Immobile KOH System The flow scheme for the immobile RFCS is shown in Fig. 3. The reactants are reduced from tank to operating pressure (4 bar) by regulators, which maintain the required differential pressure of the fuel cell system. The oxygen line dead-ends in the fuel cell, whereas the hydrogen is cycled with a jet pump to remove the product water from the FC via a condenser and a liquid/gas separator. The temperature of the FC is controlled at the inlet, while that of the condenser is controlled at the outlet with three-way mixer valves. The coolant is circulated by a coolant pump at constant flow. Excess heat is removed through a heat exchanger to the thermal control system (TCS). The internal heat exchanger keeps the part of the system not operating during eclipse or sunshine at operational temperature. The accumulator in the coolant loop compensates for volume changes caused by temperature changes, and uses the oxygen line as a pressure reference. Product water is fed by the water feed pump to the water storage tank at the pressure level of the electrolyzer (30 bar). The water feed pump operates only when actuated, controlled by the water level in the liquid/gas separator. The water storage tank uses the electrolyzer oxygen line as a pressure reference. The water is fed to the matrices of the immobile KOH electrolyzer (static feed water). The electrolyzer operating pressure and the differential pressure of the reactants are controlled by the reactant pressure controller. The temperature of the electrolyzer subsystem is controlled by the coolant loop. The coolant pump works continuously, a three-way mixer valve controlling the inlet temperature. The excess heat is removed via the internal heat exchanger and FC cooling loop to the TCS. PEM System The differences from the immobile system are: (i) that the reactant gases have to be humidified before entering the fuel cell; (ii) the hydrogen line is dead-ended; and (iii) an oxygen loop is used to remove the product water. Because the required excess flow of the oxygen is very high, a fan—instead of a jet pump or in addition to it—has to be used (see Fig. 4). The electrolyzer works according to the liquid anode feed principle. Pure water is pumped through the anode compartment to supply the process water required and to cool the cell simultaneously. Oxygen and water leaving the cell have to be separated in the gas/liquid separator—possibly a centrifuge. The electrochemical process is coupled with water transport through the membrane of approximately two moles of water per mole of converted hydrogen. Therefore a gas/liquid separator will also be necessary on the hydrogen side. The water lines, coming from the reactant/water separators, are combined. Due to a possible incomplete separation, catalytic recombiners are necessary to prevent the accumulation of an explosive mixture in the storage tanks. The recombiners can be positioned either in the water loop or in the gas lines. The waste heat to be removed from the PEM electrolyzer is greater than that for the immobile KOH electrolyzer subsystem, therefore a heat exchanger directly connected to the TCS may be necessary. The water storage tank serves as an accumulator for the water loop.
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