Composite mirror shape deviations due to temperature changes

Abstract

Composite mirrors formed of a self-supporting structure with a thin glass mirror can have advantages in comparison with traditionally used solar mirrors used in Concentrating Solar Power (CSP) applications. The intrinsic stiffness and the reduced glass thickness can allow for cost savings in the structure of a concentrator/collector and lead to increase of reflectance due to reduced losses in the glass. However, the combination of different materials may lead to mechanical stresses due to the behavior under variable temperatures or temperature distributions. Mechanical stress can lead to a deformation of the reflector surface and hence may influence the concentrator performance. As a first approach to emulate deformations of composite mirrors in the field, we assessed deformations due to temperature changes both through simulations and experimentally. The latter was possible by extensive adaptions of our measurement system. We combined a deflectometry measurement system and a chamber with tempered circulating air. The measurements were accompanied by Finite Element Method (FEM) simulations of a composite reflector model. The experimental results show the large influence that the temperature has on the reflectors reaching maximum 0.3 mm deformation, consequently this leads to paraboloid deformations and, in most of the cases, reduction in focal length of the facets with increasing temperature ranging between 143 m to 63 m, matching well with the FEM simulations.