Selection Of Mixed Working Fluids And Experimental Research Of Cascade High-temperature Heat Pump Based On Minimum Entropy Increase Method

With the approaching of carbon peak and carbon neutrality period,many industrial users with high energy consumption pay more and more attention to the recovery and utilization of waste heat.It’s urgent to improve the comprehensive utilization rate of energy through practical waste heat recovery technology.Heat pump,as a technology to improve thermal quality and transfer heat,can stepwisely recover industrial waste heat,which can not only reduce the consumption of fossil fuels and greenhouse gas emissions,but also improve the energy utilization efficiency and reduce the cost in production process.It’s an important way to recover low-temperature waste heat.However,due to low efficiency under condition of temperature lift over 65℃,one-stage heat pump can not be promoted in industrial waste heat recovery.In order to expand the scope of reuse of low-temperature waste heat and improve the performance of cascade high-temperature heat pump,theoretical and experimental researchs on water source cascade high-temperature heat pump have been done in this study,which provides guidance for its promotion and application in industrial lowtemperature heat recovery.The research contents and conclusions are as follows:A thermodynamic cycle calculation model based on the performance of cascade hightemperature heat pump system was established.According to industrial heat temperature range,basic working fluid and additive components were selected as the high-temperature and low-temperature cycle mixed working fluids.The optimization method of mixed working fluid components based on minimum entropy increase method was proposed.The entropy increase models generated by temperature difference heat transfer in low-temperature evaporator and high-temperature condenser was established respectively.The relative additional entropy increase caused by heat transfer temperature difference in low-temperature evaporator and high-temperature condenser under various working conditions was theoretically analyzed,and the optimal composition ratio of non-azeotropic mixed working fluid under each working condition was determined.A experiment test bench based on the water source cascade high-temperature heat pump system was built.The experimental conditions were set and the high-temperature and lowtemperature cycle working fluids and experimental schemes were determined.When the heat source temperature was 20-50℃,the highest temperature hot water at 150℃ was produced,which proved the feasibility of cascade heat pump in the field of low-temperature industrial waste heat recovery.The experiments were carried out under the conditions of R134 a,R134a/R245fa（9:1,8:2,7:3 and 6:4）for the low-temperature cycle,R245 fa for the hightemperature cycle,and the experimental results of the maximum cycle temperature rise up to90℃ under heat source temperature of 20,30,40℃ were analyzed.The experiments were carried out under the conditions of R142 b for the low-temperature stage cycle,R365mfc/R245fa（9:1,8:2,7:3）for the high-temperature stage cycle,heat source temperature of 20,30,40,50℃,and the maximum cycle temperature rise was 100℃,and the experimental results were analyzed.The influence of non-azeotropic mixing working fluids on the performance of water source cascade high-temperature heat pump system under different working conditions was studied: COP decreased with the increase of heating temperature.The heating capacity increasing with heating temperature was related to the decrease ratio of latent heat of high-temperature cycle working fluids and the increase ratio of mass flow.The compressor’s input power increasing with heating temperature was related to the change ratio of gas transmission coefficient and working fluid density.The compressor exhaust pressure increased with the increase of heating temperature.The appropriate proportion of low-temperature and high-temperature mixed working fluids can improve system performance by reducing the exhaust pressure and input power of compressors.The cycle performance of selected mixed working fluid was studied by analyzing COP and heat production based on experimental data under setting conditions.The relative additional entropy increase obtained from experimental data according to its nonlinear temperature-enthalpy relationship shows how the variation of working fluid ingredients in non-azeotropic mixtures affects the temperature distribution of working medium in heat exchanger.Comparing the experimental and theoretical results of relative additional entropy increase,the applicability of selecting mixed working fluid based on minimum entropy increase method was verified.It will provide reference for the selection and evaluation of non-azeotropic mixed working fluids in cascade heat pump.