3.3 HEAT ENGINE Although several advanced energy conversion devices were discussed briefly in the Introduction, only two will be considered in this report for use with the parabolic dish collector: the closed cycle Brayton engine (distributed generation), and the steam Rankine cycle (central generation). 3.3.1 Brayton Heat Engine Section 2.1.2 described the current status and availability of this engine. The performance and cost were discussed in general terms. More specific information on performance is presented in this section. Based on current capability and ongoing development programs, primarily by the Garrett AiResearch Company, near-term and advanced technologies can be identified. Small closed cycle inert gas Brayton engines (*=100 kWe) use radial compressors and recuperation, but do not incorporate reheat or compressor intercooling. Although only specific design points exist now for a small number of prototypes, an attempt will be made to generalize performance and cost for use in system optimization studies. The near-term technology assumes a 815°C turbine inlet (•= 900°C hot spot), Inconel 713 tubing in the cavity receiver, He or He-Xenon gas, a stainless steel or Hastalloy recuperator, and an Inconel 713 or Hastalloy turbine. The engine performance depends upon the electric rating. The smallest unit under development is the mini-BRU for an aerospace application and is in the 500 watt to 2 kWe range. Several programs consider units in the 7 to 30 kWe range. Figure 14 is an estimate of the engine efficiency versus size for the near- term technology defined previously. The range of 30% to 36% efficiency for a recuperated Brayton engine at about 20 kWe depends upon the capital intensiveness of the engine. The important factors are the size of the heat exchangers, magnitude of pressure drops and heat rejection temperatures. The cost estimate for the
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