Semiconductor-based Multilayer Selective Solar Absorber for Unconcentrated Solar Thermal Energy Conversion

Nathan H Thomas,Shanhui Fan,Austin J Minnich,Zhen Chen

doi:10.1038/s41598-017-05235-x

Nathan H Thomas, Shanhui Fan + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-05235-x

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Abstract

Solar thermal energy conversion has attracted substantial renewed interest due to its applications in industrial heating, air conditioning, and electricity generation. Achieving stagnation temperatures exceeding 200 °C, pertinent to these technologies, with unconcentrated sunlight requires spectrally selective absorbers with exceptionally low emissivity in the thermal wavelength range and high visible absorptivity for the solar spectrum. In this Communication, we report a semiconductor-based multilayer selective absorber that exploits the sharp drop in optical absorption at the bandgap energy to achieve a measured absorptance of 76% at solar wavelengths and a low emittance of approximately 5% at thermal wavelengths. In field tests, we obtain a peak temperature of 225 °C, comparable to that achieved with state-of-the-art selective surfaces. With straightforward optimization to improve solar absorption, our work shows the potential for unconcentrated solar thermal systems to reach stagnation temperatures exceeding 300 °C, thereby eliminating the need for solar concentrators for mid-temperature solar applications such as supplying process heat.

Highlights

Another approach for achieving spectral selectivity at moderate temperatures below 500 °C is to use semiconductors with appropriately chosen bandgaps
Variable temperature Fourier Transform Infrared Spectroscopy (FTIR) shows good stability at temperatures up to 300 °C in air, and field tests show a peak operating temperature of 225 °C that is comparable to the performance of state-of-the-art selective surfaces
The semiconductor bandgap energy must correspond to photon wavelength between 1 μm and 2 μm to absorb as much of the solar spectrum as possible

Summary

Introduction

Another approach for achieving spectral selectivity at moderate temperatures below 500 °C is to use semiconductors with appropriately chosen bandgaps. Because of the near zero absorption below the bandgap, semiconductor based selective surfaces offer the potential for thermal emittance lower than that achieved with surfaces that employ textured metals in the primary absorbing medium. Semiconductors with small bandgaps suitable for absorbing the solar spectrum (0.6 eV to 1.4 eV) have high refractive index and require elaborate antireflection coatings. The performance of semiconductor-based solar thermal absorbers has lagged that of metallic and ceramic counterparts. In this Communication, we present a semiconductor-based multilayer stack that achieves the high solar absorption and low thermal emission necessary for unconcentrated solar thermal applications. Our surface could achieve temperatures exceeding 300 °C, thereby enabling the use of unconcentrated sunlight for mid-temperature applications such as supplying process heat without need for large geometric concentrators

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