Reflections between heliostats: Model to detect alignment errors

[Display omitted] •Theoretical imaging and aerial vision are combined to assess heliostat alignment.•Validated optical model built on pinhole camera and geometric principles.•Canting deviations of 1 mrad lead to reflected image shifts around tens of pixels.•Trade-off required between camera distance...

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Bibliographic Details
Published in:Solar energy 2020-05, Vol.201, p.373-386
Main Authors: Sánchez-González, Alberto, Yellowhair, Julius
Format: Article
Language:English
Subjects:
Alhazen’s problem
Alignment
Cameras
Facet canting & focusing
Geometric transformation
Heliostat optical quality
Heliostats
Image processing
Mathematical analysis
Pinhole camera model
Pinhole cameras
Pinholes
Quality assessment
Sensitivity analysis
Solar energy
Solar heating
Solar power tower
Target recognition
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Summary:[Display omitted] •Theoretical imaging and aerial vision are combined to assess heliostat alignment.•Validated optical model built on pinhole camera and geometric principles.•Canting deviations of 1 mrad lead to reflected image shifts around tens of pixels.•Trade-off required between camera distance and focal length.•Confidence in camera position becomes the major challenge of the technique. This paper introduces a novel technique to assess the alignment quality of heliostats in operation. This technique combines theoretical imaging with aerial vision of an object heliostat reflecting the back of a neighbor target heliostat. An accurate optical model is developed to generate the theoretical images that allows to detect alignment errors by superposition on actual photographs. The resulting code is grounded in geometric transformations between systems of coordinates, and makes use of the pinhole camera model and the geometric determination of reflection points in concave mirrors. The optical model is experimentally validated with real heliostats at the National Solar Thermal Test Facility (NSTTF). In terms of pixel shifts in the target-reflected image, a sensitivity analysis of the technique is performed. The influence of several geometric factors is researched: camera position, camera focal length, heliostat-to-camera distance, and curvature of the facets. Preliminary analysis shows that canting errors as low as 0.25 mrad can be detected by facet framing.
ISSN:0038-092X
1471-1257
DOI:10.1016/j.solener.2020.03.005