Experimental investigation on an air source heat pump unit with a three-circuit outdoor coil for its reverse cycle defrosting termination temperature

Air source heat pump units could efficiently recover low grade waste heat from ambient air for indoor air heating or hot water supplying, which makes them widely applied in recent decades. For a vertically installed multi-circuit outdoor coil, a reverse cycle defrosting operation is always used to solve its frosting problem at high humidity and cold climate. Reverse cycle defrosting operation is terminated when the tube surface temperature at exit of the lowermost circuit reaching a pre-set value. It is obviously that when the pre-set temperature is higher or lower, the defrosting duration would be prolonged or more residual water left, respectively. Both of them result in potential energy waste for an air source heat pump unit, or even adversely degrade the indoor thermal comfort. However, as reported, a wide range of 10–35 °C was used as the pre-set defrosting termination temperature, without a fixed value or range given. To save energy for an air source heat pump unit, in this paper, an experimental methodology was firstly presented. Then, an air source heat pump unit with three-circuit outdoor coil was specially selected, and experimental investigation conducted. Finally, the defrosting termination temperature was concluded suitable at 20–25 °C, around 22 °C for this study. This methodology makes contributions to the control strategy optimization and energy saving for air source heat pump units.