美国NSF基金：近场增强热离子能量转换效率

Fundamental Studies of Near-field Enhancement in Thermionic Energy Conversion

In 2014, the Unites States consumed more than 97 quadrillion BTU (British thermal units) of energy. This is equivalent to the amount of energy in 3.5 billion tons of coal or 776 billion gallons (US) of gasoline. However, almost 59% of such energy consumption is being lost as waste heat. It is imperative to find an innovative way of recycling energy from a waste heat source as an emission-free and less-costly energy resource. The objective of the proposed research is to explore the near-field enhancement of thermionic emission for renewable energy recycling. Conventional thermionic energy conversion (TEC) generally requires a high cathode temperature over 1500K to thermally excite enough electrons from the cathode overcoming its binding potential, or work function, for power generation. Low efficiency is another challenging issue in TEC power generation. The research team will address this challenge by implementing a low bandgap semiconducting material as a cathode and placing it in a subwavelength distance away from a thermal emitter. The near-field enhancement of thermionic emission can reduce the required thermal emitter temperature by enhancing thermionic current generation with photoexcitation of electrons in the cathode. In addition, the energy conversion efficiency will be substantially improved because the most radiation absorbed in the cathode will benefit thermionic emission, i.e., photoexcitation from the photon energy slightly above the cathode bandgap and thermalization from the excess photon energy and sub-bandgap photon energy. The success of this project will induce a paradigm shift in thermionic energy conversion, and will spark the development of novel energy recycling technologies based on thermionic emission. The project will promote training and learning by involving students in micro/ nanofabrication, thermal and infrared characterization of nanodevices, nanoscale heat-transfer measurements, and nanoscale instrumentations. In addition, a new course focuses on Nanoscale Metrology and Experimentation will be developed and offered to students in an effort to broaden nanotechnology education. The research team will also pursue outreach to K-12 students to promote scientific learning in younger generations.

Near-field enhanced thermionic energy conversion will be examined by (1) experimentally validating the enhancement of near-field thermal radiation between plane structures within sub-micron gap distances; (2) establishing a theoretical framework for the near-field enhanced thermionic energy conversion; and (3) experimentally investigating the enhancement of thermionic emission in the near field. The researchers will thoroughly test the hypothesis that photoexcited electrons due to the absorbed near-field thermal radiation can significantly enhance thermionic current generation. They will also investigate the material properties critical for thermionic power generation, including the gap-dependence of space charge buildup, the reduction of the work function, and the high-temperature stability of low-bandgap semiconducting materials. This research will provide, for the first time, quantitative measurements of near-field thermal radiation between two macro plates within sub-100 nm gap distances. The experimental design will enable the precision plane-plane gap control with a nanometer resolution and near-field thermal radiation measurement at high temperature over 1000K. In addition, this project is the first attempt to combine near-field thermophotovoltaic and thermionic effects into a single energy conversion process. The research team will provide theoretical and experimental background for the feasibility of enhancing photo-thermionic emission with near-field thermal radiation. With the local probing of photo-thermionic electron tunneling, it will be possible to better understand the photothermoelectric behaviors of low-bandgap semiconductor materials at high temperatures, which have not been well understood to date. The accomplishments of this project will constitute a fundamental stepping-stone for the realization of a novel, thermionic-based energy recycling technology.

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