Fig. 1. Schematic diagram of the flight type fuel cell system. output power of this five cell stack was 79 W at 20 A at atmospheric pressure. The operating condition of the fuel cell system has been established successfully (1). Based upon the results of this fundamental test, a fuel cell system, consisting of an improved five cell stack, a reactant supply system and a control unit, was fabricated experimentally as a test bed, and then a functional test of the test bed was carried out by using gaseous hydrogen and gaseous oxygen as reactant and Fluorinert FC-40 as a coolant. This paper describes the construction and the results of the functional test of the test bed. 2. FUEL CELL SYSTEM A flight-type fuel cell system for the future space vehicle is planned as schematically shown in Fig. 1. Supplied gaseous hydrogen and gaseous oxygen are warmed by the preheaters, and are fed to a cell stack through the pressure regulators. The oxygen line is a blind alley except for the purge line of accumulated impurity gas; that is, gaseous oxygen is supplied just as it is consumed. Gaseous hydrogen is circulated by a separator pump. Water vapor in gaseous hydrogen (that is, the reaction product in the cell stack) is condensed in a condenser, and then condensed water is removed from gaseous hydrogen by the separator. In addition, the heat generated by the fuel cell reaction is carried away from the cell stack by circulating coolant, and then the cell stack is maintained at a constant temperature. The power of the flight-type fuel cell system is expected to range up to several kilowatts. However, a power of 80 W was planned as the target power of the test bed for developing the above fuel cell system of the space vehicle. The test bed consisted
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