Analysis Of The Performance Of A Novel Trans-Critical CO₂ Ground-Water Air Conditioning System Using An Ejector

With climate problems like global warming and ozone holes getting more and more attention,CO₂,as a natural,environmental-friendly,easily accessible and safe working medium with good thermal properties,has been more and more widely used.However,the large throttling loss of trans-critical CO₂ cycle system has become a major factor restricting system performance.Previous studies have shown that the CO₂ ejector has simple structure,no moving parts,reliable performance and high economic efficiency,introducing it into a trans-critical CO₂ system can reduce throttling loss and lift system COP.In addition,in the case of sufficient water source,CO₂ ground-water heat pump is stable,widely applicable,and has the feasibility and necessity of development.Based on the above background,a novel trans-critical CO₂ ground-water heat pump air conditioning system with ejector is proposed,which can power the primary return air processing unit to meet the heating and cooling demand in winter and summer respectively.The numerical model for performance analysis of the system under steady working condition is established based on thermodynamic principles and the ejector constant-pressure mixing model.The simulation is carried out by calculating the model using MATLAB calling REFPROPm.The impacts of fresh air ratio,temperature,relative humidity and heat moisture ratio,ground water temperature,compressor discharge pressure on the COP,the ejector efficiency and the optimal compressor discharge pressure are analyzed,and the match degree between the system and the ejector under different condition is discussed.COP is high when the temperature and ratio of fresh air is high and the relative humidity is low in summer or when the temperature of fresh air is low and air ratio is high in winter.In the case of ground water temperature below a certain number,the lower the compressor discharge pressure,the better.But when it is higher than that number,optimal compressor discharge pressure exists.low ground-water temperature in summer leads to high COP,but it’s the opposite in winter.When the summer water temperature is 21℃,the system COP reaches its peak at 4.36 at the optimal discharge pressure of 8.4MPa,and could climb to 4.32 in winter under the working condition of 10℃water temperature,-8℃fresh air temperature and 7.8 MPa discharge pressure.The compressor optimal discharge pressure fluctuates between 7.4MPa and 8.4MPa and increases with the rise of summer ground water temperature while fresh air ratio,temperature and relative humidity have a slight and large effect in summer and winter respectively.When there is preheating,low fresh air ratio together with high fresh air temperature and low ground water temperature results in high optimal discharge pressure,otherwise,the pressure will be low.When there is no preheating in winter,high fresh air ratio and low fresh air temperature lead to low optimal discharge pressure,while the influence of ground water temperature isn’t very obvious on this condition.The introduction of constant gas cooler can improve system performance in both seasons.When the supply air temperature is higher,the fresh air ratio,temperature and relative humidity and the heat moisture ratio is lower in summer,the higher performance improvement can be achieved through recovering cooling heat,which is between 10%-25%.In winter,COP without preheating is not as high as that when preheater works.The ejector efficiency is mainly affected by compressor discharge pressure and ground-water temperature.The higher the underground water temperature and the discharge pressure are,the higher the efficiency will be.The ejector efficiency increases slightly with the increase of fresh air ratio and temperature,and hardly changes with the fluctuations of fresh air relative humidity in summer and is not affected by the fresh air temperature and ratio in winter.The high efficiency operating point of the injector deviated from that of the system,especially in the summer cooling condition,but the two points are closer when the ground water temperature is higher in both seasons.