A thermophotovoltaic system that converts solar or waste heat into electricity. Photons from the thermal emitter couple to the cell in the form of evanescent modes in near-field operation.

From heat to renewable energy

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Global terawatt-scale energy needs call for renewable energy approaches operating close to thermodynamic limits. Thermodynamic analysis shows that a solar photovoltaic (PV) cell has a maximum theoretical efficiency of roughly 33%. Equivalent analysis of a thermophotovoltaic (TPV) cell, which involves an intermediate thermal emitter that transforms heat to thermal radiation, predicts a solar TPV efficiency of 54%. This boost in predicted efficiency occurs from the ability to control by design the radiative heat exchange between the thermal emitter and the cell in a TPV system, in contrast to a solar PV. TPV systems may also convert industrial waste heat into electricity, a significant portion of which is currently rejected into the environment. Compared to thermoelectrics and fluid-based heat engines, TPVs promise a mechanically reliable, solid-state, clean-energy alternative. Theory predicts that the performance of optimized TPV systems can even approach thermodynamic limits.

Although TPV energy conversion dates back to the 1960’s, the concept has been recently revived due to significant progress in photonic materials and nanofabrication. By improving the photonic properties of materials, such as creating broadband near-perfect reflectors and narrowband thermal emitters, we can significantly improve the performance of practical TPV systems. Moreover, by bringing the thermal emitter and the cell in close proximity (at distances smaller than a micrometer), in other words by operating in the near-field, thermal photons in the form of evanescent waves can deliver thermal power density that surpasses the blackbody limit. In these ways, we can tackle various practical challenges of standard far-field TPV systems, such as poor luminescence efficiency and radiation leakage.

Our research in polaritonic materials and engineering thermal emission is closely tied with our research in energy conversion. Together, these studies can pave the way for heat-to-electricity conversion with technologically relevant, large efficiencies. We aim to propose simple, practically feasible device concepts, as well as to demonstrate proof-of-concept experiments of such energy conversion schemes.

G. T. Papadakis, S. Buddhiraju, Z. Zhao, & S. Fan, “Broadening near-field emission for performance enhancement in thermophotovoltaics” Nano Letters 20, 3, 1654-1661 (2020)