Static screen deflectometry for heliostat metrology

This paper describes the design and experimental validation of a heliostat metrology system in which a large mirror's shape is obtained from measurements of its reflection of a fixed fine-scale regular pattern. A detailed map of the surface is obtained from measurements of distortion in the reflected image, using new code to correlate reflected rays with incident rays, and then deriving the surface shape by integrating the slopes obtained from the ray deviations. The method was developed to enable metrology of focusing heliostats whose reflector shape is actively altered to correct the large changes in astigmatism that arise as the solar angle of incidence (AOI) changes through the day. It was used to set and test such a reflector, made from a single 2.4 m × 3.3 m -sheet flat float glass, and bent into shape using self-twisting axes and 58 support actuators on the steel frame to focus disc images of the sun at 113 m distance for a 60ᵒ AOI. The accuracy of this method was verified by subsequent field tests directly measuring the actual solar concentration performance, which is the ultimate evaluation criterion. The heliostat formed a concentrated solar disc with a measured diameter of 1.14 m, compared to the ideal value of 1.04 m. This close agreement confirms the measured float glass surface shape error of 0.66 mrad and 0.41 mrad root-mean-square (RMS) in the x and y directions, with corresponding residual metrology uncertainties of approximately 0.37 mrad and 0.23 mrad RMS representing all unidentified error sources of the outdoor field test.