reflectivity coating. The light pipe selective surface absorber has 95 percent efficiency. The light pipe section has an inner surface, such as silvered quartz or polished metal with a silicon monoxide coating, that has low solar absorptivity, high specular reflectance, and high emissivity. High absorptivity and low emissivity characteristics are desirable for the base region. It is unknown whether such properties can be attained at thermionic temperatures with tungsten or other pure metals, or whether such performance can be attained with optical filters. If the emissivity of the base region increased from the 0. 2 assumed, the energy passing through the thermionic diodes would be reduced but the losses to the outside of the absorber/receiver would not be increased significantly. The thermionic diodes receive all the energy except the 5 percent cavity loss, the 20 percent that is absorbed by the light pipe from the visible spectrum on its way toward the base region, and that which is absorbed by the light pipe from the infrared spectrum on its way from the base region toward the mouth of the cavity. The Brayton cycle receives all the thermal energy except the 5 percent cavity loss and that which is converted to electricity in the thermionic diodes. Conduction losses through the sidewall insulation have been assumed to be negligible. Table 7-14 is a summary of the characteristics of this system. 7. 2. 3 THERMAL RADIATOR SUBSYSTEM Radiator location is a primary consideration since the radiators are large and comprise a high percentage of the total mass. Considering the options available, three locations were studied, each of which had specific advantages. These options were assessed to determine if any one had an overriding advantage. The locations assessed are indicated in Figure 7-35. Options 1 and 2 offer the smallest integration problem since the feed lines are in the vicinity of the absorber and turbomachinery. Option 2 seems to have an advantage over option 1 because part of the inner truss structure could be common with the radiator support. However, the radiator sizing is made difficult by option 1 because the view factor degrades as the required depth is increased. Because of the view angle from the reflector, the available radiator width using option 2 is much smaller. Using option 1 a 360 m width is available, while option 2 provides, at maximum, 210 m; thus, the radiator height has to be greater in option 2 than in option 1. Asa result, the feasibility of option 2 is questionable.
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