efficiency is dependent upon the degree of complexity provided in the system such as compressor intercooling, regeneration and reheat. Large units (> 400 kWe) with high pressure levels, low pressure drops, maximum complexity and axial compressors could reach an efficiency of nearly 50% at 815°C (Ref. 5). For machines in the 10 to 50 kW size, radial compressors are used; the overall efficiency is about 36% rather than 50% without reheat and intercooling. These machines have large heat exchangers with a low compressor inlet temperature. The mass produced commercial cost would be close to $400/kWe for these small machines with oversized heat exchangers (Ref. 5). Attempts to reduce cost result in an estimate of nearly $300/kW, but with a loss of efficiency. The efficiency is reduced to the 28% to 31% range with costs at $300/kW. Engines of this size would have to be developed, and an estimate of the R&D cost is approximately $25M (Ref. 5). Larger closed cycle machines (~30 MW) may be as low as $130/kW with about 46% efficiency at 800°C. Such a machine would require a development program of approximately $100M (Ref. 5). Improved performance is possible mainly by increasing turbine inlet temperatures. The current designs are limited to an 815°C turbine mixed average inlet to keep the tube hot spot temperature due to combustion products in the combustion to less than 900°C. This temperature constraint is based on using Inconel 713 as the cavity heat exchanger and turbine material. Stainless steel is used for the recuperator. Turbine inlet temperatures of 1230°C may be possible with ceramic heat exchangers, and refractory alloy (moly) turbines with advanced cooling. Ceramic materials such as SiC or Si^N^ would be used for the recuperator. Efficiency could reach 46% for small machines (< 50 kW) with costs similar to those of the 815°C machines. The hot spot would be limited to 1370°C for this advanced design (Ref. 5). In summary, relatively near-term development programs could produce 30% to 36% efficient Brayton machines costing $300/kW to $400/kW. Advanced high
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