or foamed glass which is a lightweight structural material made of glass. This latter concept is presently being developed at JPL. The optimum size collector is found by trading off the dish cost versus size with that of the Brayton engine cost versus size. 2.1.2 Brayton Engine Subsystem The Brayton heat engine is a good match with the parabolic dish collector for distributed power generation. The high temperature potential of the dish collector is suited to a gas turbine power plant. The closed-cycle gas turbine is a well established technology and related technologies are currently available for unit sizes from 5 kWe up to 250 MWe. A schematic of a closed cycle Brayton engine is shown in Figure 4. A recuperator is used to increase cycle efficiency. A single shaft BRU (Brayton rotating unit) has been selected in which the generator is enclosed in a hermetically sealed housing. An external generator could be used for near term application, or if gearing were necessary for coupling to the generator. The hermetically sealed generator with gas bearings has the potential advantage of long life without any maintenance (5 to 15 years). . Small closed cycle Brayton engines were developed for the aerospace program (Ref. 2) in the 5 to 10 kWe range. An even smaller engine (500 W to 2200 W) is currently being developed at AiResearch for NASA (Ref. 3). Devices similar to this are now used commercially for auxiliary power on DC-lOs, although they are open cycle, use jet engine combustion products and are unrecuperated. Two current commercial programs are directly relatable to this application. A 19 kWe generator for a refrigerated railroad car and a 7.5 kWe gas fired refrigeration power unit are currently being developed (Ref. 4). Both units operate at 815°C to avoid hot spot problems and ensure long life (>10 years). The expected efficiency is 28% based on its having low capital cost. The thermal
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