by Kevlar) or well insulated laminates are developed. 4.23 Solar Cell Technology Development The vast majority of solar cells currently used in space applications are made from silicon. A limited number of gallium arsenide cells are used on a few American, Japanese and Soviet satellites. Considering cells mass produced in significant volume (more than 30,000 2 by 4 cm cells per year), it is generally true that silicon cells are far cheaper, much lighter and are more reliable due to experience with their long production history. Gallium arsenide cells are heavier and more costly, but offer the advantage of higher efficiency. Under ideal laboratory conditions, gallium arsenide cells exhibit typical efficiencies of 17 to 18 percent, as compared to 13 or 14 percent for silicon cells. This advantage is much more pronounced under operational conditions, with end of life efficiencies of 15 to 16 percent for gallium arsenide versus 9 to 10 percent for silicon. 4.23.1 Development Targets After many years of apparent stagnation, sudden progress is being made in the development of both silicon and gallium arsenide solar cells. A wide variety of cell types with various features can be expected to compete for the space applications market over the next ten years. The development target for advanced silicon solar cells is 15 to 16 percent efficiency (beginning of life, air mass zero, 25°C). This is to be achieved by mid 1991. Ilie cell under development is derived from the new silicon cells developed by the groups of M. Green in Sydney and R. M. Swanson at Stanford, whose cells achieve terrestrial efficiencies of 20% (equivalent to a space efficiency of over 18%). Adapting these cells to space use will not be straightforward, and requires a significant development effort. Bifacial silicon cell development is also important. It offers a 10% increase in output power in LEO when compared to conventional configurations. Bifacial cells can be produced using advanced "high eta" technologies to provide 17% efficiency (air mass zero, 25°C, Beginning of life). Advanced silicon cell development is directed towards large area (25cm2), ultra thin conventional and bifacial devices, as well as improved on orbit lifetime and end of life power levels. Gallium arsenide and silicon cell development activities are being conducted in parallel through the various national programs. These activities are coordinated with the ESA since they are complimentary to the ESA program. There is also a separate project to investigate the suitability of terrestrial bifacial silicon cells for space applications. Development of gallium arsenide cells for space use began in the 1980's in several national programs (Italy, Germany, France and Great Britain). Anonorbit demonstration and testing of European gallium arsenide cells will fly on EURECA.
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