0191-9067/85 $3.00 + .00 Copyright ® 1985 SUNS AT Energy Council COST COMPARISONS OF SOLAR POWER SATELLITES VERSUS LUNAR BASED POWER SYSTEMS E. M. KERWIN and G. D. ARNDT NASA Lyndon B. Johnson Space Center Houston, Texas 77058, USA INTRODUCTION The solar power satellite (SPS) concept was originally proposed as a geosynchronous orbiting system, supplying continuous power to a ground receiving/rectifying antenna (rectenna). The SPS satellite had a 1 km microwave antenna transmitting 6.7 gigawatts (GW) of RF energy to a ground rectenna. After conversion losses were considered, this system supplied approximately 5 GW of power to a commercial utility grid at a cost of 46.8 mills per kWH. One of the major criticisms by the National Academy of Science of the SPS concept was the low cost assumed for producing silicon solar cells in large quantities. The cost projected for manufacturing silicon cells in ribbons or sheets was considered unrealistic in comparison to the present day costs of producing cells by growing ingots and then cutting, polishing, lapping, etc., the cells. There have been recent suggestions that the moon be used for solar power plant sites. The solar cells could be manufactured from minerals in the lunar soil, thereby achieving significant cost savings in manufacturing and transportation of the cells. In addition, part of the antenna structure could be made from lunar materials. There arises, however, the problem of transmitting the microwave power approximately ten times further (380,000 km to the moon as compared to 36,000 km to geosynchronous orbit). The purpose of this study is to compare the electricity costs of an optimized SPS system in geosynchronous orbit with an equivalent system based on the moon. The original cost model for the geosynchronous SPS as given in Ref. (1) will be modified to provide an equivalent lunar based system. GENERAL CHARACTERISTICS OF A LUNAR POWER PLANT A geosynchronous staellite remains fixed in location relative to a rectenna on the Earth and receives sunlight 100% of the time except for short periods around the two equinoxes (March 21 and September 21). In contrast, a lunar base receives sunlight approximately 50% of the time even though the same side of the moon faces the Earth at all times. In order to supply nearly continuous power to the Earth, two lunar bases, one on each extremity of the side of the moon facing the Earth, are needed. Additionally, since the Earth makes a full rotation with respect to the moon each day, three ground rectennas spaced 120° apart are required to receive the continuous power supply (Fig. 1). Thus, a lunar based power system (LPS) requires two solar
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