Conversion of solar energy into a usable form will require considerable technical skills and innovation as solar energy availability is not uniformly distributed, is low in intensity and is not continuous. Photovoltaic power conversion is considered a primary method for direct conversion of solar-to-electrical energy in space. A photovoltaic device uses thin-sheet solar cells to convert only a fraction of the total solar energy spectrum directly into electricity. The principle of photovoltaic energy is well known [1]. In short, when a photon is absorbed by a cell, voltage is induced and an electric current flows in an external circuit. The original concept of solar power satellites (SPS) was proposed in 1968 [2]. In this concept, a large array of cells collects solar energy and sends it to earth at a frequency of 2.45 GHz. A satellite is located in a geosynchronous equatorial orbit 35 800 km from Earth remaining within line-of-sight of associated microwave power receiving antennas on the Earth's surface. At the receiving antennas, microwave energy is converted into electricity. Using photovoltaic direct solar conversion for high power levels (perhaps in the gigawatt range) would require enormous arrays of cells due to their relatively low conversion efficiency. A conventional single bandgap solar cell is limited to a theoretical maximum efficiency of about 44% averaged over the normal sunlight spectrum [3], but its actual efficiency probably would not exceed half of that. For example, conversion efficiencies theoretically achieveable, range from 22-29% for silicon (Si) to 26-36% for gallium arsenide (GaAs) [4]. These efficiencies are quoted for sunlight with a power density of 1353 W/m2 just above the Earth's atmosphere. Today's best solar cells have an actual conversion efficiency range of 15-21% on the Earth's surface at total solar irradiance of 925 W/m2. The cost and weight of an SPS system can be reduced by using concentrated sunlight and by utilizing solar cells more efficiently. Several years ago, attention focused on an optical filter-reflector system as an effective concentration device. In that system, the solar spectrum is split into several spectral regimes, and each is directed to a solar cell that is optimized for that frequency regime. Onffroy et al. [5] calculated the total cell conversion efficiency and the cost of an optical filter-reflector system. They found that an efficiency of 28.4% could be attained with combined solar cell arrays of GaAs and Si. The projected cost per peak watt of this system was US $2.52. There is also interest in the utilization of solar energy to energize laser media. In this method, a working medium absorbs solar radiation and lases without the need for an intermediate form of energy. Previous reports [6, 7] described direct solar pumping of solid-state lasers such as ruby, Nd:glass and Nd:YAG. Recently, Rather [8] suggested that 100-MW gas lasers, powered by solar energy in space, are possible. However, direct conversion of solar energy into electricity, or into gas laser radiation, is an inefficient process. This is due to the narrowness of the absorption spectrum bandwidths of many potential solar cell or gas laser media compared to the effective bandwidth of the solar spectrum. Consequently, only a small fraction of the total solar energy can be utilized by direct conversion. Furthermore, absorption bands of potential working media may not lie in the same region as the peak radiant energy density of the solar spectrum. This paper presents and describes a method of efficient conversion of solar energy into electricity and into laser radiation via an intermediate blackbody cavity heated by concentrated sunlight. Such a system appears technically feasible since it requires only the combination of existing devices, such as concentrators, solar cells (or gas laser systems), power combiners, heat rejection radiators and sun-tracking mechanisms. As introduced by Yesil [9], Fig. 1 shows a simplified block diagram of such a system for indirect solar conversion. In the conversion process of blackbody radiation into
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