Performance Analysis And Optimization Design Of Transcritical CO₂ Heat Pump System

Under the background of double carbon,energy saving and emission reduction using green and environmental protection of new energy has become the trend of today’s social development.Energy saving and emission reduction using green and environment-friendly new kinds of energy sources has become the development trend of today’s social under the background of double carbon.Refrigerants in the field of air source heat pump are constantly updated based on the pressure of environmental protection.Natural working medium such as CO₂will become the main source of the future market.Compared with conventional air source heat pump,CO₂ heat pump can operate stably and reliably in low temperature environment,and is more suitable for heating in cold areas.However,the existing CO₂ heat pump equipment has the problems of low Coefficient of Performance and high emission pressure.Taking this as the entry point,this paper studies the performance,optimal discharge pressure and charging amount of CO₂ heat pump under different working conditions by the method of combining simulation and experiment,and optimizes the design of heat pump evaporator through professional heat exchanger design simulation software.The main research contents are as follows:（1）The relationship between the specific pressure heat capacity and density of the natural working medium CO₂ in the critical zone and the temperature change under different exhaust pressures is summarized,and the advantages of CO₂ as a kind of refrigerant such as high heat transfer efficiency,large mass flow rate,reducing the size of heat exchanger and saving equipment cost are revealed.The design and selection of the main components of the transcritical CO₂ heat pump are carried out,and the specification parameters of the components are determined.The mathematical model of each component is established by thermodynamic formula.（2）The mathematical model of the transcritical CO₂ heat pump are established and the trans-critical CO₂ heat pump experiment platform are builded.The influences of inlet and outlet water temperature and ambient temperature on the heat pump are studied and the results of the simulation value with the test value are compared.The results show that the increase of inlet and outlet water temperature leads to the decrease of heat production and COP,and the ambient temperature is positively correlated with heat production and COP.The increase of outlet water temperature increases the test and simulation values of exhaust temperature by 7.5 and 9.6℃,respectively,and the difference of exhaust temperature between them is about 5℃.Inspiratory superheat is negatively correlated with ambient temperature.When the ambient temperature increases from 5℃to 35℃,inspiratory superheat decreases by 63.4%,57.3%and 54.6%respectively under the effluent condition of 50,60 and 70℃.The mass flow rate of refrigerant increases significantly with the increase of ambient temperature,with an increase of more than 80%.（3）The influence of exhaust pressure on system performance is studied by simulation,and the optimal exhaust pressure of the system is analyzed.The results show that the heat production of the system decreases with the increase of outlet temperature.Taking 10.0 MPa as an example,the heat production at 50℃is 25%higher than that at70℃.Both heat production and mass flow rate decrease with the increase of exhaust pressure,while inspiratory superheat and COP increase first and then decrease,that is,there is a maximum value.The maximum superheat at the three outlet temperatures is17.4,18.9 and 20.2℃respectively,and the corresponding discharge pressure is 8.5,9.0and 10.0 MPa.Inlet temperature and ambient temperature are two important factors affecting the optimal exhaust pressure.Under the same inlet temperature,COP increases first and then decreases,and the optimal exhaust pressure is positively correlated with inlet temperature.The optimal exhaust pressure corresponding to 10,15,20,25 and 30℃is 10.0,10.5,10.8,11.0,11.3 MPa respectively.Through the simulation at-20～30℃,it is found that the optimal exhaust pressure increases gradually with the increase of ambient temperature.Combining these two important factors,this paper simulates the optimal exhaust pressure under different working conditions through a large number of data,and fits the corresponding solution formula,which provides a certain reference basis for the design and commissioning of CO₂ heat pump,and uses the CO₂ heat pump experimental platform built to test the maximum performance coefficient of the heat pump system when the CO₂ charging amount is 8 kg.（4）Coildesigner software was used to optimize the heat pump evaporator fin spacing,tube outer diameter and transverse tube spacing.The results showed that when the fin spacing increased,the evaporator heat transfer decreased.When the wind speed was 2 m/s,the fin spacing increased from 1.6 mm to 2.4 mm,and the heat transfer decreased by 13%.Heat transfer is not affected by fin spacing,and the decrease of fin spacing will lead to the increase of refrigerant side pressure drop,but the increase is small,especially when the wind speed is 1 m/s,1.2 m/s,2.2 m/s,the pressure drop basically remains unchanged;When the pipe diameter increases,the evaporator heat transfer decreases,but the range is not large,while the refrigerant side pressure drop drops sharply with the increase of the pipe diameter.When the wind speed rises from 1m/s to 2m/s,the refrigerant side pressure drop increases by 27.63 k Pa,18.59 k Pa,13.57 k Pa,9.82 k Pa,respectively;When the transverse tube spacing increases,the heat transfer of evaporator increases and the side pressure drop of refrigerant decreases slightly.Aiming at defrosting time of evaporator,this paper set defrosting time as 25min according to the degree of finning.