Electrical Power Systems
Section 4.3.3.
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Regenerative Fuel Cells

The most detailed study of Regenerative Fuel Cell (RFC) electrical storage for a lunar base is the one done by Lisa Kohout (Space Power, Volume 8, No. 4, 1989). It analyzes both a 20 kW and a 250 kW RFC system for both gas storage and cryogenic storage of the H2 and O2. (Cryogenic storage is better and less massive in both cases).

The breakdown for the 250 kW system is:

Solar array 6900 kg (assumed 22.5% efficient GaAs cells at 123 kW/kg)
Power conditioning 8500 kg (assumed 10 kg/kW)
Fuel cell total 20800 kg
Fuel cell, electrolyzer and radiator 6250 kg
Tanks 6500 kg
Reactants 3200 kg
Drying equipment 400 kg
Liquification equipment 4500 kg


250 kW for 354 hours comes to 88.5 megawatt hours. The storage mass is thus 4.25 kWh per kilogram, or 15 MJ/kg--considerably better than batteries.

The 20 kW system had 550 kg for the solar array, 670 kg for the power conditioning, and 4660 kg for the fuel cell system, for a total of 5880 kg. The system stores 7000 kWh at a specific energy of 1.5 kWh per kilogram.

D. Harris, et al., of Rocketdyne also did an analysis, using some of the first paper's numbers, but with their own fuel cell analysis. They also show a fuel cell system with cryogenic storage of the reactants to be best. Their mass breakdown for a system to provide 12.5 kW for a 354 hour lunar night is:

PV/RFC system (w/o storage) 9200 kg GaAs, 25 kW
Storage Total 5570 kg
Tanks 1200 kg
Refrigerator 300 kg
PV Array and PMAD 770 kg
Radiator 500 kg
Total 2800 kg

PMAD = power management and distribution. The PV Array and PMAD listed here produces power used to run refrigeration.

These numbers are considerably larger than Kohout's numbers for a system with nearly twice the power (12000 kg, vs. 5880 kg estimated by Kohout). The Rocketdyne crew is a group that designs spacecraft power systems; they are probably more conservative, and probably more real-world, than Lisa Kohout's analysis.

Electrical Power Systems

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