ESS - MOD 3 (1).pptx ktu eet438 ENERGY STORAGE SYSTEMS
- 2. Overview on Energy storage technologies (8) • Electrochemical energy : Batteries- Battery parameters: C-rating -SoC- DoD- Specific Energy-Specific power (numerical examples), • Fuel cells • Electrostatic energy (Super Capacitors) • Electromagnetic energy (Super conducting Magnetic Energy Storage) • Comparative analysis, Environmental impacts of different technologies.
- 3. Battery Energy Storage System • A battery energy storage system (BESS) converts electrical energy into potential chemical energy while charging, and releases electrical energy from chemical energy while discharging. • It is based on reduction and oxidation reactions (commonly called redox reactions). An electrochemical reduction reaction is one that allows the component involved to gain electrons, while an oxidation reaction allows the component to lose electrons. • Redox reactions yield new active electrochemically active substances with nonneutral electric charge, and hence ions. The flow of electrons and ions exists as long as there is an energy difference between the electro-chemically active substances involved in the reduction and oxidation reactions. • To enable this flow of ions and electrons, the battery cell has two circuits, one external and the other internal.
- 4. • The internal circuit is comprised of the battery cell itself, and provides the path through which the resultant ions flow. • The electrical circuit is closed by adding the external circuit, thus providing the path through which the electrons resulting from the redox reactions can flow. • This external path is provided by the external system (either a load or an energy source) to which the battery is connected. • The battery cell is comprised of the following components: The electrodes. • While discharged, oxidation reactions occur in the anode of the battery (the negative electrode), which is the electrode that captures the electrons lost by the component. • Conversely, reduction reactions occur in the cathode of the battery (the positive electrode), which is the electrode that provides the electrons gained by the reduced component.
- 5. Two pairs of electrochemically active substances. • There is one in the anolyte region while the other is in the catholyte region. The materials composing the anolyte electrode and the component or substance surrounding it have to react, yielding an oxidation reaction (while discharged). • Analogously, the electrochemical interaction between the materials comprising the catholyte electrode and the substance or component surrounding it yields a reduction reaction. The above defines the two pairs of electrochemically active substances. The electrolyte. • Apart from causing the two pairs of electrochemically active substances to gain or lose electrons, the redox reactions yield ions (and hence particles with a nonneutral electric charge). • To ensure the equilibrium of charge between the anolyte and catholyte electro- chemically active substances, these ions are exchanged between them. This ion transport is enabled by the electrolyte, which is a solid or liquid electronically insulating substance.
- 6. The separator. • There is an electrical potential between the electro-chemically active substances in the anolyte and catholyte regions. • The separator avoids direct contact between them, thus preventing the battery from an internal short circuit. The container. • Batteries are composed of several cells, either in series or in parallel to achieve the desired electrical characteristics. • In the container, they are all packed into a controlled and isolated environment.
- 7. • The battery cell is composed of two electrodes, made up of two materials called Y0 (for the anode) and X0 (for the cathode). They are both surrounded by the electrolyte, Z. Also, the anode (negative electrode) is surrounded by the substance or component Y1. Similarly, the cathode is surrounded by the component X1. • The materials X0–X1 and Y0–Y1 define two pairs of electrochemically active substances. The difference in energy state of the two pairs of electrochemically active substances is translated into a voltage difference. • While charged, the voltage between the electrodes of the cell is at a maximum and varies between 1 and 4 volts, depending on the technology used, yielding the so-called open- circuit voltage of the cell. • By adding an external load between the electrodes, the electrical circuit is closed. Then, the battery is being discharged, and this means that redox reactions start to occur, yielding an electric current through the load. • The electrons flow from the negative electrode (the anode, the region with the maximum energy state) to the positive one (the cathode, the region with the minimum energy state).
- 8. • These electrons, and the positive ions Y2+, are the result of the oxidation reaction between substances Y0 and Y1.The electrons are collected by the catholyte electrode, yielding a reduction reaction between substances X0 andX1, which in turn results in the ionX2−. • The internal circuit allows the ionic exchange. As a result of this process, each of the pairs of electrochemically active substances is weakened, so the electric potential between them is diminished. • The electrical potential between the two electrodes can be restored by reversing the flow of the electric current, and hence by applying an external energy source to charge the battery. • The electric potential derived from the chemical reactions by the two pairs of electrochemically active substances, measured in the full charge state of the cell when disconnected from any circuit, is called the open circuit voltage V0. • This voltage determines the free energy of reaction (the Gibbs free energy) of the electrochemical reactions in the battery cell.
- 9. Voltage Discharge Profile • Due to the internal resistance of the cell and the electric current flowing through it, there is a voltage drop in the cell, so one should not consider the cell voltage measured at the beginning of the discharging process (and hence under full charge conditions) to be equal to the open-circuit voltage. • By discharging the cell, the voltage decays from its maximum value when fully charged to the so-called cutoff voltage, which defines the usable voltage range of the cell. • From this point on, the voltage decays dramatically, strongly limiting the usability of the device.
- 10. • Manufacturers usually indicate a “nominal cell voltage.” This is just an averaged value between the maximum and the cutoff voltage. This average cell voltage, though, in conjunction with the discharge current rate, serves for the calculation of the equivalent internal resistance of the cell. • The discharge voltage profile of the cell depends on many factors, such as pressure and temperature, since they affect the performance of the chemical reactions in the cell. • The cell voltage and the discharge current rate are somewhat related, thus affecting the energy capacity. This is why manufacturers are supposed to indicate the capacity of the cells for different discharge current rates, also indicating the applied control discharge method • The higher the current discharge rate, the lower are the maximum and the cutoff voltage of the cell, thus decreasing the energy storage capacity.
- 11. Lead–Acid Batteries • There are two major types of lead–acid battery: flooded batteries, which is the most common topology, and valve-regulated batteries, which are the subject of extensive R&D. • Commonly, lead–acid battery cells are built up of several lead plates arranged in parallel. These are alternatively polarized, so that the cathodic plates are coated with lead dioxide PbO2 and the anodic plates with porous lead Pb. • The plates are immersed in the electrolyte, which is made up of sulfuric acid H2SO4. The global oxidation and reduction reactions in the cell can be summarized as follows:
- 12. • During the discharging process, the porous lead anode reacts with the sulfuric acid, yielding lead sulfate PbSO4 and an excess of electrons, which are transmitted through the external circuit of the cell (the connected external load) to the cathode. • These electrons, among with the sulfuric acid, react with the lead dioxide to also form lead sulfate PbSO4. In addition, water is formed in this process. • Since the electrolyte is consumed in the reactions, the specific gravity serves as a guide for estimating the SoC of the battery. The electrical potential between the two electrodes of the cell due to the reactions described above results in around 2.04 V. • Lead–acid batteries suffer from so-called sulfation. This occurs when the battery is deprived of periodic full-charge processes. In this case, large lead sulfate crystals are formed, which cannot be reversed in the porous lead and lead dioxide in the electrodes of the battery, thus decreasing the battery’s capacity. • Sulfation is also exacerbated by exhausting the energy stored in the battery, so very deep discharges are not recommended.
- 13. • Another common problem arises when the applied charging voltage surpasses the admissible or recommended level. In this case, the water in the electrolyte can be exhausted by forming hydrogen gas, with the consequent risk of explosion due to its high flammability. • Lead–acid batteries present the poorest cycle life, just 200–1800 cycles depending on the depth of discharge (DoD) and the operating temperature, among the different types of batteries considered in this work. • In addition, the need for periodic water maintenance (of flooded batteries) and the low energy and power densities of this type of battery. • In spite of the above-mentioned drawbacks, their use is widespread in both stationary and nonstationary applications. • One of the most important advantages of lead–acid batteries is their low cost compared to the costs of other types of batteries.
- 14. Nickel–Cadmium Batteries • Ni–Cd batteries are primarily produced using nickel and cadmium hydroxide. These materials are then polarized into nickel oxyhydroxide NiO(OH) cathodic plates, and anodic plates of porous cadmium. The battery cells are immersed in an electrolytic aqueous alkaline solution based on potassium hydroxide KOH. • To prevent short circuits between adjacent anodic and cathodic plates in the cell, a separator is used. This is generally based on polystyrene or polypropylene, but other options are suitable, such as fibrous polyamide. The selection of the separator is critical, as it can constrain the easy mobility of ions produced in chemical reactions between the electrodes. • The global oxidation and reduction reactions in the cell can be summarized as follows:
- 15. • During the discharging process, the porous cadmium Cd in the anode reacts with the ion OH−, yielding Cd(OH)2 and the electrons that are transmitted through the external circuit of the cell to the cathode. • These electrons, along with the water in the electrolyte, react with NiO(OH) to form Ni(OH)2 and the ion OH−. The electrical potential between the two electrodes of the cell due to the reactions described above results in around 1.2 V, and hence in a lower potential than in lead–acid batteries. • Ni–Cd batteries present good characteristics with regard to cyclability (more than 3500 cycles and even 50 000 cycles at 10% of DoD), high ramp power rates, and low maintenance. • On the other hand, they present three major drawbacks that limit their commercial success. First, the cost of Ni–Cd batteries is very high compared to the cost of lead– acid batteries (more than ten times). • Second, cadmium and nickel are toxic heavy metals that can cause health risks in humans. For this reason, the European Commission proposed recycling targets of at least 75% for this type of battery. • Third, Ni–Cd batteries suffer from the memory effect. When a Ni–Cd battery is repeatedly recharged before becoming fully discharged, a sudden voltage drop in the cell is experienced. The memory effect is actually regarded as a capacity fade and is experienced in sealed Ni–Cd batteries, but not in flooded ones.
- 16. Lithium-Based Batteries • The active material in the cathode (positive electrode) of Li-ion cells is usually lithium metal oxide, in the form of lithium cobalate (LiCoO2). • The negative electrode is mainly carbon (C) and lithium atoms are actually in the electrode. The electrolyte is an organic solution containing lithium-based dissolved salts, such as LiClO4 and LiPF6. • Finally, the electrode areas are separated by porous separators based on polyethylene or polypropylene. A schematic of an Li-ion battery is shown in figure • The global oxidation and reduction reactions in the cell can be summarized as follows
- 17. • During the charging process, lithium ions Li+ are extracted from the cathode of the cell and get inserted into the graphene sheets in the graphite (the negative electrode). • As always, the electrons resulting from the chemical reactions flow, in this case, from the positive electrode to the negative electrode through an external energy source, thus closing the electric circuit. The electrical potential between the two electrodes of the cell due to the reactions described above (the open-circuit voltage) reaches up to 3.7 V. • This high open-circuit cell voltage and the low weight of lithium yield a very high specific energy The energy density also proves to be very high, and thus Li-ion batteries are well suited for portable applications, such as mobile phones and electronic devices. • Other noticeable features are their fast charge and discharge capability and the relatively high roundtrip efficiency of 78%. • Among the drawbacks of the technology, it is important to note the required narrow voltage and temperature ranges for proper operation, which motivate the need for protection circuits. • In addition, the use of flammable organic electrolytes raises issues about security and environmental issues.
- 18. • Battery Capacity : Total amount of Electricity generated by electro-chemical reactions in the battery. It is determined by mass of active material contained in the battery. Its unit is ampere hour (Ah). • Cycle Life : Number of full charge and discharge cycles a battery can achieve before its capacity level drops before 80% which is considered typical end of life for most applications. Better the cycle life better the consumer experience. • Energy Density : Amount of energy a battery contains relative to its size. Also known as volumetric energy density. It is typically measured in Watt hours per litre (Wh/l). • Specific Energy : Amount of energy a battery contains relative to its weight. Also known as gravimetric energy density. It is typically measured in Watt hours per kilogram (Wh/kg) Battery Parameters
- 20. • Itis used to measure the speed at which battery is charging or discharging. • Charging at a C-rate of 1C means charging from 0 to 100% in 1 hour.
- 26. 400 Ah
- 28. Energy being
- 34. Fuel Cells • The fuel cells are comprised of two electrodes and an electrolyte, which enables ion exchange between them. • The anodic and cathodic regions are separated by a polymeric membrane. • The types of electrolyte are diverse and determine the performance of the cell; for example, the pressure of the hydrogen produced and the operating temperature. Electrolytes can be liquid or solid. • Conventional electrolyzers use liquid alkaline electrolytes, while modern ones use solid electrolytes. The latter type is known as the proton exchange membrane fuel cell (PEMFC). Depending on the sizing of the system, different types of fuel cells can be used.
- 35. • The essential functions of a fuel cell are 1.the charging (or electrolyser) function in which the chemical AB is electrochemically decomposed to A and B; 2. the storage function in which A and B are held apart; and 3. the discharge (or fuel-cell) function in which A and B are reunited, with the simultaneous generation of electricity.
- 36. • The electrochemical reactions in a fuel cell using PEM are depicted as follows: • The hydrogen gas (the fuel) stored in the tanks, or in another storage medium, is reduced in the anode. • The electrons delivered flow through the external load to which the system is connected, while the hydrogen protons travel through the polymeric electrolyte to the cathode. There, they combine with oxygen gas to form water. • As an exothermic reaction, this also delivers heat. The thermodynamic voltage in the cell is 1.227 V.
- 38. • The remarkable characteristics of this technology are its high ramp power rates, even at partial load, and its great cyclability, which is greater than the cyclability of flow batteries and conventional batteries. • It is also a non-polluting technology. • On the other hand, the high flammability of hydrogen gas must be properly addressed by adequate safety measures. • The major drawback of the technology, however, is its low energy efficiency. Assuming energy efficiencies for the electrolyzer and the fuel cell of about 60 and 70%, respectively, the round-trip efficiency of the system falls to 42%
- 39. Superconducting Magnetic Energy Storage (SMES) • In this type of system, the energy is stored in a magnetic field. This magnetic field is created by a DC current flowing through a superconducting coil at cryogenic temperatures. • Superconductor materials present almost negligible resistance while at cryogenic temperatures, so the magnetic field in the coil can be created and maintained with a very small amount of current flowing through it; very little energy is dissipated by ohmic losses. • The energy stored is determined by the self-inductance of the coil L (in henries) and the square of the electric current I (in amperes). Thus,
- 40. • There are two types of SMES systems, depending on the working temperature of the coil: SMES systems based on high-temperature coils (HTS) and low-temperature coils (LTS). • The former work at temperatures around 70 K, while the latter work at temperatures around 5 K. Therefore, a key aspect for proper operation of the latter system is to maintain these low operating temperatures. • However, due to the very low energy consumption of the system’s cryocoolers, the energy efficiency of SMES systems is very high, at around 90%. • The major advantage of SMES systems is related to their ability to inject or absorb vast amounts of energy in a very short time. • We can establish an analogy between the charge/discharge temporal profiles of supercapacitors and SMES systems: while the charging time constant of supercapacitors is proportional to the equivalent resistance of the electrolyte and the capacity of the supercapacitor cell, in SMES devices it is proportional to the resistance of the coil and its self-inductance.
- 41. • Also, the cyclability of the system is very high, at up to 105 cycles at 100% of DoD. • On the other hand, the use of SMES devices is limited to short-time storage applications, as the self-discharge rates of the system are relatively high, in the range of 10–15% of the rated energy capacity per hour. • This type of storage device becomes completely discharged in a very short time, discharging at full load (in the range of seconds up to few minutes).
- 42. The Supercapacitor Energy Storage System • Supercapacitors are based on electrochemical cells that contain two conductor electrodes, an electrolyte and a porous membrane that permits the transit of ions between the two electrodes. Thus, the presented layout is similar to the electrochemical cells of batteries. • The main difference between supercapacitors (or ultracapacitors, or double layer capacitors) and batteries lies in the fact that no chemical reactions occur in the cells, but the energy is stored electrostatically in the cell. • In supercapacitors, the electrodes and the electrolyte are electrically charged (the cathode is positively charged, the anode is negatively charged, and the electrolyte contains both positive and negative ions). • At each of the electrode surfaces there is an area that interfaces with the electrolyte, and it is in each of these areas where the phenomenon of the “electrical double layer” occurs.
- 43. • By applying a voltage between the electrodes, both the electrodes and the electrolyte become polarized. This means that the positive charge of the cathode is transferred to the area interfacing with the electrolyte, forming a layer of positive ions. • In turn, the negative ions of the electrolyte are transferred to the same electrolyte/cathode interface, forming a negative charge-balancing layer of ions.
- 44. • These two layers build up an “electrical double layer.” The mechanism behind the operating principle of such a double layer can be explained using the Helmholtz model. • The model establishes that the two layers are separated by a layer of solvent molecules of the electrolyte, called the inner Helmholtz plane. • This layer of solvent molecules actually separates the positive and negative charges of the electrode and electrolyte, thus acting as a dielectric. • Ultimately, there is a potential difference between the two layers of positive and negative ions derived from the electric field within them, and the double layer can be taken to resemble a capacitor. • Therefore, the magnitude of the electrical potential V (in volts) between the two layers of positive and negative ions at each electrode/electrolyte interface, in conjunction with the resultant capacitance F (in farads), determines the energy stored in the supercapacitor. Thus,
- 45. • The voltage generated in the cell is dependent on the strength of the electric field between the layers building up each of the “electrical double layers” • This electric field is, in turn, proportional to the amounts of positive and negative ions located at the electrode/electrolyte interface. So to avoid transfer of ions between the two layers of positive and negative ions, thus decreasing the voltage within the double layers, the breakdown voltage of the dielectric should be maximized. • This dielectric is provided by solvent molecules of the electrolyte. In this way, the selection of the electrolyte is key to ensuring the maximum energy capacity. The second factor affecting the energy capacity of supercapacitors is the capacitance of the cell. • In order to maximize the capacitance, different metal-oxide electrodes, electronically conducting polymer electrodes and activated carbon electrodes, are used in industry. • These materials are porous, so they can maximize the effective area of the electrode in which ions can be allocated.
- 46. • The most common types are the ones based on activated carbon, since they can lead to supercapacitors with a high energy density and capacitances around 5000 F; that is, capacities up to 1000 times per unit volume more than those of conventional electrolytic capacitors. • The electrolyte and electrode materials have a fundamental influence on the energy and power capacity of the supercapacitor, and also on its dynamic behavior. • To be precise, and with reference to the supercapacitor dynamics, one defining parameter is the so-called charge/discharge time constant, 𝜏. This is given by the product of the equivalent series resistance (ESR) of the supercapacitor and its capacitance. • The ESR weights the losses in the supercapacitor while charging and discharging; that is, those associated with the movement of ions within the electrolyte and across the separator. • Apart from the ESR and the capacitance, the third characteristic parameter for the supercapacitor is the leakage resistance, which weights the self-discharge of the cell.
- 47. • Supercapacitors are characterized by offering high ramp power rates, high cyclability (comparable with the cyclability of flywheels), high round-trip efficiency (of up to 80%), and a high specific power, in W/kg, and power density, in W/m3 (10 times more than for conventional batteries). • On the other hand, major drawbacks of the technology are related to its high self discharge rates (of up to 20% of the rated capacity in only 12 h) and its limited applicability to situations where high power and energy are needed. • The development of supercapacitors is mostly focused in fields such as automotive and portable devices. • Finally, it is worth noting that as a short-timescale ESS, supercapacitors are unsuitable in that they are expensive in comparison with other competitors such as flywheels. Their cost is estimated as 10 times the cost per kWh of flywheels.