5.0 SYSTEM OPTIMIZATION AND CONFIGURATION DESCRIPTION 5.1 SYSTEM OPTIMIZATION 5.1.1 Approach Section 4 described significant individual subsystems and how analyses of those subsystems led to their parametric description. In this section the generation of total SPS configurations by determining optimum values for the subsystem parameters will be described. In general the optimization was for minimum SPS mass, since transportation cost is such a dominant factor in SPS (transportation of a 100,000 metric ton SPS to GEO may cost approximately S8 billion 1976 dollars). An exception to the mass optimization approach was taken with the photovoltaic configurations, wherein cell cost can be so significant; photovoltaic SPS systems were optimized for minimum cost per kW output, including the effects of transportation costs. Many optimizations were accomplished by hand calculations or simple computer programs. However, the most significant optimization tool was the Integrated Sensitivity and Interactions Analysis, Heuristic (ISAIAH) program developed by G. R. Woodcock as an IR&D activity. ISAIAH can interact up to 100 dependent and 30 independent variables to obtain an optimum combination of values for the independent variables. Dependent variables are input as tables (one, two or three dimensional), summation product or ratio functions, or as FORTRAN expressions. ISAIAH is executed on the IBM 370; plots are outputted on request. 5.1.2 Iteration In some cases up to three iterations were used to derivation final, optimized configurations. Initial design assumptions were used to set subsystem parameters. After system (SPS level) optimization the regions of operation of the subsystems became more closely known. This allowed more detailed definition of the subsystem for the region of interest. This process was continued, guided by the goals of producing lighter, cheaper, more practical configurations. In several cases known reserves of certain materials were not sufficient to accomplish the baselined SPS program, so that alternative materials were required, causing a configuration adjustment. 5.1.3 Alternative Systems Table 5-1 (a repeat of Table 3-1) gives the systems to be described in this section. Table 5- 7. Alternative Power Systems The basic principles of each power system are explained in Section 3.1. 5.2 SOLAR DIRECT RADIATION COOLED THERMIONIC (CONCEPT 1) The thermionic diode subsystem as developed by Thermo Electron Corporation is described in Section 4.6. The emitter temperature of 1800 K (2780°F) was selected as being the practical upper limit (30 year life) for molybdenum. (Tungsten would have yielded higher performance, however the known reserves of tungsten are insufficient for the baselined SPS program). The next question is what radiator temperature to use. High radiator temperature allows more heat rejection per unit area (per T^) and a resultant trend to a lighter radiator. However, a high temperature radiator reduces the temperature differential across the thermionic diode so that the system efficiency is reduced. This tends to increase the number of diodes required, solar collection area, etc., and the amount of heat to be rejected by the radiator. (Refer to Figure 4-19.) The selected collector temperature was 1000 K (1340°F); the effective temperature of the heat pipe radiator is 900 K (1160°F). As explained in section 4.6, the voltage output of a diode panel is 129 v.d.c. (150 v.d.c. is the approximate upper limit for electrical insulation at the temperatures of the diode). 382,000 v.a.c. is baselined for power distribution. A power converter is required to affect the voltage step-up. The efficiency and specific mass of power converter are a function of their power level. Thus it must be
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