Optical analysis of solar facility heliostats

Abstract Useful insights into significant operating parameters are gained by treating the mirror collector system of a central receiver solar power station as an optical system, no matter how large. We at Sandia Laboratories have developed a method for estimating the approximate size of the solar image cast by individual heliostats. As a consequence from the Coddington equations, we have developed a simple analysis of astigmatism, the major aberration which describes optical off-axis image behavior of a focusing collector system. These predictive equations agree well with experiments performed with a spherical mirror over a range of angles of incidence exceeding 60°. Other than flat mirros, several heliostat configurations were proposed and explored and were found amenable to the same analysis. All desings which attempt to superpose the reflected energy from several mirros mounted and tracked together on the same frame are subject to the same simple rules. Although this analysis is not rigorous, it does give useful insight into the cause of the large image spread found under some operating conditions and it does indicate methods for optimizing the system. Conclusions reached are explicitly related to the off-axis performance of the mirror collectors of a central receiver solar power station.