Experimental Assessment of Simultaneous In-Situ Heliostats Calibration Methodology HelioControl at Themis Facility

An imaging method for the parallel in-situ measurement of heliostat aim points during operation in central receiver systems is assessed. In a SFERAII-access campaign, the HelioControl prototype system was integrated into the heliostat control system Hyperviseur at the Themis facility. The method, determining aim points from the cumulative flux distribution at the receiver, uses a signal of periodic movement, modulated by means of the heliostat drives. The functionality was implemented into a group of heliostats and practically assessed. For few heliostats, exemplary parallel aim point measurements at a flux target - using the methodology - show very small differences compared to a reference. Further experiments with multiple overlapping focal spots have been conducted using the small experimental cavity receiver of the system. The highly variable reflectance of the receiver area is challenging for the imaging method evaluating the reflection of the incident flux. A simplified receiver model obtained from experiments with single heliostats, however, allows for a correction and preliminary evaluation. The promising results obtained from measurements with deviations below 0.7 mrad in exemplary evaluations display the potential of the method and motivate for further and comprehensive validation in the context of commercial scale central receiver systems.