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Research Papers

A Photographic Flux Mapping Method for Concentrating Solar Collectors and Receivers

[+] Author and Article Information
Clifford K. Ho

Concentrating Solar Technologies Department,  Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-1127ckho@sandia.gov

Siri S. Khalsa

Concentrating Solar Technologies Department,  Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-1127

J. Sol. Energy Eng 134(4), 041004 (Jul 05, 2012) (8 pages) doi:10.1115/1.4006892 History: Received October 25, 2011; Accepted May 04, 2012; Published July 05, 2012; Online July 05, 2012

A new method is described to determine irradiance distributions on receivers and targets from heliostats or other collectors for concentrating solar power applications. The method uses a digital camera, and, unlike previous beam characterization systems, it does not require additional sensors, calorimeters, or flux gauges on the receiver or target. In addition, spillage can exist and can also be measured (the beam does not need to be contained within the target). The only additional information required besides the images recorded from the digital camera is the direct normal irradiance and the reflectivity of the receiver. Methods are described to calculate either an average reflectivity or a reflectivity distribution for the receiver using the digital camera. The novel feature of this new photographic flux (PHLUX) mapping method is the use of recorded images of the sun to scale both the magnitude of each pixel value and the subtended angle of each pixel. A test was performed to evaluate the PHLUX method using a heliostat beam on the central receiver tower at the National Solar Thermal Test Facility in Albuquerque, NM. Results showed that the PHLUX method was capable of producing an accurate flux map of the heliostat beam on a Lambertian surface with a relative error in the peak flux of ∼2% when the filter attenuation factors and effective receiver reflectivity were well characterized. Total relative errors associated with the measured irradiance using the PHLUX method can be up to 20%–40%, depending on various error sources identified in the paper, namely, uncertainty in receiver reflectivity and filter attenuation.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Reflection of irradiance on a small portion of a diffuse receiver toward a digital camera. The area on the receiver, AR,i , corresponds to the area captured by one pixel on the image sensor.

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Figure 2

Solid angle, dΩ, subtended by the camera iris at the receiver element

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Figure 3

Determination of ωR by comparison of angles projected onto the CCD using the sun half-angle (γ/2) as a scaling factor

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Figure 4

Schematic of vectors used to determine the cosine loss (ŝ·n̂h)

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Figure 5

PHLUX testing using heliostats at the National Solar Thermal Test Facility at Sandia National Laboratories, Albuquerque, NM

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Figure 6

Images of the heliostat beam on the tower (left) and of the sun (right) taken with a Nikon D90. Both images were taken using the same camera settings (300 mm zoom, F/32, 1/4000 s). The sun image was taken using Tiffen neutral density filters.

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Figure 7

Irradiance distribution of heliostat beam on tower calculated using the PHLUX method

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Figure 8

Irradiance distribution along vertical and horizontal transects centered within the heliostat beam on the tower

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Figure 9

Subtended angle of the sun (mrad) as a function of day of year (for Albuquerque in 2011)

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