Characteristic Study And Performance Optimization Based On CO₂ And CO₂/R170 Azeotropic Refrigerant Transcritical Cycle

Nowadays,the greenhouse effect and energy shortage have become increasingly concerned issues.CO₂,as a natural refrigerant with zero Ozone Depletion Potential（ODP）and low Global Warming Potential（GWP）,has a very broad application prospect in the refrigeration and heat pump industry.CO₂ is that its lower critical temperature（31.1℃）leads easy to realize transcritical cycle.CO₂ refrigerant has great application advantages in the heat pump water heater,i.e.,it can not only improve the maximum water outlet temperature,but also increase the output of hot water.However,compared with traditional refrigeration systems,there are still disadvantages in the CO₂system,such as high operating pressure,discharge temperature too high and relatively low energy efficiency.In order to solve above problems,in this work,refrigerant charge,mixture refrigerant replacement pure CO₂and changing circulation mode are used to detailed study on the performance of small CO₂ transcritical cycle to seek the feasible scheme and theoretical basis for enhancing the performance of CO₂system.The details are as follows.Firstly,refrigerant charges affect the performance of small CO₂ system,and the system exists the optimal refrigerant charge.For the sake of a precise theoretical calculation method,in the work,small CO₂ water source heat pump water heater system were researched.Under the optimal operating high pressure,the experimental data method,the operating method and the void fraction method（e.g.,homogeneous model,Smith model,Zivi model,L-M model,Tandon model,Yashar model,Premoli model,Harms model,Barozcy model,T-B model and Hughmark model）were used to theoretically calculate the optimal refrigerant charge of the system,respectively.Based on existing experimental rig,related experiment for the system were conducted.The research results show that the void fraction method is more suitable for transcritical cycle.Theoretical calculation results of the various void fraction models differ greatly.The deviation of other models except T-B model is within 10%,and the deviation of Zivi model and Premoli model is the smallest,only 0.9%.Secondly,the idea of adding environment-friendly refrigerant into CO₂ to form mixture refrigerant was proposed to improve the performance pure CO₂ system.Due to the composition shift of zeotropic refrigerants,which causes the operating instability of system,the thought of construct based on the CO₂ components of azeotropic refrigerants was originally presented in the paper.Through theoretical analysis,R170（ethane）,an environment-friendly refrigerant with critical parameters with CO₂,was optimum selected at last.The temperature slip of CO₂/R170 mixture under different mass ratio conditions was calculated in detail by REFPROP,and it was concluded that when the mass ratio of CO₂/R170 mixture was 0.78 to 0.22,it was azeotropic mixture.CO₂/R170 azeotropic mixture refrigerant takes the unique advantages of their respective refrigerants.In other words,flame retardant of CO₂inhibits the flammability of R170,and the superior thermodynamic performance of R170 raises the cycle efficiency of pure CO₂ system.In the work,the air-source heat pump water heater system was studied.Under the optimal high pressure,based on the first and second law of thermodynamics,the energy thermodynamic model and exergy thermodynamic model were established to simulation study the performance of the air-source heat pump water heater system with CO₂/R170 azeotropic mixture refrigerant.Meanwhile,compared with the R134a system under the same working condition,the study results show that the CO₂/R170 system has higher coefficient of performance of heating,similar heating capacity per unit,lower compressor power consumption,compression ratio and discharge temperature.In addition,exergy efficiency is improved by 31.3%.Therefore,CO₂/R170 azeotropic mixture refrigerant has the potential to substitute for R134a in the long term.Finally,it is a good idea to promote the performance of the system by changing the circulation mode.Based on CO₂ and CO₂/R170 azeotropic mixture refrigerant,energy thermodynamics models of three cycles（e.g.,single-stage compression transcritical cycle with a capillary,single-stage compression transcritical cycle with an expander and two-stage compression transcritical cycle with an expander）were established,and their performances were compared and studied.The research results show that the CO₂/R170 azeotropic mixture refrigerant can solve the lack of discharge temperature overheating in CO₂ transcritical cycle.Furthermore,among the three CO₂/R170 cycles,two-stage compression transcritical cycle with an expander has the maximum COP,the lowest discharge temperature,the maximum refrigerating capacity per unit and the minimum compressor power consumption per unit.