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Research Papers: Energy Systems Analysis

Plate Diffuser Performance in Spherical Tank Thermocline Storage System

[+] Author and Article Information
Fahad Khan

Department of Mechanical Engineering,
Worcester Polytechnic Institute,
Worcester, MA 01609
e-mail: Fahadkhan707@gmail.com

Brian J. Savilonis

Department of Mechanical Engineering,
Worcester Polytechnic Institute,
Worcester, MA 01609
e-mail: Bjs@wpi.edu

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received February 15, 2016; final manuscript received April 23, 2016; published online May 25, 2016. Assoc. Editor: Mohamed A. Habib.

J. Energy Resour. Technol 138(5), 052006 (May 25, 2016) (7 pages) Paper No: JERT-16-1093; doi: 10.1115/1.4033503 History: Received February 15, 2016; Revised April 23, 2016

Thermal energy storage (TES) systems that store sensible heat in liquid media require the use of storage tanks. Spherical tanks require less building material and insulation, which might reduce the overall cost of a TES system while providing structural rigidity. The current study investigates an optimized plate diffuser in a thermocline spherical tank storage system to possibly increase the discharge flow rate without disrupting the thermocline region and without reducing the tank thermal efficiency. For low temperature (10–90 °C heat storage applications), such as heating, ventilation, and air conditioning (HVAC) and thermal water desalination, storing hot water in a thermocline system can increase the system thermal efficiency by up to 40% when compared to a fully mixed water tank and reduce the installation cost by 30% compared to a two-tank system. This study examines using a spherical tank in a thermocline system for such applications. A computational fluid dynamic (CFD) study simulated the discharge process from a spherical storage tank thermocline water system. Thermocline thickness and temperature profile in the tank were numerically determined for Reynolds number, Re = 600 and Froude number, Fr = 1.2; results were then experimentally validated. A CFD parametric study with (500 < Re < 7500) and (0.5 < Fr < 3.3): (i) determined the influence of tank flow dimensionless numbers (Reynolds, Froude, Richardson, and Archimedes) on thermal efficiency and thermocline thickness, (ii) produced an equation to predict the tank thermal efficiency using flow dimensionless numbers, and (iii) estimated the thermocline region volume occupation as a percentage of the total volume. The study of an optimized plate diffuser produced an equation for thermal efficiency based on Re and Fr numbers and estimated a thermocline volume equal to 15% of total tank volume. Flow rate ramp up by a factor of 3 was possible after the thermocline region was formed without losing tank thermal efficiency.

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Figures

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Fig. 1

Unstructured mesh with inflation layers and inlet/exit smaller element size

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Fig. 2

Experimental setup with acrylic tank and a vertical thermocouple tree inserted from the top

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Fig. 3

Experimental setup for a plate diffuser after optimization

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Fig. 4

Dye visualization of the incoming cold water in blue

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Fig. 5

Comparing CFD (black) with experimental results (red) of thermoclines at three vertical locations measured from the bottom of the tank

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Fig. 6

Acrylic tank top cover (bottom view) with cork sheet insulation and thermocouple opening

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Fig. 7

Temperature profile at the second half of the discharge with eddy viscosity model

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Fig. 8

Comparison of experimental data, linearized CFD eddy viscosity model at the exit, and CFD laminar model (green)

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Fig. 9

Froude number versus TE in a plate diffuser

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Fig. 10

Velocity contour in the y-direction at Froude number = 3

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Fig. 11

Velocity contour in the y-direction at Froude number = 9 after 2000 s of discharge

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