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Research Papers

Numerical Study of Models for Estimating Edge Heat Loss in Solar Absorbers

[+] Author and Article Information
P. N. Nwosu

Energy Research Centre,  University of Nigeria, Nsukka 410001, Enugu State, Nigeria;paul.nwosu@unn.edu.ngEnergy Technology Department, Mechanical Engineering,  Aalto University, P.O. Box 14300, Aalto 00076, Finlandpaul.nwosu@unn.edu.ng

S. O. Onyegegbu

Department of Mechanical Engineering,  University of Nigeria, Nsukka 410001, Enugu State, Nigeria

J. Thermal Sci. Eng. Appl 4(3), 031008 (Jul 16, 2012) (11 pages) doi:10.1115/1.4006870 History: Received September 08, 2011; Accepted April 27, 2012; Published July 16, 2012

Absorbers are the prime heat transfer components of solar energy devices. The effects of variable edge loss coefficients on heat transfer performance of flat-plate solar absorbers are examined. From fundamental energy relations, an edge loss coefficient is obtained which embodies the geometric and ambient parameters for estimating the edge heat loss. This model is similar in form to that given by Kaligrou (2009, Solar Energy Engineering Processes and Systems, Elsevier, New York, pp. 156–163) but differs from that obtained by Duffie and Beckman (1991, Solar Engineering of Thermal Processes, Wiley & Sons, Inc., New York), which can be corrected by employing a correction factor given here. The results confirm that the edge loss can significantly affect the absorber performance depending on Ae/Ap ratios, and the prevailing ambient conditions. However, previous models neglect these effects. It is found that the model given by Duffie and Beckman erroneously exceeds the total energy input and overestimates the loss by as much as a factor of 6, which goes against the first law of thermodynamics. When the correction factor is applied, the model gives a better approximation.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 2

Nomenclature of the solar absorber

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Figure 3

Heat flow across the boundary of a differential element of the absorber

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Figure 4

Schematic and top view of a basic flat plate solar absorber, showing the edge insulation

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Figure 5

Finite element nomenclature of the absorber showing the transverse nodal and centerlines

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Figure 6

Evolution of temperature contours of 2D-1 model for an absorber with no edge insulation: (a) ha  = 100 W/m2 K; (b) ha  = 50 W/m2 K; (c) ha  = 5 W/m2 K

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Figure 7

Evolution of temperature contours of 2D-2 model for an absorber with no edge insulation: (a) Ta  = 278 K; (b) Ta  = 298 K; (c) Ta  = 338 K

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Figure 8

Nondimensional temperature profile of the absorber with ha  = 1 W/m2 K

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Figure 9

Nondimensional temperature profile of the absorber with ha  = 3 W/m2 K

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Figure 10

Nondimensional temperature profile of the absorber with ha  = 6 W/m2 K

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Figure 11

Nondimensional temperature profile of the absorber with ha  = 9 W/m2 K

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Figure 12

Effects of increasing h¯ on the nondimensional edge heat loss, φedge

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Figure 13

Comparisons of the heat ratios, Σφ, of different edge loss coefficients for various values of parameter h¯

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Figure 14

Effects of nondimensional edge area parameter Ae on the nondimensional edge loss (present analysis)

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Figure 15

Effects of nondimensional edge area parameter Ae on the nondimensional edge loss (Kalogirou equation [13])

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Figure 16

Theoretical thermal efficiency versus nondimensional convective heat transfer coefficient for various values of parameter κ¯

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Figure 1

(a) Thermal circuits for a solar absorber with double transparent covers and (b) equivalent circuit

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