minus the 350 EJ), may still be conservative on the high end if nuclear fusion becomes available, alternative energy sources are more actively developed on Earth, or efforts to restore the natural environment are optimistically successful. For a requirement of 550 EJ per year in the year 2030, 70,621 square kilometers of solar panels will be necessary, assuming 30 per cent efficiency for solar power conversion and 60 per cent efficiency for transmission to Earth. A number of studies have recommended power generation, at least initially, in the range of 5 to 20 gigawatts (GW) per satellite. For a satellite generating 10 GW in space, 23 square kilometers of solar panels will be needed. Allowing for losses in transmission, 3,065 such satelliteswill produce the required 550 EJ; each will produce nearly 0.2 EJ per year. Early experience with satellite construction and operation may revise decisions on size - in that case, fewer numbers of more powerful satellites may be constructed. A Solar Power Satellite (SPS) is constructed as a double cone with an axis 3.75 kilometers long, and with circular bases 9.25 kilometers and 6.75 kilometers in diameter. Eight large solar panel arrays are connected in a ring at the base of each cone. Each of the large panels is made up in turn of four medium-sized panels, individually constructed as tension/compression structures with central masts and cable systems. These medium-sized panels, 745 meters square, each are made up of four "small" 372 meter square panels containing 90,000 square meters of silicon solar cells each. Aluminum buses connect cells, small panels, medium panels and large panels, and transmit current along structural beams from the rings to the center of the double cone. There, power control equipment and an SMES, capable of storing 5 GWh of power, manage the production and transmission of the energy to Earth. Rectenna Rectennas for power reception are located in remote areas on land and sea. In shape, they are elliptical with the major axis perpendicular to the equator to conform to the intersection of the Earth's surface with the conical beam projected from an SPS. The ratio of the major to minor axes of the ellipse varies with the latitude where the rectenna is placed; longer major axes are required for rectennas at higher latitudes. Because all Solar Power Satellites must be over the equator for geosynchronous orbit, Rectennas are limited to latitudes at which their major axes will not be too long. A Rectenna at 34 degrees latitude (the latitude of Los Angeles), for example, would be 10 kilometers by 13 kilometers in size. The receiving surface of a Rectenna is made up of rows of parabolic reflec- tor/collectors parallel to the minor axis. The reflector/collector units, 25 meters long by 3 meters high, encase metal reflector wires less than 10 centimeters apart (the wavelength of the microwaves) in an extrusion that is transparent to microwaves. The encasement protects the wires from atmospheric forces (desert or ocean corrosion) and is easily cleaned by a tracked automatic cleaning machine. Para-
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