CAREER: Thermophotonics for Efficient Harvesting of Waste Heat as Energy

Purdue University, West Lafayette IN

Investigators

Abstract

In 2013, 61% of raw energy (namely, coal, natural gas, and oil) was wasted as heat because of the low efficiency of power conversion. A thermophotovoltaic (TPV) system desirable for its low maintenance and quiet, portable operation can uniquely capture this waste heat as electricity by using thermal photons (discrete units or quanta of light) whose energies match the bandgap of the photovoltaic (PV) cell. However, TPV systems emit the vast majority of thermal photons at low energies, thus greatly reducing efficiencies. To overcome this barrier, we propose to develop a highly innovative approach to TPV, which we call thermo-photonics (TPX), by redirecting thermal photons into useful energies matching the PV cell. TPX can significantly increase the efficiency of TPV converters up to 50%. What is more, this device may efficiently utilize standard silicon PV technology, thus ensuring a relatively easy transfer to commercial development when the concept is proven. Studying TPX will develop fundamental knowledge of thermal radiation, heat transfer, and the limits of photovoltaic conversion processes to make a potentially transformative contribution. Applications may include harvesting waste heat, converting solar power, and creating new light sources. The long-term career objective of this proposal is to integrate photonics-based approaches to harvesting waste heat with the education of future engineers through simulation-based learning. Simulation modules will be developed in an undergraduate research program and widely distributed to the scientific community through Purdue's NSF-supported existing nanotechnology dissemination site, nanoHUB. Courses on new device concepts will be disseminated worldwide using nanoHUB-U. Female undergraduate students will be recruited through a series of events planned with Purdue's Women in Engineering Program. The basic nanoscience and engineering concepts underlying this work will also be conveyed to local high school teachers, through an intensive professional development program known as Research Goes to Schools. Thermophotonics (TPX) is a highly innovative variation on thermophotovoltaics (TPV) where thermal radiation below the PV bandgap is strongly suppressed, while radiation above it is enhanced, thus enabling extremely high efficiencies. However, this approach requires redesign of virtually every aspect of previous TPV systems. To investigate the theoretical and experiment aspects of this problem thoroughly, we will employ a comprehensive, vertically integrated methodology utilizing the following elements: (1) a self-consistent electro-thermo-optic simulation tool that combines finite-difference time domain (FDTD) and carrier drift-diffusion (DD); (2) experimental CMOS-compatible fabrication of nanostructured thermophotonic structures via interference lithography and high-performance Si PV diodes; (3) characterization of PV receivers with a range of techniques; and (4) characterization of TPX emission and efficiency in a UHV chamber. This project will combine careful and detailed simulation of the TPX converter with innovative device fabrication and detailed characterization to verify the photonic and photovoltaic device models. Such a combination of theory, device fabrication, and experimental validation is necessary to achieve high efficiencies in this new device. Careful modeling of two new emitter designs in this work suggests that the solar spectrum can be concentrated into a narrower range of photon energies, while new work on photon recycling suggests that thin PV cells can more efficiently convert them into electricity. Combining these innovations could yield a highly efficient conversion of heat to electricity approaching 50%. This proposal will lead to a detailed understanding of the promise and feasibility of building such TPX systems by developing models, computational tools, and process flows applicable to a broad range of high-temperature nanophotonic and heat-to-electricity conversion systems. Ultimately, it will yield a demonstration of selectivity in TPX emitters, including a detailed comparison to theory and simulation, and will integrate photonics-based approaches to harvesting waste heat with the education of future engineers through simulation-based learning.