Abstract
The characteristics of the transcritical CO2 heat pump water heater (HPWH) system are; a lower inlet hot water temperature (Ti-hw) (sometimes this is lower than the water source temperature), and an outlet gas cooler temperature (To-gc) which is affected by the Ti-hw and often lower than the critical temperature. In order to study the effects of the internal heat exchanger (IHX) on the operational performance of the transcritical CO2 HPWH when To-gc is low, a transcritical CO2 water source HPWH experiment platform is established to conduct experimental research and comparative analysis on the operational performance of the transcritical CO2 water source HPWH, with or without IHX. It is found that, if only the coefficient of performance (COP) and heating at the optimal exhaust pressure of the transcritical CO2 water source HPWH were considered, COP and the heating of the non-IHX system would be slightly higher than those of the IHX system at the lower hot water flow and water source temperature, and this increase was not obvious. At the higher hot water flow rate and water source temperature, COP and the heating of the non-IHX system were also higher than those of the IHX system, and the increase was obvious. The experiment results showed that, near the optimal exhaust pressure, the variation range of COP and heating of the IHX system is relatively small, and the system has a relatively high stability.
Highlights
Traditional refrigerants, such as CFC and HCFC, have environmental problems such as destroying the ozone layer and causing the greenhouse effect
When the exhaust pressure of the basic cycle is higher than the optimal exhaust pressure, the evaporation temperature decreases obviously and is lower than that of the internal heat exchanger (IHX) cycle
A decrease in the evaporation temperature leads to a decrease in the coefficient of performance (COP), so the COP of the basic cycle starts to be lower than that of the IHX cycle as the exhaust pressure increases
Summary
Traditional refrigerants, such as CFC and HCFC, have environmental problems such as destroying the ozone layer and causing the greenhouse effect. Boewe et al [16] studied the influence of the IHX on the system performance for automotive air conditioning, and found that the IHX could improve the cooling COP by up to 25%, where the air inlet temperature on the evaporator side was 26.7 ◦ C, the air relative humidity was 40%, and the flow rate was 7.08 m3 ·min−1. Chen et al [22] established the theoretical model, and they studied the influence of IHX on the system performance They pointed out that IHX could improve the COP, and the higher the ambient air temperature, the greater the improvement effect. Zhang et al [24] theoretically studied the influence of IHX and found that when the exhaust pressure was lower than the turning point pressure and the outlet gas cooler temperature (To-gc ) was higher than the turning point temperature, the COP could be improved by using IHX.
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