2, Columbia University, New York, New York, United States
The US industries reject nearly 20%-50% of the consumed energy into the environment as waste heat. Harvesting this huge amount of heat can substantially improve the energy usage efficiency. For waste heat in the medium temperature range (~500-900 K), traditional solid-state waste heat recovery techniques like thermoelectric generators and thermophotovoltaics (TPVs) are still suffering from relatively low efficiency or power density. In this work, we analyze a near-field TPV system based on a plasmonic emitter (ITO) and a narrow-bandgap thin-film cell (InAs) that are brought to deep sub-wavelength distances for high-efficiency and high-power-density waste heat recovery. The calculations are based on a detailed balance analysis and the formalism of fluctuational electrodynamics. The thermal radiation spectrum from ITO is reshaped and enhanced toward the bandgap of the InAs cell by the photon tunneling effect between the ITO and InAs cell. We find that the near-field photon tunneling probability can be greatly enhanced by thermally excited surface plasmon resonances and waveguide modes in the thin InAs cell. We show that despite the inclusion of realistic nonradiative recombination rates and sub-bandgap heat transfer, such a near-field TPV system can convert heat to electricity with up to nearly 40% efficiency and 11 W/cm2 power density at a 900 K emitter temperature. While the dominant enhancement effect comes from the surface plasmon resonances, the waveguide modes in the thin cell play a significant role as well, especially at relatively large gap distances. We further demonstrate that the power density can be further enhanced by placing a thin metal film on the cell. This is a somewhat counterintuitive result since one might think that the thin metal film might serve to block the near-field radiation. Thus, we show that for waste heat recovery, near-field TPV systems can have performances that significantly exceed typical thermoelectric systems.