PASSIVE SOLAR REFLECTOR SATELLITE REVISITED C. Polk and J. C. Daly University of Rhode Island Department of Electrical Engineering Kingston, R. I. 02881 Principal disadvantages of the solar power satellite, as normally proposed, are its cost and low overall efficiency (about 7 per cent). To overcome conversion losses and to avoid the need for photo-voltaic cells, an alternative system has been proposed: passive light-weight reflectors in space which direct the incident solar energy to a specified location on the surface of the earth. There either photo-voltaic cells are employed or, after light concentration by another reflector system, a steam turbine alternator on a "solar tower", or a similar 'conventional1, relatively high efficiency cycle is used for electricity generation. This idea has been discarded in the past, because the small, but nevertheless significant divergence of rays at the earth-solar distance due to the finite diameter of the sun would produce a minimum spot diameter of 330 km on the earth's surface if a single passive reflector or lens is used in geostationary orbit. Spot size can be substantially reduced if the satellite is placed at lower elevation. Nevertheless, since the geostationary orbit is probably most attractive if one satellite is to provide continuous illumination for a single ground station, and since the problems arising from reduction of spot size are, in principle, the same at any sufficiently large elevation, we examine the more difficult problem of the passive reflector in geostationary orbit. If a single satellite in geostationary orbit is used, the following constraints apply to the design of the optical system: Distance from source (sun) to lens or mirror system Image distance (i.e. distance to ground station) Object size (sun diameter) J It is probably desirable that k ~ 1 (giving about 1 kW/m ), since k > ] may produce undesirable environmental effects and k < 1 would require a larger reflector area on the ground to generate a specified amount of power. Also conservation of energy requires that the power intercepted by the first aperture in space be equal to the power received on the earth Applying as first approximation purely geometric optics, we are in effect attempting to produce on the surface of the earth an image of the sun. Using for each lens or mirror
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