Thermionic converters for an Integrated Solar Upper Stage (ISUS)

Abstract

The Integrated Solar Upper Stage (ISUS) is a solar bimodal system which combines thermal propulsion and electric power generation in a single integrated system. A thermionic converter was designed and fabricated for the ISUS system. The ISUS thermionic energy converters differ from previous designs, due to the significant changes in operating temperature prior to and during an eclipse, with the emitter temperature increasing from 1900 K to 2200 K, and then back again. A complete thermal and electrical model was developed for a planar diode to determine optimum operating dimensions and parameters. The model includes an overall energy balance for the diode, and changes the interelectrode gap spacing due to thermal expansion of the parts as the emitter and/or collector temperatures change. Cesium pressure can be chosen from an external liquid reservoir, an integral reservoir using cesium intercalated into graphite attached to the collector heat pipe, or optimum cesium pressure. With optimum cesium pressure, the maximum efficiency increases from 14% to 16% as the emitter temperature increases from 1900 K to 2200 K. The improvement in efficiency is only 2% as the emitter temperature is increased, because the interelectrode gap increases from 0.007 to 0.009 due to thermal expansion. Optimum efficiency requires an external, actively controlled liquid reservoir. The peak efficiency increases significantly as emitter temperature is increased from 1900 K to 2200 K. However, the efficiency at constant power is almost constant near 12% as the emitter temperature is increased.