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Abstract
Numerical calculations are performed to determine the potential of using one-dimensional transparent photonic crystal heat mirrors (TPCHMs) as transparent coatings for solar receivers. At relatively low operating temperatures of 500 K, the TPCHMs investigated herein do not provide a significant advantage over conventional transparent heat mirrors that are made using transparent conducting oxide films. However, the results show that TPCHMs can enhance the performance of transparent solar receiver covers at higher operating temperatures. At 1000 K, the amount of radiation reflected by a transparent cover back to the receiver can be increased from 40.4% to 60.0%, without compromising the transmittance of solar radiation through the cover, by using a TPCHM in the place of a conventional transparent mirror with a In2O3:Sn film. For a receiver operating temperature of 1500 K, the amount of radiation reflected back to the receiver can be increased from 25.7% for a cover that is coated with a In2O3:Sn film to 57.6% for a cover with a TPCHM. The TPCHM that is presented in this work might be useful for high-temperature applications where high-performance is required over a relatively small area, such as the cover for evacuated receivers or volumetric receivers in Sterling engines.
Highlights
The worldwide solar thermal capacity has grown by a factor of 7.7 from 60 GWth in 2000 to 480 GWth in 2018, and these growth rates are expected to accelerate as replacing fossil fuels becomes increasingly important [1]
We assume that the receiver is a blackbody with an emissivity that is equal to 1 and define an effective emissivity, εeff = 1 − r, for the receiver when it is covered by the transparent photonic crystal heat mirrors (TPCHMs), where r is the fraction of thermal radiation that is reflected by the TPCHM back to the receiver
The work investigates the application of dielectric mirrors for the application of transparent solar selective surfaces that reflect thermal radiation
Summary
The worldwide solar thermal capacity has grown by a factor of 7.7 from 60 GWth in 2000 to 480 GWth in 2018, and these growth rates are expected to accelerate as replacing fossil fuels becomes increasingly important [1]. A solar thermal collector should be strongly absorbing over the incident solar spectral region, where ~0.3 μm < λ < ~2.5 μm, while preventing thermal radiation, with ~2.5 μm
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