development. For example, the work of Sadi Carnot, while a captain of engineers in Napoleon’s army, led to the famous Carnot principle and the foundation of thermodynamics. Similarly, generic technologies developed to meet SD1 requirements can be expected to have potential applications to the SPS. The SPS concept continues to receive consideration, judging by the growing international literature and exemplified by the Space Solar Power Review. It is the subject of meetings such as the Symposium on Energy from Space at UNISPACE 82 (7), the Space Energy symposia arranged by the Institute of Space and Astronautical Science (8), Tokyo, and the Astronautical Federation Congress, Stockholm, 1985. The SPS continues to be of world-wide interest, possibly because of the limited number of base-load power generation options that could utilize renewable resources and have potential global applications, be cost competitive, compatible with the environment and conserve terrestrial resources, particularly if lunar resources were to be used for the construction of the SPS. The following sections present an overview of specific areas where significant developments applicable to the SPS are taking place. PHOTOVOLTAIC CONVERSION During the 1980s, considerable progress had been made in the development of advanced photovoltaic materials. The improved quality and increasing size of single-crystal silicon ingots have resulted in the production of cells in the 20-cm2 area range with a thickness of 50 gm and efficiencies of about 15%. The major thrust for space applications is to reduce the mass, increase performance, and reduce the costs of solar arrays. Solar cell costs presently in the $200/W range are expected to decrease by an order of magnitude during the next decade through developments, such as (1) the back-surface field silicon solar cell of higher efficiency, longer carrier lifetime and reduced thickness, (2) high-efficiency wrap-around silicon solar cells and (3) solar cells that are resistant to space radiation. Gallium arsenide solar cells have been intensively studied for space applications because they promise the achievement of higher efficiency levels than silicon solar cells. There is a potential to construct ultra-lightweight cells with high-temperature capability and better radiation resistance achieved by in situ annealing of solar cells at modestly elevated temperatures. The objectives of the research on gallium arsenide solar cells is to demonstrate that efficiencies will exceed 20%; specific power will exceed 2.5 kW/kg; costs will fall below $40/W; radiation damage will be less than 10% after 10 years of operation in geosynchronous orbit; 100% performance recovery will be achievable by annealing the gallium arsenide solar cells. Two development approaches are being pursued: (1) a lower-cost high- performance gallium arsenide solar cell in planar form and (2) a solar cell for use in concentrators. For example, a miniature concentrator has been developed that uses gallium arsenide solar cells (4-mm2 area), with a concentration ratio of 125, an 80°C operating temperature, a solar array thickness of 1.8 cm and a demonstrated efficiency of 19%. The development of light-weight high-performance solar cells in a planar or concentrator configuration is based on a light-weight substrate (grapheoepitaxy), or a reusable substrate (peeled film). Space solar cell research is also concerned with developing cascade cells based on III—V semiconductors with the goal of achieving a 30% conversion efficiency at the operating temperature, as well as increased resistance to radiation.
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