1. What is a fuel cell?
A fuel cell is a device that generates electricity by a chemical reaction.
Anode : 2H H e-
Cathode : O2 + 4H+ + 4e H2
Overall equation : 2H2 + O H20 + Energy

2. Model Of Fuel Cell

3. Types of fuel cells Alkali fuel cells Molten Carbonate fuel cells
Phosphoric Acid fuel cells (PAFC)
Proton Exchange Membrane (PEM)
Solid Oxide fuel cells (SOFC)

4. Fuel Cells In Space Technology
For space applications energy production is cyclic
NASA power system
The Unitized Regenerative Fuel Cell (URFC)
Applications - high altitude balloons ,high altitude solar airplanes, etc.

5. Unitized Regenerative Fuel Cell System
Proton Exchange Membrane in URFC
Energy storage system.
Potential ability to store large amounts of energy
URFC – Fuel Cell + Electrolyser

6. Energy Density Energy delivered per unit mass of storage system
The Energy density is given by
Ed = (Output energy/Reactant weight)
Output Energy = n Vd Id td
Vd = Discharge cell voltage, volts
Id = Average discharge current, amps
n = Number of cells in the URFC stack
td = Discharge time, hours
Mw = Reactant weight

7. Parameters Influencing Energy Density
Where,
R = Ideal Gas Constant = (84.78 Kg-cm3)/(cm2-gmole-k)
T = Temperature in Kelvin
MOPw = Maximum operating gas pressure (Kg/cm^2)
f(Vd) = Id (Discharge current density in A/cm^2)
Kw = *106 (cm) (Water-Tank-Figure-of-Merit)
Ka = The ratio of the total cell area to stack weight=(As/w)
Where , As = Total cell area
W = Stack weight

8. Discharge Voltage And Current Density Optimization

9. URFCS EFFICIENCY
It is defined as the ratio between output energy to the input energy with regard to charging &discharging of the system

10. The Solar Airplane
The solar airplane is an ultra light flying wing aircraft
Its surface of the wing is covered with solar cells that produce electric power
Need for URFC in Solar aircraft

11. Schematic Diagram Of URFCS

12. URFC Energy Storage System Component
URFC Stack
Gas Storage Tanks
Water Tank
Condensers
Regenerative Dryers
Gas Regulators/ Check Valves
Energy Storage System Shell

13. URFC Energy Storage System Operation Description
Electrolysis Operation
Fuel Cell Operation
Electrolysis To Fuel Cell Transition
Fuel Cell To Electrolysis Transition

14. Packaging Of URFCS

15. Summary Of URFCS
URFCS capable of delivering 3500 watts during a 12 hour discharge period.
The overall energy density is 791 w-hr/kg.
Lightweight energy storage system.

16. Variations In URFCS Performance
The Size of Water Tank:
The water tank was sized to provide sufficient water storage for 3500 watts discharge power for 12 hours.
The gas tank volume:
The gas tank volumes were sized such that the fully charged tanks had a peak storage pressure of approximately 28.1 kg/cm2 (400psia)

17. URFCS Performance Parameters
URFCS Stack Electrical Performance
URFCS Reactant Mass/Pressure
Water Vapor Management

18. Conclusion
Energy densities as high as 800 w-h/kg are possible for URFCS based energy storage systems
The longer the discharge times for the URFCS the greater the energy density possible.
Over 90% of the volume could be saved from the energy storage system described in this paper.

19. Bibliography Journal of Power Source
Fred Mitlitsky, Blake Myers, and Andrew H. Weisberg, Regenerative Fuel Cell Systems’, Energy & Fuels
F. Mitlitsky, N. J. Colella, B. Myers, C. J. Anderson, “ Regenerative Fuel Cells for High Altitude Long Endurance Solar Powered Vehicles,”

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