Roadmap to Over 50% Heat to Electricity for Cars, Drones and Space Probes

Researchers achieved 29.1 ± 0.4% thermophotovoltaic power conversion efficiency, by reuse of unabsorbed subbandgap photons. They have roadmap to achieve higher efficiencies by separately considering the realistic improvements of material, device, and chamber parameters. With the improvement of these parameters, it is possible to achieve >50% power conversion efficiency using InGaAs photovoltaic cells. A highly efficient thermophotovoltaic heat engine would be an excellent choice for hybrid automobiles, unmanned vehicles, and deep space probes.

Above – An ideal regenerative thermophotovoltaic system formed by a thermal radiation chamber, and power conversion inside the chamber. (A) High-energy (blue) photons from the emitter are converted to carriers in the photovoltaic cell, while low-energy (red) photons are reflected back to the emitter and rethermalized. (B) A highly reflective rear mirror is essential since a photon will need to be reflected many times before emerging in the high-energy tail of the Planck spectrum, for absorption in the semiconductor. Other losses in the photovoltaic cell arise due to poor material quality, as well as thermalization of high-energy carriers.

In an ideal thermophotovoltaic system employing photon reuse, a hot emitter is surrounded by photovoltaic cells lining the walls of the chamber, collecting light from the emitter. For efficient recovery of unused photons, the photovoltaic cells are backed by highly reflective rear mirrors. Such mirrors are needed, in any case, to provide the voltage boost associated with luminescence extraction.

The projected thermophotovoltaic efficiency is shown and it represents a realistic efficiency projection rather than ideal Shockley−Queisser performance. The optimum bandgap increases slightly upon improving the rear reflectivity, to minimize thermalization losses from photons at the high-energy tail of the emitter Planck spectrum. With an optimal bandgap, thermophotovoltaic efficiency can reach as high as over 50%.

Figures show how they stack the material layers and achieve higher bandgaps which are the key to greater efficiency.

Thermal Solar Power conversion utilizes thermal radiation to generate electricity in a photovoltaic cell. On a solar cell, the addition of a highly reflective rear mirror maximizes the extraction of luminescence, which in turn boosts the voltage. This has enabled the creation of record-breaking solar cells. The rear mirror also reflects low-energy photons back into the emitter, recovering the energy. This radically improves thermophotovoltaic efficiency. Therefore, the luminescence extraction rear mirror serves a dual function; boosting the voltage, and reusing the low-energy thermal photons. Owing to the dual functionality of the rear mirror, researchers achieve a thermophotovoltaic efficiency of 29.1% at 1,207 °C, a temperature compatible with furnaces, and a new world record at temperatures below 2,000 °C.

Abstract

Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking solar efficiency. Now they report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high thermophotovoltaic efficiency. This mirror reflects low-energy infrared photons back into the heat source, recovering their energy. Therefore, the rear mirror serves a dual function; boosting the voltage and reusing infrared thermal photons. This allows the possibility of a practical over 50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we report a thermophotovoltaic efficiency of 29.1 ± 0.4% at an emitter temperature of 1,207 °C.

PNAS – Ultraefficient thermophotovoltaic power conversion by band-edge spectral filtering

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