A new defrosting model for microchannel heat exchanger heat pump system considering the effects of drainage and water retention

Abstract

Microchannel heat exchangers (MCHX) have been widely used in heat pump systems due to their advantages of higher efficiency and lower refrigerant charge. However, when the MCHX heat pump system works under defrosting conditions, there are problems of poor defrosting performance and long defrosting time. To study the transient characteristics of the MCHX heat pump system during defrosting, a new defrosting model was developed. The defrosting process was divided into five stages based on the variation characteristics of the frost thickness and the water film. The drainage stage was first proposed in the defrosting model to consider the influence of melted frost. In addition, the impact of retained water was also assessed in the dry heating stage. The deviation of the model and experimental data is within the acceptable range, and the transient characteristics can be accurately predicted. The visualization of the frost melt showed that defrosting was an uneven process from the top down, and the frost layer was almost completely melted at 250 s. In this study, indoor air supplied the most energy at 68.8 %. Furthermore, melting frost consumed the largest amount of energy at 33.7 %, and heating ambient air consumed the second largest amount of energy at 29.1 %. Metal energy storage (MES) had a negative impact of −7.5 % on defrosting efficiency due to a large amount of energy required to heat MCHX. Moreover, the negative influence of frost melt water and retained water on defrosting efficiency was −9.5 % and −1.9 %, respectively. It can be concluded that the key measure to improve the defrosting performance of MCHX is to reduce the effect of melted frost. This work can provide helpful insights into the defrosting simulation model development of MCHX and guide defrosting strategy optimization.