phase separation also occur, especially in the mobile KOH system where the liquid electrolyte has to be reconcentrated. Periodic operation of the subsystems also has consequences on design and operational behavior. Alkaline electrolyzers can be directly connected to the solar generator and switched off (except for the cooling loops for heat transfer) during eclipse without any pressure losses. For alkaline stacks, precious metal catalysts may be required to avoid degradation problems. In the PEM electrolyzer, the reactant pressures will decrease during eclipse due to the high gas permeability of the membrane. The humidity of the product gases is a common problem for all RFCSs. Condensation due to temperature or pressure changes has to be avoided. The exit temperature of the gases at the electrolyzer as well as the tank temperatures have to be controlled. A slight advantage for the alkaline technologies might be seen in the fact, that the vapor pressure over a KOH solution at a given temperature is lower than that over pure water. All moving parts like pumps and centrifuges have to be considered as critical. A nearly 100% reliable operation under microgravity conditions is necessary to prevent system failures. Noise and vibration have to be avoided. Contamination has to be kept to very low level to reduce the degradation of all components, especially the electrodes in the fuel cell and electrolyzer. This is a major problem for the mobile KOH technique (corrosive electrolyte) and for the PEM technique (very pure water is required). In the immobile KOH systems, the electrodes are only in contact with the gas phase, and therefore are less sensitive to contaminations. All systems require temperature, pressure and differential pressure, and phase separation. Here the need to control a number of positive components gives an advantage to the immobile KOH technology. Due to the good voltage/current density behavior at the fuel cell and electrolyzer (see Figs 7 and 9) and the low required changes in power level (e.g. ± 10% for the Columbus Free Flying Laboratory) the effect of the RFCS on power conditioning is small. A load change in this range means a stack voltage change of only approximately ±2 V. Comparison with Other Accumulators The calculated mass of the RFCS without solar array and radiator is given below for the immobile KOH system: This yields an energy storage density of approximately 36 Wh/kg for a 20 kW plant at an efficiency of 55-60%. The energy density will increase for higher power levels because many components become relatively lighter when increasing the system power. The system is operated at a maximum of 30 bar tank pressure and discharged to 7 bar tank pressure, which corresponds to a depth of discharge (DoD) of approximately 80%. The efficiencies and energy storage densities (20 kW plant) for various storage technologies are:
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