An experimental study on frosting and defrosting performances of a novel air source heat pump unit with a radiant-convective heating terminal

When the outdoor air temperature is low and relative humidity is high, frost will occur and accumulate on the outdoor coil of an ASHP unit, which may decrease the heating performances of the ASHP unit to a large degree. In addition, the defrosting performances of the commonly used defrosting method (reverse cycle defrosting) are poor due to the closed indoor air fan and insufficient energy stored in a conventional indoor coil. On the other hand, the conventional indoor coil based on forced convective heat transfer may cause strong draught sensation and dry eye problem and make users feel less comfortable than radiant heating terminals. Therefore, to improve the system defrosting performances and combine the merits of the conventional indoor coil and those of radiant heating terminals, a novel radiant-convective heating terminal was developed and integrated with an ASHP unit. The frosting and defrosting performances of the novel system were experimentally investigated, and the results showed that the average values of radiant panel surface temperature, outlet air temperature for the novel heating terminal, and system COP were 36.6 °C, 29.4 °C, and 2.78, respectively, during a 66-min frosting period (air temperature is 2.0 °C and relative humidity 84.0%). In addition, the ratio of the radiant heating capacity to natural convective heating capacity to forced convective heating capacity remained at around 1.5:1.0:7.4 during the whole frosting period. Furthermore, the experimental results demonstrated that the novel heating terminal could provide sufficient energy for the defrosting, and some energy could be provided for space heating through radiant and natural convective heat transfer during defrosting. The defrosting and resuming heating periods were 105 and 65 s, respectively.