the thermal engine ''tower top." In the tower top concept a field of steerable mirrors (heliostats) focuses energy onto a tower-mounted 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 areas receiving the most sunlight on Earth and approximately 15 times more energy than a United States location with average weather. In geosynchronous orbit 35 786 km above the Equator, a satellite has an orbital period of 24 h and remains in constant line of sight to stations on the ground. Solar power satellites in such orbits would generate electric power that would be converted to microwaves and beamed to receiving stations for distribution to consumers as conventional electric power. Figure 1-1 shows how receiving stations in various parts of the United States could be associated with a number of satellites in orbit. Thus, satellite systems can provide high availability ''baseload" power without the energy storage or backup facilities that greatly impact the cost and operational flexibility of terrestrial solar power stations. Space also offers these 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 that for ground solar stations. • There is no oxidation or corrosion. • There are no tidal waves, earthquakes, etc. • The satellite systems are far removed from demonstrators, terrorists, etc.
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