As currently envisioned, the SPS would be placed in geosynchronous orbit, 22,300 miles above the equator, where solar cell arrays would convert energy from the sun directly into electricity and feed it to microwave generators forming part of a transmitting antenna. The antenna would precisely direct a microwave beam of very low power density from the SPS to one or more receiving antennas at desired locations on Earth. At the receiving antennas, the microwave energy would be safely and efficiently reconverted into electricity and then transmitted to users. An SPS system could consist of many orbiting satellites, each beaming power to one or more receiving antennas. Assessments of the technical, economic, environmental, and societal issues associated with the SPS have not identified any single constraint that would negate the SPS concept (2). High front-end costs have been cited as a prime reason for delaying even an R&D program (3). These cost projections assumed a very large-scale program including a commitment to an integrated development and implementation program that could meet 25% of the U.S. electricity demand by 2030, and discounted the evolution of generic technologies. Based on this assumption, the U.S. Department of Energy (DOE) has not proceeded further with the SPS Concept Development and Evaluation Program (CDEP). During the early 1970s, when the SPS design was being evolved by NASA (4) for use in the CDEP, the space technologies required were in an early stage of development. Since then, significant advances in a wide range of technologies have been achieved and are being successfully applied to expanding space activities. The growing industrial infrastructure supporting these activities will strongly influence the development of space transportation systems, space stations, and similar space projects of increasing scope and complexity. The resolution of issues associated with implementation of the SPS including electrical power demand, power network interfaces, load management, receiving antenna siting, availability of material resources, and comparative assessments with other energy conversion methods was already considered as part of the CDEP, and there is an existing framework for continuing these assessments. The CDEP program was recognized by DOE as a model for the effective assessment of large-scale energy conversion projects (5). Although the depth of analysis was limited by the available funding, the studies supporting the program examined an unprecedented variety of issues that might influence development of the SPS. An explicit objective was to involve public interest groups in discussions about the SPS so that future decisions concerning the project could be based on a broad consensus rather than on narrowly defined expert opinion. SPS designs ranging from 10-5000 MW have been studied in Canada, Czechoslovakia, England, France, Japan, the Soviet Union, and West Germany, indicating the wide interest in the power generation potential. One reason for the increasing confidence in the technical feasibility of the SPS is that alternative technologies have been identified for nearly all components of the system. Most studies have been concerned with the SPS reference system which was chosen during the CDEP to provide a common basis for assessments. This “reference system,” based on assumed guidelines, was established by NASA to evaluate environmental effects, explore societal concerns, and perform comparative assessments. It is a design concept based on known technologies of the early 1970s; it does not represent a system that was expected to be actually constructed. An operational SPS, which could be developed during the next 20 years, would use some of the many alternative technologies that already have been identified for advanced SPS
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