Theoretical And Experimental Research On Heat Pump System Using CO₂Binary Mixtures Refrigerant

In this dissertation, we focus on the problem of binary mixtures refrigerant thatcomposes by CO2(R744) and "Zero-ODP" and "Low-GWP" artificial refrigerant. Themain purpose of this project is to avoid single refrigerant weaknesses, and to playtheir respective advantages of refrigerant, improving system efficiency, reducingexhaust pressure, decreasing the refrigerant charge, and reducing flammability.Adopting the theoretical analysis and experimental research combined method, thebinary mixtures refrigerant’s physical properties, combustion characteristics, additiveseffects on transcritical CO2water-water heat pump system and performance of binarymixtures refrigerant used in water-water heat pump system have been studied.The physical properties of R744/R32, R744/R41and R744/R161are analyzed.The physical properties calculated by NIST are compared with experimental data.Based on experimental data, the physical properties of R744/R32are optimized bymeans of Helmholtz equation. The maximum error of the optimization resultsdecreases from6%to3.4%.The calculation precision is significantly improved.The performances of thermodynamic cycles, including R744/R32, R744/R41,R744/R161, R744/R152a, R744/R290and R744/R1270, are analyzed. When the massfraction of CO2is higher than0.95, and the exothermic process is above the criticalpoint, therefore, the system operates with transcritical cycle. When the mass fractionof CO2is less than0.95, the system cycle convert to subcritical cycle fromtranscritical cycle. For pure CO2and binary mixtures refrigerant used in water-waterheat pump system, there is an optimal exhaust pressure and corresponding maximumCOP. When the mass fraction of CO2is less than0.85, R744/R32binary mixturesrefrigerant has the maximum system efficiency. When the mass fraction of CO2ishigher than0.85, R744/R161operates the maximum system efficiency.The experimental study of low flammability limit of R744/R32、R744/R41andR744/R161is conduced. The main purpose of this experiment is to study the effect ofinert gas on low flammability limit. Experimental results show that lowerflammability limit increase gradually with the increase of CO2mass fraction.Compared with pure R32, R41and R161, when the CO2volume ratio of binarymixtures refrigerant is about50%, lower flammability limit are increased by0.5%,0.4%and0.45%respectively.The experimental study of effect of R32, R41, and R161additives on thetranscritical CO2water-water heat pump, R32additives effect on expender, andperformance of water-water heat pump using binary mixtures refrigerant R744/R32,R744/R41and R744/R161are conducted. Experimental results show that COPh and COPc are improved obviously in some conditions when the system adds R32, R41and R161additives. The scale of increase rate of three additives is R32>R161>R41.The R744/R32and R744/R41binary mixtures refrigerant is recommended.Under heating condition, when COPh reaches the maximum value, the mass fractionof R32and R41are50%and40%, respectively. Compared with pure CO2, COPhincreases20.4%and11.9%. Under cooling condition, when COPc reaches themaximum value, the mass fraction of R32and R41are40%and40%respectively.Compared with pure CO2, COPc increases20.8%and18.5%. The discharge pressureof R744/R32and R744/R41binary mixtures refrigerant decreased with the increasingof mass fraction of R32and R41. Compared with the pure CO2, the maximumreduction rate of discharge pressure of R744/R32binary mixtures refrigerant is37.9%,and the maximum reduction rate of discharge pressure of R744/R41is15%. Based onthe experimental results, the slide temperature of R744/R161binary mixturesrefrigerant is too high, and the heating capacity and cooling capacity reduce obviously.Therefore, it is not recommend using the R744/R161binary mixtures refrigerant.