Research Papers

Slope Error Measurements of Parabolic Troughs Using the Reflected Image of the Absorber Tube

[+] Author and Article Information
Steffen Ulmer

Plataforma Solar de Almería (PSA), Institute of Technical Thermodynamics, Solar Research, German Aerospace Center (DLR), 04200 Tabernas, Spainsteffen.ulmer@dlr.de

Boris Heinz

 Institute of Thermodynamics, Technical University of Munich (TUM), Boltzmannstraße 15, 85748 München, Germany

Klaus Pottler

Plataforma Solar de Almería (PSA), Institute of Technical Thermodynamics, Solar Research, German Aerospace Center (DLR), 04200 Tabernas, Spain

Eckhard Lüpfert

 Institute of Technical Thermodynamics, Solar Research, German Aerospace Center (DLR), 51170 Köln, Germany

J. Sol. Energy Eng 131(1), 011014 (Jan 09, 2009) (5 pages) doi:10.1115/1.3035811 History: Received September 22, 2006; Revised February 25, 2008; Published January 09, 2009

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.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Measurement setup with reflected and observed rays

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Figure 2

Reflected images of the absorber tube with the reflector facing directly the camera (center) and tilted slightly up (left) and down (right). The evaluated facet row is marked with a rectangle.

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Figure 3

Result of a facet row after edge detection: (left) upper edge, (right) lower edge

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Figure 4

Slope errors of EuroTrough facets in milliradians measured with reflection method. Facets are numbered from 1 to 4 starting from the left.

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Figure 5

Picture for photogrammetric evaluation with targets on four facets, on absorber tube and module axis

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Figure 6

Transversal slope errors (in x-direction) of the facets in milliradians, from photogrammetric data

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Figure 7

Root mean square values of the slope deviations from the ideal slope computed by the absorber reflection method and the photogrammetric method




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