Several simple formula were proposed for the efficiency T|ex (see e.g. [15,16] and the references therein.) However they evaluate an upper bound which is too high to be useful for practical applications. Recently; the following more accurate upper bound formula was theoretically evaluated [17-19]: Let us consider the case of an interplanetary power station (To = 4.2 K) which use unconcentrated solar radiation. In this case the maximum efficiency T|max diminishes by increasing the distance to the Sun and the zenith angle 0 (figure 5). The influence of 0 is stronger far away from the Sun. Note the large values of r|max throughout the solar system. Figure 6 shows the influence of the concentration ratio C on the maximum efficiency rimax Only the case of normal incident solar radiation was considered. The increasing of the performance is obvious. When a surface planetary solar power station is considered the situation is entirely changed even if the only modified parameter is the ambient temperature Tq. Table 1 shows the values of Tq we used in computations [20,21], Figure 7 shows the results for some planets whose atmosphere was considered transparent to direct solar
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