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Derivation of efficiency factors for uneven irradiation on a fin absorber

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Author

  • Bengt Hellström

Summary, in English

In the equation for thermal energy output from a flat-plate solar collector (written as a function of the collector mean heat carrier temperature), both the gain and the loss terms are multiplied by the collector efficiency factor, F'. For a concentrating collector with an uneven (non-uniform) irradiation on the absorber, the efficiency factor for the gain term, here called the optical efficiency factor, F-c', is different from F' and is a function of the irradiation distribution on the absorber. If the heat loss coefficient is assumed to be constant across the fin, the optical efficiency factor for absorbed irradiation at a certain distance from the edge of the absorber is independent of absorbed irradiation at other locations and can therefore be expressed F-c'(x), where x is the distance from the edge of the absorber. Close to the edge, F-c'(x) < F' and close to the pipe, F-c'(x) > F'. In this paper formulas are derived for calculating F-c'(x) for a fin absorber with constant fin thickness. By weighting F-c'(x) with the absorbed irradiance, S-c(x), and integrating across the absorber, an absorber average optical efficiency factor, F-c,F-a', is obtained. This value replaces F' in the gain term of the equation for thermal energy output. If, instead, the energy output equation is expressed as a function of the inlet temperature, F-c,F-a' can be used for calculating a corresponding heat removal factor for uneven irradiation, F-R,F-c. Formulas for calculating the temperature distribution across the absorber for the case of uneven irradiation are also derived. (C) 2004 Elsevier Ltd. All rights reserved.

Publishing year

2004

Language

English

Pages

261-267

Publication/Series

Solar Energy

Volume

77

Issue

3

Document type

Journal article

Publisher

Elsevier

Topic

  • Building Technologies

Keywords

  • uneven irradiation
  • concentrating solar collector
  • efficiency factor
  • non-uniform irradiance

Status

Published

ISBN/ISSN/Other

  • ISSN: 0038-092X