From Stanford NEMS

Microfabricated Thermionic Energy Converters

This research program aims to create a new solid-state energy conversion technology based on microfabricated and photon-enhanced thermionics that exceeds the Shockley-Queisser efficiency limit in solar applications. We propose to apply micro/nano-fabrication materials and processes to transform thermionic energy converters (TECs) from a niche technology into the commercial mainstream. Optimized materials, surface nano-engineering and disruptive new concepts such as photon-enhanced thermionic emission (PETE) will raise efficiency and reduce operating temperatures. Such a low-cost, solid-state converter with no moving parts would provide multi-billion-dollar market opportunities in topping cycles for solar-thermal generation and residential combined heat-and-power (micro-CHP). An interdisciplinary research team from Stanford pursues three research objectives: (i) MEMS-based wafer-stack technologies for TEC and PETE fabrication, (ii) characterization of the PETE energy conversion process, and (iii) nanostructured cathodes for thermionic converters. Through this ambitious research project, we will establish the design principles and fabrication processes for micro-thermionic and PETE converters, as well as demonstrate functional components in vacuum test systems.

Figure 1. Schematic of a second-generation thermionic energy converter with nanostructure enhancements.

Due to their high operating temperatures, thermionic energy converters are uniquely suited for topping cycle applications. Thermionic converters are projected to add 20-40% to the efficiency of solar-thermal power stations, almost doubling their electricity output with only a 10% increase in installation and operating costs – reducing the cost of solar-thermal electricity below that of the lowest-cost fossil fuel generated electricity. At 50% market penetration, the CO2 emissions from the electricity sector would be reduced by 1.2B tons/year. A micro-TEC topping stage in series with a thermo-electric bottoming stage would enable solid-state micro-CHP, eliminating another 1.0B tons of CO2 emissions. These dramatic reductions in greenhouse-gas emissions are coupled with the creation of a vibrant, high-tech thermionic energy converter manufacturing industry.

Figure 2. Two types of solar thermal energy converters: solar tower (left) and parabolic dish (right) (Source: Wikipedia). Thermionic energy converters can serves as topping cycles for both of these types of converters.

Publications

1. J.W. Schwede, I. Bargatin, D.C. Riley, B.E. Hardin, R.T. Howe, Z.-X. Shen, N.A. Melosh, Photon enhanced thermionic emission (PETE) for solar concentrator systems, in press, Nature Materials (2010)

2. J.-H. Lee, I. Bargatin, J. Provine, W. Clay, J. Schwede, F. Liu, R. Maboudian, N. Melosh, Z.-X. Shen, R.T. Howe, Thermionic emission from microfabricated silicon carbide filaments, Technical Digest PowerMEMS2009 workshop (Washington, DC, December 1-4, 2009)