Heat transfer characteristics of an expanded graphite/paraffin PCM-heat exchanger used in an instantaneous heat pump water heater

•PCM heat exchanger served as condenser in instant HPWH.•Mold was firstly designed to improve thermal conductivity of PCM composite.•Temperature field dynamic distribution was recorded with thermal imager.•Dynamic expansion/contraction effect of PCM heat exchanger was studied.•A new parameter was defined to assess performance of PCM heat exchanger. This study proposed to experimentally investigate the feasibility of an expanded-graphite paraffin phase change material (PCM) heat exchanger operating as a condenser in an instant air source heat pump water heater (ASHPWH). To improve the thermal conductivity of the PCM, a mold was first designed to manufacture the PCM-heat exchanger, and then the temperature distribution and volume expansion of an expanded graphite paraffin heat exchanger (EGPHE) were investigated and tested under different inlet water flow rates. The experimental results showed that the volume expansion rate of a 16 L EGPHE was 6.25% at 76 °C. The hot water discharge differences between the EGPHE and a traditional water tank were compared in detail. At an inlet flow rate of 0.5 L/min, the hot water volume increment rate was 83.12% and 194% for supplied water temperatures of 45 °C and 40 °C, respectively. When the inlet flow rate increased, the hot water volume increment rate decreased. Thus, EGPHE applied in ASHPWH can significantly reduce traditional water tank volume. However, in the PCM heat charging/discharging process, nonuniform temperature distributions played an important role in the heat exchanger efficiency. During the heat discharging process, the exergy efficiency and the heat transfer rate of EGPHE were also analyzed using experimental data. The EGPHE efficiency could be improved by minimizing the destroyed exergy.