DOE are now nearing the end of a 3—year study of the various SPS designs described below. Once the results of those studies are available (scheduled for early 1980), a decision can be made as to whether to proceed with a prototype SPS or to shelve the concept temporarily or permanently. In January 1979, DOE and NASA published a reference design for SPS, selecting a photovoltaic system constructed in geosynchronous Earth orbit, generating 5,000 megawatts of power. In doing so, the agencies set aside designs for a solar thermal nature. For completeness, this issue brief describes both concepts. Before discussing the two most common SPS concepts, photovoltaic and solar thermal, it should be noted that two other designs are sometimes included under the rubric of SPS. The first suggests placing nuclear power plants in orbit, primarily to ease siting restrictions and environmental concerns. This system would not benefit from the greater availability or intensity of sunlight in orbit. Also, concerns about nuclear reactor—bearing satellites were heightened when a Russian satellite (Kosmos 954) accidentally deorbited and landed in northwestern Canada in January 1978. The other guasi-SPS design is a Power Relay Satellite (PRS), proposed by Krafft Ehricke (of Space Global, Inc.). As its name implies, this satellite would relay power generated by solar power plants, not produce the power itself, and therefore does not qualify in the SPS category - According to the Ehricke plan, solar power plants on the surface of the planet would generate electricity which would be converted into microwaves, then beamed up to the PRS and back down to an area requiring the power and reconverted into electricity. PRS is designed to accommodate the requirement that solar power stations be located in sunny regions (such as Arizona), while the power they produce may be needed in less sunny places like Syracuse, New York. SPS TECHNOLOGY The photovoltaic concept, or SSPS for Satellite Solar Power Station, was first proposed Dy Peter Glaser of Arthur D. Little, Inc., in 1968. His plan calls for constructing arrays of solar cells in space for the direct conversion of sunlight into electricity. a 5,000 megawatt SSPS (measured at the rectenna) would consist of two arrays, each approximately 5 by 6 Kilometers, separated by the microwave transmitter, and would generate 9,300 megawatts of power at the orbital station. An SSPS could also be designed to produce 10,000 megawatts at the rectenna. A reaction control system would be required to maintain the satellite in its desired orbit and to keep the solar arrays pointed continuously toward the Sun. Mirrors could be used to concentrate the solar energy onto the arrays. The solar cells could be made of either silicon or gallium arsenide (or other substances which may develop from future research). Silicon solar cells are the type in common usage today, and it is considered possible that these cells could attain an efficiency rating of 18% (converts 18% of light received into electricity) by the time an SSPS would be in use (compared with the 11—15% efficiency factor now achievable). Gallium arsenide cells are expected to have a higher efficiency, perhaps as high as 27%. They are also less susceptible to damage from space radiation and can provide higher efficiencies at higher temperatures, allowing concentration ratios (use of mirrors to intensify sunlight) up to 5 (silicon cells can operate efficiently with a concentration ratio of only about 2.) Until recently, gallium arsenide was also considered superior to silicon because it could operate with full performance at a thickness of 5 microns, while silicon began losing its efficiency at a thickness less than 100 microns. New research, however,
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