Study On Transcritical CO₂ Heat Pump System With Ejector In Off-design Conditions

Majorities of the synthetic refrigerants, such as CFCs, HCFCs, HFCs, are being or will be phased out due to their ozone depletion effect and impact on global warming. As one of the alternatives, for its environmental-friendliness and favorable heat transfer performance, the natural refrigerant CO2 has become a hot spot among global researches and maybe a common refrigerant in heat pump area. The transcritical CO2 heat pump system somehow overcomes the low performance of subcritical CO2 cycle. However, the large throttling loss limits the system to getting a higher coefficient of performance (COP). The ejector is considered one of the most promising item to recover some of this throttling loss in small-sized transcritical CO2 system owing to its low-cost. It was found in previous studies that the superiority of introducing an ejector in transcritical CO2 heat pump system varies in different operating conditions. The system with ejector even inferior to the conventional system in COP under some operating conditions, which means the two systems might have different applicability in off-design conditions. Focusing on these concerns, further theoretical and experimental investigations were carried out in this paper to investigate the applicability of transcritical CO2 heat pump systems in off-design conditions.The main researches are as follows:1) Impact of compressor rotation rate on refrigerant charge of the conventional transcritical CO2 heat pump systems (TCHS) and the transcritical CO2 heat pump systems with ejector (TCHSE) were experimentally investigated. TCHS showed a large variation of optimal charge amount under the changing compressor rotation rate whereas TCHSE did not shows much variation.2) In order to analyze the system applicability in off-design conditions, experiments were carried out on TCHS and TCHSE when both hot water outlet temperature and CO2 evaporating pressure were varied. TCHSE showed better applicability in stricter working conditions. However, the COP of TCHSE was lower than TCHS when hot water outlet temperature was relatively low or evaporating pressure was relatively high.3) Thermodynamic models of TCHS and TCHSE were built up to investigate the impact of CO2 gas cooler out let temperature on the optimal heat rejection pressure as well as the factors that led TCHSE had a lower COP than TCHS. Both low isentropic efficiency of ejector and liquid comes out from the gas outlet of separator have a negative effect on system performance. Whereas, only liquid CO2 comes out from the gas outlet of separator leads that THCSE inferior to TCHS in COP when they at the same high-side and low-side pressure.