Solar Thermophotovoltaics

by Makoto Shimizu, Tohoku University, Japan

Solar-Thermophotovoltaics (TPV) is a TPV system that generally uses concentrated sunlight as its heat source. It differs from general TPVs in that it requires an absorber to absorb sunlight. Since sunlight is absorbed as heat, potentially the entire wavelength range of sunlight can be used. When only monochromatic light emitted with photon energy equal to the bandgap, the PV cell functions as a Carnot engine in the open-circuit state. However, there is thermal radiation loss from the absorber itself which increases with temperature. As a result, the efficiency limit of Solar-TPV is 85% at about 2500 K, where the PV cell efficiency and the absorber efficiency are in the best balance (Davies & Luque, 1994). This is significantly higher than the Shockley-Queisser limit of 41% (Shockley & Queisser, 1961) when sunlight is directly converted by single-junction cells.

From these viewpoints, two aspects are considered in the system design: minimization of thermal radiation loss from the absorber and spectral shaping of thermal radiation from the emitter to be narrow at as high a temperature as possible.

Solar selective absorbers are useful to minimize thermal radiation loss using the mismatch between solar spectrum and the thermal radiation spectrum from the absorber. Thermal radiation loss from the absorber can also be relatively suppressed by increasing the area ratio of the emitter to the absorber. However, a large area ratio can cause a large temperature variation in the emitter surface and a decrease in the operating temperature. These factors need to be considered in a system design.

The power generation efficiency of a PV cell is maximized with monochromatic thermal radiation. Therefore, spectral shaping technology based on plasmonic and photonic structures for TPVs have been studied extensively. To realize thermal stability of the emitter which may be in conditions of above 2000 K is a future challenge. Photon recycling of the out-of-band thermal radiation can also contribute to narrowing the effective thermal radiation spectrum. It makes it possible to use emitters with a blackbody-like thermal radiation spectrum. Furthermore, technology that effectively transports thermal radiation to the PV cells has been developed such as using optical cavities. Reported solar-to-electric efficiency has improved significantly over the past decade. Furthermore, solar-TPVs have a high affinity with thermal storage systems. Therefore, future development as an advanced solar energy system is highly expected.

Outline of solar-TPV and key technologies for high efficiency conversion.

More information is available in: Shimizu et al., Solar Energy Materials & Solar Cells 245, 111878 (2022).