Optical Characterization and Modeling of Coatings Intended as High Temperature Solar Selective Absorbers

Abstract

SiCNH thin coatings synthesized by Plasma Enhanced Chemical Vapor Deposition (PECVD) are considered as high temperature ceramic materials for solar selective multilayer absorbers. As indicated by ellipsometry and spectrophotometry measurements, their optical characteristics make them suitable as solar absorbing and antireflective coatings in multilayer stacks and/or ceramic- metal composites (cermets). These measurements gave access to spectral values of complex refractive index N = n + ik, reflectance R and transmittance T of the coatings. R and T were also modeled from N, based on Fresnel equations, and a good agreement was found between simulated and measured reflectance in the visible region. A transfer matrix method was then applied to predict the optical response (R, T) of virtual multilayer stacks including SiCNH materials. W-SiCNH cermets were also considered as absorptive layers in these stacks and their refractive indices were calculated using Bruggeman effective medium approximation theory. Solar absorptance αS and thermal emittance ɛ(T) were estimated from simulated reflectance. Layer thicknesses and cermet composition were optimized using CODE software and a MATLAB in-house code, so as to maximize solar-heat conversion efficiency which relied on high αS (up to 95%) and low ɛ (down to 6% at 500°C). The corresponding global yield which could be attained in typical Fresnel and central tower solar plants using these selective coatings was also calculated and compared to current technology.