A solar dish concentrator based on ellipsoidal polyester membrane facets

Abstract

The design of a solar parabolic dish concentrator is proposed based on an array of polyester mirror membrane facets that are clamped along their edges by elliptical rims and focused by applying a slight vacuum underneath the membranes, creating an ellipsoidal shape. The axes ratio of the elliptical rims varies with the position on the dish to approach the paraboloidal shape. The elastic mirror membrane deformation under uniform pressure load is simulated by finite element structural analysis and the resulting radiative flux distribution at the focal plane of the dish is determined by the Monte Carlo ray-tracing technique. Optimization of the membrane deflection is accomplished for maximum solar flux concentration at the focal plane. Two dish geometries are examined: (i) a 1.5-m radius 3-m focal length small dish, comprising 19 facets of 0.275-m radius with four different curvatures, yielding a peak solar concentration ratio of 5515 suns and a mean solar concentration ratio of 1435 suns with an intercept factor of 90% over a 3-cm radius disk target and (ii) a 10.9-m radius 11-m focal length large dish, comprising 121 facets of 0.9-m radius with 15 different curvatures, yielding a peak solar concentration ratio of 23,546 suns and mean solar concentration ratio of 8199 suns with an intercept factor of 90% over a 10.4-cm radius disk target. The performance of the second geometry is compared to that of the more conventional design of a multi-facet dish concentrator consisting of identical circular facets and shown to reach - on the same target area - a 12% higher mean solar concentration ratio as well as a 6.6% higher intercept factor. The simulated membrane shape is experimentally verified with photogrammetrical measurements carried out on a prototype facet of the small dish.