Performance of Surface and Volumetric Solar Thermal Absorbers

[+] Author and Article Information
W. Spirkl

Sektion Physik, Ludwig-Maximilians-Universitat Munchen, Amalienstr. 54, D-80799 Munich, Germany

H. Ries

Paul Scherrer Institute, Ch-5232 Villigen, Switzerland

A. Kribus

Weizmann Institute of Science, Environmental Sciences and Energy Research Department, Rehovot 76100, Israel

J. Sol. Energy Eng 119(2), 152-155 (May 01, 1997) (4 pages) doi:10.1115/1.2887895 History: Received December 01, 1995; Revised July 01, 1996; Online February 14, 2008


Thermal surface absorbers convert all incident radiation to heat at a single local temperature. The fluid flows perpendicular to the radiation’s propagation direction. In contrast, in volumetric absorbers the fluid flows parallel to the radiation’s propagation direction, and the absorber may exhibit temperature stratification along the radiation’s direction. This raises the question whether reabsorption of parts of the thermal emission coming from the hotter absorber sections renders the volumetric absorber superior to the surface absorber. For the case of isotropic radiation, we compare the efficiency of the volumetric and the surface absorber with each other and with an isothermal absorber. We find that the nonselective volumetric absorber is less efficient than the nonselective surface absorber for the assumption of perfect heat transfer between absorber and fluid. Thus we conclude that in practical high-flux applications, the superiority of volumetric absorbers stems from the enhanced heat transfer area. If the fluid flows against the direction of radiation propagation the volumetric absorber is slightly more efficient than if it flows with this direction. We also discuss the effect of the two-flux approximation on simulation of isotropic volumetric absorbers.

Copyright © 1997 by The American Society of Mechanical Engineers
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