radiation. As we see the efficiency T]max has considerably smaller values as compared to the case of interplanetary power stations. Even if they are far away from the Sun, the outer planets are characterized by larger values of the maximum efficiency t]max This is due to their reduced ambient temperature. By increasing solar radiation concentration a considerable improvement of the performance is obtained (figure 8). Again, the ambient temperature has a more important influence on tlmax as compared to the distance to the Sun. Conclusions For a better accuracy the flux of solar energy must be computed by considering the Sun as a source of non-isotropic radiation. The isotropic approximation is not recommended at small distance to the Sun or when strongly concentrated radiation at large values of the zenith angle is considered. Solar radiation is relatively abundant in the region of inner planets (including Mars and asteroids) and can be used with today's technology. However, the large scale utilization of solar energy in the area of the outer planets seems to be a question to be addressed in the far future. The efficiency of solar radiation conversion into available work has large values in case of space solar power stations. Then surface planetaiy power stations are considered, the above efficiency is reduced. In both cases a significant improvement of the performances can be obtained by solar radiation concentration. References [1] . T. Gaudin, SPS and the next century, Proc, of the 2nd Int. Symp. on Space Power Satellites SPS 91, Paris, August 27-30, 1991, p. 27-30. [2] . P Glaser, Solar Power Satellites option re-examined, Proc, of the 2nd Int. Symp. on Space Power Satellites SPS 91, Paris, August 27-30, 1991, p. 59-64. [3] , V. Prisnyakov, SPS interest and studies in USSR Proc, of the 2nd Int. Symp. on Space Power Satellites SPS 91, Paris, August 27-30, 1991, p. 36-44. [4] , D. Criswell, Lunar system to supply solar electric power to Earth, Proc, of the 25th Intersociety Energy Conference, vol. 1, August 1990, p. 61-70. [5] , A. Coher, Human exploration of space and power development, Proc, of the 2nd Int. Symp. on Space Power Satellites SPS 917 Paris, August 27-30, 1991 p. 31-35. [6] , J.A. Angelo, D. Buden, The nuclear power satellite (NPS) - key to a sustainable energy economy and solar system civilization, Proc, of the 2nd Int. Symp. on Space Power Satellites SPS 91. Paris. August 27-30, 1991, p. 117-124.
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