| The conventional Organic Rankine Cycle(ORC),with pure working fluid as a circulating fluid,although it has several advantages of energy saving,low carbon,environmental protection,but it has disadvantages of low cycle efficiency and low economic value.Mixed Fluid has the potential to improve the efficiency of ORC heat engine.Therefore,this paper has presented simulation and effect of Mixed Fluid on the efficiency of ORC heat engine.In this paper,the ORC heat engine of 6kW power generation is considered as an object,the cold source temperature is 25 ?,heat source temperature varying from 70 ? to 140 ?and the efficiency of heat engine is studied.The main research work includes the following:1)Selection of working fluid:four pure refrigerants R123,R142b,R600a,R245fa and the mixture of two groups R123/R142b and R600a/R245fa were considered as the research object.2)Based on the Redlich-Kwong-Soave equation of state and thermodynamic general relation,a thermodynamic model was established by using Visual Basic language for the selected pure and mixed working fluid(using molar concentration).3)The mathematical modeling of the ORC system is composed of evaporator,expander,condenser,working fluid pump,and ORC system;4)Simulation of ORC system using pure working fluid;5)Simulation of ORC system using mixed working fluid;6)Analyze and comparison of efficiency characteristics of the mixed and pure working medium in ORC system;The results of this paper are as follows:(1)Rational use of mixed refrigerant can improve the efficiency of the cycle:with the heat source range of 70 0C?140 ?,when the concentration ratio of the mixed refrigerant R600a/R245fa is 0.6/0.4,then the maximum efficiency of cycle is 16.2%higher than that of R600a,which is higher than the maximum value of R245fa by 29.2%,with an average increase of 22.7%for both pure working fluids.(2)The capability of improving the cycle efficiency of different mixed working fluid is different.When the concentration ratio of mixed working fluid R123/R142b is 0.5/0.5,the maximum cycle efficiency of the system is 22.7%which is higher than the maximum value of R123,and higher than the maximum value of R142b by 5.3%,with an average increase of 14.0%for both pure working fluids.The capability of cycle efficiency is improved by the influence of slip temperature.The slip temperature of R123/R142b(0.5/0.5)is 6.7K which is lower than the slip temperature of R600a/R245fa(0.6/0.4)by 7.8K,so the cycle efficiency improvement potential is lower than that of R600a/R245fa(0.6/0.4).(3)There exist an optimum value of heat source temperature for each pure working fluid due to which the cycle efficiency is reduced when the temperature is lower or higher than this optimum temperature value.This optimum temperature is different for the different working fluid and higher than the maximum isentropic efficiency corresponding to the heat source temperature.For example pure working fluid R123,the maximum cycle efficiency is 0.07 corresponds to the heat source temperature of 115? which is higher than the maximum isentropic efficiency corresponding to the heat source temperature of 93 ?.(4)The mixed refrigerant also has the optimum heat source temperature similar to that of the pure working fluid.Taking the mixed working fluid R123/R142b concentration ratio as 0.5/0.5 as an example,the maximum circulating efficiency of the system is 0.076 with the heat source temperature of 120 ? which is higher than that of maximum isentropic efficiency corresponds to a heat source temperature of 85 ?,35K.The heat source temperature continued to rise from 120? but also thermal efficiency is decreased.In this paper,the comparative study on the cycle efficiency of ORC system using mixed and pure working fluid has a certain guiding significance on the theoretical research and technical development of how to improve cycle efficiency of the ORC heat engine. |
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