regenerative fuel cell, which exhibits the highest energy density of all the non-nuclear systems for storage periods exceeding 2 h. The primary components of the conventional RFC subsystem (Fig. 1) include a fuel cell unit, an electrolysis unit, reactants and reactant tankage. The fuel cell and electrolysis unit masses scale with power level, which is a function of electrode area, while the reactant and associated tankage masses scale with energy, which is a function of reactant volume. During the eclipse portion of a mission, gaseous hydrogen and oxygen are delivered to the fuel cell unit at regulated pressure. Electrical power and heat are generated as the hydrogen and oxygen reactants are combined to form water. The water leaves the fuel cell stack and is stored in a tank. During the daylight portion of the mission, the stored water is pumped to an electrolysis unit, which is supplied with electrical power from an outside source (the photovoltaic array) to electrolyze the water and regenerate the gaseous hydrogen and oxygen reactants. Conventional RFCs store the reactants as gases in pressurized tanks, typically in the range 0.7-2.4 MPa (100-350 psia). Common tank materials include Inconel (a nickel-base alloy) and lightweight filament-wound materials such as Kevlar/epoxy. It was determined during the course of this study that, for short storage periods such as those associated with a low earth orbit application (~0.5 h storage), tankage accounts for only 5.5% of the total power system mass (Fig. 2) for reactants stored in Inconel tanks. However, as the storage time increases, an increasing percentage of the RFC subsystem mass lies in tankage. For lunar missions, where the storage requirements approach 350 h, the tankage comprises an overwhelming 82.5% of the total system mass. Substitution of lightweight Kevlar/epoxy tanks for Inconel tanks reduces this percentage only slightly, from 82.5 to 64.6% (Fig. 3). Since the tankage does not directly contribute to the power and energy output of the system, any reduction in its mass would be advantageous. This is especially significant considering the high cost of delivering a payload to the lunar surface. One option for reducing tankage mass is to liquefy and store the reactant streams as cryogens rather than pressurized gases. However, cryogenic storage requires a
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