Heat engines convert thermal energy into mechanical energy which can be converted to electricity by a generator. Solar power systems based on heat engines use concentrated solar radiation to heat a working fluid. Some of the energy of the working fluid is extracted by a mechanical device such as a turbine and the remainder is rejected by a radiator. Use of these systems in space can offer good efficiencies but can also present problems with complex machinery, temperature limitations, and maintenance. Three types of heat engines were evaluated in this study: Brayton cycle, Rankine cycle, and Stirling cycle. A fourth type of heat engine, the magnetohydrodynamic (MHD) generator concept, appeared attractive due to mechanical simplicity and high theoretical efficiency. This concept was not thoroughly investigated, as it was felt that the high temperature required in the working fluid would make constructing such an engine from lunar materials impractical. A variation on the MHD concept, the liquid metal MHD generator, may prove suitable for construction from lunar materials after further development. The Brayton cycle was evaluated at two ranges of temperatures. The lower temperature range had lower non-lunar mass despite lower efficiency and higher total mass. The high temperature cycle required many non-lunar materials in hot and/or fast moving parts of the engine. Neither has a clear cost advantage, assuming a cost ratio of fifty to one for non-lunar vs. lunar materials. Many estimates of lunar material substitution for the Brayton system were based on intuitive assumptions, since detailed information on complex high-temperature machinery often could not be found. The Rankine cycle was studied using two different working fluids: potassium and water. The steam cycles gave better efficiencies but required more mass. The least non-lunar mass for the Rankine was achieved with the high temperature steam cycle. For both working fluids, the low temperature cycles required more non-lunar mass and more total mass. As for the Brayton system, many estimates of lunar material substitution in the Rankine were based on judgement rather than data. The Stirling engine has only recently been seriously considered for space power systems. It is potentially easier to construct from lunar materials than either Brayton or Rankine, but is more massive overall than either. Because relatively little has been done with the Stirling as a space power system, this study considers only one temperature cycle of the Stirling engine.
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