Thermodynamic analyses and performance improvement on a novel cascade-coupling-heating heat pump system for high efficiency hot water production

In the background of global warming and reducing carbon emission, air-source heat pump water heater is used as an energy efficient technology to improve the efficiency of building hot water production, which may account for 21–41% of building energy cost. However, there are still space for the energy efficiency improvement of heat pump water heater, as there is large temperature difference between water and refrigerant, which may limit the energy performance (especially at low ambient temperature) but receives little attention. Therefore, a novel cascade-coupling-heating heat pump system with less water-refrigerant temperature different is proposed in this paper, where part of the heat is first provided at the low-temperature stage, reducing the heat load at the high-temperature stage and the high-temperature stage compressor power, and increasing system energy efficiency. To study the system, a theoretical model was established and verified through experiments. Experimental and theoretical results show that: in the ambient temperature range of 243.15 K ∼ 323.15 K, the average and maximum improvement in the coefficient of performance of the new system reach 34.8% and 107.7%, respectively, comparing with cascade heat pump; and in the ambient temperature range of 248.15 K ∼ 323.15 K, when comparing with single-stage heat pump, the new system reveals average and maximum improvement in the coefficient of performance as high as 25.8% and 55.6%, respectively. This study provide a promising potential technology for efficiency water heating, and lay the basis for further study and application of the new system.