these techniques as this subject is extremely complex and involved, but rather, to describe several concepts which should be considerd in system selection. Absorber design techniques are directed mainly toward decreasing radiation and reflection losses through the aperture, as well as minimizing losses through the walls and structure of the absorber. Proper absorption of the high heat fluxes involved and transfer of energy to the working fluid are accomplished through judicious system geometry choices and proper selection of materials. The cavity-type absorber is the only type considered in this investigation, as it provides significantly higher thermal efficiency than any other type due to reduced reflection and re-radiation losses, and it allows the achievement of the high heat flux requirements and control of same within the cavity. The cavity walls are insulated (the Boeing concept uses Alumina/Silica fiber insulation^), and the working fluid is pumped through columbium tubing within the absorber. Use of refractory metals is common in absorbers of this type. Future technological breakthroughs are predicated on the use of ceramic materials in the cavity to allow even higher cycle temperatures (contingent upon advances in high-temperature turbocomponent technology). The solar reflector, while integrally related to the high-temperature absorber in its design, is the lowest temperature component of the system. The higher its reflectivity, the lower its operating temperature. The small amount of heat absorbed in the reflector system is radiated directly to space while the reflector concentrates large amounts of energy toward the aperture of the absorber. For this application the perfect reflector or concentrator, of course, is a parabola, and thus all concentrator designs must approach a paraboloidal geometry to achieve the high temperatures required for the thermal engine system. While rigid collectors of high optical quality are preferred, structural problems limit the degree of rigidity which can be achieved, and weight considerations force the designer to accept reflector materials such as the Boeing aluminized plastic film with optical qualities inferior to those of a good silvered mi rrorlO. Several reflector configurations were examined in the study, including inflatable, inflatable rigidized, petal, and faceted types. These configurations are shown in Figure IV-B-l-c-9. Figure IV-B-l-c-9a shows a spherical collector which is inflated with a gas at a very low pressure. The power conversion device and spherical absorber are mounted at the center of the sphere. This system requires no attitude control, as it gathers and concentrates sunlight in any orientation. Solar energy penetrates the transparent sections, while the opaque sections have an internal reflective coating to focus the solar energy toward the center of the sphere. Problems with this concept are (1) inflation gas leakage; (2) spherical absorber design problems; (3) remote radiator location on opaque sections, with associated installation problems after inflation is accomplished; and most importantly (4) achievable concentration ratios are low.
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