Photogrammetry: A Powerful Tool for Geometric Analysis of Solar Concentrators and Their Components

Abstract

Digital close range photogrammetry has proven to be a precise and efficient measurement technique for the assessment of shape accuracies of solar concentrators and their components. The combination of high quality mega-pixel digital still cameras, appropriate software and calibrated reference scales in general is sufficient to provide coordinate measurements with precisions of 1:50,000 or better. The extreme flexibility of photogrammetry to provide high accuracy 3-D coordinate measurements over almost any scale makes it particularly appropriate for the measurement of solar concentrator systems. It can also provide information for the analysis of curved shapes and surfaces, which can be very difficult to achieve with conventional measurement instruments. The paper gives an overview of quality indicators for photogrammetric networks, which have to be considered during the data evaluation to augment the measurement precision. A selection of measurements done on whole solar concentrators and their components are presented. The potential of photogrammetry is demonstrated by presenting measured effects arising from thermal expansion and gravitational forces on selected components. The measured surface data can be used to calculate slope errors and undertake raytrace studies to compute intercept factors and assess concentrator qualities.Keywords: Photogrammetry, Quality Control, Concentrator Analysis, Parabolic Trough Collector, Ray-Tracing INTRODUCTIONThe optical performance of solar concentrating collectors is very sensitive to inaccuracies of components and assembly. Because of a finite sun-shape and extant imprecisions of the collector system (e.g. tracking, receiver alignment, mirror alignment, mirror shape and mirror specularity) the interception of light at the focal receiver is reduced. High precision photogrammetry is an appropriate tool to measure 3D-coordinates of concentrator support points and mirror surfaces, especially for the analysis of large concentrators [1,2,3]. In contrast to measurement tools for monitoring solar flux in the focal region [4,5], the photogrammetric method directly delivers coordinates of selected test points and thus allows performance assessments of the concentrator to be made. Whereas other surface evaluation methods are limited to special shapes, e. g. to point focusing devices [6] (such as the (V)SHOT-method [7,8] or the SCCANmethod [9]), or to linear parabolic concentrators (indoor [10,11] or outdoor laser ray trace [12]), photogrammetry is a universal method for testing almost any type of concentrator or structure.

Development of a modular concrete heliostat prototype

“…Then, the mirror elements are applied. To evaluate the optical efficiency, the mirror surface will be photogrammetrically measured [13] and qualified.…”

Laser Profilometry on Micro-PTC

“…Photogrammetry [15,16], which is essentially the interpretation of photographic images, can be a powerful tool for the geometrical analysis of collectors and solar components. The setup requires applying a grid of targets that the camera can automatically detect on the surface to be measured.…”