the exhaust pressure, superheating, or using higher maximum pressure. However, these methods require compromises in turbine efficiency and may significantly increase blade erosion. Concepts such as reheat cycle and regeneration cycle offer efficiency improvements without the compromises on the turbines. These concepts, however, may require more massive cycles. Overall design of a suitable Rankine cycle depends on the specific requirements of the power system concerning the efficiency, mass, and availability of the materials. The designs presented in this section are based on a NASA MSFC report.(1) This document provided the mass statements of Rankine cycles designed for an output power of 17 GW. The primary requirement of the NASA study was the efficiency of the cycle, with low total mass as the other important requirement. The data provided in the NASA report was used to estimate the component masses of the Rankine cycles presented there. Substitutions for the non-lunar materials were made where ever possible. 2.5.2 Design Description The Rankine cycle studied by NASA uses potassium as the working fluid. Solar energy is concentrated on the cavity heat absorber, where the working fluid is heated. The radiator is a heat pipe radiator which utilizes the working fluid of the cycle, eliminating the need for a cooler. The generator converts the mechanical energy of the turbine to electrical output. The pump is run by power from the generator. The use of a • recuperator in this cycle is not possible since the necessary temperature potential between the compressor output and the turbine output does not exist in this cycle. Four Rankine cycles were investigated in this study. Two cycles use potassium, one with a turbine inlet temperature of 1600 K and the other with 1422 K. The other two cycles use steam as working fluid with turbine inlet temperatures of 1644 K and 811 K. The thermodynamic variables of these cycles were selected to give the highest cycle efficiency without unrealistic sacrifices in masses of the components. The cycle characteristics of these cycles are shown in Table 2.5-1. TABLE 2.5-1: RANKINE CYCLE CHARACTERISTICS
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