electricity. Primary concepts for electric power production on Earth are photovoltaic (solar cell arrays or “farms”) and the thermal engine “tower top.” In the tower top concept a field of steerable mirrors (heliostats) focuses energy onto a towermounted heat absorber. This heat can provide steam or some other fluid to turn turbogenerators. Solar power plants on Earth suffer from the diffuse nature of solar radiation (insolation), reduction in insolation from clouds, haze, etc., the varying angle of the sun’s rays and, of course, nightfall. A power plant located in space can receive nearly direct, unfiltered sunshine almost without interruption. For a given reception area, a space system will receive six times more energy per year than will the “sunniest” areas on Earth, and about 15 times more energy than a U.S. location with “average” weather. In geosynchronous orbit 35,786 kilometers (22,236 statute miles) above the equator, a satellite has an orbital period of 24 hours and so remains in constant line of sight to stations on the ground. Solar power satellites in such orbits would generate electric power which would be converted to microwaves and beamed to receiving stations for distribution to consumers as conventional electric power. Receiving stations in various parts of the U.S. could be associated with a number of satellites in orbit. Thus satellite systems can provide high availability “base load” power without the energy storage or backup facilities which greatly impact the cost and operational flexibility of terrestrial solar power stations. Space offers other advantages: • Thermal pollution from the power generation process is released in space rather than to the biosphere. ® The low gravity potential permits low-mass construction of the large areas necessary to intercept the solar energy. Consequently the total amount of resources used is less than for ground solar stations. • No oxidation or corrosion. • No tidal waves, earthquakes, etc. • Far removed from demonstrators, terrorists, etc. Other potential advantages of the SPS concept include contribution to U.S. energy independence, possibility of export, reduced pollution and improved economic stability (from reduction of inflationary pressures).
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