Figure 7-33. Optimum number of turbomachines per absorber. 40 to 50 turbomachines would be more optimum than the current 544 units. Also, there are indications that it may be somewhat advantageous to have a small number of turbomachines per absorber. This secondary conclusion could change as the pointing requirements for the concentrators are further studied. This analysis shows that the total weight of the thermionic-Brayton SPS could be reduced by approximately 20 percent primarily because of reductions in the turbomachinery weight. The thermionic-Brayton cascaded system has merit only if the thermionic diodes can operate at temperatures well above the maximum possible Brayton turbine inlet temperatures. For 1985 technology, a Brayton turbine inlet temperature limit of 1026°C was projected. The performance efficiency of the thermionic diode projected for the same time period was 17 percent at an emitter temperature limit of 1826°C and a collector temperature of 1026°C. The Brayton cycle conversion efficiency is approximately twice that of the thermionics for approximately the same AT. If higher Brayton turbine inlet temperatures could be allowed (up to 1426°C), the complexity of adding the thermionics in a cascaded system could not be justified on a performance basis. However, with the projected 1985 technology and the proposed hybrid thermionic/ Brayton configuration, an effective overall cycle efficiency of 45. 5 percent can
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