A sustainable defrosting technology for air source heat pump systems

Air source heat pump (ASHP), as a sustainable energy-saving technology, has been widely utilized all over the world. Currently, it has been demonstrated that the thermal energy storage defrosting (TESD) method can significantly improve the defrosting energy efficiency. However, the energy stored by the energy storage component usually occupies a part of the system heating capacity, which may have some negative impacts on the heating performance. Therefore a sustainable defrosting technology based on the TESD method is proposed, which has few negative impacts on the heating performances of an ASHP under the normal heating mode. The binary mixture of 65% capric acid and 35% lauric acid with a phase change temperature of 19.0°C is selected as the phase change material for the sustainable defrosting technology. After 4000 thermal cycles, the phase change temperature and latent heat decrease by 0.8°C and 7.9%, respectively. Compared to the traditional reverse cycle defrosting (RCD) mode, there are no predefrosting and postdefrosting stages in the sustainable defrosting modes because they do not need the reversing operation of four-way valve and shutdown. The total defrosting times of sustainable defrosting Modes 1-3 are shortened by 73.7%, 75.8%, and 76.8% when compared to the traditional RCD mode. For the traditional RCD mode and sustainable defrosting Mode 1, there is no space heating operation during the defrosting process. By contrast, the total space heating energy amounts in sustainable defrosting Modes 2 and 3 are 111.4 and 336.6kJ, respectively. In general, the sustainable defrosting Mode 2 shows the best defrosting performance because it takes into account both energy consumption and indoor thermal comfort. The results of this chapter may help to better understand the proposed sustainable defrosting ASHP system.