Elsevier

Applied Energy

Volume 87, Issue 5, May 2010, Pages 1495-1504
Applied Energy

Development of a novel two-stage liquid desiccant dehumidification system assisted by CaCl2 solution using exergy analysis method

https://doi.org/10.1016/j.apenergy.2009.08.048Get rights and content

Abstract

Air conditioning system based on liquid desiccant has been recognized as an efficient independent air humidity control HVAC system. To improve thermal coefficient of performance, a novel two-stage liquid desiccant dehumidification system assisted by calcium chloride (CaCl2) solution is developed through exergy analysis based on the second thermodynamic law. Compared with the basic liquid desiccant dehumidification system, the proposed system is improved by two ways, i.e. increasing the concentration variance and the pre-dehumidification of CaCl2. The exergy loss in the desiccant–desiccant heat recovery process can be significantly reduced by increasing desiccant concentration variance between strong desiccant solution after regeneration and weak desiccant solution after dehumidification. Meanwhile, the pre-dehumidification of CaCl2 solution can reduce the irreversibility in the regeneration/dehumidification process. Compared to the basic system, the thermal coefficient performance and exergy efficiency of the proposed system are increased from 0.24 to 0.73 and from 6.8% to 23.0%, respectively, under the given conditions. Useful energy storage capacity of CaCl2 solution and LiCl solution at concentration of 40% reach 237.8 and 395.1 MJ/m3, respectively. The effects of desiccant regeneration temperature, air mass flux, desiccant mass flux, etc., on the performance of the proposed system are also analyzed.

Introduction

In recent years, more and more attention of researchers has been attracted to the alternatives of the conventional vapor compression system due to depleting energy resources and serious environmental pollution. As one of the alternatives, air conditioning system based on liquid desiccant is an efficient independent humidity control HVAC system and is attractive for utilizing waste energy from industrial processes, geothermal energy, solar energy, etc. [1], [2], [3], [4]. Moreover, liquid desiccant dehumidification system uses natural substance water as working fluid, which is environment-friendly and does not cause ozone depletion.

Research on the design, development and analysis of dehumidifier/regenerator which are the prominent components in liquid desiccant dehumidification system is very active. The fundamental heat and mass transfer processes was broadly investigated by experimental method [5], [6], [7], [8], [9], as well as numerical simulation method which is a good complementary to the experimental study. Bulk flow model [8], [10] and finite differential model [11], [12], [13], [14] are commonly used to simulate the heat and mass transfer between liquid desiccant and air. By these methods, effects of variables including air and desiccant flow rates, air temperature and humidity, and desiccant temperature and concentration on dehumidification and regeneration process are analyzed. Xiong et al. found that it was more efficient to run the system in a high desiccant variance mode [15]. However, few studies on the effect of the liquid desiccant concentration variance on the performance of dehumidification/regeneration cycles are reported to date.

Exergy analysis based on the second thermodynamic law has been recognized as an effective way to improve the system performance. Reports are limit about exergy analysis on liquid desiccant dehumidification system, and most of the reported studies to improve the liquid desiccant dehumidification systems are based on the first thermodynamic law. Assouad [16] pinpointed and quantified the exergy consumption of a solar powered liquid desiccant system using solar collector/regenerator by carrying out an exergy analysis. It was found that exergy was mainly consumed in the pre-heater, the regenerator and the absorber. Ahmed et al. [17] did exergy investigation on a hybrid system incorporating absorber and dehumidifier based on liquid desiccant, calculated the irreversible losses of the hybrid cycle and attempted to optimize the desiccant flow rate of the partly closed solar regenerator. Li [18] analyzed the operating condition for the reversible dehumidification process. Nevertheless, no work is reported on the optimization of the liquid desiccant dehumidification system by exergy analysis method.

The objective of this paper is to improve the liquid desiccant dehumidification system in the view of exergy analysis. Especially, investigation on a novel design – two-stage liquid desiccant dehumidification system assisted by CaCl2 solution will be carried out.

Section snippets

Basic liquid desiccant dehumidification

A basic liquid desiccant dehumidification system is depicted in Fig. 1, as well as its desiccant solution cycle. The system is composed of a dehumidifier, regenerator, and several heat exchangers. Its working principle was well described in the Refs. [19], [20] and is explained through its desiccant solution cycle here. As shown in Fig. 1b, the process 1–2 is a dehumidification process followed by the process 2–3 which is a heat recovery process. Then in the process 3–4, the liquid desiccant

Two-stage liquid desiccant dehumidification system assisted by CaCl2

To increase the performance of liquid desiccant dehumidification system, a two-stage liquid desiccant dehumidification system assisted by CaCl2 solution is proposed, as shown in Fig. 2a. Process air is first dehumidified by CaCl2 solution and then further dehumidified by LiCl solution to the desired air humidity. On the contrary, in regeneration process, air is first used to regenerate LiCl solution and then to regenerate CaCl2 solution. This arrangement is made according to the property of

Performance of the two-stage liquid desiccant dehumidification system assisted by CaCl2

The performance of the two-stage liquid desiccant dehumidification system assisted by CaCl2 solution is shown in Table 2 (the 2nd column) in accompany with that of the basic liquid desiccant dehumidification cycle (the 4th column). Besides, the 3rd column in Table 2 shows the performance of the new system with CaCl2 cycle off, which reflects the effect of the high desiccant concentration variance method. The improvement is obvious. The exergy efficiency is increased from 6.8% to 18.7% ant COPt

Optimization of the two-stage liquid desiccant dehumidification system

Important parameters such as the regeneration temperature (desiccant temperature at the inlet of the regenerators), air mass flux and desiccant mass flux are studied in this section. The operating parameters shown in Table 2 are adopted. Regeneration temperature is a prominent parameter to liquid desiccant dehumidification system. In the two-stage liquid desiccant dehumidification system, LiCl and CaCl2 could be regenerated at different temperatures. The effect of the regeneration temperature

Validation of the simulation results

A single stage liquid desiccant dehumidification system has been built and tested. It is operated in a mode similar to that of the two-stage liquid desiccant dehumidification system assisted by CaCl2 solution when CaCl2 cycle is off. In the experiment, dehumidification process and regeneration process are separated. By this way the concentration variance of the desiccant solution can be tested and calculated. The results are shown in Table 3 with the test instruments shown in Table 4. The

Conclusion

The second law of thermodynamics is applied to the liquid desiccant dehumidification system to improve the performance. Based on the analysis results, two-stage liquid desiccant dehumidification system is proved to be an efficient system. The following conclusions are made.

  • (1)

    It is found that the exergy efficiency of basic liquid desiccant dehumidification system is very low, only 6.8% due to the high temperature difference between hot strong liquid desiccant solution after regeneration and cool

Acknowledgement

This work was supported by National Key Technologies R&D Program under the contract No. 2006BAA04B03 and the State High Technologies R&D Program under the contract No. 2008AA05Z420.

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