sustainable energy science and engineering...
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Sustainable Energy Science and Engineering Center
Energy Storage
Storage modes are determined by the particular end-use
applications
Anjane Krothapalli, September 20, 2006
http://www.sesec.fsu.edu
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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 ≡Wre cov ered
Win
= ηinηout
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Electrochemical systemsbatteries and flow cells
Mechanical systemsfly-wheels and compressed air energy storage (CAES)
Electrical systemssuper-capacitors and super-conducting magnetic energystorage (SMES)
Chemical systemshydrogen cycle (electrolysis -> storage -> power conversion)
Thermal systemssensible heat (storage heaters) and phase change
Generic Storage Systems
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Ragone Plot
Source: Tester et. al. Sustainable energy, MIT Press
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Energy Density
2/m3Compressed Air38Hydrogen0.9Flywheel, Fused Silica0.2Flywheel, Carbon Fiber0.05Flywheel, Steel0.3/m3Hydrostorage0.04Lead Acid Batteries14GasolinekWh/kgMethod
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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
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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
Performance Factors
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Rechargeable Battery Characteristics
http://www.afrlhorizons.com/Briefs/Feb04/PR0306.html
>2,000>3.6300150Lithium
>10,0001.20.0655Nickel hydrogen
>10001.20.0835Nickel Cadmium
40020.0835Lead acid
Cycle LifeVoltageWh/m3Wh/kgProperties
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400 Wh/kg Goal
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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
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
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Potential Energy Storage
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%)
Pumped hydropower:
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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
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Source: Toshiba
Pump Turbine Progress
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Compressor train Expander/generator train
Fuel (e.g. natural gas, distillate)
Intercoolers
Heat recuperator
PC PG
AirExhaust
AirStorage
Aquifer,salt cavern,
or hard mine
hS = Hours ofStorage (at PC)
PC = Compressorpower in
PG = Generatorpower out
Compressed Air Energy Storage SystemPotential Energy Storage
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Wind capacity factor can be significantly improved
Wind farm Transmission
CAES plant
Undergroundair storage
PWF = Wind Farm max.power out(rated power)
PT = Transmission linemax. power
PWF PT
Wind/CAES
Source: Toward optimization of a wind/ compressed air energy stoSource: Toward optimization of a wind/ compressed air energy storage (CAES) power system rage (CAES) power system Jeffery B. Jeffery B. GreenblattGreenblatt, , Samir SuccarSamir Succar, David C. , David C. DenkenbergerDenkenberger, Robert H. Williams, , Robert H. Williams,
Princeton UniversityPrinceton University
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Compressed Air Energy Storage
Huntorf plant
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Compressed Air Energy Storage
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Compressed Air Energy Storage
Energy needed to compress gases: the adiabatic compression work
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
W =γ
γ −1poVo
p1
po
⎛
⎝ ⎜
⎞
⎠ ⎟
γ −1( )γ
−1⎡
⎣
⎢ ⎢ ⎢
⎤
⎦
⎥ ⎥ ⎥
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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 - $40/kW
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Flywheel ParametersStored energy:
ω = rotational velocity
I = moment of inertia (ability of an object to resist changes in its rotational velocity)
= kmR2
k: inertial constant depends on shape; m: mass; R: radius
Wheel loaded at rim (bike tire); k = 1: solid disk of uniform thickness; k = 1/2 solid sphere; k = 2/5; spherical shell; k = 2/3; thin rectangular rod; k = 1/2
KE =12
Iω 2 =12
kmR2ω 2
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In order to optimize the energy-to-mass ratio, the flywheel needs to spin at the maximum possible speed.
Rapidly rotating objects are subject to centrifugal forces that can rip them apart.
Centrifugal force =
While dense material can store more energy it is also subject to higher centrifugal force and thus fails at lower rotational speeds than low density material. Therefore the tensile strength is more important than the density of the material.
Efficiency ~ 80%
Flywheel Parameters
mRω 2
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Technical Challenges: Light weight high tensile strength rotors (e.g. Fused Silica or composite material rotors) , low density and reduced
friction using evacuated housing with magnetic bearings
Specific Energy Density
Specific energy density =KErotational,max
m=
kσ max
ρm
σmax= maximum allowable tensile stress of the material
ρm = volumetric mass density (kg/m3)
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3600-300Ultracapacitors
1500-650SMES
12750CAES
500350Flywheels
200300Li-Ion
12800Pumped Hydro
US$/kWhUS$/kWStorage Type
Estimated Costs
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Alternative Ragone Plot
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Supercapacitors
Electrical energy storage in the form of confined electrostatic charges in a device consisting of conductive plates separated by dielectric medium.
Power Density =
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.
0.5V2
RA2
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Supercapacitors - State of the Art
Source: CAP-XX Pty Ltd, Australia, 2005
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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.
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Storage Technology Costs
Source: iti Scotland Ltd, 2005
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Hydrogen Storage
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Energy Sources Typical Chemical Energy Density
Hydrogen 142.0 MJ/kgEthanol 29.7 MJ/kgAmmonia 17.0 MJ/kgAutomotive Gasoline 45.8 MJ/kgMethane 55.5 MJ/kgMethanol 22.7 MJ/kg
(Source: Chemical Energy, The Physics Hyper text Book)
Potential Fuel
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Fuel Typical Stored ChemicalEnergy Density
Hydrogen 7.1 MJ/kg @ 5wt%Ethanol 26.7 MJ/kg @ 90wt%Ammonia 13.6 MJ/kg @ 80wt%Automotive Gasoline 41.2 MJ/kg @ 90wt%Methane 44.5 MJ/kg @ 80wt%Methanol 20.4 MJ/kg @ 90wt%
Energy Density
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ObjectiveTo achieve adequate stored energy in an efficient, safe and
cost effective system.
Source: Oak Ridge National Laboratory Hydrogen Storage Work Shop, May 2003
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Source: Ian Edwards, ITI Energy, May 24th, 2005
Energy Storage
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Hydrogen pellets
Mg(NH3)6Cl2
Hydrogen Storage
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Technology Status
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Storage Methods
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14 13 13 13 13 12 12 11 12 13 12 129
1113
2018 18 17 16 15
1212 9 8
4
0
10
20
30
40
cetan
e (dies
el)
octane (
gasolin
e)hep
tane
hexan
epen
tane
butane
ethan
epro
pane
ethan
olmeth
ane
methan
olam
monialiq
. hyd
rogen
hydrid
ewate
rEn
ergy
Den
sity
(MJ/
liter
)
carbon
hydrogen
Hydrogen density range
Energy Densities (LHV) in Liquid state
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Compressed Gas Storage Density (300 K, LHV)
0
1
2
3
4
5
0 2000 4000 6000 8000 10000
Pressure (psi)
Ener
gy D
ensi
ty (M
J/lit
er)
350 bar 700 bar
Gasoline: 13 MJ/L
Compressed Gas
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• 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
Compressed Gas
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Carbon fiber wrap/polymer liner tanks are lightweight and commercially available.
weight specific energy6 wt.% 7.2 MJ/kg7.5 wt.% 9.0 MJ/kg10 wt.% 12 MJ/kg
Energy density is the issue:
Pressure Gas density System density350 bar 2.7 MJ/L 1.95 MJ/L700 bar 4.7 MJ/L 3.4 MJ/L
Compressed Gas Cylinders
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Liquid Hydrogen EOS
0
100
200
300
400
500
600
20 30 40 50 60 70 80 90 100
Temperature K
Pres
sure
bar
High Pressure Cryogenic Tank
• reduces temperature requirements• eliminates liquifaction requirement• essentially eliminates latency issue
S. Aceves, et al 2002
Estimated energy density: 4.9 MJ/L (Berry 1998)
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Requires cryogenic systems
• Equilibrium temperature at 1 bar for liquid hydrogen is ~20 K.• Estimated storage densities1
Berry (1998) 4.4 MJ/literDillon (1997) 4.2 MJ/literKlos (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
Liquid Storage
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Gaseous Hydrogen Storage
Higher Heating value of Hydrogen: 142 MJ/kg
Hydride storage of hydrogen may be compared to the compression of
hydrogen
Source: The Future of Hydrogen Economy: Bright or Bleak? Baldur Eliasson and Ulf Bossel, ABB Switzerland Ltd.
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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.
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Hydrogen Delivery - Pipelines
Source: The Future of Hydrogen Economy: Bright or Bleak? Baldur Eliasson and Ulf Bossel, ABB Switzerland Ltd.
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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
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Alanates
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Complex Hydrides
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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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Source: Ali T. Raissi, FSEC
Complex Hydrides
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Fuel Tank Problem
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Current Status
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Compressed Gas System Requirements
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Improvements
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US DOE Targets
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Comparative Volumes and WeightsComparative Volumes and Weightsof a FCEV Hydrogen Storage Systemof a FCEV Hydrogen Storage System
(Capable of 560 km (350 mi) Range (Capable of 560 km (350 mi) Range –– Compact Sedan)Compact Sedan)
0
100
200
300
400
500
600
700
800
ICE Case (Reference)248 bar H2
Compressed Hydrogen (345 bar)
Liquid H2(< 300 mm dia.)
Liquid H2 (>540 mm dia.)
Metal Hydride (Mg)Sodium Borohydride
Lite
rs
kg
Lite rskg
FCEV Storage System
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LH2 Tank Configuration
filling port
inner vessel
gaseous hydrogen(+20°C up to +80°C)
liquefied hydrogen253°C)
suspension
safety valve
outer vessel
shut off valve
cooling waterheat exchanger
electrical heater
reversing valve(gaseous / liquid)
liquid extractiongas extraction
filling line
level probe
super insulation
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Storage Systems
CompressedGas (5,000 psi)
Cryogenic Liquid H2
Cryo - LiquidCompressed H2
RechargeableMetal Hydride
Carbon Adsorbtion
Chemical Hydride
SystemWeight
SystemVolume
ExtractionComplexity
SystemCost
FuelCost
Dormancy Safety
Good Acceptable Problem