NiCr–MgF2 spectrally selective solar absorber with ultra-high solar absorptance and low thermal emittance

Abstract

NiCr–MgF2 spectrally selective solar absorber, which consists of the tandem films of Au, high-metal-volume-fraction (HMVF) NiCr–MgF2, low-metal-volume-fraction (LMVF) NiCr–MgF2 and MgF2, is designed and fabricated for ultra-high solar absorptance and low thermal emittance. The performance of Au/NiCr–MgF2 (HMVF)/NiCr–MgF2 (LMVF)/MgF2 is simulated using the effective medium theory and film matrix method, and the optimized NiCr–MgF2 spectrally selective solar absorber has a normal solar absorptance (αsn) of 0.981 and a normal thermal emittance (εn) of 0.042 at 25 °C. The tandem films of Au/NiCr–MgF2 (HMVF)/NiCr–MgF2(LMVF)/MgF2 are fabricated by magnetron sputtering and measured by UV-VIS-IR spectrometer, and it has an ultra-high near normal solar absorptance of αsn = 0.976 and a low near normal thermal emittance of εn = 0.045 at 25 °C, which approaches very closely to the simulated limit. Furthermore, NiCr–MgF2 spectrally selective solar absorber allows a wide incident angle (θ) with its angular solar absorptance αs(θ)≥0.957 and angular thermal emittance ε(θ)≤0.054 if θ ≤ 50°. Such spectrally selective solar absorber is an outstanding candidate for solar thermal applications.