Design and Optimization of Metallic Photonic Crystal Emitter to Improve the Spectral Efficiency of Solar Thermo-photovoltaic System

Abstract

In this work, metallic photonic crystal emitter was designed by using COMSOL MULTIPHYSICS software to enhance broadband emissivity at selective wavelength to improve the performance of a solar thermo-photovoltaic system. The ef ects of structural parameters on the emissivity of a square array of cylindrical micro cavities on tungsten slab filled with air have been studied. It has been shown that the physical nature of the emissivity enhancement depends on the structural parameters of the micro cavities. In the case of narrow and shallow micro cavities with radius ≤ 0.2µ and depth ≤ 0.5µ ; the emissivity has the same behavior as that of flat tungsten. Thermally excited surface plasmon polaritons cause a sharp peak in the emissivity of micro cavities. In the case of wide and deep micro cavities with r ≥ 0.4 µm and d > 0.5 µm; there are anomalous peaks in the emissivity which could be matched with external quantum ef iciency of photovoltaic cell. At wavelengths shorter than periodicity, the Bragg dif raction from the surface of periodic micro cavities reduces the emissivity. We have optimized cavity parameters at r=0.67µm, d=8.9µm, a=1.4µm and cut of wavelength=2.3µm for an emitter and r=0.45µm, d=7.9µm, a= 1µm and =1.5µm for an absorber. At average emissivity and absorptivity of 95% and 93% was obtained respectively at an optimized parameters for normal incidence of electromagnetic waves on surface of structured tungsten photonic crystal. The designed photonic crystal has high spectral ef iciency relative to black body and other previous studies. For instance, at 1500K designed tungsten photonic crystal has spectral ef iciency of 87.2% but black body has spectral ef iciency of 45% in the wavelength range of 750nm to 5000nm which shows selectivity of proposed emitter. Generally the obtained results show that to have a favorable selective thermal emitter from tungsten photonic crystal micro cavities with emissivity/absorptivity more than 90%, the periodicity should be as small as possible, the cavity depth should be large enough and its radius should be selected based on band gap of photovoltaic cell.