1. 2. 2 PHOTOVOLTAIC POWER GENERATION In this potential system solar cell arrays are connected in series and in parallel as required to provide the high voltage necessary for the transmitter. Several types of solar cells may be considered as candidates: silicon (used on current spacecraft), gallium arsenide heterojunction, and several thin film types such as cadmium sulphide. Solar cells require several advances to provide maximum benefit for power satellite use. These advances are increased efficiency of conversion of sunlight to electricity, lighter weight (i. e., thinner), increased resistance to space radiation, and, of great significance, lower cost. One approach to cost reduction is the use of solar concentrators. Mirrors of thin metal or metallized plastic film would shine additional solar energy onto the cells. This causes increased cell output, so that fewer cells per satellite are required. Concentration ratio selection must, however, take into account the heating of the cells which tends to occur, since heating reduces efficiency. Cell cooling, possibly with metal fins, is an additional possibility. A lightweight structure of limited flexibility is required to support the cells. This structure will probably also be called upon to act as a power distribution system, i. e., carry electric power from the cells to the transmitter. An attitude control system is also required to align the solar arrays to the sunlight, providing a ''base" from which to point the transmitter antenna. Since the arrays face the Sun and the transmitter must face the Earth, a rotating joint must be provided. The electric power must cross this joint on its way to the transmitter. 1. 2. 3 THERMAL ENGINE POWER GENERATION In this concept reflecting mirrors, probably in the form of a paraboloid, focus solar energy into a cavity absorber, i. e., the rays enter a hole in a sphere. This insulated sphere contains a heat exchanger assembly composed of an array of tubes. Gas (probably helium) flowing through these tubes picks up thermal energy. The gas flow expands through turbines which turn the electric generators. The turbines also turn compressors which route the gas through the remainder of the system and back to the cavity absorber to collect more energy.
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