Thermionic conversion is unique amongst power conversion approaches in its ability to generate power efficiently with high temperature heat rejection. This feature has made thermionics an attractive choice for space power systems; and it has substantial potential for terrestrial advanced energy conversion, if certain recently identified technological developments are realized in a low cost manner. Thermionic energy converters are well suited to a modular approach. Thermionics is a passive system without moving pa&. Thermionic energy conversion is able to use heat at the highest temperatures available, and to reject waste heat at temperatures high enough to be efficiently used by other energy conversion systems. For example, a thermionic converter can utilize heat at a high temperature from a flame or other heat source, convert some of it to electricity, and deliver its waste heat at a temperature high enough to run a steam generator. The combination of the thermionic converter and steam generator could produce as much as 40% more electricity from the fuel than the steam generator alone. Other terrestrial applications include cogeneration and a possible power source for a hybrid, low-emission electric vehicle. These terrestrial applications require advances in efficiency and power density in order to operate with lower emitter temperatures than space power thermionic systems.Recently it has been shown that close spaced thermionic converters can achieve the performance goals necessary to meet these attractive new applications. The paper addresses the progress in this regard and describes approaches for engineering practical closed spaced converters for large scale applications. Clearly the potential for thermionic energy conversion is great. Every effort must now be made to use this technology to help solve the world's energy problems.Investments in the manufacturing infrastructure necessary to make thermionic energy conversion cost effective are needed to reach this goal.
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