be achieved for the hybrid system, compared to 36. 7 percent for a Brayton cycle alone. This results in approximately 20 percent less thermal output required from the concentrator/absorber and 30 percent less primary radiator area required for the hybrid system in comparison to a Brayton only system. 7. 2. 2 HYBRID THERMIONIC-BRAYTON SYSTEM DESCRIPTION Some recent studies [7] have based the concentrator design on the use of several thousand orientable ''flat facets" that reflect the Sun's image to a common point, a classical ''black body" cavity. The cavity is shared by several Brayton engines. Smaller modules, double curved ''spherical facets," a light pipe selective surface absorber, and a single thermionic-Brayton cascaded conversion system coupled to each absorber were examined in this study. The unique characteristics of this concept will become apparent in the following description. Figure 5-4 depicts the overall thermal SPS concept. The structure is a hexagonal honeycomb composed of struts and cables with the absorber/ receiver located on one face of the hexagon and the reflector facets mounted across the opposite face. Each of the 37 required facets is assumed to have a ring around the perimeter that supports a thin shell reflector. Two facet types may be used, a pure reflector or a combination reflector/radiator facet. While a pure reflector facet surface may be a highly polished metal or metallized surface with low emissivity, the reflector/radiator facet must have a specular reflecting, low absorptivity, high emissivity surface, such as rear metallized teflon or acrylic film, or a metal surface with a silicon monoxide overcoat. One approach to this construction of the reflector facet uses the ECHO II technique of a laminate of polyester film and aluminum foil. In forming, the aluminum is stretched, and thereby work hardens, and results in a rigid thin shell. The baselined radiator is a conventional tube-fin radiator. The definition of a lightweight radiator with a liquid manifold about the perimeter and heat pipe distribution throughout a thin shell reflector/radiator was deemed beyond the scope of this study. The 37 facets have a curvature and are oriented to have overlapping images at the absorber/receiver location. Each facet has a concentration ratio of approximately 56, and the combination of 37 results in a concentration ratio or more than 2000. A block diagram and energy balance for the system is shown in Figure 7-34. Characteristics are given for one of the required 544 modules. The dc-ac rotary converter (not shown) is a necessary part of the thermionic diode power circuit before the energy goes onto the satellite power bus. The reflector efficiency is assumed to be 90 percent with silver or an enhanced
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