lecture 2 - PHES.pdf (energy storage systems)

Energy Storage Systems
PUMPED-HYDRO ENERGY
STORAGE

3
Potential Energy Storage
 Energy can be stored as potential energy
 Consider a mass, m elevated to a height, ℎ
 Its potential energy increase is:
E = mgh
 Lifting the mass requires an input of
work equal to (at least) the energy
increase of the mass
  We put energy in to lift the mass
  That energy is stored in the mass as
potential energy

4
Potential Energy Storage
 If we allow the mass to fall back to its
original height, we can capture the
stored potential energy
  Potential energy converted to
kinetic energy as the mass falls
  Kinetic energy can be captured to
perform work
  Perhaps converted to rotational
energy, and then to electrical energy

5
Pumped-Hydro Energy Storage
 Potential energy
storage in
elevated mass is
the basis for
pumped-hydro
energy storage
(PHES)
  Energy used to
pump water from
a lower reservoir to an upper reservoir
  Electrical energy input to motors is converted to
rotational mechanical energy
  Pumps transfer energy to the water as kinetic, then
potential energy

6
Pumped-Hydro Energy Storage
 Energy stored in
the water of the
upper reservoir is
released as water
flows to the lower
reservoir
  Potential
energy is
converted to
kinetic energy
  Kinetic energy of falling water turns a turbine
  Turbine turns a generator
  Generator converts mechanical energy to electrical
energy

7
History of PHES
 PHES was first introduced in Italy and Switzerland
in the 1890’s
  Favorable topography in the Alps
  Four-unit systems
◼ Turbine
◼ Generator
◼ Motor
◼ Pump

8
History of PHES
 First PHES plant in the US:
  Rocky River hydro
plant, New Milford, CT
  Water from the
Housatonic River pumped
up into Candlewood Lake
  230 feet of head
  6 billion ft3 of water
  Two-unit (binary) system
◼ Reversible pump/turbine –
one of the first
  29 MW of generating
power

9
Pumped-Hydro Storage Today
 PHES accounts for 99% of worldwide energy storage
  Total power: ~127 GW
  Total energy: ~740 TWh
  Power of individual plants: 10s of MW – 3 GW
 In the US:
  ~40 operational PHES plants
  75% are > 500 MW – strong economies of scale
  Total power: ~23 GW
◼ Current plans for an additional ~6 GW
  Total energy: ~220 TWh

11
PHES Fundamentals
 Two storage
reservoirs
  Upper and lower
  Lower reservoir
may be a river or
even the sea
 Separated by a height, ℎ
  The hydraulic head
  Assume ℎ ≫ depth of the upper reservoir
◼ ℎ remains constant throughout charge/discharge cycle
 Upper reservoir can store a volume of water, Vu

12
PHES Fundamentals - Energy
 Total stored energy (assuming it is all at a height, h)
 The energy density – energy per unit volume – of the stored water is
therefore

13
PHES Fundamentals – Hydrostatic Pressure
 The energy density of the stored water is also the
hydrostatic pressure at the level of the lower
reservoir
  This is the energy density of the water at the
turbine

14
PHES Fundamentals - Power
 The rate at which energy is transferred to the
turbine (from the pump) is the power extracted
from (delivered to) the water
 This is the total power available at the turbine
  Less than the power actually delivered to the turbine
(from the pump), due to inefficiencies

15
A Generalized Power Relation
 Note that power is given by the product of a driving
potential, or effort, 𝑝,and a flow, 𝑄

16
A Generalized Power Relation
 Also similar to an electrical system

17
Energy & Power vs. Head
 The total stored energy and available power are
 Both are proportional to head, ℎ
  Large vertical separation between lower and upper reservoirs is
desirable
  Limited by topography
  Limited by equipment – pump and turbine
 Specific energy is also proportional to head:
 As is energy density:

18
Specific Energy & Energy Density vs. Head
 Most PHES plants have head in the range of 100 – 1000 m
 Using 300 m as a representative head, gives:
  Energy density for h = 300 m
  Specific energy for h = 300 m

19
Specific Energy & Energy Density
 Comparison of PHES energy density and specific energy
with other energy storage/sources
PHES
h = 100 m
PHES
h = 500 m
PHES
h = 1000m
Li-ion
Battery
Natural
Gas Gasoline Units
Energy
Density
0.273 1.36 2.73 400 10.1 9,500 Wh/L
Specific
Energy
0.273 1.36 2.73 150 15,400 13,000 Wh/kg
 Even at high heads, PHES has very low energy density
  Large reservoirs are required

20
PHES Applications
 Pumped hydro plants can supply large amounts of
both power and energy
 Can quickly respond to large load variations
 Uses for PHES:
 Peak shaving/load leveling
  Help meet loads during peak hours
◼ Generating while releasing water from upper reservoir
◼ Supplying expensive energy
  Store energy during off-peak hours
◼ Pumping water to the upper reservoir
◼ Consuming inexpensive energy

21
PHES Applications
 Frequency regulation
  Power variation to track short-term load variations
  Helps maintain grid frequency at 60 Hz (50 Hz)
 Voltage support
  Reactive power flow control to help maintain
desired grid voltage
◼ Varying the field excitation voltage of the generator/motor

22
PHES Applications
 Black start capability
  Ability to start generating without an external
power supply
  Bring the grid back online after a blackout
 Spinning reserve
  Capable of responding quickly – within seconds
to minutes – to the need for additional generation

24
Components of a PHES Plant
K. Webb ESE 471

25
PHES Components – Reservoirs
 Upper and lower
reservoirs separated by
an elevation difference
 Two configurations:
  Open-loop:
◼ At least one of the
reservoirs connected to a
source of natural inflow
◼ Natural lake, river, river-fed reservoir, the sea
  Closed-loop:
◼ Neither reservoir has a natural source of inflow
◼ Initial filling and compensation of leakage and evaporation
provided by ground water wells
◼ Less common than open-loop

26
PHES Components – Penstock
 Penstock
  Conduit for water flowing
between reservoirs and to the
pump/generator
  Above-ground pipes or below
ground shafts/tunnels
◼ 5 -10 m diameter is common
◼ One plant may have several penstocks
◼ Typically, steel- or concrete-lined, though may be unlined
  Flow velocity range of 1 – 5 m/s is common
  Tradeoff between cost and efficiency for a given flow rate, Q
Larger cross-sectional area:
◼ Slower flow
◼ Lower loss
◼ Higher cost

27
PHES Components
 Tailrace tunnel
  Typically, larger
diameter than penstocks
  Lower pressure
  Lower flow rate
  Downward slope from
lower reservoir to
pump/turbine
◼ Inlet head helps prevent
cavitation in pumping mode
 Surge tanks
  Accumulator tanks to absorb high pressure transients
during startup and mode changeover
  May be located on penstock or tailrace
  Especially important for longer tunnels
  Hydraulic bypass capacitors

28
PHES Components – Power House
 Power house
  Contains
pump/turbines and
motor/generators
  Often underground
  Typically below the
level of the lower
reservoir to provide
required pump inlet
head
  Three possible configurations
◼ Binary set: one pump/turbine and one motor/generator
◼ Ternary set: one pump, one turbine, and one motor/generator
◼ Quaternary set: separate pump, turbine, motor, and generator

30
Power Plant Configurations – Quaternary Set
 Quaternary set
  Pump driven by a motor
  Generator driven by a turbine
  Pump and turbine are
completely decoupled
  Possibly separate
penstocks/tailrace tunnels
  Most common configuration
prior to 1920
  High equipment/infrastructure
costs
  High efficiency
◼ Pump and turbine designed to optimize
individual performance

31
Power Plant Configurations – Ternary Set
 Ternary set
  Pump, turbine, and
motor/generator all on a single
shaft
◼ Pump and turbine rotate in the
same direction
  Turbine rigidly coupled to
the motor/generator
  Pump coupled to shaft
with a clutch
  Popular design 1920 – 1960s
  Nowadays, used when head exceeds the usable range of a
single- stage pump/turbine
◼ High-head turbines (e.g., Pelton) can be used
  Pump and turbine designs can be individually optimized

32
Power Plant Configurations – Ternary Set
 Ternary set
  Generating mode:
◼ Turbine spins generator
◼ Pump decoupled from the shaft
and isolated with valves
  Pumping mode:
◼ Motor turns the pump
◼ Turbine spins in air, isolated with
valves
  Both turbine and pump
can operate simultaneously
  Turbine can be used for pump startup
◼ Both spin in the same direction
◼ Turbine brings pump up to speed and synchronized with grid, then
shuts down
◼ Changeover time reduced

33
Power Plant Configurations – Binary Set
 Binary set
  Single reversible
pump/turbine coupled to a
single motor/generator
  Most popular
configuration for modern
PHES
  Lowest cost configuration
◼ Less equipment
◼ Simplified hydraulic pathways
◼ Fewer valves, gates, controls, etc.
  Lower efficiency than for ternary or quaternary sets
◼ Pump/turbine runner design is a compromise between pump and
turbine performance

34
Power Plant Configurations – Binary Set
 Binary set
  Rotation is in opposite
directions for pumping and
generating
  Shaft and
motor/generator must
change directions when
changing modes
◼ Slower changeover than for ternary or quaternary units
  Pump startup:
◼ Pump/turbine runner dewatered and spinning in air
◼ Motor brings pump up to speed and in synchronism with the
grid before pumping of water begins

36
Turbines
 Hydro turbine design selection based on
  Head
  Flow rate
 PHES plants are typically sited to have large head
  Energy density is proportional to head
  Typically 100s of meters
 Reversible Francis pump/turbine
  Most common turbine for PHES applications
  Single-stage pump/turbines operate with heads up to 700 m
 For higher head:
  Multi-stage pump/turbines
  Ternary units with Pelton turbines

38
Francis Turbine – Components
 Volute casing (scroll casing)
  Spiral casing that
feeds water from the
penstock to the turbine
runner
  Cross-sectional
area decreases
along the length of
the casing
◼ Constant flow rate
maintained along the
length
Francis turbine casing – Grand Coulee:

39
 Guide vanes and stay vanes
  Direct water flow from the casing into the runner
  Stay vanes are fixed
  Guide vanes, or wicket gates, are adjustable
◼ Open and close to control flow rate
◼ Power output modulated by controlling flow rate
◼ Set fully open for pumping mode
Source: Stahlkocher Source: Stahlkocher

40
 Turbine runner
  Reaction turbine
◼ Pressure energy is extracted from the flow
◼ Pressure drops as flow passes through the
runner
  Flow enters radially
  Flow exits axially
  Typically oriented with a vertical
shaft
 Draft tube
  Diffuser that guides exiting flow to the
tailrace
Source: Voith Siemens Hydro Power

41
High-Head PHES
 Options for heads in excess of
700 m:
  Two-stage Francis
pump/turbines
◼ Typically, no wicket gates in two-stage
configuration
◼ No mechanism for varying generating
power
  Ternary unit with Pelton turbine
Two-stage pump/turbine:
Source: Alstom

42
Pelton Turbines
 Pelton Turbine
  Suitable for heads up to 1000 m
  Impulse turbine
◼ Nozzles convert pressure energy to kinetic
energy
Source: Alstom
◼ High-velocity jets impinge on the runner at
atmospheric pressure
◼ Kinetic energy
transferred to the
runner
◼ Water exits the turbine
at low velocity
  Cannot be used
for pumping
◼ Used as part of a
ternary set
Source: BFL Hydro Power

44
Motor/Generator – Fixed-Speed
 Pump/turbine shaft connects to a motor/generator unit
  Above the turbine runner in typical vertical configuration
 Motor/generator type depends PHES category:
  Fixed-speed pump/turbine
  Variable-speed pump/turbine
 Fixed-speed pump/turbine
  Motor/generator operates at a fixed speed in both
pumping and generating modes
  Synchronous motor/generator
◼ Rotation is synchronous with the AC grid frequency
◼ Stator windings connect to three-phase AC at grid frequency
◼ Rotor windings fed with DC excitation current via slip rings

45
Motor/Generator
 Variable-speed (adjustable-speed) pump/turbine
  Rotational speed of motor/generator is adjustable
  The preferred configuration for large (>100 MW) PHES
plants nowadays
 Advantages of variable-speed plants
  Pump and turbine speeds can be independently varied
to optimize efficiency over range of flow rate and head
  Pumping power can be varied in addition to generating
power

46
PHES for Frequency Regulation
 Frequency regulation
  Tracking short-term load
variations to maintain grid
frequency at 60 Hz (or 50 Hz)
 PHES plants can provide frequency
regulation
  Different for fixed- or variable-speed plants
 Fixed-speed plants
  Generating mode
◼ Frequency regulation provided by rapidly varying power output
◼ Power varied by using wicket gates to modulate flow rate
◼ Same as in conventional hydro plants
  Pumping mode
◼ Pump operates at rated power only – power input cannot be varied
◼ No frequency regulation in pumping mode

47
Frequency Regulation – Variable-Speed
 Variable-speed plants
  Pump speed can be varied
over some range, e.g. ±10%
  Pump power is proportional
to pump speed cubed
◼ For ±10% speed variation, power is adjustable over ±30%
  Power variation in pumping mode can track rapid
load variations
  Frequency regulation can be provided in both
modes of operation

48
Frequency Regulation – Ternary Sets
 Fixed-speed ternary sets
  Generating mode
◼ Wicket gates in turbine control flow rate to vary power
output
◼ Pump disconnected from shaft
  Pumping mode
◼ Hydraulic short circuit provides power modulation
◼ Pump and generator both turn on the shaft
◼ Pump operates at full load
◼ Generator operates at variable partial load

49
Hydraulic Short Circuit
 Kops II PHES plant in Austrian Alps:
Source: Vorarlberger Illvwerke AG

51
PHES System Efficiency
 Round-trip efficiency:
  Typical round-trip efficiency for PHES plants in the range of 70% – 80%
 PHES loss mechanisms
  Transformer
  Motor/generator
  Pump/turbine
  Water conduit

52
PHES Losses
 Transformers
  Pumped hydro plants connect to the AC electrical grid
◼ Transformers step voltage between high voltage on the grid side
to lower voltage at the motor/generator
  Transformer loss mechanisms:
◼ Winding resistance
◼ Leakage flux
◼ Hysteresis and eddy currents in the core
◼ Magnetizing current – finite core permeability
  Power flows through transformers on the way into
the storage plant and again on the way out
  Typical loss: ~0.5%

53
PHES Losses
 Motor/generator losses
  Electrical resistance
  Mechanical friction
  Typical loss: ~2%
 Pump/turbine
  Single runner in binary sets
◼ Typically, lower efficiency, particularly for fixed-
speed operation – design of both compromised
  Separate runners in ternary, quaternary sets
◼ Higher efficiency
  Typical loss: ~7% - 10%

PHES Losses
 Penstock
  Frictional loss of water flowing through the
conduit
◼ Major losses along penstock
◼ Minor losses from bends, penstock inlet, turbine inlet, etc.
  Dependent on
◼ Flow velocity
◼ Penstock diameter
◼ Penstock length
◼ Penstock lining – steel, concrete, etc.
  High head is desirable, but long penstocks are not
◼ Steeper penstocks reduce frictional losses for a given head
◼ Typical length-to-head ratio: 4:1 – 12:1
  Typical loss: ~1%

PHES Losses
 Typical losses for PHES:

Pumping-Mode Efficiency
 Efficiency of the pumping operation is given by

Pumping-Mode Efficiency
 The volume of water pumped to the upper reservoir is

Generating-Mode Efficiency
 Efficiency of the generating operation is given by

Generating-Mode Efficiency
 Generating mode efficiency is

Pumping and Generating Times
 Due to losses, charging/discharging times differ, even for equal grid-side
power input/output
  Energy flows in from the grid faster than it is stored in the upper reservoir
  Energy flows out of storage faster than it is delivered to the grid

Pumping and Generating Times
 Ratio of generation to pumping time:
  That is, the ratio of discharging to charging time is equal to
the round-trip efficiency

Disadvantages of PHES
 Disadvantages of PHES
  Environmental issues
◼ Water usage
◼ River/habitat disruption
  Head variation
◼ Pressure drops as upper reservoir drains
◼ Efficiency may vary throughout charge/discharge cycle
◼ Particularly an issue for lower-head plants with steep, narrow upper
reservoirs
  Siting options are limited
◼ Available water
◼ Favorable topography
◼ Large land area
 Possible alternative potential energy storage:
  Rail energy storage

Rail Energy Storage
 Rail energy storage
  Electric-motor-driven railcars
  Weights are shuttled up and down an incline between upper
and lower storage yards
  Power input drives motors to move weights up the track
  Regenerative
braking on the way
down supplies
power to the grid
  Weights are
loaded and
unloaded at
storage yards
◼ Large quantities of
energy can be
stored with few
trains

Advantages of Rail Energy Storage
 More siting options than for PHES
  Open space
  Elevation change
  No need for water or
topography conducive to
reservoirs
 Lower capital cost than PHES
 Easily scalable
 Efficient
  RT efficiency: 78% - 86%
  Constant
efficiency,
independent of SoC
 No standby losses
  No evaporation/leakage

Rail Energy Storage
 Three categories of rail energy storage plants proposed
by ARES:
  Small
◼ 20 – 50 MW
◼ Ancillary services only
  Intermediate
◼ 50 – 200 MW
◼ Ancillary services,
integration of
renewables
  Grid-scale
◼ 200 MW – 3 GW
◼ 4 – 16 hours of storage at full power

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