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Research Papers

Numerical Investigation of Thermal Separators Within the Evacuated Tubes of a Water-in-Glass Solar Water Heater

[+] Author and Article Information
Asif Soopee

Mechanical and Production
Engineering Department,
Faculty of Engineering,
University of Mauritius,
Reduit 80837, Mauritius
e-mail: amsoopee@gmail.com

Abdel Anwar Hossen Khoodaruth

Mechanical and Production
Engineering Department,
Faculty of Engineering,
University of Mauritius,
Reduit 80837, Mauritius
e-mail:a.khoodaruth@uom.ac.mu

Anshu Prakash Murdan

Electrical and Electronic Engineering Department,
Faculty of Engineering,
University of Mauritius,
Reduit 80837, Mauritius
e-mail: a.murdan@uom.ac.mu

Vishwamitra Oree

Electrical and Electronic Engineering Department,
Faculty of Engineering,
University of Mauritius,
Reduit 80837, Mauritius
e-mail: v.oree@uom.ac.mu

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 April 14, 2018; final manuscript received June 26, 2018; published online July 24, 2018. Assoc. Editor: Gerardo Diaz.

J. Sol. Energy Eng 140(6), 061014 (Jul 24, 2018) (13 pages) Paper No: SOL-18-1172; doi: 10.1115/1.4040758 History: Received April 14, 2018; Revised June 26, 2018

The effects of thermal separators within the evacuated tubes of a water-in-glass solar water heater (SWH) were numerically investigated using the commercial computational fluid dynamics (CFD) software ANSYS fluent. To validate the three-dimensional (3D) model, an experiment was performed for the passive operation of the SWH for a fortnight period, of which 3 h of recorded data was selected. The Boussinesq's approximation was employed, and the respective solar irradiance and ambient temperature profiles were incorporated. A maximum deviation of only 2.06% was observed between the experimental and numerical results. The model was then adapted for the case where thermal separators are inserted within the evacuated tubes of the SWH and both cases were run for two tilt angles, 10 deg and 40 deg. The temperature and velocity profiles within the evacuated tubes were analyzed alongside the temperature contours, thermal stratification, and overall thermal efficiency of the SWH. At a 40 deg tilt, without thermal separators, the flow streams within the evacuated tubes are restrained, and a chaotic thermal behavior was observed, thereby restricting thermal distribution to the water stored in the SWH tank. A lower tilt angle (10 deg) provided a more desirable thermal distribution. With thermal separators, however, the tilt angle preference was reversed. A faster and more uniform thermal distribution was achieved within the water tank, with a sizeable reduction in the thermal stratification at a 40 deg tilt. The overall thermal efficiency of the SWH was improved by 4.11% and 4.14% for tilt angles of 10 deg and 40 deg, respectively.

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Figures

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Fig. 1

Typical working principle of a solar thermal collector

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Fig. 2

(a) Passive tracking ability of ETSCs, (b) cross section of evacuated tube showing the two concentric glass tubes, with water within the inner tube, (c) cross section of a SWH showing the buoyant-driven water flow due to the thermosiphon phenomenon

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Fig. 3

(a) Schematic diagram of the experimental setup with temperature sensors T1 and T2, (b) experimental setup showing positioning of the temperature sensors T1 and T2, and the data logger station

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Fig. 4

Measured temperatures T1, T2, and To, during the experiment. T1 and To are referenced to the scale of the left, while T2 to that on the right.

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Fig. 5

Measured irradiance, I, during the experiment, and the calculated heat flux absorbed by the ETSC, Iτσ¯. I is referenced to the scale of the left, and Iτσ¯ to that on the right.

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Fig. 6

Ambient temperature and heat flux input profiles employed as thermal boundary conditions for the numerical model. To is referenced to the scale of the left, and Iτσ¯ to that on the right.

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Fig. 7

The computational domain considered is sectioned as indicated by the rectangle

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Fig. 8

Domain discretization with hexahedral cells of the actual SWH design

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Fig. 9

Boundary conditions employed for all simulations

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Fig. 10

Results of solar shading analysis for the solar times 08:00, 09:00, 10:00, and 11:00

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Fig. 11

Validation of the numerical model by comparing temperature readings from the temperature sensors T1e and T2e obtained from experimental data and their respective temperature points T1a and T2a recorded from numerical data (Fig. 7)

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Fig. 12

(a) Schematic of SWH with a thermal separator within the evacuated tube and (b) mesh discretization of part of the SWH with a thermal separator

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Fig. 13

Cross section of an evacuated tube to define the average tube velocity

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Fig. 14

(a) Temperature readings from numerical analysis over 3-h interval. T1 and T2 represent temperature sensors as from experiment. T3-T8 represent temperature points in Tube 1 from numerical data. (b) Temperature readings from numerical analysis over 3-h interval. T9-T14 represent temperature points in tube 2 from numerical data. A-10 and A-40 represent the data for the actual evacuated tube design at 10 deg and 40 deg tilts, respectively. N-10 and N-40 represent the data for the novel evacuated tube design at 10 deg and 40 deg tilts, respectively.

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Fig. 15

Temperature contours for the actual evacuated tubes (without thermal separators)

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Fig. 16

Temperature contours for the actual evacuated tubes (with thermal separators)

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Fig. 17

Axial velocity profiles based on arbitrary XYZ′ coordinates in the Z′-direction. Dimensionless value of u* is calculated based on Eq. (7), with umin = 0 ms−1 and umax = 0.03 ms−1 when considering all the cases above.

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