1. Introduction
The liquid desiccant cooling system (LDCS) has been considered as a promising alternative to the traditional vapor compression cooling system (VCS) due to its various advantages, such as being energy efficient and environmentally friendly [
1,
2]. Unlike the VCS, which is heavily dependent on electricity consumption, the LDCS can take advantage of low grade energy, such as solar energy, during the regeneration process of liquid desiccant. In addition, it deals with the sensible and latent load separately to avoid reheating or overcooling, which commonly occurs in VCS. Therefore, the LDCS can not only make use of renewable energy, but also meet people’s increasing pursuit of indoor thermal comfort more efficiently. As a result, it draws increasing attention worldwide in recent years [
1,
2,
3,
4]. Compared with the intense attention on the absorber [
3,
4], the regenerator has relatively limited research. Even though these two components have similar working principle, many differences exist in the actual operating conditions, such as the concentration of solution, working temperature, etc. Therefore, more investigation is necessary to support the design of regenerator for LDCS.
Fumo et al. [
5] experimentally investigated the regeneration performance of a packed tower regenerator. They found that the desiccant temperature and solution concentration had greater impact on the regeneration performance than air flow rate and desiccant flow rate. They also built a mathematical model for the regeneration process whose results agreed well with the experimental data. Zhang et al. [
6] also studied the mass transfer characteristics of a structured packing regenerator/dehumidifier. From the experimental data, they concluded that both the air flow rate and solution flow rate had a positive effect on the mass transfer coefficient. The influence of solution temperature on mass transfer coefficient was adverse due to the property of desiccant solution. Finally, a dimensionless overall mass transfer coefficient empirical correlation was developed with the prediction deviation less than 20%. Yin [
7] explored the performance of a packed tower for absorber or regenerator under different operation parameters. He obtained that the average mass transfer coefficient of the regenerator was 4 g/m
2·s. However, he did not show much comparative results under different working conditions. Different from the above researchers who employed lithium chloride as the liquid desiccant, calcium chloride was adopted by Bassuoni [
8] to study the mass transfer performance in a structured packing tower. Four criteria were chosen to identify the influence of regeneration performance: the mass transfer coefficient, moisture removal rate, effectiveness and coefficient of performance. They found that some of these criteria showed different tendencies for dehumidification and regeneration under the same influencing parameters. Finally, the economic analysis on the LDCS was also presented. In addition to the experimental exploration of the regeneration performance of a counter flow packed bed regenerator [
9], Cannistraro et al. [
10] used regenerators in hospital air treatment systems, where more ventilation flow rates and considerable energy consumption were required. Liu et al. [
11] also analyzed two kinds of regeneration modes: hot air and hot desiccant. She suggested that the hot desiccant mode, i.e., using regeneration heat to heat liquid desiccant rather than air, had better mass transfer performance. In their another study [
12], she presented the analytical solutions for the parallel flow, counter flow and cross flow adiabatic regenerator/dehumidifier.
The aforementioned packed bed regenerator is usually operated in adiabatic conditions, i.e., there is no heat exchange between the absorber/regenerator and the outer circumstance. However, during the regeneration process, due to the difference of the partial water vapor pressure between air and liquid desiccant, water in the liquid desiccant would evaporate and absorb heat from the desiccant simultaneously. As a result, the temperature of liquid desiccant would decrease along with the regeneration and the mass transfer performance would deteriorate because of the reduction of mass transfer driving force. Moreover, due to the high packing density of packing materials, the pressure loss of air through the packed bed regenerator is quite large and more pump power for air is required. Finally, the air velocity in the packed bed regenerator should be high. Consequently, it is very likely to entrain the liquid desiccant into the air which is a great threat to the indoor air quality. To alleviate the performance deterioration under the adiabatic conditions as well as overcome the drawbacks of packed bed regenerator mentioned above, internally cooling falling film regenerator was introduced [
13,
14,
15,
16]. Compared with the adiabatic one, it has the advantages of higher efficiency, lower pressure drop and less possibility of liquid entrainment.
There are generally two types of falling film regenerator: the shell-tube type and the plate type. Compared with the former one, the plate type regenerator can be fabricated with smaller size, bigger heat and mass transfer area and lower price [
17,
18,
19]. Therefore, in the present study, a single channel plate regenerator was chosen for detailed study. Yin et al. [
13,
14,
15] experimentally and numerically investigated the regeneration performance of a regenerator. They deduced from the experimental results that the regeneration efficiency of internally-heated one was higher than that of adiabatic one [
14]. The validated mathematical model also verified this viewpoint in Yin’s other studies [
15]. Liu et al. [
16] studied the regeneration performance of an internally-heated regenerator made of plastic. They concluded that the plastic internally-heated regenerator performed better than the metal ones and had the advantage of anti-corrosion as well.
The most commonly used material for the fabrication of dehumidifier/regenerator is metal [
13,
14,
15,
16]. Unfortunately, due to the special chemical properties of liquid desiccant, e.g., lithium chloride, corrosion on metal is an inevitably serious problem.
Figure 1 shows some of the metal corrosion phenomenon obtained from previous studies [
20] for stainless steel 304 and copper, and the present observation for normal aluminum. Even though plastic is an alternative of metals [
16], the inherent low conductivity and poor wettability make it hard to take the place of metal. Therefore, it is necessary to explore other technologies related to metal anti-corrosion for regenerator fabrication.
In present study, we firstly introduced an anodized aluminum plate in the manufacture of regenerator. A single channel anodized aluminum plate regenerator was experimentally investigated with the size of 500 mm × 500 mm (Length × Width). The corrosion characteristics of normal aluminum regenerator and anodized one were identified and compared by electrochemical method. Quantitative experiments were carried out under different influencing parameters to compare the regeneration performance with and without internal heating on the purpose-built test bench. The wettability and surface energy of different plates were also investigated.