Review of heliostat calibration and tracking control methods

TLDR

Large‑scale central receiver systems use thousands to over a hundred thousand heliostats, each requiring precise alignment to bisect the sun–receiver angle, but tracking errors make sub‑mrad accuracy unrealistic without a calibration system. The paper argues that a calibration system is essential to improve aiming accuracy and reduce spillage in large‑scale heliostat arrays. The review surveys existing central receiver systems, identifies tracking error sources, and classifies calibration systems by their signal‑flow components and measurement devices. The authors present comparative tables and an assessment of each method’s strengths and weaknesses.

Abstract

Large scale central receiver systems typically deploy between thousands to more than a hundred thousand heliostats. During solar operation, each heliostat is aligned individually in such a way that the overall surface normal bisects the angle between the sun's position and the aim point coordinate on the receiver. Due to various tracking error sources, achieving accurate alignment ≤1 mrad for all the heliostats with respect to the aim points on the receiver without a calibration system can be regarded as unrealistic. Therefore, a calibration system is necessary not only to improve the aiming accuracy for achieving desired flux distributions but also to reduce or eliminate spillage. An overview of current larger-scale central receiver systems (CRS), tracking error sources and the basic requirements of an ideal calibration system is presented. Leading up to the main topic, a description of general and specific terms on the topics heliostat calibration and tracking control clarifies the terminology used in this work. Various figures illustrate the signal flows along various typical components as well as the corresponding monitoring or measuring devices that indicate or measure along the signal (or effect) chain. The numerous calibration systems are described in detail and classified in groups. Two tables allow the juxtaposition of the calibration methods for a better comparison. In an assessment, the advantages and disadvantages of individual calibration methods are presented.