Replacing silver by aluminum in solar mirrors by improving solar reflectance with dielectric top layers

Abstract

Silver is a strategic material for renewable energy. Among other applications related to PV, it is used as reflective layer in solar mirrors for concentrated solar power (CSP), thanks to its high reflectance in the whole solar range. CSP plants require large mirror areas, typically in the 0.1 to 1 km2. However the estimated reserves of silver do not exceed 20 years. To sustain the deployment of CSP as a valuable input in the future energy mix, silver must be replaced with a less critical material, ideally low cost. In this theoretical study we propose to replace silver by aluminum, adding a dielectric multilayer coating on top, to reach similar optical performance. This design is inspired by Bragg mirrors but differs in that the stack parameters (layer thicknesses) are individually optimized, so as to maximize solar reflectance. A thin aluminum layer with MgF2/TiO2 aperiodic stack on top can theoretically reach solar reflectance of 96.1%, higher than silver (95.5%). Other materials are also found suitable: SiO2, ZnO, ZrO2. A reasonable number of layers (〈20) not only enhances the optical performance of metallic reflectors, but also allows tuning said performance, which is not possible with classical mirrors relying on a single metal reflective layer. For instance, one can optimize the stack to reach a better performance than silver at typical mean incidence angles of solar light in CSP plants, for different locations. A solar mirror without rare or precious materials and more effective than traditional silver mirrors is thus obtained.