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Research Papers: Integrated Sustainable Equipment and Systems for Buildings

Domestic Hot Water Storage Tank: Design and Analysis for Improving Thermal Stratification

[+] Author and Article Information
Nathan Devore

Equinix
255 Caspian Dr.,
Sunnyvale, CA 94089
e-mail: nathan_devore30@hotmail.com

Henry Yip

Stratedigm, Inc.,
19 Great Oaks Boulevard,
San Jose, CA 95119
e-mail: hyip@stratedigm.com

Jinny Rhee

Department of Mechanical Engineering,
San Jose State University,
One Washington Square,
San Jose, CA 95192-0087
e-mail: Jinny.Rhee@sjsu.edu

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received January 9, 2013; final manuscript received September 11, 2013; published online October 17, 2013. Assoc. Editor: Moncef Krarti.

J. Sol. Energy Eng 135(4), 040905 (Oct 17, 2013) (7 pages) Paper No: SOL-13-1013; doi: 10.1115/1.4025517 History: Received January 09, 2013; Revised September 11, 2013

Experimental designs for a solar domestic hot water storage system were built in efforts to maximize thermal stratification within the tank. A stratified thermal store has been shown by prior literature to maximize temperature of the hot water drawn from the tank and simultaneously minimize collector inlet temperature required for effective heat transfer from the solar panels, thereby improving the annual performance of domestic solar hot water heating systems (DSHWH) by 30–60%. Our design incorporates partitions, thermal diodes, and a coiled heat exchanger enclosed in an annulus. The thermal diodes are passive devices that promote natural convection currents of hot water upward, while inhibiting reverse flow and mixing. Several variations of heat exchanger coils, diodes and partitions were simulated using ansys Computational Fluid Dynamics, and benchmarked using experimental data. The results revealed that the optimum design incorporated two partitions separated by a specific distance with four diodes for each partition. In addition, it was discovered that varying the length and diameter of the thermal diodes greatly affected the temperature distribution. The thermal diodes and partitions were used to maintain stratification for long periods of time by facilitating natural convective currents and taking advantage of the buoyancy effect. The results of the experiment and simulations proved that incorporating these elements into the design can greatly improve the thermal performance and temperature stratification of a domestic hot water storage tank.

Copyright © 2013 by ASME
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References

Figures

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Fig. 1

Sketch of thermal diode (source: F. de Winter [12])

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Fig. 2

Illustration of coiled heat exchanger

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Fig. 3

Illustration of ANSYS-CFX mesh elements used in grid independence study: (a) default mesh, (b) high quality mesh, and (c) close-up of default mesh

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Fig. 4

Demonstration of grid independence for current numerical study

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Fig. 5

Benchmark comparison of thermocline in a fully mixed tank to Zachar and Aszodi [15]

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Fig. 6

Illustration of heat exchanger designs in current study: (a) coiled heat exchanger only, (b) coiled heat exchanger with an “express elevator” channel to the top of the tank, and (c) coiled heat exchanger with “express elevator” and restricted bottom opening to minimize entrainment of cold fluid.

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Fig. 7

Comparison of thermal stratification in hot water store after 2 h of charging for three heat exchanger designs in current study

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Fig. 8

Hot water store designs with (a) one partition and (b) two partitions

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Fig. 9

Resulting thermal stratification from varying number of partitions. (Reference profile is for a fully mixed tank.)

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Fig. 10

Vertical temperature profiles resulting from varying thermal diode diameter

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Fig. 11

The effect of varying the number of thermal diodes per partition on thermal stratification in the hot water store

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Fig. 12

Dependence of thermal stratification on length of thermal diodes

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Fig. 13

Comparison of temperature profiles after 2 h of charging, followed by 6 h of standby, for the one partition design and the optimal design

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Fig. 14

(a) Coiled copper heat exchanger, (b) PVC conduit around coiled heat exchanger and two foam partitions (thermal diodes have not been installed yet), and (c) assembled hot water store with insulation

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Fig. 15

Thermocouple locations for experimental hot water storage tank

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Fig. 16

Comparison numerical and experimental temperature stratification of hot water store

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