Advancements in anti-frosting and defrosting techniques for air source heat pumps: A comprehensive review of recent progress

Air source heat pumps (ASHPs) have emerged as up-and-coming alternatives to fossil fuel heating systems due to their energy efficiency, environmental friendliness, and installation flexibility. However, the performance of ASHPs can be compromised by frost accumulation on the outdoor evaporator in low-temperature and highhumidity environmental conditions. This article reviews the recent advancements in anti-frosting and defrosting technologies of the ASHP, explicitly focusing on developments from 2018 to 2023. Firstly, the characteristics of frost layer growth and the influential factors that affect the frosting process were introduced. Next, various anti-frosting methods were examined, including modifications to the evaporator structure and surface wettability, optimization of operational principles, and design of operating parameters for ASHP systems. Finally, the recent developments in reverse cycle defrosting, hot gas bypass defrosting, hot liquid defrosting, and thermal storage defrosting methods were discussed. In addition, the defrosting control strategies based on machine learning, mathematical models, and image processing techniques were analyzed. The results demonstrate that the solar-assisted ASHP system and the superhydrophobic and superhydrophilic moistening heterosexual hybrid surfaces exhibit superior frost suppression performance and warrant further investigation. Regarding defrosting methods, utilizing multiple outdoor heat exchangers for rotational hot liquid defrosting and utilizing phase change materials for thermal storage defrosting can provide continuous room heating during the defrosting process, thereby improving thermal comfort during defrosting. Therefore, future defrosting methods should focus on achieving uninterrupted heating. This comprehensive review establishes a theoretical foundation and provides valuable references for selecting anti-frosting and defrosting methods in ASHPs, thus promoting their technological advancement.