EXPERIMENTAL STUDY OF A LIQUID DESICCANT. SYSTEM FOR SOLAR COOLING AND. DEHUMIDIFICATION

K. Gommed and G. Grossman

Faculty of Mechanical Engineering Technion, Israel Institute of Technology, Haifa 32000,

ISRAEL

Growing demand for air conditioning in recent years, even in European countries with no air conditioning tradition, has caused a significant increase in demand for primary energy resources. Solar cooling has been recognized as one of the environmentally-friendly techniques toward alleviating the problem. A promising method is through the use of a liquid desiccant system, where humidity is absorbed directly from the process air by direct contact with the desiccant. The desiccant is then regenerated, again in direct contact with an external air stream, by solar heat at relatively low temperatures. The liquid desiccant system has many potential advantages over other solar air conditioning systems and can provide a promising alternative to absorption or to solid desiccant systems.

Earlier work included theoretical simulations and preliminary experiments on the key components of the liquid desiccant system. The objective of the present study has been to construct a prototype system based on the knowledge gained, to monitor its performance, identify problems and carry out preliminary design optimization. A 12 kWt system was installed at the Energy Engineering Center at the Technion, in the Mediterranean city of Haifa. The system comprises a dehumidifier and a regenerator with their associated components operating together to dehumidify the fresh air supply to a group of offices on the top floor of the building. LiCl-water is employed as the working fluid. The system is coupled to a solar collector field and employs two methods of storage — hot water and desiccant solution in the regenerated state. The performance of the system was monitored for five months during the summer of 2003 under varying operating conditions. The paper describes the operation of the experimental system and presents the measured data and the calculated performance parameters.

Background

The growing demand for air conditioning, particularly in hot and humid climates such as in Mediterranean countries, has caused a significant increase in demand for energy resources. Electric utilities have their peak loads in hot summer days, and are often barely capable of meeting the demand, with brown-out situations. With suitable technology, solar cooling can help alleviate, if not eliminate the problem. It is a good application for solar energy due to the fact that the greatest demand for air conditioning occurs during times of highest insolation (Grossman and Johannsen, 1981; Grossman, 2002).

Conventional closed-cycle absorption chillers require heat source temperatures that are significantly higher than the temperatures of corresponding heat sinks. Thus, they have to be operated with high-grade heat extracted from natural gas, steam, concentrating solar collectors and the like. A promising alternative is the use of an open absorption system, where humidity is absorbed directly from the air to be treated by direct contact with the absorbent. The absorbent is then regenerated, again in direct contact with an external air stream, at relatively low temperatures of the heat source. The entire operation takes place at atmospheric pressure, thus eliminating vacuum vessels and the like.

Earlier work has been conducted on liquid desiccant systems for cooling and dehumidification, using solar energy for regeneration. In several cases, direct regeneration of the solution in the sun has been considered, using a special type of collector. Wood and co-workers at Arizona State University (Ameel et al., 1995; Nelson and Wood, 1989a, 1989b, 1989c) have constructed and tested a full-scale liquid desiccant system, employing LiCl as well as a mixture of LiCl and CaCl2 as liquid sorbents. Kessling (1997) studied a LiCl-water system operating at a large concentration difference between the strong and weak desiccant, to facilitate cold storage by means of a regenerated solution. Kababaev et al.(1976, 1977, 1981) report on the operation of a large scale air-conditioning system employing LiCl-water, where both direct regeneration in open collectors and cold storage in the form of regenerated solution have been attempted. Noteworthy are also the liquid desiccant system analyses of Collier (1979), Haim et al. (1992), and Gandihdasan and Al — Farayedhi (1995).

The objective of this project has been to design, build, test and evaluate a fully — instrumented liquid desiccant cooling system, to provide a demonstration and supply operating data under varying conditions. The system is capable of using as its source of power low-grade solar heat from low-cost flat plate collectors and has the potential to provide both cooling and dehumidification in variable ratios, as required by the load. The significance of this work lies in the potential to provide solar powered cooling, dehumidification and air conditioning for residential and commercial applications.

As part of the design phase of the liquid desiccant system, a complete system simulation was conducted, in order to predict trends and attempt a preliminary optimization. The lack of reliable data on heat and mass transfer coefficients in the absorption and desorption processes had been a serious impediment in earlier simulation studies to obtaining a good prediction of the system’s performance. Particularly critical are the performances of the dehumidifier (absorber) and regenerator (desorber), forming the two key components of the liquid desiccant system. Such data has now been obtained through the experimental work described by Gommed, Grossman and Ziegler, (2002). This made it possible to conduct an extensive parametric study of the overall system behavior(Gommed and Grossman, 2002). The computer simulation yielded the temperature and humidity of the air at the system outlets as well as heat duties of the various system components as functions of the specified conditions at inlets and other operating conditions.