The Free Piston Stirling Engine (FPSE) is currently being evaluated for space applications. The Space Power Demonstration Engine by Mechanical Technology Inc. (MTI) is slated to test the major technology issues in mid- 1985. The demonstration engine contract was awarded to MTI by NASA Lewis for design, fabrication, and testing of an FPSE. The FPSE is still in the early stage of development but may prove to be reliable, efficient, and suitable for manufacture with little non-lunar material. The FPSE has only two moving parts (displacer and power piston), hydrostatic gas bearings and noncontacting seals to avoid lubrication and wear of moving parts, and the potential for a hermetically sealed power module. The planned final maximum operating temperature will be between 950 K and 1300 K. The demonstration engine goals (3,4) are: T(hot) » 630 K T(cold) - 315 K 25 kw output 252 efficiency (heat to electricity) 8 kg/kw less than 0.008 cm vibration amplitude along any axis. Because the Stirling can operate efficiently at low temperatures, it has promise as a power source on deep space probes which must generate power fron waste heat. 2.6.2 Design Description The Stirling SPS design configuration is similar to the modular design of the Brayton or Rankine SPS. In each module, a large concentrator provides thermal energy for thousands of double cylinder engines, which reject waste heat into a single large radiator. Integrated into each engine is a linear alternator; thus no motional feedthroughs or rotary joints are required. Each engine/generator produces 25 kW of electrical power at 252 efficiency. The hot side temperature of the engines is assumed to be 720 K, and the cold side temperature is 360 K. Helium is used as the working fluid. The engines are mostly made of steel, with part of the engine using a high temperature nickel steel. It is assumed that absorber/concentrator modules like those of the Brayton design are used. In the Brayton and Rankine designs, each module produces up to 1.1 GW. For the Stirling design, power per module is reduced to 1.0 GW, so 40,000 Stirling engines are required in each module. 2.6.3 Design Summary The mass statement for a Stirling system design which produces 9 GW at the SPS bus is shown in Table 2.6-1.
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