Heating and energy storage characteristics of multi-split air source heat pump based on energy storage defrosting

In recent decades, multi-split air source heat pump (M-ASHP) unit has been widely used for space heating. Similar to the split-ASHP unit, frost would accumulate on its outdoor coil surface under frosting condition, which significantly deteriorates its heating performance. In the previous study, a novel reverse-cycle defrosting method based on phase change energy storage was developed and investigated, and the study demonstrated that the novel defrosting method could significantly improve defrosting performance. To provide guidelines for designing the M-ASHP units with phase change energy storage, it is necessary to investigate the characteristics of heating and energy storage for M-ASHP units, which is presented in this paper. Experimental results showed that there was no frost on outdoor coil when only the indoor unit with heating capacity of 7.1 kW was turned on under frosting condition. In addition, the energy storage period increased as the number of operating indoor unit increased, namely the load rate. Furthermore, the optimal starting time of the energy storage operation was around 10 min after the start-up of M-ASHP unit. Even though, compared with the conventional heating mode with three turned-on indoor units, the energy storage mode shortened the heating period by 8 min and decreased the average heating capacity and COP by 4.4% and 1.9%, respectively. The above negative impacts of energy storage operation on the heating performance of the M-ASHP unit could be neglected by considering its positive impacts on defrosting performance, for a periodic heating-defrosting condition.