Accurate Design of Solar Selective Absorber Based on Measured Optical Constants of Nano-thin Cr Film

Zheng-Yong Wang,Er-Tao Hu,Liang-Yao Chen,Qing-Yuan Cai,Jing Wang,Hua-Tian Tu,Wei Wei,Ke-Han Yu

doi:10.3390/coatings10100938

Abstract

Solar selective absorbers have significant applications in various photothermal conversion systems. In this work, a global optimization method based on genetic algorithm was developed by directly optimizing the solar photothermal conversion efficiency of a nano-chromium (Cr) film-based solar selective absorber aiming to work at the specified working temperature and solar concentration. In consideration of the semi-transparent metal absorption layer employed in multilayered solar selective absorbers, the optical constants of ultrathin Cr film were measured by spectroscopic ellipsometer and introduced into the optimization process. The ultrathin Cr film-based solar selective absorber was successfully designed and fabricated by the magnetron sputtering method for the working temperature at 600 K and a solar concentration of 1 Sun. The measured reflectance spectra of the sample show a good agreement with the numerical simulations based on measured optical constants of ultrathin Cr film. In comparison, the simulated results by using the optical constants of bulk Cr film or literature data exhibit a large discrepancy with the experimental results. It demonstrates the significance of considering the actual optical constants for the semi-transparent metal absorption layer in the design of nano-metal film-based solar selective absorber.

Highlights

Solar–thermal conversion systems, which can convert abundantly available solar energy into heat, have been extensively studied in the past decades due to its advantages of high energy-conversion efficiency, simple device structure, and appealing energy storage functionality
To reduce the fitting parameters, the void fraction was set to 50%, and the thickness of the roughness layer was set to be the same as the root mean square (RMS) value of the sample, as revealed by the AFM results (Supplementary Materials, Figure S1)
The film thickness for the bulk Cr layer was fixed at 165 nm as measured by the step profiler, while for the ultrathin Cr film, its film thickness was treated as the fitting parameter

Summary

Introduction

Solar–thermal conversion systems, which can convert abundantly available solar energy into heat, have been extensively studied in the past decades due to its advantages of high energy-conversion efficiency, simple device structure, and appealing energy storage functionality. Most of them are limited to just optimize solar absorptance and thermal emittance [2,10,24,25,26], rather than focusing on the solar PTCE, which is more practical for solar selective absorbers working at a particular temperature and solar concentration [27,28,29]. The particle-swarm optimization method [5] and genetic algorithm [9] have been developed to directly optimize the PTCE of the solar selective absorber In their simulations, the optical constants were obtained from Palik, corresponding to the bulk optical materials [18,30]. The optical constants of ultrathin Cr film were measured in advance and introduced into the optimization process

Numerical Design

Experimental Details

Results and Discussion

Conclusions