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Research Papers

Buoyancy Driven Mass Transfer in a Liquid Desiccant Storage Tank

[+] Author and Article Information
Josh A. Quinnell

e-mail: quinnell@me.umn.edu

Jane H. Davidson

e-mail: jhd@me.umn.edu
Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the Journal of Solar Energy Engineering. Manuscript received October 13, 2012; final manuscript received April 12, 2013; published online July 2, 2013. Editor: Gilles Flamant.

J. Sol. Energy Eng 135(4), 041009 (Jul 02, 2013) (8 pages) Paper No: SOL-12-1282; doi: 10.1115/1.4024249 History: Received October 13, 2012; Revised April 12, 2013

A new concept for long-term solar thermal storage is based on the absorption properties of aqueous calcium chloride. Water, diluted and concentrated calcium chloride solutions are stored in a single tank. An immersed heat exchanger and stratification manifold are used to preserve long-term sorption storage, and to achieve thermal stratification. The feasibility of sensible heating the tank via large-scale natural convection without mixing salt solutions is demonstrated via measurements of velocity, CaCl2 mass fraction, and temperature in a 1500 l prototype tank. Experiments are conducted over a practical range of the relevant dimensionless parameters. For Rayleigh numbers from 3.4 × 108 to 5.6 × 1010 and buoyancy ratios from 0.8 to 46.2, measured Sherwood numbers are 11 ± 2 to 62 ± 9, and the tank is thermally stratified. Convective mixing between salt layers is inhibited by the presence of a steep density gradient at the interface between regions of differing mass fraction. The predicted storage time scales based on mixing via natural convection for the reported Sherwood numbers are 160–902 days.

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Figures

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Fig. 1

Closed-cycle absorption heating system

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Fig. 2

Schematic of absorption/sensible storage tank: (a) illustration of immersed heat exchanger and stratification manifold in an unmixed storage with water, diluted and concentrated aqueous calcium chloride and (b) anticipated convective flow patterns during sensible charging

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Fig. 3

Top and side views of the tank including dimensions and the location of the PIV/PLIF imaging plane. Initial temperature, mass fraction, and the fluid height of the two regions are indicated. Solid circles indicate the position of thermocouples and the open circle indicates the position of conductivity probe in region 2.

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Fig. 7

Vertical temperature distributions during transient heating for experiment No. 8

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Fig. 6

CaCl2 mass fraction as a function of time for experiments Nos. 4, 8, and 10

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Fig. 5

CaCl2 mass fraction adjacent to the interface for experiment No. 8 at t = 6.55 h

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Fig. 4

PIV/PLIF data near the density interface for experiment No. 8 at (a) t = 0.20 h and (b) t = 4.62 h

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