Air-source heat pumps have been widely used for room heating and air conditioning because of their stable performance and easy installation. The greatest performance limitation is the frosting and defrosting problem of the outdoor heat exchanger during winter. This study presents a review of the frosting conditions

frost-free method

and different defrosting methods for air-source heat pump systems. In particular

the system and principles of the application of heat storage technology for heat pump defrosting are summarized. The applied phase change material (PCM) and structures of heat storage exchangers based on different studies are also presented in this work. Finally

future research directions for realizing frost-free or effective defrosting and continuous heating of air-source heat pump products are discussed. These include the development of reliable adsorption materials and regeneration methods to overcome problems posed by frost. Methods of high-voltage field and ultrasonic defrosting must be further considered to improve stability and economy. The application of PCM for defrosting has good market prospects

but the design of heat pump systems and the structure of PCM exchangers must be optimized

and PCM should be further studied.