CAREER: Enhanced Power Generation in a Nanoscale-Gap Thermophotovoltaic Device due to Radiative Heat Transfer Exceeding the Blackbody Limit

University Of Utah, Salt Lake City UT

Investigators

Abstract

CBET 1253577 Mathieu Francoeur, University of Utah Approximately 58% of the energy consumed annually in the US is lost to heat. Direct thermal-to-electrical energy conversion via thermophotovoltaic power generators can contribute significantly to recycling this large amount of waste heat in various systems such as combustion chambers, photovoltaic cells and personal computers. Conventional thermophotovoltaic systems are limited by the blackbody spectrum. By separating the heat source and the cells converting heat into electricity by a nanosize gap, radiation heat transfer exceeds the blackbody predictions by a few orders of magnitude due to energy transport by evanescent waves. Enhanced energy transfer by evanescent wave tunneling can thus lead to a significant increase of thermophotovoltaic power generation. This research will demonstrate experimentally that power generation in a nanoscale-gap thermophotovoltaic (nano-TPV) device can be enhanced by a factor of 20 to 30 compared to conventional thermophotovoltaic systems. This will be accomplished by measuring radiative heat flux and nano-TPV electrical power output and conversion efficiency in a device involving planar surfaces separated by a gap as small as 20 nm. The nanosize gap will be maintained via spring-like spacers. The application of an electrostatic force between the surfaces combined with the knowledge of the spring constant of the spacers will allow precise control and measurement of the gap thickness. This research will provide for the first time well-controlled radiative flux measurements between planar surfaces separated by a nanosize gap. This will allow the verification of the d-2 and d-3 near-field thermal radiation regimes, where d is the gap thickness, predicted respectively for optically thick and thin materials supporting resonant surface waves. The research will provide the first quantitative experimental nano-TPV performance analysis at nanosize gaps. The spectral effects will be considered by testing various materials for the radiator, such as indium tin oxide supporting surface plasmon-polaritons in the near infrared band. Nano-TPV performances will be systematically quantified as a function of the gap thickness and the temperatures of the radiator and the cells, and will be compared against predictions based on a coupled near-field thermal radiation, charge and heat transport model. The impacts of heat dissipation within the cells due to lattice and free carrier absorption, thermalization and non-radiative recombination of electron-hole pairs will also be analyzed in great detail. This project will enhance fundamental knowledge in near-field thermal radiation by measuring fluxes between surfaces spaced by sub-wavelength gaps and in evanescent wave-based energy conversion by experimentally analyzing nano-TPV performances. The research is an important step toward the establishment of miniature waste heat recovery devices that could be used in personal computers and systems harvesting heat from the human body. The project will also promote training and learning through the involvement of high school, undergraduate and graduate students in the activities. Fundamentals of near-field thermal radiation and its application to power generation will be disseminated via the development of a new elective course at the University of Utah dedicated to both undergraduate and graduate students, where the class content will be made freely available to the general public. K-12 outreach will be performed via the Utah Science Olympiad. Departmental scholarships and research fellowships will be offered to high school students participating in this event. Direct thermal-to-electrical energy conversion will be promoted via the conception of an interactive, portable demo-kit that will be presented at the Utah Science Olympiad and in high schools. These activities will assist the efforts of the College of Engineering in recruiting high quality students in science and engineering programs.