concept. Except for the conversion of sunlight to electricity, this system is very similar to the solar photovoltaic concept and has similar microwave, transportation, and support considerations. a. Power Conversion Methods Three power conversion methods, with one method having two * approaches, have been studied. The first method is the closed Brayton cycle system which utilizes rotating machinery. The second method is the thermionic system, one approach for which is passive except for coolant pumps, if required, while the other approach involves active cooling. The third method is a cascaded system employing thermionic devices and the Brayton cycle in series, i.e., the devices are cooled by the Brayton cycle, with each extracting a portion of the solar energy available. A brief description of each of these possibilities is given in the following four subsections. (1) Solar Thermionic, Direct Radiation Cooled In a thermionic diode, electrons are produced at the emitter (cathode) due to its elevated temperature, and travel to the lower temperature collector (anode). The circuit is completed through the load. Several processes within the emitter-collector gap tend to reduce the efficiency of power generation from the applied thermal energy. For example, the electrons in the gap tend to repel those being produced at the emitter. The diodes are mounted in the wall of the solar cavity absorber; the emitters are heated by the concentrated solar energy. By allowing the collectors to dissipate waste heat to space the temperature differential required for operation is produced. Fins are added to the collectors to improve cooling. Individual diodes have outputs of approximately 0.8 volts, and it is not practical (due to insulation breakdown) to use series strings to produce the 20,000 volts required by the transmitter. Therefore, rotary converters are used to step up the voltage. * The basic information for this subsection has been obtained from Reference A17.
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