The resource of solar plants with PVs (photovoltaic arrays) will be determined by the rate of their degradation. For solar energy plants of power greater than 100 kW, solar thermal is more efficient (per unit mass). Higher efficiency and the corresponding decrease of the concentrator surface of the solar-thermal plants (machine cycle or SEP - Th) permit power levels of 0.3 to 1.0 MW. Danger of accidents is the essential deficiency of nuclear power plants. However, certain types of missions (e.g. expeditions to Mars) require the use of nuclear propulsion. At present nuclear power plants produce about 25 kW and last about 5 years. A Mars expedition would need about 15 MW. The high power plants may have a modular construction. Examining the diagram of increasing the maximum power used in the energy plants, is shown in Figure 2. It is necessary to point out the tendency for rectilinear increase of power on a semilogarithmic scale along the time coordinate. Two latter types are not alternatives for different space problems. It is natural, that as the range of uses for power expands, the types of required power plants expands. Radioisotope thermoelectric and thermoemissional, electrochemically based heating element power plants have been added to storage batteries and solar cells. Nuclear energy high power plants and solar plants with the dynamic cycle at middle power level are considered in perspective.
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