ESA Document - Emits - ESA

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HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 304 of 422
Figure 4-51: Primary FC with Air+H2 as reactants
There are five types of fuel cells:
• Phosphoric Acid Fuel Cell (PAFC): phosphoric acid is used as an electrolyte. It needs to
operate around 200°C and the cathode performance is inefficient.
• Proton Exchange Membrane Fuel Cell (PEMFC): The mechanism is the same as PAFC.
They differ in that PEMFCs operate at relatively low temperatures (about 100°C). They
have high power density and can vary their output quickly to meet shifts in power
demand
• Molten Carbonate Fuel Cell (MCFC): An alkali metal carbonate (Li, Na, K) is used as
the electrolyte. It needs to operate at about 600°C.
• Solid Oxide Fuel Cell (SOFC): solid, nonporous metal oxide electrolytes are used. The
cell operates at about 800-1000°C with an efficiency that can reach 60%.
• Alkaline Fuel Cell (AFC): AFC uses alkaline potassium hydroxide as the electrolyte.
These cells can achieve power generating efficiencies of up to 70%.
PEMFC and SOFC have the best maturity and European strength and fit the best with the
Martian surface requirement.
Due to the relatively low efficiency of fuel cells, the thermal dissipation will be important and
can be used for thermal regulation of fuel cells themselves but also for the rest of the spacecraft.
Using H2/O2 fuel cells has an important added value: water is the product of the reaction and can
be used for the life support. Hence, the trade-off of the power system on the Martian surface
should also include the water for the life support.
4.3.5.4.3.2 Regenerative fuel cells
Some fuel cells called “regenerative FC” can also be used in a reverse way: when power is
supplied, an electrolysis process take place and the fuels are produced.