Improving reverse cycle defrosting performance of air source heat pumps using thermal storage-based refrigerant sub-cooling energy

When an air source heat pump (ASHP) unit is used for space heating at a low ambient temperature in winter, frost may be formed on its outdoor coil surface. Over time, frost accumulation on coil surface may become sufficient to restrict air passage through the coil and increase the heat transfer resistance between ambient air and coil surface, leading to performance degradation for the outdoor coil, or even the shutdown of the ASHP unit. Therefore, periodic defrosting is necessary. Currently, the most widely used standard defrosting method for an ASHP unit is reverse cycle defrosting. During a standard reverse cycle defrosting (SRCD) operation, the indoor coil in an ASHP unit actually acts as an evaporator. Because the indoor fan is usually turned off during defrosting, there is an insignificant amount of energy from indoor coil, leading to a number of associated operational problems such as a longer defrosting duration and the risk of having a lower air temperature inside a heated space during defrosting, etc. To solve the fundamental problem of insufficient heat available during defrosting while ensuring the efficient and safe system operation, a novel reverse cycle defrosting (NRCD) method which is thermal energy storage-based using sub-cooling energy of refrigerant for ASHPs has been developed. Comparative experiments using both the SRCD method and the NRCD method were carried out on an experimental ASHP unit with a nominal 2.5kW heating capacity. Experimental results clearly suggested that when using the NRCD method, the discharge and suction pressures were increased by 0.56 and 0.12 MPa, respectively. Furthermore, the defrosting duration and heating-resumption duration were shortened by 30.8% and 25.0%, respectively, and the defrosting energy consumption was reduced by 13.9%, compared to those when using the SRCD method. Practical application: ASHPs have been widely used worldwide due to their significant energy-saving potentials. However, frost can be formed on the outdoor coil surface when an ASHP unit operates for space heating. Frost accumulation on coil surface reduces the coil's efficiency, or causes even the shutdown of the ASHP unit. Reverse cycle defrosting is the most widely used standard defrosting method. This paper presents the reverse cycle defrosting performance for an ASHP unit with a PCM-HE acting as both a sub-cooler during heating operation and a heat source during defrosting operation, which could help to ensure the safe operation and achieve a higher defrosting efficiency of the ASHP unit.