Comparative study of air-source heat pump water heater systems using the instantaneous heating and cyclic heating modes

A comparative experiment prototype of an air-source heat pump water heater (ASHPWH) system was established for the comparative study between instantaneous and cyclic heating modes. The ASHPWH system worked at ambient temperatures of (19 ± 0.5 °C) in Hefei, China. From the initial temperature of 20 °C, 176 kg of water was heated to achieve final set temperatures of 30, 35, 40, 45, 50, and 55 °C in the two modes. After calculating the coefficient of performance (COP) of the ASHPWH system, it was found that the average COP of the instantaneous heating mode was 25.6, 19.0, 20.0, 21.1, 22.9, and 24.0% higher than that of the cyclic heating mode in the obtained final set temperatures of 30, 35, 40, 45, 50, and 55 °C of heated water, respectively. The results show that the instantaneous heating mode not only has a higher COP, but also a higher heating capacity, saving power consumption and decreasing heating time. The condensation pressure, the compression ratio, the maximum compressor discharge temperature and other important parameters of the instantaneous heating mode were also superior to those of the circulation heating mode. The exergy efficiency of the major components in the two modes was also analyzed, and according to the results, the instantaneous heating mode had higher exergy efficiency.