A layer of photovoltaic cells facing the cavity receives the blackbody radiation and converts it into electricity. Since the blackbody cavity is at a lower temperature than the sun, the blackbody radiation has reduced intensity in the short wavelengths where the solar cell is inefficient. In addition, the majority of the unabsorbed longer wavelength light is returned to the blackbody cavity walls by means of a reflective back surface on the cell. Thus, a TPV device can convert solar energy into electricity more efficiently than conventional photovoltaic systems. The feasibility of a solarelectric TPV system with a spherical cavity utilizing Si cells has been analyzed by Bracewell & Swanson [10], in which cell conversion efficiency was predicted to be in the range 30-50%. However, experimental results showed that the actual maximum cell conversion efficiency was only 12%. They concluded that this low efficiency was due mostly to high parasitic absorption inherent in the design of their TPV system. In 1979, Hertzberg & Sun [11] described a study on the feasibility of the TPV power cell concept on a space shuttle. They estimated that an overall system with an efficiency of 11.4% could be designed into a 2.5 cubic foot payload envelope, assuming a Si TPV cell efficiency of 30% and a concentration ratio of 6626. In 1983, Yesil [9] introduced a cylindrical solar-electric TPV system utilizing GaAs and Si cells with an estimated cell conversion efficiency in the order of 50%. In this work, a TPV system similar to that of Ref. [9] but with an improved cooling and heat rejection system is introduced. This system may have an overall cell conversion efficiency of 50% or more. With increased system efficiency and with reduction in photovoltaic cell area by more concentration, it is possible to increase further the specific power (W/kg) output of the cell. Thus, there are potential opportunities for weight and cost reduction in an SPS system for a given power level. 2.2 Description of the Solar-Electric TPV System Model The proposed solar-electric TPV configuration, in which sunlight is concentrated and focused into a blackbody cavity (converter or receiver) through a Cassegrainian aperture system, is shown in Fig. 2. This design consists of optics, blackbody cavity, solar cell (and laser) system, sun-tracking device and heat rejection system. The optics employ a large paraboloid primary mirror and a small hyperboloid secondary mirror. It is reported [10] that a total concentration ratio of 20000 (5000 by the primary mirror and 4 by the secondary mirror) into the converter entrance could be possible with a configuration similar to that shown in Fig. 2. This two-stage concentration reduces aberration losses and surface errors compared to a single higher-concentration paraboloid mirror for illuminating the converter. The optics also include a reflective cone (compound parabolic concentrator) mounted on top of the converter hole to enhance sunlight collection efficiency. Without this optical device, some sun rays could completely miss the cavity hole. In addition, it aids in minimizing re-radiation from the blackbody cavity. This is done by selecting a thin entry window (sapphire is a good material for the window because it is highly transparent, inert, has a high melting point and good mechanical strength) coated internally to maximize reflection of radiation back into the cavity. The blackbody receiver captures the solar energy reflected from the concentrators and transfers it to the solar cells at the required conditions. The receiver configuration is a closed cylinder of solar cells, insulative material and cooling systems (graphite felt, vacuum, coolant, etc.) surrounding an interior cylindrical cavity with a length-to- diameter ratio of 2 (Fig. 2). The inner surface of the cavity is lined with graphite
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