# Review of Hygroscopic Coating on Aluminum Fin Surface of Air Conditioning Heat Exchanger
## 1 Introduction  
With the advancement of science and technology, people's lives have improved significantly, and they are pursuing a more comfortable living environment. Temperature, humidity, and air cleanliness are the main parameters to evaluate the indoor air environment. When the moisture of the building exceeds a certain level, it will not only affect people's comfort but also result in the growth of fungus stirring up, not conducive to people's production and life. At present, more than 90% of air dehumidification systems are based on condensing moisture by the air conditioner to dehumidify the air [1]. To meet the indoor humidity comfort, the conventional air conditioning system generally maintained the evaporative cooling temperature much lower than the air dew point temperature. It is necessary for the air after cooling and dehumidification to go through a reheating process to ensure indoor thermal comfort [2]. There is a serious waste of energy and cannot meet the development requirements of green buildings. It was reported that air conditioning system accounts for over 50% of the building energy consumed in developed countries [3]. Air conditioning load generally includes latent heat load and sensible heat load, and the former is usually more than twice the latter [4]. Therefore, the improvementof air conditioning system dehumidification performance has great potential for building energy conservation. Aluminum fins are widely used in air conditioner heat exchangers, due to their high thermal conductivity, low cost, and wide source of raw materials. However, there are some drawbacks about the ordinary aluminum fins: Weak adsorption to water molecules limits dehumidification capacity; during the refrigeration process, water molecules condense into droplets adhering to the surface of the fins, increasing the wind resistance of the heat exchanger, resulting in a decrease of the heat transfer efficiency, and hindering the continuous condensation of water molecules. To solve the above problems, researchers have proposed several novel dehumidification methods, which were divided into two categories: (1) Coating desiccants on the fins, it should be mentioned that the porous adsorption desiccants are mainly considered here when the process air flows over the fin side of the heat exchanger, the water molecules will be absorbed into the desiccant pores. Using a two-unit heat exchanger system, one unit is saturated and then enters the regenerative state, switching to another unit to continuously cool and dehumidify [5]. This method can dehumidify indoor air without lowering the evaporative cooling temperature below the air dew point temperature, which is of great significance in energy-saving and indoor comfort. (2) Changing the physical or chemical properties of the heat exchanger fin surface to form wettability surfaces, including hydrophobic surfaces, hydrophilic surfaces, and hybrid hydrophobic-hydrophilic surfaces [6]. This method reduces the evaporative cooling temperature to the air dew point temperature and dehumidifies by condensation. With the same amount of dehumidification, the evaporative cooling temperature of the heat exchanger with wettability aluminum fins can be much higher than that of conventional aluminum fins, with no need for air preheating. In this paper, a comprehensive review of the key developments in the field of desiccants coating and hydrophobic/hydrophilic coating on the surface of aluminum fins in air conditioning heat exchangers must be presented, respectively. Based on current studies, the challenges of using these approaches and future work will be further analyzed. Other review articles exist about coating heat exchangers to control building humidity [4; 7; 8], but usually introduced a single type of coating dehumidification method, without combining the desiccants coating and the hydrophobic/hydrophilic coating.