Chris_Mi_handout
.pdf
A fuel cell
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The First System
zIn the world that uses an SOFC fuel cell coupled with a gas turbine was developed at Siemens Westinghouse in Pittsburgh, Pennsylvania. The 220kW power plant converts nearly 60 % of the energy contained in natural gas into electric power
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Useful links
zNYSERDA
zElectric Power Research Institute
zU.S. Environmental Protection Agency
zFuel Cells 2000
zNational Fuel Cell Research Center
zU.S. Department of Energy
zU.S. Fuel Cell Council
zThe Hydrogen & Fuel Cell Investor's Newsletter
zNational Hydrogen Association
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Fuel Cell Applications
zVehicle Applications: Require low temperature operation
zStationary Applications: Rapid operation and cogeneration is desired
zResearch: new materials for electrodes and electrolytes
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Fuel Cell Characteristics
zFuel cell theoretically operates isothermally
-=> all free energy in a chemical reaction should convert to electrical energy
zH fuel does not burn, bypassing thermal to mechanical conversion
-=> direct electrochemical converter
zIsothermal operation: Not subject to limitations of Car, not subject to cycle efficiency imposed on heat engines.
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Fuel Cell Characteristics
zVoltage/Current Output of a hydrogen/oxygen fuel cell.
1.0 |
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Practical |
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Cell potential, V |
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0.5 |
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Current density, A/cm2 |
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z1V is the theoretical Prediction, but not achievable in a practical cell
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Fuel Cell Characteristics
zWorking voltage falls with increasing current
zSeveral cells are stacked in series to get desired voltage
zMajor advantage: Lower sensitivity to scaling (system efficiency similar from kW to MW range).
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Fuel Cell Types
zSix Major Fuel Cell Types:
-Alkaline Fuel Cell (AFC)
-Proton Exchange Membrane (PEM)
-Direct Methanol Fuel Cell (DMFC)
-Phosphoric Acid Fuel Cell (PAFC)
-Molten Carbonate Fuel Cell (MCFC)
-Solid Oxide Fuel Cell (SOFC, ITSOFC)
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Fuel Cell Comparison
Fuel Cell |
Fuel |
Electrolyte |
Operating |
Efficiency |
Applications |
Variety |
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Temperature |
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Phosphoric |
H , reformate |
Phosporic acid |
~2000C |
40-50% |
Stationary |
Acid |
2 |
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(>250kW) |
(LNG, |
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methanol) |
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Alkaline |
H |
Potassium |
~800C |
40-50% |
Mobile |
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2 |
hydroxide |
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solution |
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Proton |
H , reformate |
Polymer ion |
~800C |
40-50% |
EV/HEV, |
Exchange |
2 |
exchange film |
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Industrial up to |
(LNG, |
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Membrane |
methanol) |
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~80kW |
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Direct |
Methanol, |
Solid polymer |
90-1000C |
~30% |
EV/HEVs, small |
Methanol |
ethanol |
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portable devices |
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(1W-70kW) |
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Molten |
H , CO (coal |
Carbonate |
600-7000C |
50-60% |
Stationary |
Carbonate |
2 |
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(>250kW) |
gas, LNG, |
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methanol) |
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Solid Oxide |
H , CO (coal |
Yttria- |
~10000C |
50-65% |
Stationary |
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2 |
stabilized |
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gas, LNG, |
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methanol) |
zirconia |
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Hydrogen Storage
zHydrogen is not very dense at atmospheric pressure
zCan be stored as compressed or liquefied gas
-Lot of energy required to compress the gas
-Generation of liquid hydrogen requires further compression
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Fuel Cell Controller
zFuel cell characteristics as a function of flow rate
Stack |
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Stack |
potential, V |
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power, kW |
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Power for |
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Base Flow |
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.25 Base |
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.75 Base |
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.5 Base |
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.75 Base |
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Base Flow |
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Current, A |
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Fuel Cell Operation
zFuel Cell Operation
-Low Voltage/High Current make it sensitive to load variations
-Fuel Cell Controller regulates flow of hydrogen into fuel cell to maximize performance while minimizing excess hydrogen venting
-Pulling too much power without compensation in hydrogen flow may damage fuel cell membrane
-Controller avoids operation in current limit mode to maintain a decent efficiency
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Fuel Cell Operation
zFuel Cell Operation
-Due to slow response characteristics a reserve of energy is kept to ensure uninterrupted operation
-At 100% hydrogen usage, Fuel Cell goes into current limited mode due to internal losses
-By-product of Fuel Cell is water and (steam) and excess H
-Steam can be used for heating in the vehicle, but excess hydrogen is wasted
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Ultra-Capacitors
zElectrochemical energy storage systems
zDevices that store energy as an electrostatic charge
zHigher specific energy and power versions of electrolytic capacitors
zStores energy in polarized liquid layer at the interface between ionically conducting electrolyte and electrode
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Ultra-Capacitors
zMore suitable for HEVs
zCan provide power assist during acceleration and hill climbing, and for recovery of regenerative energy
zCan provide load leveling power to chemical batteries
zCurrent aim is to develop ultra capacitors with capabilities of 4000 W/kg and 15Whr/kg.
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How an Ultra-Capacitor Works
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Charger |
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Polarizing |
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Collector |
electrodes |
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Collector |
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Separator |
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Electric double layers |
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Energy = |
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CV 2 |
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Equivalent Circuit
z Three major components: |
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Series resistance |
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iL |
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iC |
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VC |
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C RL |
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resistance
- 
Vt =VC − Ri
dV
C dtC = −iC = −iL +i
V iL = RC
L
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Typical Discharging of Ultra-capacitor
z2600F capacitance
z2.5V cell voltage
2.5
2.0 |
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1.5 |
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I=50A |
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100 |
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1.0 |
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200 |
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0.5 |
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300 |
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400 |
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600 |
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0 0 |
20 |
40 |
60 |
80 |
100 |
120 |
140 |
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Discharge time, Sec. |
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52
Useful Energy and SOC
Useful Energy : Eu = 12 C(VCR2 −VCb2 )
SOC = |
0.5CV 2 |
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V 2 |
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Cb |
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Cb |
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0.5CV 2 |
V 2 |
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CR |
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CR |
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Efficiency, when |
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neglecting iL |
η |
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ICVC |
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VC |
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C |
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ItVt |
Vt |
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Charging: |
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η |
d |
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ItVt |
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Vt |
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ICVC |
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VC |
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Discharging |
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Technical Specifications
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BCAP0010 |
BMOD0115 |
BMOD0117 |
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(Cell) |
(Module) |
(Module) |
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Capacitance (Farads, -20% /+20%) |
2600 |
145 |
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435 |
maximum series resistance ESR at 25oC (m ) |
0.7 |
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Voltage, (V) Continuous (peak) |
2.5 (2.8) |
42 (50) |
14 (17) |
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Specific power at rated voltage (W/kg) |
4300 |
2900 |
1900 |
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Specific energy at rated voltage (Wh/kg) |
4.3 |
2.22 |
1.82 |
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Maximum current (A) |
600 |
× |
×600 |
600 |
Dimensions (mm ) (referance only) |
60 ×172 |
195 165 |
415 |
195×265 ×145 |
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(Cylinder) |
(Box) |
(Box) |
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Weight (kg) |
0.525 |
16 |
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6.5 |
Volume (Liter) |
0.42 |
22 |
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7.5 |
Operating temperature* (oC) |
-35 to +65 |
-35 to +65 |
-35 to +65 |
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Storage temperature (oC) |
-35 to +65 |
-35 to +65 |
-35 to +65 |
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leakage current (mA) 12 hours, 25oC |
5 |
10 |
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* Steady state case temperature
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