Experimental study on the melted frost influence on the metal energy storage during an air source heat pump defrosting

When an air source heat pump works at reverse cycle defrosting mode, the metal temperature of its indoor coil decreases and that of outdoor coil increases. The energy storage of two coils’ metal at heating mode would influence the system defrosting efficiency. For a vertically installed outdoor coil having multiple circuits, meanwhile, the melted frost downwards flowing from upside circuit(s) also delay the defrosting process of downside circuit(s). To quantitatively evaluate the influence from two parameters, metal energy storage and melted frost, four experimental cases were designed and carried out in this study, based on a specially made three-circuit outdoor coil. Three circuits are working at the frosting mode, while two and three are used at the defrosting mode in four cases. As concluded, the melted frost influence on defrosting efficiency and metal energy storage effect would be increased from 4.87% to 10.45% and from 0.77% to 1.49%, respectively, after the outdoor coil changed from two- to three-working circuit. Outcomes of this study are meaningful for the structure optimization and energy saving of air source heat pump units at defrosting mode.