Study On Performance Of Transcritical CO₂ Heat Pump System And Optimal Discharge Pressure

Heat pump system has been widely used in air conditioning,heating,and refrigeration.It is of great significance to the development of heat pump system for the natural refrigerants,because the artificial refrigerants have serious effects on the global warming and ozone layer destruction.The natural refrigerants that can be used in heat pump systems include NH₃,C₃H₈,C₄H₁₀,and CO₂.Because the first three are flammable or toxic,CO₂ has become the most potential natural refrigerant.This study performed numerical simulation and experimental research on the performance of conventional transcritical CO₂ heat pump system and main components（compressor,gas cooler,internal heat exchanger,expansion valve,evaporator）.A new compression/injection transcritical CO₂ heat pump for simultaneous cooling and heating was designed through the above results,and numerical simulations and related experimental studies were performed.The main research contents and results are as follows:Experimental study of the effect of discharge pressure on conventional transcritical CO₂ heat pump system,and analyzed the related efficiency of compressor,gas cooler,internal heat exchanger,expansion valve,and system COP;Based on the experimental research results,the calculation methods of compressor isentropic efficiency,volumetric efficiency,and mechanical efficiency were revised,and the calculation method of the internal heat exchanger efficiency was proposed;A numerical simulation study of the conventional transcritical CO₂ heat pump system was carried out,a modified calculation model of the compressor,gas cooler,and internal heat exchange is proposed by comparing the simulation and experimental results,which improves the calculation accuracy of the system model;A dimensionless correlation for predicting the optimal discharge pressure was obtained by the Buckingham PI theorem,and the prediction results are in good agreement with the experimental results.Aiming at the problem of the large throttling loss in the conventional transcritical CO₂ heat pump system,a new compression/injection transcritical CO₂ heat pump for simultaneous cooling and heating which is add an ejector and low-temperature evaporator was designed.Based on the modified model of the conventional transcritical CO₂ heat pump system,a simulation study of the new system was conducted,and it was found that the new system could significantly improve the system COP.In addition,the dimensionless correlation for predicting the optimal discharge pressure proposed above are verified.Based on the numerical simulation,a compression/injection transcritical CO₂ heat pump experimental system for simultaneous cooling and heating was set up.Two different experimental studies were carried out,which were divided into the performance comparison between with and without ejector system and the study of the compression/injection system performance change under different compressor frequency and discharge pressure.In the experimental study of performance comparison between with and without ejector,it was found the ejector can effectively reduce the input work of compressor,and the heating COP of the new system is 14%higher than the conventional system.Those two results prove the performance advantages of the compression/injection transcritical CO₂ heat pump experimental system for simultaneous cooling and heating.In the experimental study of the performance change under different compressor frequency and discharge pressure,it was found that with the increase of the compressor frequency and the discharge pressure,the water outlet temperature of gas cooler can reach above 75℃,the water outlet temperature and the air outlet temperature of the low-and high-evaporators can be as low as 5℃and 20℃.Those results prove that the system can meet the needs of heating,cooling,and air conditioning.In addition,as the discharge pressure increases,there is a peak in the variation of the system COP.Comparing the prediction results of the dimensionless correlation for predicting the optimal discharge pressure proposed above,it proves that the correlation is applicable to different transcritical CO₂ cycle systems.