Research On CO₂/Low GWP Mixture Heat Pump Systems On Basis Of Pinch Point Temperature

Heat pump,an energy lifting device,plays a significant role in energy conservation and environmental protection,and CO₂ is concerned because of its environmentally-friendly nature.CO₂ heat pump is difficult to widely used because of it high operating pressure and low system performance.Based on the external working conditions and the pinch point temperature,this paper studied the CO₂/low GWP mixed working fluid heat pump and researched the mixed working fluid selection and system optimization so as to achieve to protect environmental,reduce system operation pressure and improve system performance.According to the working fluid screening criteria and purposes of this study,five candidate second component refrigerants,R41,R1234ze,R161,R152a and R290,were received.Taking CO₂/R161 and CO₂/R152a for example,the variation characteristics of the specific pressure,the dynamic viscosity,the thermal conductivity and other parameters of the CO₂ mixed refrigerant with temperature were analyzed and the thermodynamic model of the regenerative system was established.The effects of condensing pressure on the output parameters of basic heat pump systems with CO₂ and mixed-refrigerant were discussed respectively,and the changes of number and location of the pinch points in heat exchangers at different condensing pressures were analyzed and compared.The result showed that the there is an optimal condensing pressure in CO₂ and mixed refrigerant heat pump systems to make the COP of systems maximum.At the optimum condensing pressure,there are two pinch points in the condenser to achieve the best temperature matching,but there is only one narrow pinch points at other condensing pressures.With the increase of condensing pressure,there is only one narrow point of the evaporator of the CO₂ system,which is located at the inlet of working fluid.There is also one narrow point of the evaporator of the mixture system,which is located at the inlet of the working fluid firstly,but then becomes two,located in the two ports,and finally changes one,located in the outlet of working fluid.The influence of mixed components on the optimal performance of the system was also analyzed.The result revealed there is an optimal mass mixing ratio to makes the system COP maximum,the COP of CO₂/R161（10/90）system was 4.37,which is the largest among them,and the corresponding optimal condensing pressure was 2.25MPa.Compared to CO₂ system,the COP of CO₂/R161（10/90）was improved by 0.32 and the optimal condensing pressure was reduced by7.1MPa,so CO₂/R161（10/90）was the best mixed working fluid.The influence of pinch point temperature on CO₂ and CO₂/R161（10/90）systems was also studied.It is found that the maximum COP and the optimal condensing pressure decrease with the increase of the pinch point temperature of the evaporator.When the pinch point temperature of the condenser increases,the maximum COP of the system declines,while the optimal condensing pressure rises.Taking the basic system and the regenerative system of CO₂ and CO₂/R161（10/90）as the research objects,the influence of the condensing pressure on the system performance of the regenerative systems were analyzed in detail.The results showed that the regenerator can reduce the optimal condensing pressure and improve the system COP,and the effect of regenerators is different with different working fluids.There is a critical condensing pressure,9.33MPa for CO₂ and 2.24MPa for CO₂/R161（10/90）.Only at the condenser pressure lower than critical condensation pressure,can the regenerator improve the system COP.The influence of inlet temperature of hot source and outlet temperature of hot water on the system optimal output parameters were analyzed.The results show that the higher the inlet temperature of the heat source,the greater the COP and the optimal condensing pressure.When the inlet temperature of the heat source is higher than 22℃,the regenerator has no effect on the CO₂/R161（10/90）system.Besides,the higher the hot water outlet temperature,the smaller COP,and the higher optimal condensing pressure.Finally,based on the second law of thermodynamics,the total irreversible loss and the component irreversible loss of four systems at the optimal condensing pressure were analyzed.The results show that mixed working,compared with CO₂,can reduce the total irreversible loss and the regenerator can reduce throttle and compressor irreversible loss.Through the research and analysis of this paper,the conditions for obtaining the best performance of the system are obtained.In addition,the mixed working fluid and the regenerator have significant effect on reducing the operating pressure to improve the system performance.The research results of this paper can be used as the theoretical guidance of the practical application of the system and have some significance for the system engineering.