inner pressure-vessel and a concentric aluminum outer shell with 90 layers of multilayer insulation and two vapor-cooled shields placed between the inner and outer spheres. The vapor-cooled shields, together with the Joule-Thomson valve and pressure-vessel wall heat exchanger, comprises a thermodynamic vent system which provide thermal protection from radiant heat flux. For purposes of characterization, the tank volumes for both the 250 and 20 kWe systems were determined based on the volume of required reactants plus a 5% reactant residual. An additional 10% tank volume was also added to accommodate the maximum filling level achievable [6]. Fluid expulsion techniques were not considered in the scope of this study. However, a couple of options have been identified. A pressurized line, running from the dry gas streams to the respective cryogen tanks, could be used to provide pressurized expulsion of the fluid from the tank. Another option is to utilize fuel cell waste heat to provide the energy required for fluid expulsion. A comparison of the tank dimensions for both the baseline gaseous system and the cryogen system is given in Table II. As for the gaseous system, the reactant water was stored at 2.2 MPa (315 psia) in tanks made from filament-wound Kevlar 49/epoxy matrix. A working stress of 233 MPa (33 750 psi) was assumed in modeling the tank. Radiators Radiators are required for the fuel cell, drying, and hydrogen and oxygen liqeufaction subsystems. Radiator characteristics for both the 250 and 20 kWe systems are listed in Table III. The effective emissivity accounts for the actual surface emissivity at end-of-
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