Closed Loop Optical Tracking of Heliostats

Accurate tracking of heliostats is a key requirement for cost-effective commissioning and operation of central receiver concentrating solar thermal plants. We explore sensor-based options to enable pointing accuracy of +/- 1 mrad or better. A driver for this work is the concept of a 'Drop-in' heliostat, which has been developed as part of the Australian Solar Thermal Research Initiative (ASTRI). The 'Drop-in' heliostat is designed for rapid installation, no field wiring, self-powered, autonomous calibration and autonomous operation. Using this system, the orientation of the heliostat mirror was monitored using a single-chip sensor consisting of an accelerometer, magnetometer and gyroscope. Tracking using orientation sensors also requires knowledge of the time of day, heliostat location and receiver location, which were derived using real-time kinematic GPS. Closed loop optical tracking requires identifying and tracking the heliostat sun reflection at the receiver, which we demonstrate using a low-cost camera on the heliostat and an array of retroreflectors on a target. This is then used to derive an error signal that can be used to servo the heliostat.