Elsevier

Solar Energy

Volume 72, Issue 4, April 2002, Pages 351-361
Solar Energy

Study of an aqueous lithium chloride desiccant system: air dehumidification and desiccant regeneration

https://doi.org/10.1016/S0038-092X(02)00013-0Get rights and content

Abstract

Desiccant systems have been proposed as energy saving alternatives to vapor compression air conditioning for handling the latent load. Use of liquid desiccants offers several design and performance advantages over solid desiccants, especially when solar energy is used for regeneration. For liquid–gas contact, packed towers with low pressure drop provide good heat and mass transfer characteristics for compact designs. This paper presents the results from a study of the performance of a packed tower absorber and regenerator for an aqueous lithium chloride desiccant dehumidification system. The rates of dehumidification and regeneration, as well as the effectiveness of the dehumidification and regeneration processes were assessed under the effects of variables such as air and desiccant flow rates, air temperature and humidity, and desiccant temperature and concentration. A variation of the Öberg and Goswami mathematical model was used to predict the experimental findings giving satisfactory results.

Section snippets

INTRODUCTION AND BACKGROUND

Liquid desiccant cooling systems have been proposed as alternatives to the conventional vapor compression cooling systems to control air humidity, especially in hot and humid areas. Research has shown that a liquid desiccant cooling system can reduce the overall energy consumption, as well as shift the energy use away from electricity and toward renewable and cheaper fuels (Öberg and Goswami, 1998a). Burns et al. (1985) found that utilizing desiccant cooling in a supermarket reduced the energy

EXPERIMENTAL FACILITY AND PROCEDURE

A schematic of the experimental facility is shown in Fig. 1. The packed bed absorption tower was constructed from a 25.4 cm (24 cm I.D.) diameter acrylic tube to allow for flow visualization. The height of the tower is constant and equal to 60 cm. The packings used were 2.54 cm (1 in.) polypropylene Rauschert Hiflow® rings with specific surface area of 210 m2/m3. Fresh, unused lithium chloride was stored in a tank, and its temperature was adjusted by circulating cold or warm water through a submerged

THEORETICAL HEAT AND MASS TRANSFER MODEL OF THE PACKED BED TOWER

Öberg and Goswami (1998b) developed a finite difference model based on the model for adiabatic gas absorption presented by Treybal (1969) with the exception that the resistance to heat transfer in the liquid phase was neglected. For their model they assumed adiabatic absorption; concentration and temperature gradients in the flow direction (Z-direction, referring to Fig. 2) only; only water is transferred between the air and the desiccant; the interfacial surface area is the same for heat

Air dehumidification

Table 1 presents the experimental results, while Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8 show the experimental results for dehumidification together with the theoretical modeling results. Uncertainties of the experimental measurements were calculated using the method by Kline and McClinton (1953). Error bars obtained from these calculations are also shown in the figures. It is seen from the figures that the adapted finite difference model shows very good agreement with the experimental

CONCLUSIONS

Reliable sets of data for air dehumidification and desiccant regeneration using lithium chloride were obtained. The influence of the design variables studied on the water condensation rate from the air and evaporation rate from the desiccant can be assumed linear. Therefore, the slope of the curves in Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14 give a measurement of the impact of the variable on the water condensation and evaporation

NOMENCLATURE

    at

    specific surface area of packing (m2/m3)

    aw

    wetted surface area of packing (m2/m3)

    cp

    specific heat (kJ/kg °C)

    D

    diffusivity (m2/s)

    Dp

    nominal size of packing (m)

    FG

    gas phase mass transfer coefficient (kmol/m2 s)

    FL

    liquid phase mass transfer coefficient (kmol/m2 s)

    G

    superficial air (gas) flow rate (kg/m2 s)

    g

    acceleration of gravity (m/s2)

    hG

    gas side heat transfer coefficient (kJ/m2 s)

    kG

    gas phase mass transfer coefficient (kmol/m2 s Pa)

    kL

    liquid phase mass transfer coefficient (m/s)

    L

    superficial desiccant

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