Thermophotovoltaic systems that use a photovoltaic cell (PVC) to convert thermal radiation from a hot object into electricity are attracting attention as they potentially offer a high energy density that is comparable to a fuel cell. However, to work well, the emission spectrum of the thermal emitter should match the absorption band of the PVC. In reality, this is often not the case, because the thermal emission spectrum described by Planck’s black-body radiation formula is much broader than that of the PVC’s absorption, leading to inefficient operation.
Now, Masahiro Suemitsu and co-workers from Kyoto University and Osaka Gas Company Limited in Japan have developed a thermophotovoltaic system based on a custom-designed silicon (Si)-rod photonic crystal (PC) thermal emitter (pictured). Significantly, the system offers a record-high efficiency of operation (ratio of output power to ingoing heat flux) of 11.2% at an emitter temperature of 1,338 K (ACS Photon. https://doi.org/10.1021/acsphotonics.9b00984; 2019).
The emitter was fabricated by processing a polycrystalline Si thin film on a 500-μm-thick MgO substrate. In order to prevent chemical reaction of Si with MgO at the high temperature of operation, a 20-nm-thick HfO2 layer was deposited between them. Silicon rods (diameter of 360 nm, height 825 nm) were obtained by dry etching and arranged in a rectangular lattice with a lattice constant of 700 nm. “We designed the Si-rod PC in order to suppress thermal emission in the wavelength region where photons are not absorbed by the PVC”, Suemitsu said.
A power generation test was implemented in a vacuum chamber. The emitter was suspended with Pt wires and InGaAs single-junction PVCs were placed either side of the emitter at a distance of less than 500 μm away. An M-shaped Pt/Ti heater was attached to the MgO substrate of the Si-rod PC. The directly generated emission from the heater was negligible with respect to that from the emitter since the heater’s area is merely 0.6% of that of the emitter. The actual system efficiency gradually increased with the temperature, reaching 11.2% at 1,338 K.
“We believe further increase of the actual system efficiency is feasible,” Takashi Asano of Kyoto University, who is the corresponding author of the paper, told Nature Photonics. The key strategies in my mind are the finding of a novel thermal radiation material, the development of a novel PC structure and the introduction of near-field thermal radiation transfer.”
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Horiuchi, N. Efficient thermophotovoltaics. Nat. Photonics 14, 66 (2020). https://doi.org/10.1038/s41566-020-0585-1