Practical Field Alignment of Parabolic Trough Solar Concentrators

[+] Author and Article Information
Richard B. Diver

 Sandia National Laboratories, P.O. Box 5800, MS1127, Albuquerque, NM 87185rbdiver@sandia.gov

Timothy A. Moss

 Sandia National Laboratories, P.O. Box 5800, MS1127, Albuquerque, NM 87185tamoss@sandia.gov

J. Sol. Energy Eng 129(2), 153-159 (Jun 05, 2006) (7 pages) doi:10.1115/1.2710496 History: Received November 11, 2005; Revised June 05, 2006

In this paper a new technique for parabolic trough mirror alignment based on the use of an innovative theoretical overlay photographic (TOP) approach is described. The technique is a variation on methods used to align mirrors on parabolic dish systems. It involves overlaying theoretical images of the heat collection element (HCE) in the mirrors onto carefully surveyed photographic images and adjustment of mirror alignment until they match. From basic geometric principles, for any given viewer location the theoretical shape and location of the reflected HCE image in the aligned mirrors can be predicted. The TOP approach promises to be practical and straightforward, and inherently aligns the mirrors to the HCE. Alignment of an LS-2 mirror module on the rotating platform at the National Solar Thermal Test Facility (NSTTF) with the TOP technique along with how it might be implemented in a large solar field is described. Comparison of the TOP alignment to the distant observer approach on the NSTTF LS-2 is presented and the governing equations used to draw the theoretical overlays are developed. Alignment uncertainty associated with this technique is predicted to be less than the mirror slope error.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 3

Theoretical overlay photograph of the LS-2 upper outer row (a) before and (b) after TOP alignment. Alignment is accomplished by adding shim washers on the side of the mirror in the direction the image needs to be moved.

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Figure 7

Distant observer photographs of the LS-2 module at the NSTTF (a) prior to and (b) after TOP alignment. Because the bottom mirror rows reflect the sky and the top mirror rows reflect the ground, misaligned regions appear blue on the bottom and brown on top. The distance from the camera to the trough vertex is approximately 460m(1507ft).

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Figure 1

Schematic illustrating the TOP alignment technique. Accurately positioned cameras on an alignment fixture measure the position of the HCE image in the mirror. The boresight camera is used to align the fixture, HCE, and trough concentrator. The mirror is aligned to superimpose the HCE image onto the theoretically calculated image position.

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Figure 2

Photograph of a “boresight gauge” used to boresight the middle camera, HCE, and trough. A foam pad squeezes between the center two mirrors and accurately holds the color sight in position. The green center contrasts with the red edges and is 0.147m long. The green center was sized to just be hidden from the camera by the HCE when on axis, but can be readily seen when off axis.

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Figure 4

Theoretical overlay photograph of the LS-2 bottom outer row (a) before and (b) after TOP alignment. The TOP alignment called for more shims on the left side than the right side of the middle mirror to correct the slanted image.

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Figure 5

Schematic showing the coordinate system and locations of the mirror, HCE, target, focus, camera, and the solar rays for calculating the lower receiver edge position in the mirrors.

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Figure 6

Photograph of the alignment fixture and LS-2 concentrator on the rotating platform at the NSTTF. The camera is mounted at the boresight position.



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