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. Therefore, the SPS deserves to be reexamined as one of the few long-term global energy supply options. An SPS R&D program suitably integrated with future space projects will have significant benefits for both government and private sector participants in space and energy projects. THE SOLAR POWER SATELLITE CONCEPT EVOLUTION 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. In 1972, the Solar Energy Panel of the National Science Foundation outlined a plan for an SPS research and development program. In that year, NASA also started a program to evaluate the feasibility of the SPS concept. In that study, a reference system design was adopted to provide a continuous power output of 5 million kW on Earth. The program investigated structural design and control requirements, examined microwave power transmission techniques, and identified technological, environmental and economic issues. Extensive system definition studies were started in 1975. The following year, the principal responsibility for the SPS program was transferred to the Energy Research and Development Administration (ERDA). On the basis of its own appraisal, ERDA recommended that a more complete evaluation be made. Concurrently, ERDA was reorganized and absorbed within the new DOE. In 1977, DOE, working closely with NASA, started a three-year CDEP, with a budget of $19 million, to develop an understanding of the technical feasibility, economic viability, and societal and environmental acceptability of the SPS concept. 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
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