Review: Survey of the control strategy of liquid desiccant systems

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Abstract

This paper presents the survey of various models of the control strategy of liquid desiccant systems. A basic description of liquid desiccants concept is given first and some specifications of the main liquid desiccants are summarized and listed in table. Next, heat and mass transfer process and liquid desiccant cooling systems are described briefly. Furthermore, current research on control strategies for liquid desiccant systems is then discussed in detail. Finally, summary and conclusions are presented according to the collected information about the control and optimization of liquid desiccant systems.

Introduction

The main purpose of the desiccant is to attract the water vapor from the air because of the difference in vapor pressure between the air and the surface of the desiccant solution. Dehumidification process is said to occur when the vapor pressure of the surface of the desiccant is less than that of air and continues until the desiccant reaches equilibrium with air [1] (Fig. 1). Desiccants can be regenerated at low temperature, from approximately 50 °C to 80 °C [2]. Thus, the regeneration process could be driven by heat sources with a relatively low temperature of approximately 70 °C, such as solar energy, waste heat, and geothermal power.

The desiccants can be classified into solid and liquid desiccant. Several types of solid materials can hold of water vapor; they are silicas, polymers, zeolites, aluminas, hydratable salts, and mixtures. Other available liquid desiccants are: calcium chloride, lithium chloride, lithium bromide, Tri-ethylene glycol, and a mixture of 50% calcium chloride and 50% lithium chloride. Fig. 2 shows the main types of solid and liquid desiccants. These liquid desiccants have general properties, but their requirements are not fully answered by any single desiccant. These requirements include low vapor pressure, low crystallization point, high density, low viscosity, low regeneration temperature, and low cost [3] as shown in Table 1. Many studies have examined the performance of liquid desiccants types under various parameters, such as inlet mass flow rate, temperature, and humidity ratio of the air as well as the inlet mass flow rate, temperature, and concentration of the desiccant solution. Hassan and Salah [4] proposed a desiccant with a mixture of 50% weight of water calcium chloride and 20% calcium nitrate. They studied the physical properties of the mixture, such as viscosity, vapor pressure, density, and heat, and the mass transfer process. The results of their study showed a significant increase in vapor pressure of approximately 14.7, 20.6, 34.4, and 47.3 mmHg at 30, 40, 50, and 60 °C, respectively. Li et al. [5] proposed a novel method that mixed lithium calcium chloride and lithium chloride. The experimental results showed that the dehumidification effect of the mixture was 20% more than the lithium chloride solution alone.

The control, efficiency, capacity, and economics of a desiccant cooling and dehumidification system largely depend on thermal energy management within the system. When properly applied, a liquid desiccant system can save energy and lower humidity independent of the air temperature. To ensure normal and continuous operation as well as to adapt to changing cooling, dehumidification, and ventilation loads, liquid desiccant system requires reliable control. The main objective of this study is to survey a literature review of research works done by many researchers on control strategies for liquid desiccant systems.

Section snippets

Heat and mass transfer process and operating parameters

After the air is dehumidified by being brought into contact with strong liquid desiccant, vapor compression, vapor absorption, direct or indirect evaporation cooler are used to provide sensible cooling to dehumidification air. When the solution is weakened by absorption of moisture, it is send direct to regeneration process to release the moisture by using external heat resource. This is called “reactivating” the desiccant [6]. The typical cycle of the desiccant is made up by three processes as

Liquid desiccant systems

The Kathabar system was the first liquid desiccant air conditioning system introduced in the United States, in 1910, and the other is the Hygrol system manufactured by Niagara Blower Company, in the 1930s, that consists of two components, the absorber component and regenerator component and used the Tri-ethylene glycol as a desiccant to remove frost from evaporator coil in refrigeration systems . Among the current development technology of the liquid desiccant systems, there are different kinds

Control performance of liquid desiccant systems

In the past 30 years, many studies have examined the development of liquid desiccant air conditioning systems and their components. However, few studies investigate control issues for such systems. Xiaohua et al. [24] presented the working principle of a liquid desiccant system combined with a refrigeration/heat grid system with independent humidity control (IHC) and energy consumption strategies. The independent humidity control (IHC) system consists of liquid desiccant and cooling/heating

Summary of control strategy of liquid desiccant systems

The summary of control strategy of liquid desiccant systems are presented in Table 2, which may help the researchers to select the best control parameters for investigating a good dehumidification capacity and achieving significant energy savings, low energy consumption and low operating cost. The table includes system description method, working fluid, control parameters and performance of the systems.

Conclusions

The review found that desiccant dehumidification systems can easily achieve independent control of the temperature and humidity ratio of the process air and the operation of systems depending on the values of various parameters that affect system performance (i.e., air and solution temperature, air and solution flow rate, air humidity ratio, and solution concentration). The following conclusions can be used for control and optimization strategy applications in liquid desiccant systems:

  • 1.

    The

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