The performance of a collector is defined by the quality of its single components as well as their alignment and behavior under various load conditions. Our testing methods are designed to measure and assess the following quality features of parabolic trough collectors:
The Institute of Solar Research at DLR mainly uses close range photogrammetry such as the measurement systems “TARMES” and “QFly” for these kinds of analyses.
This technique determines the coordinates of points of interest from a set of images taken from several positions. The points of interest are highlighted by special markers. The PG method can be applied in any collector orientation and allows for sufficient spatial resolution to detect characteristic shape deviations of glass mirrors and absorber tubes as well as changes of the rotation axis. It is especially suitable for deformation analyses of prototypes and in cases where no reflective mirror surfaces are mounted. The spatial resolution depends on the density of markers. The photogrammetry measurement system is a flexible, portable and non-tactile tool for the accurate geometry evaluations of large structures and surfaces.
TARMES achieves high accuracy and high spatial resolution and produces slope error maps of the mirror in a parabolic trough collector from a set of photos. Normal vectors of the mirror surface are determined from the spatial coordinates of the absorber tube edges, the position of the reflected absorber-tube image on the mirror, and the position of the camera. The measurement principle of TARMES is shown in Fig. 2.
The measurement method QFly was developed at DLR and is based on the same concept as TARMES. The main difference is the possibility to measure large collector fields in short time. Instead of a stationary camera, QFly uses an airborne camera vehicle which allows for a completely automated and fast measurement of large numbers of collectors without affecting the operation of the solar field.