Theoretical And Experimental Study On Optimal Discharge Pressure And Peak COP Of Transcritical CO₂ Heat Pump

CO₂as a refrigerant have come back into people’s vision and become a hotspot research in recent years because of its high-temperature water heating capacity,good low temperature performance and environmentally friendly.However,due to the transcritical cycle,the requirements for the design,manufacture and operation control of heat pump equipment are more stringent.So far,the commercialization of transcritical CO₂heat pumps has developed slowly in China.Therefore,improving performance of transcritical CO₂heat pump system is helpful to promote the development of CO₂heat pump,accelerate refrigerant replacement work in China and meet the needs of clean heating of Chinese people.In this paper,based on the detailed and comprehensive research,the influencing factors of transcritical CO₂heat pump were studied from two aspects of CO₂cycle parameters and operating conditions,the control method of discharge pressure was put forward,and the matters needing attention in strengthening equipment performance in the future were given.The main research contents are as follows:1.The theoretical analysis model of optimal discharge pressure was established,and the effects of superheat,IHX efficiency,isentropic efficiency,evaporation temperature and gas cooler outlet temperature on optimal discharge pressure were quantitatively studied by using multivariate ANOVA（analysis of variance）method.The results show that the gas cooler outlet temperature,evaporation temperature and isentropic efficiency are the main factors affecting the optimal discharge pressure and peak COP.2.The thermodynamic model of compressor and the distributed heat transfer model of gas cooler based on multi-objective optimization were presented.Combined with the optimal discharge pressure model,the self-optimization simulation calculation program of discharge pressure was proposed.The simulation results show that the optimal discharge pressure increases with the increase of compressor frequency,evaporation temperature and inlet water temperature,and decreases with the increase of superheat.The peak COP decreases with the increase of compressor frequency and inlet water temperature,increases first and then decreases with the increase of evaporation temperature,and increases with the increase of superheat.In practice,the heat transfer performance of the gas cooler should be improved to reduce the gas cooler outlet temperature.The reason why the superheat and evaporation temperature differ from the theoretical analysis model is that the superheat and evaporation temperature change the mass flow of CO₂during the operation of theequipment.3.The influence of compressor frequency,inlet water temperature and inlet water flow rate on the optimal discharge pressure and peak COP is experimentally studied,and the simulation conclusion is verified.According to the calculation model of the optimal discharge pressure,a method to determine the optimal discharge pressure based on the discharge pressure deviation was developed,and the accuracy under nine different working conditions were verified.It is found that the average error of the predicted value of the optimal discharge pressure is 0.56%,and the COP loss is 0.54%.