An experimental study on defrosting performance of an air source heat pump unit with a multi-circuit outdoor coil at different frosting evenness values

•A special experimental rig was built and its details are reported.•Defrosting at different FEVs (82.6%, 90.6% and 96.6%) was comparatively studied.•Uneven frosting accumulation has negative effects on defrosting performance.•Defrosting performance improved 6.8% when FEV increased from 82.6% to 96.6%.

For an air source heat pump (ASHP) unit with a vertically installed multi-circuit outdoor coil, it is rarely possible to make the frost accumulations on each circuit's surface equal, leading to different frosting evenness values (FEVs) as a reverse cycle defrosting operation starts. On the other hand, uneven defrosting phenomenon was found and reported as many methods were developed to improve defrosting performance of ASHP systems, which may be attributable to the reverse cycle defrosting starting at an uneven frosting. Understanding of defrosting performance of an ASHP unit with a multi-circuit outdoor coil at different FEVs is of importance for ASHP units' application, but studies are scarce in the open literatures. In this paper, we report an experimental study on defrosting performance when frost accumulated on the surface of outdoor coil at different FEVs. This paper presents details of an experimental ASHP unit and the experimental conditions, followed by results, and conclusions. Finally, an increase of 6.8% in defrosting efficiency was confirmed when the FEV changed from 82.6% to 96.6%.