Heliostat surface estimation by image processing

Solar field performance is dependent upon the ability of each heliostat to project a concentrated beam shape on the solar receiver. The specified tolerances for the heliostat surface are very tight since a deviation of even tenths of a milliradian is significant for performance. A precise method of measuring the actual heliostat surface is critical both for quality assurance and in order to calculate the actual flux that the solar field will apply to the solar receiver. The measurements will be used as part of the solar field control so that the flux can be distributed as required for solar steam generation. BrightSource has developed a method for estimating the heliostat's shape after installation in the solar field. The measurement system is based on a heliostat control system, a visual range camera, the heliostat itself and the sun. The camera captures a rapid sequence of images as the heliostat moves in a precisely-defined path. In parallel, the system captures the exact time of each image. Throughout the sequence, the system also records the movement of the heliostat in both elevation and azimuth directions. The captured images, the timestamps and the matching recorded movements are post-processed using a complex algorithm to calculate the surface of the heliostat mirrors. The algorithm divides the mirror surface into many different elements. The algorithm identifies each element in each of the sequence of images and uses the recorded data to calculate the normal for that element. On the basis of all the elements, the algorithm generates a unified mirror surface. This method can be used both as a quality control method (sample of solar field after construction) and as a method of calculating the baseline characteristics of each heliostat. (C) 2015 The Authors. Published by Elsevier Ltd.