Research Papers: Thermal Systems

Improved Model for Calculating Instantaneous Efficiency of Flat-Plate Solar Thermal Collector

[+] Author and Article Information
Oussama Ibrahim

Faculty of Engineering,
Lebanese University,
Hadath 1003, Lebanon
e-mail: oibrahimul@hotmail.com

Farouk Fardoun

Department GIM,
University Institute of Technology,
Lebanese University,
Saida 1600, Lebanon
e-mail: ffardoun@ul.edu.lb

Rafic Younes

Faculty of Engineering,
Lebanese University,
Hadath 1003, Lebanon
e-mail: ryounes@ul.edu.lb

Mohamad Ibrahim

Universite Grenoble Alpes, INES,
Grenoble F-38000, France;
Department of Solar Technologies,
Le Bourget du Lac F-73375, France
e-mail: mohamad.ibrahim@cea.fr

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received February 7, 2017; final manuscript received November 7, 2017; published online February 27, 2018. Assoc. Editor: Ali Khounsary.

J. Heat Transfer 140(6), 062801 (Feb 27, 2018) (8 pages) Paper No: HT-17-1067; doi: 10.1115/1.4038827 History: Received February 07, 2017; Revised November 07, 2017

The performance of a flat-plate solar collector is usually assessed by its efficiency. This efficiency is normally defined on a steady-state basis, which makes it difficult to correctly track the instantaneous performance of the collector in various case-studies. Accordingly, this paper proposes an improved definition of instantaneous efficiency of a flat-plate solar collector used as a part of a solar water heating system. Using a predeveloped model by the authors for such a system, the proposed efficiency-definition is examined and compared with the conventional one for specific case studies. The results show that the improved definition of efficiency records reasonable values, i.e., no over-range values are observed contrast to the case of conventional efficiency-definition. Furthermore, this suggested efficiency approximately coincides with the conventional one at a wide range of time, as long as the system is operating in the so-called trans-steady-state phase or when the system is off-operational provided that the instantaneous rate of heat stored in the heat transfer fluid (HTF) is less than or equal to zero. As a result, the improved efficiency-definition yields more realistic results in reflecting the performance of a flat-plate collector in an active solar water heating system and is recommended to be used.

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Fig. 1

Schema of an active solar water heating system employing a flat plat collector

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Fig. 2

Cross section of the flat-plate solar collector

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Fig. 3

Min-values of ambient temperature, wind speed, and solar radiation for June 21 and December 21 in Beirut, Lebanon

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Fig. 4

Hourly hot water consumption profile for a family of four persons during summer and winter days in Lebanon

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Fig. 5

Discretization scheme of a single tube of the flat-plate solar collector

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Fig. 6

Variations with respect to time on June 21 of (a) instantaneous conventional efficiency, (b) temperatures of side-plate, center-plate, tube-wall, HTF and ambient, (c) rate of heat stored in side-plate, center-plate, tube-wall and HTF, and (d) zoom in figure (c)

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Fig. 7

Schematic of energy flows in the studied flat-plate solar collector

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Fig. 8

Variation of the improved efficiency with respect to time on June 21

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Fig. 9

Variation of improved and conventional instantaneous efficiencies with respect to time (a) on June 21; (b) in interval I; (c) in interval II; (d) at beginning of interval VI

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Fig. 10

Variation of the difference between improved and conventional instantaneous efficiencies with respect to time on June 21

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Fig. 11

Variation of improved and conventional instantaneous efficiencies with respect to time on December 21



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