has shown that, although the efficiency of silicon cells does decrease somewhat as the cell becomes thinner than 100 microns, the efficiency increases again as the cell becomes even thinner. It is now expected that a silicon cell 5 microns in thickness would have comparable efficiency to the gallium arsenide cells. End—to—end efficiency of an SSPS system is expected to be about 7.06% for a silicon ceil design, and 6.97% for a system using gallium arsenide, assuming a cell efficiency of 16.5% for silicon and 18.2% for gallium arsenide. The major factors in favor of silicon cells are familiarity in working with them and a possible supply problem with gallium. In the DOE/NASA reference design, no choice was made between the two solar cell options. Solar thermal systems involve focusing solar energy with mirrors into a cavity containing a heat absorber (helium, potassium vapor, or liquid cesium) which becomes heated and drives a turbine which produces electricity. The solar thermal concepts most often discussed are the closed Brayton cycle and the Rankine cycle. (A third concept, using thermionic diodes, was dropped from active consideration because of its low end-to-end efficiency — about 4% — and high weight requirements.) These power plants would probably be built on a modular concept, with each module containing a solar concentrator (mirrors), a cavity heat absorber, turbomachinery, and other subsystems. Both the Brayton and Rankine cycles depend on high temperatures to heat the cavity heat absorber, requiring a concentration ratio of about 1000 (compared with 2 to 5 for the photovoltaic systems). A large area of mirrors would therefore be needed to concentrate the solar energy, a disadvantage both in terms of weight requirements and construction time. In a closed Brayton cycle SPS, helium is compressed and then heated by the solar energy, causing the gas to expand. This process generates power to run the turbines, after which the helium flows through a radiator where the heat is rejected into space, and then the cycle begins anew. Temperatures of about 1380 degrees centigrade are required for this system to be competitive with an SSPS. The Rankine cycle operates in a manner similar to the Brayton cycle, but uses liquid cesium or potassium vapor as the cavity heat absorber, and can be competitive with SSPS using lower temperatures, about 1038 degrees centigrade. Initially, the Boeing Company sponsored a Brayton cycle SPS called Powersat, which would have consisted of 16 modules and produced 10,000 megawatts at the rectenna with an end-to-end efficiency of about 9-10%. Boeing later concluded an SPS study showing a silicon photovoltaic system would be preferable to the Brayton cycle. Rockwell International has proposed a Rankine cycle SPS using liquid cesium as the cavity heat absorber and a steam bottoming cycle, with an expected end-to-end efficiency of 9.3%. Boeing's second choice for an SPS is a Rankine cycle using potassium vapor. Rectenna design would be essentially the same for either the photovoltaic or solar thermal system. Each rectenna for a 5,000 megawatt SPS would be approximately 13 by 10 kilometers and would require about 270 square kilometers of land area, depending on its latitude (as latitude increases, a “flashlight” effect occurs requiring larger, more elliptical rectennas). A 10,000 megawatt SPS would use two rectennas, rather than a single, large one. The orbiting microwave transmitters would consist of 7,220 subarrays, necessitating an automatic phasing system to ensure that the total power output from the transmitter would remain constant. A pilot signal emanating
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