reduce the diffusion of gas through the tank wall. The tanks were sized for the volume of reacting gas plus approximately 28% residual gas needed to maintain the tank pressures at the fuel cell operating pressure of 0.4 MPa (60 psia) to the end of the cycle. The water storage tank was also assumed to be constructed of filament-wound Kevlar 49/epoxy matrix and was designed for a storage pressure of 2.2 MPa (315 psia). In modeling the cryogenic system, the gaseous reactant storage tanks were removed from the baseline system, as indicated by the broken line in Fig. 1, and replaced with a refrigeration plant and storage facility, including driers for the reactant gas streams, liquefaction units, and cryogen storage tanks (Fig. 4). Additional PV array area was also included to provide power to the refrigeration plant. A 250 kWe system was chosen as the initial design point because of a previous study [4] which addressed the definition and preliminary design, including component mass estimates, of space-based cryogenic processing for an on-orbit fuel depot. The process flow rate for the depot corresponded to the flow rate required for a 250 kWe RFC. After the 250 kWe system was defined, the system was scaled down to 20 kWe. This lower power level is representative of the minimum power level envisioned for an installation such as a lunar observatory. Brief descriptions of each subsystem, as well as the methods employed for subsystem scaling, are given below. Fuel Cell/Electrolyzer Subsystem The fuel cell/electrolyzer (FC/EU) subsystem was modeled using a code that was developed at the NASA Lewis Research Center [2], The code calculates mass and performance characteristics based on input design parameters, which, for this study, were set to state-of-the-art values (Table I). These parameters were the same for both the 250 and 20 kWe systems.
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