An experimental study on defrosting performance for an air source heat pump unit at different frosting evenness values with melted frost local drainage

A previous experimental study reported that the defrosting performance could be increased when the defrosting process started at a higher frosting evenness value (FEV) for an air source heat pump (ASHP) unit with a vertically installed multi-circuit outdoor coil. In that study, to calculate the FEVs and thus fix the opening degrees of stop valves, melted frost of each circuit was collected by using water collecting trays installed between circuits. Thereafter, these trays were taken away to model a regular multi-circuit outdoor coil. However, during defrosting, the downward flowing melted frost from up circuit(s) along the surface of coil due to gravity would adversely affect the defrosting performance of down circuit(s). Different negative effects of downward flowing melted frost would make the effects of FEV on defrosting performance hard to qualitatively and quantitatively confirmed. Therefore, in this study, an experimental study on defrosting performance for an ASHP unit at different FEVs with melted frost local drainage was carried out. Finally, experimental results indicated that the defrosting duration could be shortened by about 11.2% and defrosting efficiency increased by about 5.7% as the FEV increased from 79.4% to 96.6%.