Exergy Analysis of Liquid Desiccant Dehumidification System

Abstract

Liquid desiccant dehumidification system has been recognized as an efficient independent humidity control HVAC system. To provide a thorough understanding of the liquid-desiccant-based system and improve its performance, analysis using the second thermodynamic law is carried out. Exergy performances of the dehumidifier and the regenerator are first studied to find out the proper ranges for the important operating parameters such as the desiccant concentration, air humidity ratio, flow rate ratio of desiccant to air, and regeneration temperature. Exergy analysis of the basic liquid desiccant dehumidification system is carried out with appropriate parameters. Exergy efficiency of the basic system is only 6.3%. It is found that desiccant–desiccant heat exchanger (HE), hot-water–desiccant HE, and cooling-water-desiccant HE contributes to 75.9% of the entire exergy loss because of the high temperature difference between the hot regenerated liquid desiccant and the warm diluted liquid desiccant, and the incomplete heat recovery. Furthermore, to improve the system performance, adjustable liquid desiccant concentration method and calcium chloride (CaCl₂) predehumidification method are proposed. The system exergy efficiency can be improved by using these two methods to 19.1% and 21.9% respectively. Useful energy storage capacity is increased by adjustable liquid desiccant concentration method, besides the less exergy losses in heat recovery between hot regenerated desiccant solution and warm dilute desiccant solution. The CaCl₂ predehumidification method significantly reduces the irreversibility in the dehumidifier by using a pre-dehumidification process by CaCl₂ solution before the dehumidification process by lithium chloride solution.

Keywords:

• Exergy analysis
• Liquid desiccant
• Dehumidification
• Adjustable concentration

ACKNOWLEDGMENT

The funding provided by Project of Scientific and Technical Supporting Programs of China during the 11th Five-year Plan (No.2006BAA04B03) is gratefully acknowledged.