System Mass Comparison The mass of the components and the total system, including the penalties for power demand and heat rejection, are important criteria for space applications. In a first approximation, it is assumed that the mass and power requirements of the peripherals do not depend on the applied current densities (efficiencies) of fuel cell and electrolyzer. The mass of the electrolyzer and fuel cell combined with the necessary solar array and radiators has been optimized. The mass of the electrolyzer decreases with increasing current density under a fixed load. Due to the decreasing efficiency with increasing current density the mass of the necessary solar array (input power) increases. Thus, there is a minimum system mass at a particular current density. For the fuel cell, including the waste heat radiators, a minimum mass (versus current density) can be calculated in the same way. Combined with the minimum for the electrolyzer and solar generator, the mass minimum of the total RFCS, including the mass of the peripherals and reactant storage units, can be calculated. The results of these calculations for all systems considered are shown in Fig. 11. Basic information used in the calculations is as follows: The immobile RFCS has the lowest mass, followed by the mixed system. The mobile and PEM systems are much heavier, mainly due to the lower efficiency of the corresponding fuel cells. The RFCS mass, including solar generator and radiator, differs by up to 400 kg. The mass of the peripherals, including storage units, is in the range 120 kg for the immobile to 160 kg for the mobile system. The differences are mainly caused by the reconcentrator loop and the number of pumps and separators. The mass of the fuel cell and the electrolyzer is primarily influenced by the applied nominal current density, and only secondarily by the specific mass of the cell. The minimum masses of the RFCS occur at very different nominal current densities for the fuel cells and electrolyzers (Fig. 11). One cannot consider only the mass of fuel cell, electrolyzer and peripherals, because this mass decreases with increasing current density, as mentioned above. The minimum in total system mass also depends on the efficiency. A lower mass for the stacks and peripherals, as well as higher efficiency, moves the minimum to lower current densities. An exception is the PEM electrolyzer. Due to the characteristics of the membrane, the minima will always occur at relatively high current densities. Operational Behavior Comparison The operational behavior of the RFCS is mainly influenced by four characteristics: —operation under microgravity; —interrupted operation of fuel cell and electrolyzer; —handling of H2 and O2, and possibly water or electrolyte; —lifetime >30 000 cycles.
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