Research On Characteristics Of R744-based Mixtures Used In Heat Pump System

Nowadays, HCFC22 is the mainly used refrigerant in China for heat pump systems, despite the fact it is harmful to the environment (ODP=0.05, GWP=1810). With the stricter environmental protection requirements imposed, alternatives, such as HFC 134a, HFC410A and HFC407C, have been gradually taking its place. These alternatives have the zero ODP; however, they still belong to high GWP gases, which leads to the never-ending researches at home and abroad for more environmentally friendly replacements. Therefore, the natural working fluid R744 (or CO2) has regained a new life in heat pump applications, and in fact it has already been commercialized in some developed countries, but still in research and development phase in mainland China. The reason is that there are still some problems with the currently used R744 heat pump, such as the high heat rejection pressure. In this research, in order to decrease the heat rejection pressure, and increase the system efficiency as well, R744 as the basic component is mixed with other working fluid in an appropriate amount.According to the literature on R744 mixtures, the current studies have been mainly aimed at the cascade or autocascade refrigeration system for low temperature and the cooling performance for air conditioning system. The investigations on the heating perfonnance of heat pump using R744 mixtures as refrigerants, especially for experimental studies, are still scarce and at the initial stage in the published literature. More theoretical and experimental researches on R744 mixtures should be carried out. Based on the researches carried out by our team on the heat pump system using single component R744 as a refrigerant, this project focuses on a second component working fluid selection, heat pump system characteristics using R744 mixtures. And this thesis focuses on the following elements:(1) For heat pump with specific temperature for heating, the second component fluid which is suitable to mix with R744 is selected according to the fluids’ environmental performance, safety performance, reduced heat rejection pressure and temperature glide properties. The suitable component candidates and the suitable cycles are given. Based on the pinch point of heat transfer, the numerical models of heat pump cycle using R744-based mixture are developed. For different R744 mixture candidates, the operation characteristics in subcritical cycle and transcritical cycle are calculated and discussed. Based on a comprehensive consideration of COPh, optimum heat rejection pressure, volumetric heating capacity, discharge temperature, flammability and explosive property, the binary mixture R744/R290 is determined as the best working fluid for specific heat pump application.(2) For R744/R290 with different mass fraction, it discusses respectively how the R744 varies with the mass fraction of R744 based mixture’s performance, safety performance, temperature glide property, physico-chemical properties and heat pump cycle performance. And the optimum mass fraction range of 80/20～100/0 for R744/R290 is obtained, and the influences of superheating degree and internal heat exchanger are analyzed to achieve a high COPh.(3) The experimental testrig is designed and set up for the transcritical cycle for heat pump system. The experimental researches with different R744 mass fraction are carried out, which conduct a study on the variations of heat pump performance, component’s mass fraction and working fluid charge. The optimum working fluid charges for different mass fraction are given, and the optimum mass fraction of R744/R290 is found to be 95/5, with which the optimum heat rejection pressure is decreased while COPh of the system is increased. The experimental results validated the R744/R290 natural mixture proposed in theory. The experimental results provide useful reference on the optimization and improvement of R744/R290 heat pump testrig.(4) For R744/R290 with optimum mass fraction of 95/5, the influences of operation parameters on the optimum heat rejection pressure are discussed by the theoretical simulation and experimental research. The thermodynamic model for the basic cycle is proposed to study the property which will influence on the optimum heat rejection pressure. A regression analysis to the simulation results is made and then the correlation of the optimal heat rejection pressures in terms of main influencing parameters is developed. Then through experimental researches, the working fluid temperature at the exit of gas cooler is validated to be the main impact factor for the optimum heat rejection pressure.