To increase the absorption of the thin film selective solar absorber, multilayer anti-reflection coating is designed and optimized, which consists a layer of SiO2 followed by TiO2. Local optimization is performed for 20 um Si and at 550C. The optimal thicknesses for SiO2 and TiO2 are found to be 100nm and 50nm respectively, which yields 76.59% thermal transfer efficiency at 100 suns concentration. The selective absorber exhibits sharp selectivity and flat absorptivity below cutoff wavelength which is around 97% from 400nm to 1100nm. The influence of temperature is also analyzed for 20 um Si. The optimal operation temperature is around 450C at high concentration, and it gradually decreases as the concentration decreases. Finally, the dependence of efficiency on Si thickness is examined under 550C. The optimal Si thickness decreases as we decrease the concentration. In conclusion, the two-layer anti-reflection coating (SiO2+TiO2) design increases the absorption of the selective absorber as compared with one-layer design (Si3N4), while having little influence on the thermal re-radiation. Therefore, this structure produces higher thermal transfer efficiency, and in the same time, is simple and easy to fabricate.
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EE 115, Purdue University, West Lafayette, IN