2 LITERATURE REVIEW
2.1 INTRODUCTION
Liquid desiccant dehumidification systems have been used for many years in specialized applications.
The performance of liquid desiccant systems lies heavily on the heat and mass transfer characteristics of the two critical components: the dehumidifier and the regenerator. The purpose of this chapter is to provide a comprehensive review of the heat and mass transfer correlations developed to mathematically model the adiabatic absorption and desorption process. There has been an attempt to describe the most important characteristics of each research, such as the method adopted, the assumptions used, the validation of the data, as well as the most important results. It was found that most work considers the desiccant solution flowing counter currently with the air and the use of structured packing instead of random. Fewer researchers considered the co flow configuration or desiccants other than salts.
Liquid desiccant air conditioning systems have attracted more and more attention in recent years due to their environmentally friendly technology and promising utilization of low‐grade thermal energy provided by solar flat‐plate collector or waste heat [1]. In liquid desiccant cooling cycles, a sorbent solution is employed to dehumidify the air, circulating between the two critical components;
the dehumidifier and the regenerator.
As the strong desiccant solution is sprayed on top of the dehumidifier, it flows down by gravity and comes in contact with the process air. The air can be moving in parallel, counter or cross flow with the solution. The desiccant solution which, by definition, has a strong affinity for water vapor, absorbs moisture from the air. As the water vapor condenses and mixes with the desiccant, heat is released. This heat equals to the latent heat of condensation for water plus the chemical heat of mixing between the desiccant and water. In an adiabatic dehumidifier (Figure 2‐1) this heat would raise the temperature of the desiccant and decrease its ability to remove water vapor from the air.
An internally cooled dehumidifier (Figure 2‐2), by means of cooling water derived from an evaporative cooler, would be a solution to this problem. The cooling water restricts the temperature changes for both the desiccant and air and therefore, increases the dehumidification capacity. The end of the process finds the air cool and dehumidified and the solution diluted.
Figure 2‐1 Hydraulic scheme of an adiabatic liquid desiccant cooling system.
Figure 2‐2 Hydraulic scheme of an internally cooled liquid desiccant cooling system with plate heat exchangers.
2 LITERATURE REVIEW
The role of the regenerator is to retrieve the initial concentration of the diluted desiccant solution that exits the dehumidifier. The regeneration is a very important process, since it requires high temperatures and therefore, affects both cooling capacity and energy utilization efficiency of air conditioning systems [1]. The regenerator device can have the same configuration with the dehumidifier; however, the process occurring is just the opposite. The hot and diluted desiccant solution comes into contact with the ambient air.
Since the vapor pressure of the desiccant is now higher than that of the air, moisture is evaporated from the solution and transferred to the exhaust air stream. In an adiabatic regenerator (Figure 2‐1), the heat is supplied to the desiccant solution before entering the regenerator. This results in the cooling of the solution with the progress of regeneration, so the performance is gradually decreased. Internally heated regeneration (Figure 2‐2), by means of hot water derived from a low temperature source, is thus preferred. At the end of the process, the hot humid air is rejected to the ambient and the concentrated solution is driven to the dehumidifier. A liquid‐to‐liquid heat exchanger is used to precool the warm concentrated solution using the cool dilute solution from the outlet of the dehumidifier. This reduces the heat input to regenerator by 10‐15% and improves the system performance [2].
The complex heat and mass transfer phenomena, occurring both in the dehumidifier and regenerator, have been the subject of a great amount of research, especially within the last 25 years [3, 4]. During dehumidification (absorption) and regeneration (desorption) process, heat and mass are transferred through and between the liquid and vapor phases. The driving force for heat transfer between the liquid desiccant solution and the air is their temperature difference, while the driving force for the mass transfer is the difference between the vapor pressure of the desiccant and the partial pressure of water vapor in the air. However, these driving forces change as the process progresses, due to changes in the local temperature and concentration and due to changes in the liquid‐vapor interface equilibrium condition [3].
Equilibrium condition is achieved when the air temperature is equal to that of the liquid desiccant solution and the partial pressure of water vapor in the air is equal to the saturation pressure of the solution [5]. The heat and mass transfer phenomena are thus coupled and so, the accuracy of the mathematical models depends on the method and correlations used to predict these heat and mass transfer processes.
This chapter attempts to provide a wide review on the mathematical models developed for the coupled heat and mass transfer processes during dehumidification and regeneration in adiabatic liquid desiccant systems. This review includes the most common device geometries, flow configurations and desiccant solutions used. However, models which have
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