2.4 The SP-100 With Stirling Power Conversion For this study, the SP-100 Stirling power system shown in Figure 2.9 and Figure 2.10 is assumed to be composed of the same components as the SP-100 thermoelectric power system except for the method used to convert thermal energy to electric energy. We assumed that the SP-100 Stirling power system uses six free-piston Stirling engines (Figure 2.11) with linear alternators instead of thermoelectric modules. As explained in the Ground Rules reliability section, six engines are required to provide 20% redundancy. Based on information from NASA (Ref. 9) we assumed that the specific mass of the Stirling engines is constant with power level. Our calculations assumed the use of two different free piston Stirling engines; one a superalloy engine with temperature limits of 1050 K, and the other a refractory engine which would permit operation up to 1350 K. Heat energy from the reactor is carried to the Stirling engines by a flowing lithium coolant. Helium is the engine working fluid. Since the Stirling cycle approaches a Carnot cycle, high cycle efficiencies, typically around 30%, are achievable. System optimization for refractory
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