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Research Papers

Investigation of Reverse Thermosyphoning in an Indirect SDHW System

[+] Author and Article Information
Cynthia A. Cruickshank, Stephen J. Harrison

Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON, Canada K7L 3N6

J. Sol. Energy Eng 133(1), 011001 (Nov 23, 2010) (9 pages) doi:10.1115/1.4002556 History: Received September 19, 2009; Revised September 06, 2010; Published November 23, 2010; Online November 23, 2010

Thermal energy storages with thermosyphon natural convection heat exchangers have been used in solar water heating systems as a means of increasing tank stratification and eliminating the need for a second circulation pump. However, if the storage system is not carefully designed, under adverse pressure conditions, reverse thermosyphoning can result in increased thermal losses from the storage and reduced thermal performance of the system. To investigate this phenomenon, tests were conducted on single tank and multitank thermal storages under controlled laboratory conditions. Energy storage rates and temperature profiles were experimentally measured during charge periods, and the effects of reverse thermosyphoning were quantified. Further objectives of this study were to empirically derive performance characteristics, to develop numerical models to predict the performance of the heat exchanger during reverse thermosyphon operation, and to quantify the relative magnitude of these effects on the energy stored during typical daylong charge periods. Results of this study show that the magnitude of the reverse flow rate depends on the pressure drop characteristics of the heat exchange loop, the system temperatures, and the geometry of the heat exchanger and storage tank. In addition, the results show that in the case of a multitank thermal storage, the carryover of energy to the downstream thermal energy storages depends on the effectiveness of the exchangers used in the system.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of a simple SDHW system

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Figure 2

Measurement points and reference heights used in the calculation of the pressure head for natural convection flow

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Figure 3

Schematic of an indirectly charged, multitank SDHW system, equipped with an individual NCHE on each tank (14)

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Figure 4

Apparatus used to conduct heat exchanger characterization tests

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Figure 5

Hypothetical charge profile used in the “daylong” experimental evaluation

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Figure 6

Plot of experimental results showing the dependence of the thermosyphon flow rate on the pressure head

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Figure 7

Experimentally derived modified effectiveness as a function of modified capacity ratio

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Figure 8

A liquid storage tank divided into sections representative of (a) the physical apparatus and (b) the numerical model

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Figure 9

Net heater input power and charge temperatures for 2 day high input power test for both test arrangements

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Figure 10

Variation in thermosyphon flow rate during test sequence

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Figure 11

Variation in hydrostatic pressure head during test sequence

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Figure 12

Plot of experimental results showing the dependence of the thermosyphon flow rate on the pressure head

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Figure 13

Temperature profile of storage tanks during charging for 2 day high input power test

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Figure 14

Individual charge rates across each heat exchanger for 2 day high input power test

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Figure 15

Heat exchanger temperatures at primary heat exchanger during test sequence

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Figure 16

Effect of heat exchanger height on the magnitude of the reverse thermosyphon flow rate

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Figure 17

Effect of carryover for a multitank storage for a εmod value (a) less than 1 and (b) equal to 1

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