An experimental study on the defrosting performances of an ASHP having a novel dual-fan outdoor coil under different levels of frosting evenness along airflow direction

•Defrosting performances of an ASHP at different frosting evenness were carried out.•Defrosting of an ASHP was always unsynchronized along refrigerant flow direction.•Frosting evenness improved from 0.8 to 0.05, frosting duration lengthened by 44.9%.•Positive impacts of even frosting overweighted its negative effects in defrosting.•The evener the frosting level of outdoor coil, the lower the defrosting efficiency. Uneven frosting along the airflow direction is a common phenomenon that can accelerate deteriorating the heating efficiency of ASHP units under frosting conditions. To effectively alleviate this uneven frosting phenomenon, an experimental ASHP unit having a novel dual-fan outdoor coil (ASHP / DFOC) was developed in a previous experimental study. The ability of the DFOC on achieving even frosting along the airflow direction, and the operating performances of the experimental ASHP/DFOC unit at different levels of even frosting evenness were experimentally and numerically investigated. However, the defrosting performances of the experimental ASHP/DFOC unit at different levels of frosting evenness remain to be evaluated, which was nonetheless essential for developing an on-demand defrosting initiation method for achieving higher defrosting efficiency. Therefore, the defrosting performances of the experimental ASHP/DFOC unit at different levels of even frosting were experimentally investigated in this paper. Five experimental cases with different levels of even frosting were designated, and the defrosting performances of the experimental ASHP/DFOC unit, including the defrosting time duration at different defrosting stages and the energy consumption during defrosting, were analyzed in great detail. The results demonstrated that during a single frosting-defrosting operating cycle, the defrosting always proceeded from one side to the other along the refrigerant flow direction. The evener the frosting level, the longer the frosting and defrosting duration, and the lower the defrosting efficiency. However, during multiple frosting-defrosting cycles, the evener the frosting level, the higher the comprehensive heating performances of the experimental ASHP/DFOC unit. The results from this study have provided a valuable basis for developing a more accurate defrosting initiation method with a higher defrosting efficiency for ASHP/DFOC units.