# Review of Hygroscopic Coating on Aluminum Fin Surface of Air Conditioning Heat Exchanger
## 2 Desiccant-Coated Heat Exchangers
### Desiccant Materials  
The material of the desiccant is a decisive factor that affects the performance of DCHE. Finding the optimum desiccant suitable for DCHE is a hot research topic at present [25]. Profiting off high sorption capacity and low regeneration temperature desiccant materials was the key to developing coated heat exchangers [26; 27]. At first, the desiccant material should have a large saturated adsorption capacity and be easy to reactivate [28]. Besides, regeneration is the most energy consumed in the whole cycle of desiccant. Low regeneration temperature can effectively utilize low-grade energy and renewable energy to improve the efficiency of the DCHE system. The porous physical desiccant of silica gel has the advantages of high porosity, low cost, and stable adsorption performance; the currently studied DCHE system mainly uses silica gel as the coating desiccant material [29; 30]. [PERSON] et al. [31] constructed an open dehumidification cycle using a silica gel coated heat exchanger, and the results showed that the dehumidification capacity was 46% higher than that of the traditional device. [PERSON] et al. [32] conducted an experimental comparison between a silicone-coated finned tube heat exchanger and a polymer-coated finned tube heat exchanger, and the results showed that the effective dehumidification time of silicone-coated DCHE was longer, and the average dehumidification capacity was stronger. However, it was reported that when the partial pressure of water vapor is low, the adsorption capacity of silica gel decreases rapidly with the increase of temperature [33; 34]. Figure 1: Fin tube heat exchanger: (**a**) Without desiccant coating; (**b**) with desiccant coating [22]. The pursuit of advanced materials, improving traditional desiccant materials is the main way to prepare superior water-absorbing desiccant materials, for example, immerse hygroscopic salts, such as LiCl and CaCl\({}_{2}\), in conventional desiccants [35; 36]. It was proposed that, compared with single silica gel, composite desiccant has higher dehumidification capacity and lower regeneration temperature [37; 38; 39; 40]. And composite silica gels not only are more hygroscopic than pure silica gels, but also can enhance thermal conductivity and dynamic sorption properties [41; 42]. [PERSON] et al. [43] prepared a novel composite desiccant material by let silica gel acts as a host matrix and inorganic salt like LiCl is impregnated into its pores. Through experimental testing, they found that DCHE based on composite desiccant materials has higher dehumidification capacity, especially at lower relative humidity. [PERSON] et al. [29] found that the dehumidification capacity of composite LiCl and silica gel was 25-45% higher than that of pure silica gel. [PERSON] et al. [44] proposed that increasing the LiCl fraction in the binary salt will increase the water adsorption capacity. [PERSON] et al. [45] found that potassium formate and silica gel coated heat exchangers have 20% higher dehumidification capacity and 50% higher total cooling capacity than single silica gel coated heat exchangers. [PERSON] et al. [46] found that the finned tube heat exchanger coated with silica gel-supported lithium chloride desiccant can save up to 31% of energy compared with conventional air conditioning heat exchangers by establishing a mathematical model. [PERSON] et al. [47] prepared new composite desiccant materials for coated heat exchangers, and the experimental testing results show that the dehumidification capacity of silica gel and sodium acetate coated heat exchanger is 30% higher than the single silica gel coated heat exchanger. [PERSON] et al. [48] prepared sodium alginate and silica gel composite desiccant, which has twice the water absorption of silica gel, and its dehumidification capacity is 34% to 60% higher than that of silica gel coated heat exchangers. In addition, [PERSON] et al.