SPS Background There is a lot of commonality between SPS and STS, and the development of an STS system could facilitate the eventual deployment of SPS, since the economically exploitable hydropower resources will eventually all be used and energy demand will continue to grow. At that point the addition of SPS derived power to the system will be logical and technically straightforward since the receiving and distribution systems will already be in hand. A brief description of SPS is thus in order. Space Power Satellites are in geostationary orbit above the earth's equator-the same orbit typically used by communications satellites. This orbit is at an altitude of 35,800 km. The satellites intercept solar radiation and convert it to electricity. This is typically done with photovoltaic cells and the arrays of cells are held perpendicular to the sunlight for maximum efficiency. This power is then converted to microwaves (typically 2450 Mhz) which is beamed to the ground from a circular antenna which always points at the earth. This system is shown schematically in Fig. 3 and Fig. 4. The typical scale of an SPS as described in various sources [2] is shown in Fig. 4. The solar array is 4 by 10 km, and the trusswork to ensure rigidity is 470 m thick. The circular antenna is 1 km in diameter. The entire structure weighs 50,000 tons. The receiving antenna on earth is typically 10 by 13 km. The receiving antenna rectifies the microwaves to direct current which is then converted to alternating current for transmission. During the equinoxes there are periods of about three weeks when the SPS is in daily eclipse for about an hour. This reduces power availability by about 100 hours per year. The energy balance of the design described above is as follows: • assumed sunlight conversion efficiency of 12%; • 8,180 MW of DC to the microwave generator; • 6,960 MW of microwaves from the generator to the antenna;
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