Improved indoor thermal comfort during defrost with a novel reverse-cycle defrosting method for air source heat pumps

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. Frosting affects its operational performance and energy efficiency, and therefore periodic defrosting is necessary. Currently, the most widely used standard defrosting method for ASHP units is reverse-cycle defrost. During a standard reverse-cycle defrosting process, the indoor coil in an ASHP unit actually acts as an evaporator, therefore, no heating is provided and hence indoor air temperature in a heated space can drop. Furthermore, a longer period of time is needed before space heating can become available immediately after the completion of defrosting. Consequently, occupants' thermal comfort may be adversely affected. To improve the indoor thermal comfort for occupants during reverse-cycle defrosting, a novel thermal energy storage (TES) based reverse-cycle defrosting method has been developed and the improvement to occupants' thermal comfort experimentally evaluated and is reported in this paper. Comparative experiments using both the novel TES based reverse-cycle defrosting method and the standard reverse-cycle defrosting method were carried out. Experimental results and the evaluated indoor thermal comfort indexes clearly suggested that when compared to the use of standard reverse-cycle defrost, the use of the novel reverse-cycle defrosting method can help achieve improved indoor thermal comfort, with a shorter defrosting period and a higher indoor supply air temperature during defrosting.