NASA is responsible for the conduct of the systems definition activity, and the Department of Energy is responsible for the other three activities. The largest effort to date has been directed toward the systems definition activity and associated cost studies. The other areas are now receiving increased emphasis under the direction of the Department of Energy. 2. SYSTEMS DEFINITION AND EXPLORATORY RESEARCH Within the systems definition activity, a number of approaches have been explored. There is obviously more than one way the system could be implemented, and technology programs will be required to provide definitive data in order to select an optimum system. The studies to date (8-11) have been valuable in helping to identify the strengths and weaknesses, or questions, related to various approaches and thereby to define the needs of research and technology programs. In order to illustrate the systems definition activity and the emerging research requirements, four major elements involved in the concept will be considered. These are: Energy conversion in space Power transmission to earth Space transportation Space construction A typical system configuration for the Solar Power Satellite is presented in Fig. 2. This configuration provides 5000 MW of electrical power to a ground receiving station for introduction into a commercial electrical power grid. The mass of the satellite is estimated to be 45,000 Mg (50,000 tons), with a solar collector area of 50 km2 (19 square statute miles). This large size is directly related to its large power output. It will require multiple launch vehicle flights and construction in space. Energy conversion A number of energy conversion approaches have been studied. Photovoltaic devices — specifically silicon and gallium aluminum arsenide — have received considerable attention. These two approaches are shown in Fig. 3. The solar collector illustrated on the left utilizes single-crystal silicon cells with a basic cell efficiency of 17.3% at 298 K (25°C) and a concentration ratio of one. A system employing gallium aluminum arsenide cells is depicted on the right of the same figure. This approach utilizes concentrators to focus the solar energy on the cells. Consequently, although the overall size of the two collectors is approximately the same, the area of the gallium aluminum arsenide cells is one-half the silicon cell area. This reduced cell area is achieved at the expense of a somewhat more complex construction arrangement involving the use of the concentrators. The gallium arsenide cell offers the potential of higher efficiency, less degradation of output at high temperatures, and less susceptibility to damage by the natural radiation in space than the more common single-crystal silicon cell. The disadvantages of gallium arsenide cells are less certain availability of gallium in the quantities needed for a large SPS program and a present lower state of development than silicon cells. Other promising photovoltaic cell candidates include thin-film cadmium sulfide
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