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Research Papers

Analytical and Experimental Investigation to Determine the Variation of Hottel–Whillier–Bliss Constants for a Scaled Forced Circulation Flat-Plate Solar Water Heater

[+] Author and Article Information
U. C. Arunachala

Department of Mechanical
and Manufacturing Engineering,
Manipal Institute of Technology,
Manipal University,
Manipal 576 104, India
e-mail: arunchandavar@yahoo.co.in

M. Siddhartha Bhatt

Energy Efficiency & Renewable Energy Division,
Central Power Research Institute,
Bangalore 560 080, India
e-mail: msb@cpri.in

L. K. Sreepathi

Department of Mechanical Engineering,
JNN College of Engineering,
Shimoga 577 204, India
e-mail: sreepathi_lk@hotmail.com

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received November 25, 2014; final manuscript received July 24, 2015; published online August 18, 2015. Assoc. Editor: Gilles Flamant.

J. Sol. Energy Eng 137(5), 051011 (Aug 18, 2015) (8 pages) Paper No: SOL-14-1352; doi: 10.1115/1.4031213 History: Received November 25, 2014; Revised July 24, 2015

Fixed tilt flat-plate solar thermal collectors, popularly known as solar water heaters, still remain as one of the most interesting technologies for utilization of solar energy. The system performance deteriorates due to scaling because of the continuous use of hard water as feed water. The present study deals with the experimental and analytical approach to determine the variation of Hottel–Whillier–Bliss (H–W–B) constants (which compactly represent the efficiency characteristics of a solar water heater) due to variation in solar power input and degree of scaling in case of forced circulation system (FCS) without considering the variation of input power to the circulating pump. Indoor tests are performed with a copper tube to investigate the flow characteristics. This forms a part of conventional FCS, in place of the usual nine-fin tube array in a full-fledged collector. In indoor tests, electrical heating is favored to simulate solar radiation level. Various energy parameters are determined and compared by incorporating the developed numerical code FLATSCALE. Variation between experimental and analytical mass flow rate, overall heat loss coefficient, and H–W–B constants with simulated solar radiation level is plotted. In scaled condition, the drop in instantaneous efficiency is due to both scale thickness and reduced water flow rate. Scale thickness acts as an additional thermal conductive resistance between absorber plate and flowing water. Overall heat loss coefficient increases as absorber plate temperature is high during reduced flow rate. The maximum deviation observed is 21.68% in mass flow rate, 14.64% in absorber plate mean temperature, 7.86% in overall heat loss coefficient, and 12.04% in instantaneous efficiency. Compared to a clean tube, a highly scaled tube of 3.7 mm scale thickness indicates a drop of 4.76% in instantaneous efficiency and 40.28% in mass flow rate. It is concluded that the growth of scale in FCS does not affect the instantaneous efficiency significantly because of the margin in heat carrying capacity of water in spite of high drop in the flow rate.

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Figures

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Fig. 1

Various losses in FSWH

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Fig. 2

Performance characteristics test of pump

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Fig. 3

Pump performance characteristic curves

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Fig. 4

Block diagram of the experimental set up

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Fig. 5

Pressure terms in the flow system: 1, friction loss in riser; 2, friction loss in delivery pipe; 3, friction loss in suction pipe; 4, riser head; 5, delivery pipe head; and 6, suction pipe head

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Fig. 6

Flow chart to determine solar radiation level

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Fig. 7

Comparative mass flow rates

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Fig. 8

Experimental and analytical mass flow rate

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Fig. 9

Experimental and analytical overall heat loss coefficient

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Fig. 10

Experimental and analytical a0

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Fig. 11

Comparison of experimental and analytical a1

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Fig. 12

Variation of experimental and analytical instantaneous efficiency with scale thickness

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