alized society to develop high technology could be applied to the development of solar energy conversion methods in space on a scale which may not be possible on Earth. In the 1960's, the logical soundness of using the synergism of solar energy conversion technology and space technology led to the concept of the SPS (2). As conceived, the SPS would convert solar energy into electricity and feed it to microwave generators forming part of a planar, phased-array transmitting antenna. The antenna would precisely direct a microwave beam of very low power to one or more receiving antennas at desired locations on Earth. At the receiving antennas, the microwave energy would be safely and efficiently reconverted to electricity and then transmitted to users. An SPS system would comprise a number of satellites in GEO, each beaming power to one or more receiving antennas. Furthermore, a major advantage of solar energy conversion in space for use on Earth is that solar radiation in geosynchronous orbit (GEO) — unlike solar radiation received on Earth — is available during most of the year. Solar radiation in GEO will be interrupted by the Earth's eclipses of the Sun from 22 days before to 22 days after equinoxes for a maximum period of 72 min a day when the Earth as seen from a GEO position is near local midnight. Overall, eclipses will reduce the solar energy received in an orbital position in GEO by only about 1% of the total available during a whole year. A solar energy conversion system in GEO will collect at least four times the solar energy that would be available to it on Earth, even in favorable locations, because of interruptions caused by weather and the diurnal cycle. Therefore, terrestrial photovoltaic systems are more likely to be useful for displacement of electric utility capacity because such systems could be operated intermittently with appropriate utility- provided system storage to meet peak load demands, rather than be called upon to generate baseload power which would require storage capacity in the utility system. In contrast, the SPS will be capable of generating baseload power with either none or very limited utility-provided system storage. Furthermore, a hypothetical comparison of an SPS in GEO vs an idealized terrestrial photovoltaic system (i.e. one with no storage and retrieval losses) shows that the latter under average weather conditions, even in a geographically favored location would require an area at least twice the size of the SPS's receiving antenna site. In recognition of the potential of the SPS as a large-scale global method of supplying power to the Earth, the challenges posed by the SPS concept are being explored through feasibility studies of the technical, economic, environmental, social and international issues by the U.S. Department of Energy and NASA (3). The status of the SPS development to date has been reviewed and the issues which require resolution highlighted in a position paper issued by the American Institute of Aeronautics and Astronautics (4). As originally conceived (5), an SPS can utilize current approaches to solar energy conversion, e.g. photovoltaic and thermal-electric, and others likely to be developed in the future. Among these conversion processes, photovoltaic conversion represents a useful starting point because solar cells are already in wide use in satellites. An added incentive is the substantial progress being made in the development of low-cost, reliable photovoltaic systems and the increasing confidence in the capabilities of achieving the required production volumes (6). Because the photovoltaic process is passive, it could reduce maintenance requirements and achieve at least a 30-year or even a several hundred year operating lifetime for an SPS. Micrometeoroid impacts are projected to degrade 1% of the solar cell array area over a
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