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Technical Briefs

Spatially Varying Extinction Coefficient for Direct Absorption Solar Thermal Collector Optimization

[+] Author and Article Information
Todd P. Otanicar

Department of Mechanical Engineering, Loyola Marymount University, Los Angeles, CA 90045todd.otanicar@lmu.edu

Patrick E. Phelan, Robert A. Taylor

School of Mechanical, Aerospace, Chemical and Materials Engineering, Arizona State University, Tempe, AZ 85287

Himanshu Tyagi

Department of Mechanical Engineering, India Institute of Technology-Ropar Rupnagar, Punjab 140001, India

J. Sol. Energy Eng 133(2), 024501 (Mar 22, 2011) (7 pages) doi:10.1115/1.4003679 History: Received August 26, 2010; Revised February 10, 2011; Published March 22, 2011; Online March 22, 2011

Direct absorption solar thermal collectors have recently been shown to be a promising technology for photothermal energy conversion but many parameters affecting the overall performance of such systems have not been studied in depth, yet alone optimized. Earlier work has shown that the overall magnitude of the extinction coefficient can play a drastic role, with too high of an extinction coefficient actually reducing the efficiency. This study investigates how the extinction coefficient impacts the collector efficiency and how it can be tuned spatially to optimize the efficiency, and why this presents a unique design over conventional solar thermal collection systems. Three specific extinction profiles are investigated: uniform, linearly increasing, and exponentially increasing with the exponentially increasing profile demonstrating the largest efficiency improvement.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic representation of direct absorption collector heat loss

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Figure 2

Variation in collector efficiency for uniform extinction coefficient profile (labels represent condition for bottom surface)

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Figure 3

Variation in collector efficiency for linearly increasing extinction coefficient profile

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Figure 4

Variation in collector efficiency for exponentially increasing extinction coefficient profile

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Figure 5

Variation in the divergence of radiative flux as function of the spatial variation in the extinction coefficient (optical depth=10.0, back surface perfect reflector)

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Figure 6

Collector temperature profiles. (a) Perfect absorber bottom, no volumetric absorption. (b) Perfect reflector bottom, exponentially varying volumetric absorption.

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