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Research Papers

A Method for Measuring the Optical Efficiency of Evacuated Receivers

[+] Author and Article Information
Charles F. Kutscher

Center Director
National Renewable Energy Laboratory,
15013 Denver West Parkway,
Golden, CO 80401
e-mail: chuck.kutscher@nrel.gov

Judy C. Netter

Engineer III
National Renewable Energy Laboratory,
15013 Denver West Parkway,
Golden, CO 80401
e-mail: judy.netter@nrel.gov

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received December 31, 2012; final manuscript received December 16, 2013; published online January 8, 2014. Editor: Gilles Flamant.

J. Sol. Energy Eng 136(1), 010907 (Jan 08, 2014) (5 pages) Paper No: SOL-12-1347; doi: 10.1115/1.4026335 History: Received December 31, 2012; Revised December 16, 2013

Evacuated receivers used with parabolic trough solar collectors can be characterized in terms of their heat loss and optical efficiency. The optical efficiency is the ratio of the energy collected to the incident solar radiation when operating at ambient temperature. If one restricts attention to the active portion of the absorber receiving incident sunlight, this is equal to the product of the transmittance of the glass cover (τ) and the absorptance of the absorber surface (α). This paper describes a new outdoor transient test method for measuring the optical efficiency. An aluminum tube is inserted into the center of the stainless steel absorber tube, and the annulus between the two tubes is filled with a measured mass of aluminum shot. The receiver is precooled and then exposed on an outdoor test rig to solar radiation. The slopes of the temperature versus time curves for the aluminum filler and the steel tube are taken during a period of steady solar radiation and near the point at which the average glass temperature is close to the average absorber temperature (i.e., when there is minimal heat loss from the absorber tube to ambient). The slopes are then used to determine the optical efficiency. This method has the advantage of using the actual solar spectrum and has an uncertainty of about ±3%, which can be improved upon if a reference receiver is used for comparison. When this method was applied to the active section of the receiver tube, measurements of an actual receiver tube yielded a τα that, to within experimental uncertainty, is consistent with the manufacturer's values of τ and α.

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Figures

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Fig. 1

Example plot of absorber and fluid temperatures from a model of a transient experiment using water for the thermal mass. Replacing water with aluminum would yield a similar looking graph.

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Fig. 2

(a) Schematic of test rig showing 3 receivers. (b) Photo of actual shrouded commercial receiver under test. (Photo: C. Kutscher)

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Fig. 3

Dots in the schematic above indicate thermocouple locations inside receiver tube

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Fig. 4

Select thermocouple temperatures versus time for receiver thermal transient test performed on December 26, 2012

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Fig. 5

Measured direct normal irradiance and total in the plane of the receiver under a similar mask as the receiver

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