SBIR-STTR Award

It will be demonstrated in Phase I by analysis and experiment that electrodes for thermionic converters having heat pipes incorporated in the electrode structure accommodate high interelectrode heat flux without distortion. These results will enable closespaced converters with electrode gaps of 5 ,um or smaller. Such a converter can be 13% efficient with an emitter temperature of only 1300 K. This is a new approach to enhanced energy conversion efficiency which can significantly increase overall efficiency of power generation at competitive cost. The analysis of the electrodes will consider heat transfer, mechanical deformation, and thermionic performance. Trade-off of heat flux and physical dimensions will lead to an optimum design. Experiments will use laser interferometry to characterize and confirm the capabilities of the isothermal electrode. An application study will quantify the commercial benefits that can reasonably be expected from this improved performance, which can be commercialized in this decade. Follow-on work would include fabrication of an operating module of close-spaced thermionic converters with the isothermal electrodes. Anticipated Results /Potential Commercial Applications as described by the awardee: Thermionic conversion at high efficiency and relatively low temperatures can be incorporated as topping cycles to steam and/or gas turbine electric generation, as solar thermal generators, and for cogeneration. Significantly enhanced overall efficiencies mean benefits economically and in resource utilization.

Thermionic converters with close-spaced interelectrode gaps are capable of substantial performance improvements over conventional ignited mode diodes. At very close gaps (2 to 10 microns), a thermionic converter operates in the collisionless Knudsen mode with nearly double the output voltage of conventional diodes and concomitant increases of efficiency. In Phase I a special electrode structure which can accommodate high heat flux without distortion was investigated. This type of electrode is needed in order to maintain an interelectrode gap of about five microns and to avoid short circuiting by thermal expansion deformation. In Phase I a laser interferometric technique was also developed to measure the thermal expansion deformation of electrodes with high heat fluxes passing through them. In Phase II the electrode investigation will be further extended with new features to increase power density and higher operating temperatures. The higher power density operation is attractive for utility topping cycles and cogeneration. Additional experiments using laser interferometry will be carried out in Phase II, and complete thermionic converters with novel features to produce substantially increased efficiency and power density will be tested. Prototypic devices with silicon carbide hot shells will be constructed and operated in an effort to develop a practical diode for commercial applications. Although electrical heat sources will be used for experimentation in Phase II, the type of converters which will be built would be suitable for operation with flame heat sources or solar concentrators operating in atmospheric air.Anticipated Results/Potential Commercial Applications as described by the awardee: An anticipated result is the demonstration of new milestones for thermionic converter performance in terms of efficiency and power density. Successful development of these devices will have commercial applications for remote power systems, cogeneration applications, topping cycles of central station power plants, hybrid electric vehicles, and solar thermal power systems for both space and terrestrial operation. Very large savings in fuel costs and reduction in air pollution could result from pervasive adoption of successful close-spaced thermionic converter applications.