Numerical Performance Evaluation of Aqueous LiCl and CaCl₂ Solutions as Liquid Desiccants in Dehumidification Systems ^†

Abstract

This study aimed to determine the transfer performance of aqueous LiCl and CaCl₂ liquid desiccants by numerical means. This research was conducted numerically by simultaneously solving heat and mass transfer equations explicitly using the finite difference method, with the changes in property values of the liquid desiccant in each control volume. The differences in fluid characteristics and property values between the aqueous LiCl and CaCl₂ solutions remarkably affected the reduction water content from the air. When using the CaCl₂ solution, performance decreased by 15% compared to the LiCl solution. The temperature of the liquid desiccant increased by around 13%.

1. Introduction

Air quality is not just about being clean and comfortable. The right level of humidity is also a crucial aspect for comfort and health. Invisible to the naked eye, the air contains water vapor [1]. The amount of water vapor affects the humidity level in the air. In tropical countries like Indonesia, the humidity level is generally relatively high. With a relatively constant temperature, the humidity level does not change much throughout the year. Drastic changes usually occur during the transition between the rainy season and the dry season. According to data from the National Statistics Agency in Indonesia, the average relative humidity level in Indonesia ranges from 70% to 80%. These data indicate that the challenges to regulate humidity and air temperature in a room to achieve comfort still remain high. When fresher air is introduced into a room, the cooling load and indoor humidity will increase. Additionally, the more strenuous the activities of the people in the room, the higher the cooling load capacity and indoor humidity will be, and vice versa [2].

Based on these climatic conditions, people living in tropical countries flock to use air conditioning systems (AC). The AC system demands a significant portion of the total energy consumed by the building sector [3]. The energy consumption of HVAC (heating, ventilation, and air conditioning) systems in buildings accounts for 40% of global energy consumption. The amount of energy required depends on the load and the HVAC system. Additionally, the use of refrigerants in HVAC systems can sometimes have adverse environmental impacts, such as ozone layer depletion potential and global warming potential. Currently, many efforts have been made to develop green HVAC systems. These efforts can include the addition of auxiliary equipment, the replacement of existing HVAC systems [4], and proposing new working fluids [5,6,7,8]. In recent years, new innovations in household air conditioning using the concept of liquid desiccant have emerged [9]. Concerns about electricity costs, grid stability, and global warming are growing, leading to alternative energy sources and a primary focus on energy use efficiency [10]. Liquid desiccant generally has an automatic cleaning system designed to induce a flow of inert-rich vapor, where the water in the air is separated in a trap while the vapor is returned to the system [11,12]. Subsequent research by [13] discusses a practical model for the simultaneous heat and mass transfer process in an internally cooled dehumidifier paired with a newly developed ionic liquid desiccant from Evonik Industries using experimental methods and a numerical approach. Heat and mass transfer are estimated using the heat and mass transfer coefficient analogy from the literature, incorporating variable film thickness and partial wetting models, thus accounting for changes in film thickness and wet area in each control volume.

This study aimed to test a system capable of reducing humidity levels in the air using a drying liquid system, thereby improving air quality and reducing the workload of air conditioners. LiCl and CaCl₂ solutions were used as liquid desiccant, and their performance was observed. The properties were calculated using the equations from the other studies [14,15]. This research is resolved using the finite difference method explicitly, referencing the numerical method framework to [16].

2. Method

The proposed system in this research is a regenerative dehumidification cycle using liquid desiccant, which in this cycle can transfer moisture and heat between the supply air streams to the liquid desiccant. The moisture absorption occurs on a flat vertical plate coated with a flowing liquid desiccant film with dimensions of 0.5 m in height and 1 m in width. The humid air enters at the top of the plate and exits through the bottom, parallel to the direction of the liquid desiccant flow. As the air passes through the system, moisture is absorbed by the liquid desiccant due to concentration differences, and heat transfer occurs due to temperature differences. The schematic of the liquid desiccant and air flow is illustrated in Figure 1.

To numerically simulate the transfers processes, the dehumidification system is divided into several control volumes. The heat and mass transfer processes in a control volume are shown in Figure 2. A couple of energy and mass balance equations, which are used to determine the outlet conditions of a control volume, were utilized. This is followed by the procedure for determining the outlet conditions of the absorbent from the control volume. In this study, the absorption process was assumed to occur adiabatically. An adiabatic process is a thermodynamic process in which there is no heat exchange between the control volume and its surroundings.

The balance of mass and energy is an important concept in thermodynamic analysis, as it indicates a condition where there is no accumulation of mass and energy within the control volume in a steady state condition.

When the air is divided into two components, dry air and water vapor, the energy balance at the air side and desiccant side can be written as Equations (1)–(4):

$$H_{a,e}=H_{a,i}-{\Delta H}_v-{\Delta q}_c$$

(1)

$$m_{a,e}{\int }_{T_0}^{T_{a,e}}{Cp}_{a,e} dT=m_{a,i}{\int }_{T_0}^{T_{a,i}}{Cp}_{a,e} dT-{\Delta m}_v{h}_v-{\Delta q}_c$$

(2)

$$H_{d,e}=H_{d,i}+{\Delta H}_v+{\Delta q}_c$$

(3)

$$m_{d,e}{\int }_{T_0}^{T_{d,e}}{Cp}_{d,e} dT=m_{d,i}{\int }_{T_0}^{T_{d,i}}{Cp}_{d,e} dT+{\Delta m}_v{h}_v+{\Delta q}_c$$

(4)

The mass balance at the air side and desiccant side can be expressed as Equations (5)–(7):

$$m_a=m_{da}+m_v$$

(5)

$$m_{a,e}=m_{a,i}-\Delta m_v$$

(6)

$$m_{d,e}=m_{d,i}+\Delta m_v$$

(7)

3. Results and Discussions

In this section, the results of the simulation obtained through the numerical calculations are explained. The explanation is divided into several subsections according to the variation conducted. The results are compared in terms of the reduction in moisture content. Figure 3 shows the validation of the present results in comparison with previous research at the same operating conditions. The system used in the validation process utilizes a parallel flow system and constant property values. The type of liquid desiccant used was LiCl. The results of all calculations are presented in the form of graphs. The graphs present variations in the mass flow rate of inlet liquid desiccant. Figure 3 shows that the decrease in air humidity ratio using the validation model reached approximately 56% of the initial air humidity ratio, and from the three models, the initial air humidity ratio was the same at 15.228 g_w/kg_da. Figure 3 indicates that the higher the liquid mass flow rate of the desiccant, the greater the decrease in humidity ratio. The difference in the final humidity ratio between the highest and lowest liquid mass flow rate inlets was around 16%. The first validation was conducted based on the study by [10], resulting in a maximum error value of 4%. The second validation was performed based on the study by [9], resulting in a maximum error value of 6%.

Based on the above validation data analysis, with maximum error values of 4% and 6%, the present research using the explicit finite difference numerical method and employing a parallel flow direction system can be deemed valid.

Figure 4 shows the air and desiccant solution temperature distribution along the plate in a parallel flow system, utilizing LiCl as the liquid desiccant. It shows the temperature changes in the liquid desiccant and air during the water vapor absorption process from the air. When the first water vapor was introduced into the liquid desiccant, LiCl salt in the solution reacted with water molecules. Figure 4 shows the increase in temperature of the liquid desiccant from the effect of the exothermic reaction. A chemical reaction occurred in the LiCl liquid desiccants when water vapor was dissolved in the solution, wherein water molecules interacted with Li⁺ and Cl⁻ ions in the solution to form LiCl + H₂O.

Similarly, in the CaCl₂ liquid desiccant, an exothermic process occurred when absorbing water due to the soluble reaction between chloride ions (Cl⁻) and calcium ions (Ca²⁺) with water molecules. When water was added to the CaCl₂ liquid desiccant, these ions reacted with water molecules to form a solution complex of CaCl₂ + H₂O. During this process, the energy released due to the formation of bonds between ions and water molecules was greater than the energy required to separate water molecules into water atoms. Consequently, energy was released in the form of heat, resulting in an increase in the solution’s temperature.

Figure 5 shows the change in water vapor concentration in two components: (a) the air and (b) the liquid desiccant. The calculation process uses a parallel flow system and employs LiCl as the type of liquid desiccant: (a) The decrease in water concentration in air caused by the dehumidification process. As the water passes through the plate, its water concentration in air decreases due to the absorption of water vapor from the air. The decrease in water concentration in the air when it passed through the dehumidification system using the liquid desiccant was around 0.67%. (b) The increase in water concentration in solution caused by the dehumidification process. The increase in water concentration in the solution when air passed through the dehumidification system using the liquid desiccant was around 2.7%.

Figure 6 shows the performance difference between two types of liquid desiccants, LiCl and CaCl₂. Both liquid desiccants work in a dehumidification system using a parallel flow system with changing properties values. The differences in fluid characteristics and properties of the aqueous LiCl and CaCl₂ solutions remarkably affect the reduction water content from the air. Figure 6 indicates that the water absorption performance of an aqueous LiCl solution with a concentration of 40% (wt. of LiCl) is better than that of the aqueous CaCl₂ solution, 15% higher compared to the LiCl solution. This is evident from the higher absorption rate of the LiCl solution compared to CaCl₂ under various temperature and humidity conditions. The LiCl solution showed better stability in maintaining its dehumidification capability despite changes in the solution’s properties, whereas the CaCl₂ solution tended to experience a more significant decline in performance. Additionally, the ability of the LiCl solution to absorb water more quickly is also a determining factor in the overall performance of the dehumidification system. Based on observations, the LiCl solution can maintain its performance longer, making it more efficient for use in dehumidification applications that require continuous moisture reduction.

4. Conclusions

This research model can predict the heat and mass transfer processes in a dehumidification system using flat plates and parallel flow direction with a liquid desiccant. In the present work, the performance of a dehumidification system with a liquid desiccant was analyzed numerically using the two most common liquid desiccants, CaCl₂ and LiCl [17]. The results of the study were compared with previous research and were found to be valid. The LiCl solution exhibited higher performance in absorbing water vapor from the air compared to the CaCl₂ solution. This is because the LiCl solution has a lower saturation pressure than the CaCl₂ solution. The differences in fluid characteristics and properties between the aqueous LiCl and CaCl₂ solutions remarkably affected the reduction in water content from the air. The performance of an aqueous LiCl solution with a concentration of 40% (wt. of LiCl) in absorbing water vapor from the air was better than that of the aqueous CaCl₂ solution. Apart from its performance, the corrosive nature of LiCl and the relatively cheaper price of CaCl₂ means that can be considered a fairly reliable alternative liquid desiccant. Next, in practical applications, the use of CaCl₂ solution can enhance cost efficiency in dehumidification systems, even though its performance is slightly below that of the LiCl solution. However, for applications requiring high absorption efficiency, the LiCl solution remains the primary choice. It is also important to consider maintenance aspects and potential material damage due to corrosion when using LiCl, as this can affect the equipment’s lifespan. Therefore, the selection of the drying solution should be tailored to the specific needs of the application, including economic, environmental, and operational efficiency considerations.