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.
A new and fast method for optically measuring the reflector slope of parabolic troughs with high accuracy has been developed. It uses the reflection of the absorber tube in the concentrator, as seen from some distance, and is therefore called “absorber reflection method.” A digital camera is placed at a distant observation point perpendicular to the trough axis with the concentrator orientated toward it. Then, a set of pictures from the absorber tube reflection is taken with slightly different tilt angles of the concentrator. A specially developed image analysis algorithm detects the edges of the absorber tube in the reflected images. This information, along with the geometric relationship between the components, the relative collector tilt angles, and the known approximately parabolic shape of the concentrator, is used to calculate the slopes perpendicular to the trough axis. Measurement results of a EuroTrough segment of four facets are presented and verified with results from a reference measurement using high-resolution close-range photogrammetry. The results show good agreement in statistical values as well as in local values of the reflector slope. Compared to the existing photogrammetric method, the new technique reduces drastically the time measurement.
In order to optimize the solar field output of parabolic trough collectors (PTCs), it is essential to study the influence of collector and absorber geometry on the optical performance. The optical ray-tracing model of PTC conceived for this purpose uses photogrammetrically measured concentrator geometry in commercial Monte Carlo ray-tracing software. The model has been verified with measurements of a scanning flux measurement system, measuring the solar flux density distribution close to the focal line of the PTC. The tool uses fiber optics and a charged coupled device camera to scan the focal area of a PTC module. Since it is able to quantitatively detect spilled light with good spatial resolution, it provides an evaluation of the optical efficiency of the PTC. For comparison of ray-tracing predictions with measurements, both flux maps and collector geometry have been measured under identical conditions on the Eurotrough prototype collector at the Plataforma Solar de Almería. The verification of the model is provided by three methods: the comparison of measured intercept factors with corresponding simulations, comparison of measured flux density distributions with corresponding ray-tracing predictions, and comparison of thermographically measured temperature distribution on the absorber surface with flux density distribution predicted for this surface. Examples of sensitivity studies performed with the validated model are shown.
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