Thin-film solar cells have also been considered for use in space in view of the successful development of thin-film solar cells on Earth, e.g. amorphous silicon. The challenges faced in the development of thin-film space solar cells include long-life stability, absence of interlayer movement, automated production, control of I-V properties, effective encapsulation and mitigation of the potentially deleterious effects of spacecraft charging. Solar arrays for satellites are being developed to meet specific application requirements, including solar cells integrated with the satellite structure, solar array wings which can be folded compactly and fastened to the side of a satellite, expandable solar arrays, e.g. to a 30x84-foot 25-kW solar array. Operation of an expandable array prototype was successfully demonstrated on a Space Shuttle mission. Solar array concepts are being developed for space power sources in the 300- to 1000-kW range, based on the self-deployable planar concept. Such solar arrays can be assembled to form a platform of approximately 200x 190 feet to generate about 430 kW, and they are capable of being transported into orbit in a single Space Shuttle. SOLAR DYNAMIC CONVERSION Solar dynamic conversion systems for use in space have been studied since the 1960s. The modest power outputs required by satellites up to the present have favored the application of photovoltaic conversion systems. However, as requirements increase into the megawatt range during the next decade, solar dynamic conversion systems will be reexamined as an alternative power generation source for the space station and beyond. A generic solar dynamic conversion system consists of a light-weight solar concentrator, a cavity to absorb the concentrated solar radiation and transfer it to a heat engine, and efficient thermal radiators to reject waste heat to space. Light-weight solar concentrators were developed in the 1960s and 1970s, with a variety of designs used to provide acceptable optical surfaces that had the required concentration factors. Adaptive optics and accurate attitude controls can ensure that solar radiation will be focused into the cavity absorber. Stirling, Brayton, or Rankine-cycle engines can be used to convert heat into power with efficiencies in the 20-30% range. Waste heat can be rejected to space by means of space radiators operating in conjunction with heat pipes or circulating fluids. Recent developments in two-phase fluid flow show considerable promise for increasing the effectiveness of space radiators. The selection of the most suitable materials and the operation of rotating machinery with high reliability for extended periods are difficult challenges to overcome, even for terrestrial solar dynamic systems. However, the growth of the space industrial infrastructure will lead to the development of automated maintenance, servicing and retrieval capabilities that will permit solar dynamic conversion systems to be serviced on a planned schedule and thus achieve reliable operations. There is every reason to expect that solar dynamic conversion will grow in importance as an alternative to photovoltaic energy conversion. GENERIC SPACE TECHNOLOGY DEVELOPMENTS Space experiments performed in conjunction with space missions are essential for
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