near-term technology is shown in the bottom half of Figure 14. The more expensive (solid line) engine has the higher performance. These units are low pressure devices with about 100 psi turbine inlet conditions. Also shown is a Garrett Corporation projection of the performance of advanced designs. Implied in this figure are turbine inlet temperatures of 1230°C (1370° hot spot) and refractory alloy (molybdenum) turbines and ceramic heat exchangers; the ceramic materials would probably be SiC or Si3N4. Efficiency is computed at about 46% for even these small machines (< 50 kWe), and costs are estimated to be similar to those of the 815°C machine. These projections for performance and costs are based upon building a new plant and paying it off at current economic factors after producing enough small Brayton engines for a total generating capacity of 1000 MWe in 1000 working days (~4 years). The cost data are based on the complete Brayton engine, including electrical generator and cavity heat exchanger. It is recognized that there is significant uncertainty in these cost-performance estimates. A major factor is the annual production rate. The DC-10 auxiliary power supplies are produced at a rate of about 60/year, while turbochargers, which are nearly a complete engine with external combustion in the 650°C to 815°C range, are produced at the rate of about 250,000/year. The turbocharger units cost from $l/kWe to $20/kWe based on equivalent electrical ratings. With the completion of the two current commercial development programs*, more reliable cost and performance data should become available. The Brayton engine, as with any heat engine, is sensitive to hot side temperatures. Figure 15 shows Brayton engine efficiency versus turbine inlet temperature for a specific low-pressure closed-cycle system. The maximum cycle pressure is 110 psi a; the compressor inlet temperature is 38°C; and the regeneration effectiveness is nearly 90% with no intercooling or reheat, typical of a 10 to 20 *AGA sponsored 7.5 kWe rooftop refrigeration demonstration unit, and PFE 20 kWe refrigeration railcar generator set demonstration unit.
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