Dynamic photogrammetry applied to a real scale heliostat: Insights into the wind-induced behavior and effects on the optical performance

Wind loads may decrease a heliostat's optical performance by affecting its shape and orientation. As small scale wind tunnel or numerical models cannot reflect a heliostat's dynamic behavior entirely, in this study we investigated a real scale Stellio heliostat installed on the DLR heliostat testing platform where it was exposed to the natural wind. The wind-induced response was captured with a dynamic photogrammetry system that combines a high spatial and high temporal resolution. As its application to a heliostat is novel, the first focus of this paper is to validate the applied setup and the accuracy of measured displacements. The second focus is to discuss the wind-induced Stellio behavior. A method is presented which allows for separating the total wind-induced behavior into a trackingand a slope-relevant part. Based on this separation, the wind induced tracking deviation (0.44 mrad RMS) during the investigated measurement period (mean wind speed approximate to 4.8 m/s, mean turbulence intensity approximate to 26 %) reached a level of approximately one third of a heliostat's typical total tracking deviation. Likewise, during the time step of largest deformations, the wind-induced slope deviation (0.75 mrad RMS) of the most affected facet reached a level of approximately one third of a Stellio facet's total slope deviation. However, wind-induced slope deviations occurred only locally and temporally. Furthermore, wind-excited eigenfrequencies were revealed to have a negligible impact on both the tracking and slope deviation in case of the Stellio. Oscillations and deformations related to frequencies below the eigenfrequencies were rather found to have a predominant impact on the optical performance.