Performance and thermal charging/discharging features of a phase change material assisted heat pump system in heating mode

At low ambient temperature, air-source heat pump suffers from decrease in both heating capacity and coefficient of performance (COP), and increase in compressor's pressure ratio. A parallel triple-sleeve energy storage exchanger with phase change material (PCM) for storing thermal heat was designed to ensure the reliable operation of a heat pump under various weather conditions and enhance the system performance at low ambient temperature. The innovative device also includes a solar thermal collector loop for utilizing free solar energy. Thermal heat can be transferred into and stored by the PCM using water as the heat transfer medium. Controlled experiments were carried out to investigate the performance and the simultaneous thermal charging and discharging behavior of this enhanced heat pump system. Transient operating characteristics, including the temperatures, pressures and heat transfer rate, were analyzed. The COP value in an operating mode with a constant heat transfer fluid water temperature and flow rate increased until the system entered into the stable operating stage. The final COP can reach up to 3.9 when the three heat transfer mediums (water/PCM/refrigerant) achieved the steady-state. The experimental results show an interesting phenomenon that the heat transfer process between water and refrigerant with a PCM interlayer was a dynamic balance. The PCM temperature and the difference of the water temperature at the inlet and outlet of the evaporator regularly fluctuated around some balance points. In this study, the measured PCM temperatures in the dynamic steady-state were 12.8 and 14.2 C. The difference of the water temperature at the inlet and outlet of the evaporator balanced around 2.8 C. The findings in the research implied more study is needed to explore the PCM charging/discharging mechanisms and improve the operation of the PCM assisted thermal system.