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Research Papers

Validation of Two Optical Measurement Methods for the Qualification of the Shape Accuracy of Mirror Panels for Concentrating Solar Systems

[+] Author and Article Information
Tobias März, Christoph Prahl, Steffen Ulmer, Stefan Wilbert

German Aerospace Center (DLR),  Institute of Technical Thermodynamics, Solar Research, Plataforma Solar de Almería, Ctra. de Senés s/n km 4, Apartado 39, 04200 Tabernas, Spainsteffen.ulmer@dlr.de

Christian Weber

CSP Services GmbH, Office Spain: Paseo de Almería 73, 2, 04007 Almería, Spain

J. Sol. Energy Eng 133(3), 031022 (Aug 16, 2011) (7 pages) doi:10.1115/1.4004240 History: Received January 09, 2011; Revised May 03, 2011; Published August 16, 2011; Online August 16, 2011

The solar field is the major cost component of a solar thermal power plant and the optical quality of the concentrators has a significant impact on the field efficiency and thus on the performance of the power plant. Measuring slope deviations in the parabolic shape of the mirror panels in the accuracy and resolution required for these applications is a challenge as it is not required with the same characteristics in other industries. Photogrammetry and deflectometry are the two optical measurement methods that are typically used to measure this shape accuracy of mirror panels used in CSP applications. They have been compared and validated by measuring a typical mirror panel under optimal conditions. Additionally, a flat water surface has been measured as an absolute reference object using deflectometry. The remaining deviations between the results of both methods and to the reference object are discussed and possible sources of errors during the measurement are identified. A detailed error analysis is conducted for both methods and compared to the experimental findings. The results show that both methods allow for surface slope measurement with the necessary accuracy for present CSP applications and that among the two, deflectometry exhibits advantages in speed, measurement accuracy, and spatial resolution. However, for obtaining correct results several sources of errors have to be addressed appropriately during measurement and postprocessing.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Left: Setup of automatic QDec measurement system. Center: Measured panel with reflected sinusoidal stripes. Right: Result graph “Slope deviations in x” of DF measurement, color bar in Mrad.

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Figure 2

Uncertainties of the measured slope angles αx (left) and αy (right) for an inner panel of RP3 geometry. Color bars in Mrad.

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Figure 3

Left: Measurement setup for PG measurement with approximately 3000 targets. Center: Camera positions of PG measurement as a part of the evaluation process. Right: Result graph showing height deviations of panel in millimeters.

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Figure 4

Difference of DF result to an ideal flat water surface in x-direction (left) and y-direction (right). Color bars in Mrad.

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Figure 5

Differences between DF and PG in slope in x- (left) and y-direction (right) in Mrad

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Figure 6

Difference between the DF results from before and after PG measurement in x-direction (left) and y-direction (right). Color bars in Mrad.

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