Continuous heating of an air-source heat pump during defrosting and improvement of energy efficiency

Abstract

During winter operation, an air-source heat pump extracts heat from the cold outside air and releases the heat into the living space. At certain outside air conditions, when it operates in heating mode, frost can form on the air-cooled heat exchanger, decreasing the heating performance. Conventionally, reverse-cycle defrosting (RCD) has been the common method of frost removal. But this method requires the interruption of heating during defrosting, as well as a period of time to reheat the cooled pipes of the indoor units after defrosting. In this study, a new technology called continuous heating was developed, which utilize only a hot gas bypass valve to remove the frost from the outdoor heat exchanger and thus enabling the supply of hot air to indoors without any interruption. For this, a new high temperature and low pressure gas bypass method was designed, which is differentiated from the common high pressure hot gas bypass methods by its use of low pressure. Various refrigerant mass flow distributions were examined, and the most effective defrosting mass flow was 50% in this case. Heating capacity was increased by 17% because of continuous heating, and the cumulated energy efficiency was increased by 8% compared to the traditional reverse cycle defrosting over 4h including two defrost operations. Also, cumulated energy efficiency was increased by 27% compared to electronic heaters that supply the same heating capacity during defrosting. By this new technology, it has been proved that continuous heating and energy savings could be achieved without adopting expensive technologies.