Sustainable Energy Science and Engineering Center
Energy Storage
Storage modes are determined
by the particular end-use
applications
http://www.sesec.fsu.edu
Anjane Krothapalli, September 20, 2006
Sustainable Energy Science and Engineering Center
Main Parameters
Energy density: The amount of energy that can be stored.
Recovery rate: The efficiency at which the energy can be
recovered.
Hydrogen has for example has one of the highest storage
densities (kWh/kg) of 38 as compared to that of lead acid
batteries, 0.04.
The efficiency of work exchange processes:
ηcycle ≡
W re cov ered
= ηinηout
W in
Sustainable Energy Science and Engineering Center
Generic Storage Systems
Electrochemical systems
batteries and flow cells
Mechanical systems
fly-wheels and compressed air energy storage (CAES)
Electrical systems
super-capacitors and super-conducting magnetic energy
storage (SMES)
Chemical systems
hydrogen cycle (electrolysis -> storage -> power conversion)
Thermal systems
sensible heat (storage heaters) and phase change
Sustainable Energy Science and Engineering Center
Ragone Plot
Source: Tester et. al. Sustainable energy, MIT Press
Sustainable Energy Science and Engineering Center
Energy Density
Method
Gasoline
Lead Acid Batteries
Hydrostorage
Flywheel, Steel
Flywheel, Carbon Fiber
Flywheel, Fused Silica
Hydrogen
Compressed Air
kWh/kg
14
0.04
0.3/m3
0.05
0.2
0.9
38
2/m3
Sustainable Energy Science and Engineering Center
Battery Storage
Designed for load leveling
Large number of batteries charged during low demand
periods
One acre could store 400 MWh of energy, deliver 40 MW
for 10 hours
Batteries require environmentally damaging chemicals
Typical installation: Southern California Edison
8000 lead acid battery modules to deliver up to 10 MW of
power for four hours of continuous discahrge
Sustainable Energy Science and Engineering Center
Performance Factors
1. Life time (maximum number of charge and discharge
cycles)
2. Overall cycle efficiency
3. Depth of discharge per cycle (deep cycle - less instant
energy but longer term energy delivery: e.g.: Golf cart
battery)
4. Cost of unit of power or energy stored
Sustainable Energy Science and Engineering Center
Rechargeable Battery Characteristics
Properties
Wh/kg
Wh/m3
Voltage
Cycle Life
Lead acid
35
0.08
2
400
Nickel Cadmium
35
0.08
1.2
>1000
Nickel hydrogen
55
0.06
1.2
>10,000
Lithium
150
300
>3.6
>2,000
http://www.afrlhorizons.com/Briefs/Feb04/PR0306.html
Sustainable Energy Science and Engineering Center
400 Wh/kg Goal
Sustainable Energy Science and Engineering Center
Battery Storage Requirement
Individual Americans use about 1.5 kWh of electricity every hour
Typical storage requirement: 15 kWh
Battery capacity: 400 Wh/kg
Batteries weight: 37.5 kg; Cost: $3000
Typical compact automobile power requirement: 50 KW
Typical driving distance: 500 km
Average speed: 100 km/hr
Required stored energy: 250 kWh
Battery capacity: 400 Wh/kg
Batteries weight: 625 kg; Cost: $50,000
Sustainable Energy Science and Engineering Center
Potential Energy Storage
Pumped hydropower:
Use excess energy to pump water uphill from a lower reservoir to
higher reservoir:
Energy recovery depends on total volume of water and its height
above the turbine location
The efficiency of pumping: 80%
Net efficiency: 0.8 x 0.8 = 0.64
The typical coal power plant efficiency ~ 40%
The efficiency of the recovered energy =
0.64 x 0.4 ~ 0.25 (25%)
Sustainable Energy Science and Engineering Center
Pumped-Storage Plant
Source: NREL
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Hydropower Storage Facility
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Pumped Stored Energy
Energy stored is predominantly Nuclear Power
The water head ranges widely: 25 - 600m
Large range in capacity: 4 MW - 2100 MW
USA storage capacity: 20 GW
Europe: 32 GW
Others: 34 GW
Opportunity to store renewable energy (especially wind)
when it is produced and deliver when needed.
Ideally suited in mountainous regions.
Total hydro capacity: 420 GW
Typical pumping up cost: $14 ~ $21/MWh
Sustainable Energy Science and Engineering Center
Pump Turbine Progress
Source: Toshiba
Sustainable Energy Science and Engineering Center
Potential Energy Storage
Compressed Air Energy Storage System
Compressor train
Expander/generator train
Air
Exhaust
PC
PG
Intercoolers
PC = Compressor
power in
PG = Generator
power out
Aquifer,
salt cavern,
or hard mine
Heat recuperator
Fuel (e.g. natural gas, distillate)
Air
Storage
hS = Hours of
Storage (at PC)
Sustainable Energy Science and Engineering Center
Wind/CAES
PWF
PT
CAES plant
Wind farm
PWF = Wind Farm max.
power out
(rated power)
Transmission
Underground
air storage
PT = Transmission line
max. power
Wind capacity factor can be significantly improved
Source: Toward optimization of a wind/ compressed air energy storage
storage (CAES) power system
Jeffery B. Greenblatt,
Greenblatt, Samir Succar,
Succar, David C. Denkenberger,
Denkenberger, Robert H. Williams,
Princeton University
Sustainable Energy Science and Engineering Center
Compressed Air Energy Storage
Huntorf plant
Sustainable Energy Science and Engineering Center
Compressed Air Energy Storage
Sustainable Energy Science and Engineering Center
Compressed Air Energy Storage
Energy needed to
compression work
compress
gases:
the
adiabatic
(γ −1)
⎡
⎤
γ
⎛
⎞
γ
⎢ p
⎥
W =
poVo ⎢⎜ 1 ⎟
−1⎥
γ −1
⎝p ⎠
⎢⎣ o
⎥⎦
W: specific compression work (J/kg); po: initial pressure; p1: final
pressure; Vo: initial specific volume (m3/kg); γ: ratio of specific heats
Net work = Wnet= Wturbine - Wcompressor
ηoverall = ηturbineηcompressor
Sustainable Energy Science and Engineering Center
Flywheels
A flywheel is a mechanical battery storing energy mechanically in the form of
kinetic energy. It uses an electric motor to accelerate the rotor up to a high
speed and return the electrical energy by using the same motor as a
generator. The rotational energy is delivered until friction overcomes it.
Flywheel has a higher energy density over chemical energy storage. The rate
at which energy can be exchanged into or out of the flywheel is limited only by
the motor-generator design. Hence, it is possible to withdraw large amounts of
energy in a far shorter time than with traditional chemical batteries. As such
they are widely used in automobile applications.
Flywheels store energy very efficiently and can provide high specific power
(kWh/kg) compared with batteries.
Flywheel purchase costs: $100/kW - $300/kW
Installation costs: $20/kW - $40/kW
Sustainable Energy Science and Engineering Center
Flywheel Parameters
Stored energy:
1 2 1
2 2
KE = Iω = kmR ω
2
2
ω = rotational velocity
I = moment of inertia (ability of an object to resist changes in its rotational velocity)
= kmR2
k: inertial constant depends on shape; m: mass; R: radius
Wheel loaded at rim (bike tire); k = 1: solid disk of uniform thickness; k = 1/2
solid sphere; k = 2/5; spherical shell; k = 2/3; thin rectangular rod; k = 1/2
Sustainable Energy Science and Engineering Center
Flywheel Parameters
In order to optimize the energy-to-mass ratio, the flywheel needs to spin
at the maximum possible speed.
Rapidly rotating objects are subject to centrifugal forces that can rip them
apart.
Centrifugal force =
mRω 2
While dense material can store more energy it is also subject to higher
centrifugal force and thus fails at lower rotational speeds than low density
material. Therefore the tensile strength is more important than the density
of the material.
Efficiency ~ 80%
Sustainable Energy Science and Engineering Center
Specific Energy Density
Specific energy density =
KE rotational,max kσ max
=
m
ρm
σmax= maximum allowable tensile stress of the material
ρm = volumetric mass density (kg/m3)
Technical Challenges: Light weight high tensile strength rotors (e.g.
Fused Silica or composite material rotors) , low density and reduced
friction using evacuated housing with magnetic bearings
Sustainable Energy Science and Engineering Center
Estimated Costs
Storage Type
US$/kW
US$/kWh
Pumped Hydro
800
12
Li-Ion
300
200
Flywheels
350
500
CAES
750
12
SMES
650
1500-
Ultracapacitors
300
3600-
Sustainable Energy Science and Engineering Center
Alternative Ragone Plot
Sustainable Energy Science and Engineering Center
Supercapacitors
Electrical energy storage in the form of confined
electrostatic charges in a device consisting of conductive
plates separated by dielectric medium.
Power Density =
2
V5.0
2
AR
V: applied voltage; R: total effective resistance of the
capacitor, A: nominal surface area of the conducting plate
or electrode
Very high surface area activated carbon (nanopores)
electrodes and charge separation distances in nanometers.
Attractive for Regenerative breaking and other power
needs in electric and hybrid vehicles.
Sustainable Energy Science and Engineering Center
Supercapacitors - State of the Art
Source: CAP-XX Pty Ltd, Australia, 2005
Sustainable Energy Science and Engineering Center
Superconducting Magnetic Energy
Storage (SMES)
A device for storing and instantaneously discharging large quantities of power. It
stores energy in the magnetic field created by the flow of DC in a coil of
superconducting material that has been cryogenically cooled. The SMES recharges
within minutes and can repeat the charge/discharge sequence thousands of times
without any degradation of the magnet. Recharge time can be accelerated to meet
specific requirements, depending on system capacity.
In low-temperature superconducting materials, electric currents encounter almost no
resistance.
Sustainable Energy Science and Engineering Center
Storage Technology Costs
Source: iti Scotland Ltd, 2005
Sustainable Energy Science and Engineering Center
Hydrogen Storage
Sustainable Energy Science and Engineering Center
Potential Fuel
Energy Sources
Typical Chemical
Energy Density
Hydrogen
Ethanol
Ammonia
Automotive Gasoline
Methane
Methanol
142.0 MJ/kg
29.7 MJ/kg
17.0 MJ/kg
45.8 MJ/kg
55.5 MJ/kg
22.7 MJ/kg
(Source: Chemical Energy, The Physics Hyper text Book)
Sustainable Energy Science and Engineering Center
Energy Density
Fuel
Typical Stored Chemical
Energy Density
Hydrogen
Ethanol
Ammonia
Automotive Gasoline
Methane
Methanol
7.1 MJ/kg
26.7 MJ/kg
13.6 MJ/kg
41.2 MJ/kg
44.5 MJ/kg
20.4 MJ/kg
@ 5wt%
@ 90wt%
@ 80wt%
@ 90wt%
@ 80wt%
@ 90wt%
Sustainable Energy Science and Engineering Center
Objective
To achieve adequate stored energy in an efficient, safe and
cost effective system.
Source: Oak Ridge National Laboratory Hydrogen Storage Work Shop, May 2003
Sustainable Energy Science and Engineering Center
Energy Storage
Source: Ian Edwards, ITI Energy, May 24th, 2005
Sustainable Energy Science and Engineering Center
Hydrogen Storage
Hydrogen pellets
Mg(NH3)6Cl2
Sustainable Energy Science and Engineering Center
Technology Status
Sustainable Energy Science and Engineering Center
Storage Methods
Sustainable Energy Science and Engineering Center
Energy Densities (LHV) in Liquid state
40
carbon
Energy Density (MJ/liter)
hydrogen
30
Hydrogen
density range
20
18
18
17
20
16
15
12
12
9
8
4
10
14
13
13
13
13
12
12
11
12
13
12
12
11
13
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Sustainable Energy Science and Engineering Center
Compressed Gas
5
Energy Density (MJ/liter)
Compressed Gas Storage Density
(300 K, LHV)
Gasoline: 13 MJ/L
4
3
2
1
700 bar
350 bar
0
0
2000
4000
6000
Pressure (psi)
8000
10000
Sustainable Energy Science and Engineering Center
Compressed Gas
•
•
•
increased pressure (>700 bar)
– stronger, lighter composite tanks (cost)
– hydrogen permeation
– non-ideal gas behavior
conformable tanks
– maximum volume gain ~20% (cylind./rect. volumes)
– some increase in weight
microspheres
– multiple shell volumes
– close-packed packing density ~60% of volume
– hydrogen release/reload mechanism
Sustainable Energy Science and Engineering Center
Compressed Gas Cylinders
Carbon fiber wrap/polymer liner tanks
are lightweight and commercially
available.
weight
6 wt.%
7.5 wt.%
10 wt.%
specific energy
7.2 MJ/kg
9.0 MJ/kg
12 MJ/kg
Energy density is the issue:
Pressure
350 bar
700 bar
Gas density
2.7 MJ/L
4.7 MJ/L
System density
1.95 MJ/L
3.4 MJ/L
Sustainable Energy Science and Engineering Center
High Pressure Cryogenic Tank
600
Liquid Hydrogen EOS
Pressure bar
500
400
300
200
100
S. Aceves, et al 2002
0
20
30
40
50
60
70
80
90
100
Temperature K
• reduces temperature requirements
• eliminates liquifaction requirement
• essentially eliminates latency issue
Estimated energy density:
4.9 MJ/L (Berry 1998)
Sustainable Energy Science and Engineering Center
Liquid Storage
Requires cryogenic systems
•
•
•
Equilibrium temperature at 1 bar for liquid hydrogen is ~20 K.
Estimated storage densities1
Berry (1998)
4.4 MJ/liter
Dillon (1997)
4.2 MJ/liter
Klos (1998)
5.6 MJ/liter
Issues with this approach are:
– dormancy.
– energy cost of liquifaction.
1
J. Pettersson and O Hjortsberg, KFB-Meddelande 1999:27
Sustainable Energy Science and Engineering Center
Gaseous Hydrogen Storage
Hydride storage of
hydrogen may be
compared to the
compression of
hydrogen
Higher Heating value of
Hydrogen: 142 MJ/kg
Source: The Future of Hydrogen Economy: Bright or Bleak?
Baldur Eliasson and Ulf Bossel, ABB Switzerland Ltd.
Sustainable Energy Science and Engineering Center
Hydrogen Storage - Liquefaction
Total energy requirement for
liquefaction of 1 kg of H2
Source: The Future of Hydrogen Economy: Bright or Bleak?
Baldur Eliasson and Ulf Bossel, ABB Switzerland Ltd.
Sustainable Energy Science and Engineering Center
Hydrogen Delivery - Pipelines
Source: The Future of Hydrogen Economy: Bright or Bleak?
Baldur Eliasson and Ulf Bossel, ABB Switzerland Ltd.
Sustainable Energy Science and Engineering Center
Hydrides
Chemically bond hydrogen in a solid material
•
•
•
This storage approach should have the highest hydrogen packing
density.
However, the storage media must meet certain requirements:
– reversible hydrogen uptake/release
– lightweight with high capacity for hydrogen
– rapid kinetic properties
– equilibrium properties (P,T) consistent with near ambient
conditions.
Two solid state approaches
– hydrogen absorption (bulk hydrogen)
– hydrogen adsorption (surface hydrogen)
including cage structures
Sustainable Energy Science and Engineering Center
Alanates
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Complex Hydrides
Sustainable Energy Science and Engineering Center
Hydrogen Storage Volume
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Hydrogen System Weight
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Future
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US DOE Strategy
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US DOE Strategy
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Complex Hydrides
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Complex Hydrides
Source: Ali T. Raissi, FSEC
Sustainable Energy Science and Engineering Center
Fuel Tank Problem
Sustainable Energy Science and Engineering Center
Current Status
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Compressed Gas System Requirements
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Improvements
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US DOE Targets
Sustainable Energy Science and Engineering Center
FCEV Storage System
Comparative Volumes and Weights
of a FCEV Hydrogen Storage System
(Capable of 560 km (350 mi) Range – Compact Sedan)
800
700
Liters
kg
600
500
L ite rs
kg
400
300
200
100
0
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0
30
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m
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Sustainable Energy Science and Engineering Center
LH2 Tank Configuration
inner vessel
super insulation
outer vessel
level probe
filling line
gas extraction
liquid extraction
filling port
suspension
liquefied hydrogen
253°C)
safety valve
gaseous hydrogen
(+20°C up to +80°C)
shut off valve
electrical heater
reversing valve
(gaseous / liquid)
cooling water
heat exchanger
Sustainable Energy Science and Engineering Center
Storage Systems
System
Weight
System
Volume
Extraction
Complexity
System
Cost
Fuel
Cost
Dormancy
Compressed
Gas (5,000 psi)
Cryogenic
Liquid H2
Cryo - Liquid
Compressed H2
Rechargeable
Metal Hydride
Carbon
Adsorbtion
Chemical Hydride
Good
Acceptable
Problem
Safety