The role of draft facets in temperature effects in silicone-on-glass Fresnel lens applications
If Silicone-On-Glass (SOG) Fresnel lenses are applied in CPV power plants, one of the major contributions to optical losses is geometrical light spillage of a SOG compound due to temperature variant refractive index of silicone and the mismatch in coefficient of thermal expansions (CTE) of silicone and glass substrate leading to deformations of the lens structure. Although the involved thermal behavior is well understood, a closer look into optical losses beside geometrical light spillage is necessary in order to point out another important temperature dependent Fresnel lens design aspects: optically not usable aperture area of a Fresnel lens associated to the draft facets and their thermal behavior. We show that the effective draft angle changes with temperature due to the deformations of the lens structure. The rate of change of effective draft angle with temperature can be displayed as a function of prism aspect ratio and amounts to approximately -0.2 to -0.6 mrad/K for the investigated constant height lens designs. The detailed understanding of this effect can be used to optimize Fresnel lens designs with respect to their temperature behavior.