Research Papers

Heliostat Shape and Orientation by Edge Detection

[+] Author and Article Information
Marc Röger, Christoph Prahl, Steffen Ulmer

 German Aerospace Center (DLR), Institute of Technical Thermodynamics Solar Research, Plataforma Solar de Almería (PSA), Apartado 39, E-04200 Tabernas, Spain

J. Sol. Energy Eng 132(2), 021002 (Apr 29, 2010) (7 pages) doi:10.1115/1.4001400 History: Received September 17, 2008; Revised December 18, 2009; Published April 29, 2010; Online April 29, 2010

The heliostats of central receiver solar power plants reach dimensions up to 150m2 with focal lengths up to 1000 m. Their optical properties and tracking accuracy have great influence on the power plant efficiency and need to be monitored both at plant start up and during operation. Up to now, there are few efficient and fast measurement techniques that allow the heliostat properties to be measured. Flux density measurements and close-range photogrammetry are possible approaches, yet they do not fulfill the requirement to be accurate, inexpensive, and fast at the same time. In this paper, we present a noncontact measurement principle, which uses edge detection to extract the heliostat and facet vertices. This information is used to calculate the surface normals. Furthermore, the corners can replace retroreflective targets generally used in close-range photogrammetry, thus, enabling a fast and completely automatic evaluation of the three-dimensional heliostat structure. The pictures are provided by a digital camera, which is mounted on a pan tilt head on top of the central receiver tower, offering visibility to all heliostats and allowing the automated qualification of whole heliostat fields in a short period of time. It is shown that measurement uncertainties in heliostat orientation for the investigated heliostat are below 4 mrad in 80% of the relevant heliostat positions. Heliostat orientation is available within three minutes. Photogrammetric measurements based on edge detection at a 40m2 CESA-1 heliostat at the Plataforma Solar de Almerìa exhibit an accuracy of 1.6 mrad for a single-facet normal vector with the results being available within 30 min. The reduced measurement time allows the economic characterization of entire heliostat fields. The lower accuracy compared with manual photogrammetry with retroreflective targets is still sufficient to detect facet misalignments in existing heliostat fields.

Copyright © 2010 by American Society of Mechanical Engineers
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Figure 1

(a) 40 m2 CASA heliostat with detected edges, corners and gearbox and (b) single-facet with detected items and additional retroreflective targets

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Figure 2

(a) Heliostat as observed from the camera on tower top and (b) close-up of the upper left heliostat facet and the derived characteristic values. For details see Table 2.

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Figure 3

Sensitivity of camera and heliostat position, facet dimensions and focal length to heliostat azimuth (a) and elevation (b) heliostat No. 514. For details see Table 2.

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Figure 4

Real and measured heliostat orientation with estimated uncertainty: heliostat 514 (a) and heliostat 506 (b); 24-facet heliostat with averaging over between two and five images

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Figure 5

Measurement uncertainty in heliostat azimuth (a) and elevation (b) relative to optical axis of camera (in mrad) for a 24-facet heliostat with averaging over five images

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Figure 6

Deviation of facet surface normals between new photogrammetric method and conventional reference measurement

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Figure 7

Deviations of facet surface normals from design paraboloid before (a) and after (b) canting

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Figure 8

Uncertainty due to limited resolution of Hough transformation (a and b). Uncertainty estimate based on parallelism deviation of observed edges (c).



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