttavia soggette all'imperio vi doveva essere il visconte, e per conseguenza il conte. Lo stesso si praticava in Francia. Veramente i Longobardi soleano chiamar giudici i governatori delle loro città, come consta dalle lor leggi. Contuttociò talvolta ancora questi giudici portano il nome di conte. L'ordinario poi significato del titolo di duca competeva a quei solamente che comandavano a qualche provincia, ed avevano sotto di sè più conti. Trovansi nondimeno duchi d'una sola città. Ma di queste cose ho io abbastanza trattato nelle Antichità estensi [Antichità Estensi, cap. 1, part. 1.] e nelle Antichità italiane [Antiq. Italic., Dissert. VIII.]. Quello ancora ch'è da notare, non era per anche nato in questi tempi il titolo di marchese; e però la bolla che il Rossi, per quanto accennai di sopra, riferisce data da s. Gregorio a Mariniano arcivescovo in Ravenna, si scuopre falsa al vedere fatta ivi menzione dei marchesi, nome nato circa due secoli dipoi. Penso io che al presente anno appartenga la notizia di uno sbarco fatto dai Longobardi nell'isola di Sardegna, di cui siam debitori ad una lettera di san Gregorio [Greg. Magnus, lib. 9, ep. 4.], scritta ne' primi mesi della Indizione seconda, cominciata nel settembre di quest'anno. L'aveva già preveduto il buon pontefice, senza lasciare di portarne per tempo colà l'avviso, acciocchè si facesse buona guardia, ma non gli fu creduto nè ubbidito. Ora colla presente lettera, scritta a Gennaro vescovo di Cagliari, significa che finalmente era riuscito all'abbate Probo, inviato da esso papa al re Agilolfo, d'intavolar la pace. Ma perchè ci voleva del tempo, prima che ne fossero sottoscritte le capitolazioni da tutte e due le parti, perciò lo esorta ad ordinar una miglior guardia delle mura e ne' siti pericolosi, affinchè non venga voglia ai nemici di tornare in questo mentre a visitarli. Convien poi credere che nascesse qualche difficoltà, per cui paresse intorbidata la speranza d'essa pace; perciocchè da lì a poco (se pure non v'ha sbaglio nell'ordine e nella distribuzion delle lettere di s. Gregorio) torna egli a scriver al medesimo vescovo [Gregor. Magnus, lib. 9, ep. 6.], che finita
- 77. questa pace Agilolfo re de' Longobardi non farà la pace: parole scure all'intendimento nostro. Forse era seguita una tregua, e si temeva che terminata questa non vi avesse da essere pace. Pertanto gl'inculca la necessità di stare all'erta, e di fortificare e provvedere di viveri più che mai la città di Cagliari e gli altri luoghi della Sardegna, per deludere gl'insulti de' nemici. Così il santo pontefice, indefesso in accudire anche alla difesa delle terre lontane dello imperio romano pel suo nobil genio, ed eziandio, come si può credere, perchè Maurizio Augusto gli avea data la incumbenza di vegliare e soprintendere ai suoi affari per tutta l'Italia.
- 78. Anno di Cristo DXCIX. Indizione II. Gregorio I papa 10. Maurizio imperadore 18. Agilolfo re 9. L'anno XVI dopo il consolato di Maurizio Augusto. Finalmente in quest'anno fu conchiusa la pace fra il re Agilolfo e Callinico, esarco di Ravenna. Ne fa menzione Paolo Diacono [Paulus Diaconus, lib. 4, cap. 13.], e l'anno si ricava dalle lettere scritte sotto la presente indizione seconda da san Gregorio papa [Greg. Magnus, lib. 9, ep. 42 et 43.], non solo alla cattolica regina Teodelinda, ma anco ad esso re Agilolfo, forse tuttavia ariano; non apparendo ch'egli avesse peranche abbracciata la religion cattolica. Ringrazia dunque Agilolfo della pace fatta, il prega di ordinare ai suoi duchi che la osservino e non cerchino dei pretesti per guastarla. Il saluta ancora con paterna carità: parole che paiono indirizzate ad un re cattolico, ma che sembrano poi non accordarsi coll'altre che egli soggiugne alla regina. Perciocchè dopo averla ringraziata dell'efficace mano che ella aveva avuta per condurre alla pace il regal consorte, l'esorta, ut apud excellentissimum conjugem vestrum ita agatis, quatenus christianae reipublicae societatem non rejiciat. Nam sicut ei vos scire credimus, multis modis est utile, si se ad ejus amicitias conferre voluerit. Queste parole paiono significare, desiderarsi dal papa una lega dei
- 79. Longobardi coll'imperadore; ma può anche sospettarsi desiderio nel pontefice che la regina s'ingegni di tirare il marito al cattolicismo: il che per molte cagioni gli sarebbe riuscito di profitto, perchè certo tanti Cattolici suoi sudditi non miravano di buon occhio un principe ariano, e molto meno i Cattolici non suoi sudditi. Anche secondo l'umana politica sarebbe tornato il conto ad Agilolfo l'unirsi colla Chiesa cattolica; e questo punto l'intese bene Clodoveo il grande re de' Franchi e Recaredo re dei Visigoti, principi che abbracciarono la fede cattolica romana, e meglio con ciò si stabilirono nei loro regni. E che così facesse anche il re Agilolfo l'abbiamo da Paolo Diacono [Paulus Diaconus, lib. 4, cap. 6.], là dove scrive ch'egli mosso dalle salutevoli preghiere della regina Teodelinda, catholicam fidem tenuit, et multas possessiones Ecclesiae Christi largitus est, atque episcopos, qui in depressione et abjectione erant, ad dignitatis solitae honorem reduxit. Ma ciò dovette seguire più tardi, siccome vedremo più abbasso. Intanto certa cosa è che il re Agilolfo, cattolico o ariano che si fosse in questi tempi, non inquietava punto per conto della religione i Cattolici, e lasciava tutta la convenevole libertà ai vescovi di esercitare il sacro lor ministero, di comunicare colla santa sede, e di passare, occorrendo bisogni ecclesiastici, a Roma e a Ravenna, tuttochè città nemiche. In somma s'egli non avea per anche abjurato l'arianismo, almeno per le premure di Teodelinda piissima e cattolica regina, amorevolmente trattava i professori del cattolicismo. Non so io poi intendere come san Gregorio dopo avere scritte le lettere suddette, in una altra indirizzata ad Eulogio patriarca [Greg. Magnus, lib. 9, ep. 78.], sotto la stessa Indizione II, gli dica di trovarsi oppresso dai dolori della podagra e dalle spade dei Longobardi. Se la pace era fatta, come poi lagnarsi della guerra che suppone fatta dai Longobardi ai Romani? Ciò mi fa dubitare se a questa lettera sia stato assegnato il suo convenevol sito. Ma è ben degna di attenzione un'altra lettera scritta da questo glorioso pontefice a Teodoro curator di Ravenna [Idem, ibid., ep. 98.], ministro che cooperato avea non poco alla conclusion della pace. Gli fa dunque sapere che Ariolfo duca di Spoleti non avea voluto sottoscrivere la pace puramente, come il re Agilolfo avea fatto, con avervi apposto due condizioni, cioè ch'egli l'accettava, purchè dalla
- 80. parte dei Romani non si commettesse in avvenire eccesso alcuno contra de' Longobardi, nè potessero i Romani far guerra ad Arichi, ossia Arigiso duca di Benevento, confinante col ducato di Spoleti e collegato di esso Ariolfo. Nell'edizione di san Gregorio è scritto Arogis, ma si ha da scrivere Arigis. Questa maniera di giurar la pace con tali riserve comparve a san Gregorio insidiosa e furbesca, affinchè restasse aperto l'adito a nuove rotture, non mancando mai pretesti per far guerra a chi ha in odio la pace. E tanto più trovava egli delle magagne in questo aggiustamento, perchè Varnilfrida (forse moglie d'esso Ariolfo, non parendo questo un nome di maschio, che sarebbe stato Varnilfrido) non l'avea voluto sottoscrivere. Aggiunge che gli uomini mandati dal re Agilolfo a Roma esigevano che dal medesimo papa fossero sottoscritti i capitoli della suddetta pace: segno della considerazione e stima che quel re avea del romano pontefice, oppure che, non fidandosi dei Romani, esigesse per sigurtà lo stesso pontefice. Ma san Gregorio abborriva di farlo, sì perchè gli erano state riferite da Basilio, uomo chiarissimo, delle parole ingiuriose proferite da esso re contra della sede apostolica, e dello stesso papa Gregorio, benchè Agilolfo negasse a spada tratta di averle dette; e sì ancora, perchè se mai si fosse mancato da lì innanzi contro i patti, egli non voleva averne da render conto, premendogli di non disgustare un principe, di cui avea troppo bisogno pel governo di tante chiese poste sotto il di lui dominio. Però si raccomanda affin d'essere esentato da quella sottoscrizione. Stendeva in addietro il vescovo di Torino la sua giurisdizione nella valle di Morienna e di Susa. Furono occupati questi paesi da Guntranno re di Borgogna, allorchè i Longobardi fecero le irruzioni nelle Gallie, come raccontammo di sopra, ed uniti al suo regno della Borgogna. Ciò fatto, non piacendo ad esso re che que' popoli neppure pel governo spirituale fossero sottoposti al vescovo di Torino, cioè di una città sottoposta ai Longobardi, fece creare un nuovo vescovo della Morienna. Se ne dolse Ursicino vescovo di Torino con san Gregorio, il quale sopra ciò scrisse due lettere [Gregor. Magnus, lib. 9, ep. 95 et 96.], l'una a Siagrio vescovo d'Autun, e l'altra a Teoderico e Teodeberto re de' Franchi, con
- 81. pregarli che non fosse recato pregiudizio ai diritti del vescovo torinese. Ma egli cantò a gente sorda; il vescovato di Morienna sussistè, e tuttavia sussiste. E da una d'esse lettere apparisce che il vescovo di Torino avea patito dei saccheggi nelle sue parrocchie, e che il popolo era stato condotto (certamente dai Franchi) in ischiavitù negli anni addietro. Rapporta l'Ughelli [Ughellius Italia Sacr., tom. 4, in Episcop. Bobiens.] una carta d'oblazione fatta da san Colombano abate del monistero di Bobio a san Gregorio papa anno pontificatus domni Gregorii summi pontificis et universalis papae IV, Indictione III sub die III mensis novembris. L'indizione terza cominciata nel settembre mostra appartener quella carta all'anno presente. Ma il lettore osservando che non correva in quest'anno l'anno quarto di san Gregorio, e che non fu in uso di que' tempi il chiamare il romano pontefice, benchè capo della Chiesa di Dio, papa universale: (titolo che lo stesso san Gregorio impugnò cotanto nel patriarca di Costantinopoli); e che questa carta discorda dall'altre antiche memorie che fanno, siccome diremo più abbasso, fondato molto più tardi il monistero di Bobio; e che non si fa menzione degli anni dell'imperadore, come era il costume, benchè la carta si supponga scritta in Roma: non saprà, dissi, il lettore prestar fede ad un sì fatto documento.
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