An analysis of meteoroid effects indicated no problems for such concentrators; however, radiation effects could not be quantized and are therefore considered a potential problem. Thermal Engines Turbomachine systems were investigated by the Garrett Corporation, a subcontractor. Available and ''emerging" materials were analyzed as candidates for the high temperature/high stress elements. Most promising were Astar 811C, a tantalum alloy, and silicon carbide, a ceramic. All portions were designed for a 30 year running life. Gas bearings are used throughout. The investigation also included two important portions of a closed Brayton cycle system: the recuperator (a gas-to-gas heat exchanger) and the cooler (a gas-to-liquid metal heat exchanger which interfaces the Brayton gas loop to the liquid metal loop of the radiator). Parametric descriptions of the above were developed for use in the Brayton cycle optimization process. A baseline engine size of 300 MW received primary emphasis; a 10 MW engine for pilot plant use will also be investigated. Thermionic Converters These passive devices directly convert high temperature thermal energy into electricity. Thermionic diodes are mounted in the wall of the solar cavity absorber so that their electron emitters are exposed to the concentrated solar energy. The electron collectors with their associated cooling fin are exposed to space for heat dissipation. The voltage level of a series string of diodes must be kept low to avoid insulation breakdown. Rotary converters are used to step up the output to the level required by the transmitter. Initially tungsten was selected as the electrode material; however, all known reserves of this material would be required for the baseline SPS program. A diode of nearly equal performance was later baselined using a molybdenum emitter and a molybdenum-coated nickel collector. Subcontractor for thermionics analyses was the Thermo Electron Corporation. Cavity Absorbers The cavity absorber is a hollow sphere with an opening to admit the concentrated solar energy from the concentrator. The tubular heat absorber assemblies for the Brayton cycle system or thermionic converters are mounted
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