Through the promotion policy of energy saving and emission reduction in China

space heating has been gradually transformed from the traditional small coal-fired boiler to a clean heating mode. As a key technology of clean heating

the application and development of air-source heat pump is growing rapidly due to its energy conservation

high efficiency and environment protection. However

frost occurs on the surface of the finned-tube heat exchanger when the surface temperature is below both the dew point temperature of ambient air and the freezing point temperature. Frosting on the surface of the evaporator increases the heat transfer resistance of the heat exchanger and then decreases its heat transfer coefficient and the COP of the system so that the application and popularization of the air source heat pump system is seriously limited under the clean heating policy of substituting coal with electricity in China. Thus

it is of great significance that the frosting mechanism on a cold surface is investigated to explore the defrosting technology of high efficiency and long-term-effective anti-frosting technology for the development of air-source heat pump technology and the promotion of clean heating policy. In the present paper

firstly

the research status of frosting mechanism on the cold surface is briefly described

as based on the growth law of a frost layer and its physical properties. Various factors

including air temperature

air relative humidity

air flow rate

surface characteristic

surface temperature

structure of finned-tube heat exchanger

etc.

affecting the frosting process and the corresponding defrosting/anti-frosting method

such as reverse cycle

hot gas bypass

electric heater

low-frequency oscillation

hot water spray

ultrasonic vibration

electric field

magnetic field

changing the characteristics of inlet ambient air

changing surface temperature

surface treatment methods and so on are summarized. The research progresses of three kinds of common defrosting methods

including reverse cycle defrosting

hot gas bypass defrosting and electric heating defrosting are reviewed. The electric heating defrosting method can improve the indoor comfort during defrosting in comparison with the other two methods. The anti-frosting effects are analyzed based on changing air parameters

cold surface temperature

and heat exchanger structure. The research progress of anti-frosting technology is summarized for changing surface characteristics methods. Finally

the limitation of the thermal conductivity model of frost layers and the problems existing in defrosting/anti-frosting technology are pointed out. Based on the previous review and analysis

it is suggested that effects on defrosting technology of indoor comfort and energy consumption should be further studied; several measures should be taken to explore the anti-frosting technology

which can strongly inhibit different stages of frosting process

such as condensation of water vapor

condensate water freezing

frost layer melting and collapse. In addition

because of the limitation of the length of the article

research progress and existing gaps of other defrosting and anti-frosting techniques are not provided. This paper can be used for guiding the future design and system optimization for air source heat pump.