2. Solar concentrator efficiency and mass are a function of the number of reflector facets used. 3. The total weight includes the solar concentrator, cavity absorber, diode assembly, radiator and radiator pumps. The optimum radiator temperatures were: inlet, 868 K (1102°F), outlet 388 K (238°F). The "effective" radiator temperature (uniform temperature of an area equal to that of the radiator having the same emissivity and which rejects any equal power) is 589 K (600°F). Some diode collectors will be along their NaK tubes such as to be near the radiator inlet and will consequently be at approximately 900 K (1160°F); at the other end (the radiator outlet) the collector temperature will be only 405 K (269°F). The average collector temperature will be 605 K (629°F). Table 5-3 is a mass statement for the optimized configuration. The total busbar power produced by this SPS is 17.5 GW; 0.64 GW of this is the power required to pump the radiator system. This requirement is not offset by the slightly higher diode efficiency (when comparing this system to the direct radiation cooled variant). However, the diodes can be placed quite close together, due to the remote radiator system, allowing short interelectrode busbars. Consequently 17,000 diodes are grouped on a 20x20 m (65.6 ft.) panel. In order that the power conversion panels may be passively cooled without a remote radiator system, only one diode panel may be used per power conversion panel (to keep the dissipation within the capability of the area of the power conversion panel). This SPS concept is seen to be far more massive than the direct radiation cooled thermionic SPS; it was consequently not pursued in the study extension phase. 5.4 SOLAR CLOSED CYCLE BRAYTON (CONCEPT 3) Derivation of parametrics for the turbomachines, solar concentrators, radiator and cavity absorber. SYSTEM DESCRIPTION: THERMAL ENGINE SPS The basic configuration of this SPS is composed of four power generating modules and one microwave transmitter. The power generating modules consist of solar concentrator assembly of 16,500 metallized plastic film reflectors supported by a graphite-epoxy framework. The cavity absorber assemblies located at the focal points of these concentrators are hollow spheres 160 meters in diameter; the apertures are 100 meters in diameter. The solar concentrators are hexagonal in form with edge members 2482 meters long. Figure 5-12 shows a single solar Brayton power module; the cavity absorber is held at the focal point of the concentrator by six support arms. The radiator is shown inclined by 11.75°. This positions the radiator midway in the apparent arc traveled by the sun from equinox to equinox. (The entire SPS is rolled 180° about the axis to the sun on each equinox.) As a result, less sunlight falls on the radiator, the radiator casts a smaller shadow on the concentrator, and the radiator sees a smaller effective meteoroid flux. Table 5-3. Liquid Cooled Thermionic SPS Mass Fig. 5-12. Brayton Module
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