Termination Control Temperature Study for an Air Source Heat Pump Unit during Its Reverse Cycle Defrosting

For an air source heat pump (ASHP) unit with a vertically installed multi-circuit outdoor coil, a reverse cycle defrosting (RCD) operation is always terminated when the tube surface temperature at the exit of the lowest circuit reaches a pre-set value. It is obviously that when the pre-set temperature is higher, the defrosting duration would be prolonged. Not only more energy would be consumed on heating cold ambient air, but also the occupants' thermal comfort adversely affected. However, if the pre-set temperature is lower, more residual water would be left on the downside surface of fin in an outdoor coil, which degrades the system performance during the next frosting/defrosting cycle. In order to find a suitable DTT or its range, in this paper, an experimental investigation on RCD operation for an ASHP unit with a multi-circuit outdoor coil was conducted and reported. As concluded, DTT is suitable at the range of 20-25 °C, around 22 °C.