Unsteady characteristics of sleeping thermal comfort during defrosting of a T-ASHP system

Abstract

Defrosting is necessary for the normal operation of air source heat pump (ASHP) units to solve the frosting in the winter, which deteriorates the performance of ASHP. During this defrosting period, the ASHP cannot provide heat to the indoor space, leading to adverse thermal comfort, which may reduce the sleep quality. Hence, a task/ambient ASHP (T-ASHP) system was designed in this study, and the transient thermal environment was investigated through a computational fluid dynamic (CFD) method. The duration of t oz (average air temperature in the occupied zone) increase period was 4.33 times as long as t oz decrease period, although the duration of the phase 1 heating period was 1.9 times as that of defrosting period. During the defrosting procedure, the PMV value decreased to the lowest value of −0.83, and the period of feeling cool and slightly cool lasted for 45.4 min. The hand, foot and leg parts were much cooler than the whole body, and the non-uniformity of the PMV value on body parts was obviously enlarged during the defrosting period. Higher draught risk around 22% near the head and trunk regions was found during the defrosting period and was significantly reduced in the follow-up heating period. Practical application: The results from this study emphasize the sleeping thermal comfort during defrosting of air source heat pumps. The present study gave a map of transient thermal comfortable levels influenced by the operating of defrosting and re-heating. These results should serve as a recommendation to air source heat pump designers to determine suitable defrosting technology to maintain normal indoor thermal comfort.