3.0 ALTERNATIVE POWER GENERATION APPROACHES 3.1 CONCEPTS INVESTIGATED The alternative satellite power systems shown in Table 3-1 were investigated: The last concept does not generate power in space; a mirror system in geostationary orbit would reflect sunlight to an area on Earth, potentially allowing night operation of ground solar power plants. 3.2 SOLAR THERMIONIC, DIRECT RADIATION COOLED (CONCEPT 1) 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. Table 3-1. Alternative Power Systems 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 converter/transformer assemblies are used to provide the DC necessary to energize the transmitter, up the voltage. An AC to DC converter is used to provide the DC necessary to energize the transmitter. The solar thermionic direct radiation cooled system is shown in Figure 3-1. 3.3 SOLAR THERMIONIC, LIQUID COOLED (CONCEPT 2) In this configuration a liquid metal cooling loop is used to remove waste heat from the diode collectors. In effect, the coolant loop couples the diodes to a greater radiating area than is practical for fins directly attached to the diodes, thereby producing a lower collector temperature, a greater temperature differential across the diode and greater electrical output. Thus the diodes are more efficient, so that fewer diodes are required; however, active cooling uses power drawn from the diodes and requires a liquid metal loop with thermal radiator. Converter/transformer assemblies are used to step- up the diode output voltage. An AC to DC
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