square kilometer receiving rectenna. To amortize the cost of the large antennas required, a 5,000 MW (output to user) power level was proposed. An integrated SPS design using the same sized rectenna and transmitting aperture as the baseline, assuming the same system efficiency, would allow a minimum power level of only 33 MW [using the 1990s assumptions above] or 100 MW [using the 2000+ assumptions]. For higher power levels (i.e., a larger transmitter aperture), a smaller rectenna could be used. For example, at the 5 GW level proposed for the 1980 reference SPS [4], the integrated array/transmitter would allow the rectenna area to be reduced by a factor of 50 to 150, resulting in a required rectenna area of only 2 or .67 km2 respectively using the far-term and near term assumptions. Such power levels, although representing absolute intensity of only a few times that of the sun, could be too high for single user operation; but multiple users, each with these smaller receivers, could share the power either by “spot beam hopping” or by simultaneous transmission as discussed. The use of an integrated design, with potentially very large transmitter apertures, thus enables new system architectures providing power to smaller receivers at multiple sites. Applications The main application of the solar power satellite envisioned by Glaser and by many of the later advocates of satellite solar power was to provide baseline electrical power for terrestrial use. However, it is quite likely that some of the most important applications, and certainly some of the initial applications, will be in space. Here atmospheric attenuation does not limit the frequency choices and transmission distances may be less. Further, because of the high total mass of the power systems (including storage, PMAD, thermal control and structures) and the high transportation costs, existing power sources for use in space provide power at a considerably higher effective price ($800/kW-hr) than terrestrial power sources ($.10/kW-hr) [17]. By building integrated solar power satellites, remote power sources would be able to serve several critical needs. These include: (1) Beamed power for lunar bases, rovers, remote instruments, outposts, etc. [7] (2) Remote powering of electric propulsion vehicles, e.g., for interorbital transport vehicles using ion or magnetoplasmadynamic engines. (3) Power for Earth-orbital stations [6]. (4) Support for Mars missions and solar system exploration and exploitation. Developments Required So far we have only discussed the potential advantages, and not the problems. The concepts outlined above have been schematic, not detailed engineering designs of how such a system could be built. Many problem areas remain to be addressed.
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