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Research Papers

Slope Measurements of Parabolic Dish Concentrators Using Color-Coded Targets

[+] Author and Article Information
Steffen Ulmer

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

Peter Heller, Wolfgang Reinalter

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

J. Sol. Energy Eng 130(1), 011015 (Dec 28, 2007) (5 pages) doi:10.1115/1.2807193 History: Received September 22, 2006; Revised March 21, 2007; Published December 28, 2007

A new fast and highly accurate method for measuring the slope errors of parabolic dish concentrators has been developed. This method uses a flat target with colored stripes placed close to the focal plane of the concentrator and a digital camera located at an observation point on the optical axis at some distance from it. A specially developed image analysis algorithm detects the different colors of the images of the reflection of the target in the concentrator and assigns them their known position on the color target. This information, along with the geometry of the measurement setup and the theoretical parabolic shape of the concentrator, is used to calculate the normal vectors of the concentrator surface. From these normal vectors, the radial and tangential slopes can be calculated and compared to the design values of the concentrator. The resulting slope errors not only give the total concentrator error for general characterization of the dish but also indicate systematic errors from fabrication and installation with a high spatial resolution. In order to verify the quality of the results obtained, a ray-tracing code was developed that calculates the flux distribution on planes perpendicular to the optical axis. Measured slope errors of a 8.5m dish concentrator are presented and the calculated flux distributions are compared to measured flux distributions. The comparison shows excellent agreement in the flux distribution on the absorber plane. This verifies the promising potential of this fast and highly precise new method for measuring imperfections in dish concentrator shape.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Sketch of the measurement setup with coordinate system and variable designations

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Figure 2

Images of the reflected color target in the DISTAL-2 concentrator as seen from the camera: (a) horizontal stripes, (b) vertical stripes

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Figure 3

Surface slopes in radians in x and y directions of the DISTAL-2 concentrator under study. Positive values in (a) denote inclinations to the left. Positive values in (b) denote inclinations downward.

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Figure 4

Slope errors in milliradians (compared to the ideal parabola) of the DISTAL-2 concentrator under study in radial and tangential directions. Positive values in (a) denote inward deviations and positive values in (b) denote deviation angles in a clockwise direction.

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Figure 5

Calculated flux distribution in the focal plane (a) and measured flux distribution in the focal plane (b)

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Figure 6

Calculated flux distribution on the receiver plane (a) and measured flux distribution on the receiver plane (b) . The eccentricity of the focal spot in the measured data is due to a measurement device offset.

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