TECHNOLOGY ALTERNATIVES SOLAR ENERGY CONVERSION As originally conceived, the SSPS can utilize a number of approaches to solar energy conversion3 — thermionic, thermal electric, photovoltaic conversion, and others likely to be developed in the future. Among these conversion processes, photovoltaic energy conversion was chosen as a starting point because solar cells represent a demonstrated technology as a result of widespread use in the space program. Solar cells are used widely in space power supply systems whereas earlier efforts based on solar thermal and nuclear power were not as successful. In addition ERDA's National Photovoltaic Program has as its objectives to develop low-cost reliable photovoltaic systems and to stimulate the creation of a viable, industrial, and commercial capability. The photovoltaic process, since it is a passive one, could reduce maintenance requirements and lead to increased reliability during the desired 30-year operational lifetime of the SSPS. Present communication satellites (e.g., INTELSAT IV) already have a projected lifetime of 10 years. Because the space environment is benign compared to the terrestrial environment it should be possible to extend the lifetime of solar cells beyond 30 years by processes such as annealing, or recycling in a space manufacturing facility. Solar thermal conversion is of interest primarily because machinery operating on thermodynamic cycles; e.g., the Brayton Cycle, has had a long and successful history in terrestrial applications. Furthermore, the development of orbital solar power plants could be based on the development of optical focussing systems and central receivers for solar thermal plants which could be adapted for use in the SSPS.4 Novel techniques to achieve geometric perfection desired for the solar concentrators through active mirror-shaping controls are being investigated. If successful, these efforts will result in the large concentration factors required to achieve elevated temperatures for high thermodynamic efficiency. Progress in gas-bearing technology holds promise that the reliability of rotating machinery could be extended beyond the few thousand hours associated with most terrestrial applications of rotating machinery. Rejection of waste heat through active systems, including radiators, still poses a major challenge because of the mass required for a large-area space radiator and for operational reliability. Thermionic energy conversion, which has been investigated in the space program but not yet applied, may be useful as part of a topping cycle in combination with thermal conversion systems. Considerable technical and economic analyses will be required to establish which of the potential approaches for solar energy conversion will be optimal. However, the fact that there are several promising approaches to meet the
RkJQdWJsaXNoZXIy MTU5NjU0Mg==