# Review of Hygroscopic Coating on Aluminum Fin Surface of Air Conditioning Heat Exchanger
## 2 Desiccant-Coated Heat Exchangers
### Applications in Air Conditioning Systems  
The desiccant-coated heat exchanger has received great attention because it can reduce the latent heat load in the room by absorbing water molecules, and then regenerate desiccants by using renewable energy or low-grade energy, which effectively reduces air conditioning energy consumption.  
[PERSON] et al. [54] integrated a regenerative evaporative cooler, and a desiccation coated heat exchanger, a self-cooling solid dryer cooling system was developed, as shown in Figure 2. The system consists of four groups of working fluids (regeneration air, process air, heating water, cooling water) and three main components (cross-flow falling film evaporative cooler, sensible heat exchanger, desiccant-coated heat exchanger). It operates and switches between dehumidification mode and regeneration mode. The desiccant-coated heat exchanger in the dehumidification process is cooled by the chilled water produced by the regenerative evaporative cooler. In addition, the coated desiccant material is regenerated by hot water heated by low-grade thermal energy. Reprinted with permission from ref. [54]. Copyright 2021 Copyright Clearance Center Elsevier.  
[PERSON] et al. [55] proposed a DCHE cooling system driven by solar energy, as shown in Figure 3. The system consists of a desiccant-coated heat exchanger, a solar collector heating system, and an indirect evaporative cooling system. By controlling the valve, the switch can change the cold water, and hot water flows through the heat exchanger, to control the heat exchanger dehumidification and regeneration. [PERSON] et al. [56] found that for air conditioning systems with silicone coated heat exchangers driven by evacuated tube solar water heaters, pre-cooling can achieve a better dehumidification rate, and it is beneficial to cooling capacity and COPth after the dehumidification process. Reprinted with permission from ref. [55]. Copyright 2021 Copyright Clearance Center Elsevier.  
To compare the heat and mass transfer characteristics of the conventional heat exchanger and desiccant-coated heat exchanger, [PERSON] et al. [57] found that the heat transfer capacity of DCHE was reduced by 30%, pressure drop was increased by 60% (based on the designed experimental testing). This was because of the heat resistance of the desiccant coating. Besides, without changing other structural parameters, doubling the depth of fins increases the average moisture removal by 40% and COPth by 10%. In order to study the performance of desiccant-coated heat exchanger air conditioning system in winter, [PERSON] et al. [58] built a test bench, and the experimental principle is shown in Figure 4; they found that higher ambient air humidity will lead to an increase in the humidity ratio of the incoming indoor air, the temperature of the ambient air has little effect on the incoming  
Figure 2: Schematic of self-cooled desiccant-coated heat exchanger cooling system [54].  
indoor air, and choosing the appropriate regeneration temperature and switching time, the DCHE air-conditioning system can be used for winter operation.