Optical evaluation of heliostat mirrors using caustics

Solar thermal power generation is based on the concept of concentrating solar radiation to provide high temperature heat for electricity generation via conventional power cycles. The high relative cost of optical subsystems necessitates a careful study of their components. Solar collector fields are typically modeled by ray-tracing or convolution methods; however, no general method is available for engineering analysis. We propose the use of caustics to predict the image of the sun reflected by an arbitrary mirror of focal length F and aperture a on a target. The mirror surface is described parametrically by a quadric and placed at a distance L from the target on a heliostat. The method of caustics was validated against SolTRACE, a ray-tracing code developed by U.S. National Renewable Energy Laboratories. We show that there is a value of a F which depends on a L, for which the incident average energy flux to the target reaches a maximum. The method of caustics allows the efficient computation of the image on an arbitrary target surface and obviates many of the difficulties with ray-tracing methods.