| In the face of increasingly serious energy and environmental problems,the use of organic Rankine cycle(ORC)technology,recycling of renewable energy and low-temperature thermal energy in industrial waste heat is one of the effective ways to achieve energy conservation and emission reduction.At present,the research on ORC mainly focuses on the performance analysis of the system,the cycle parameters optimization and the working fluid selection,seldom involves the research on revealing the relationship between the physical properties of working fluid and the performance of system.However,the physical properties of working fluid have an important effect on the performance of ORC.In order to further reveal the relationship between the working fluid physical properties and the performance of ORC,it is necessary to further study the thermal characteristics of working fluid in each process of ORC and its influence on the performance of system.Therefore,based on the thermal characteristics of working fluid in the cycle,this paper studies the relationship between the physical properties of working fluid and the thermodynamic performance of ORC,and the influence of the physical properties of working fluid on the comprehensive economy of ORC system.Based on the analysis of the thermodynamic processes and diagram of ORC,4 cycle characteristic parameters that can characterize the thermal characteristics of working fluid in the ORC were established:The temperature corresponding to the maximum specific entropy of saturated vapor Tsmax;Imaginary temperature difference ?Tim,that is,the difference between the imaginary maximum temperature of the cycle and TSmax;The isentropic degree coefficient B,indicated the degree of proximity between dry and isentropic working fluid;The isothermal heat addition coefficient C,indicated the degree of proximity between average temperature during preheating under evaporation temperature of Tsmax and Tsmax-Based on the cycle characteristic parameters,the expressions for average temperature of heat addition and heat rejection were derived.By comparing the data in the literature,the accuracy of calculating exergy efficiency and thermal efficiency of ORC based on cycle characteristic parameters was verified.According to the expressions of the average temperature of heat addition and heat rejection,the effect of the cycle characteristic parameters on the average temperature of heat addition and heat rejection was studied under the heat sources of 358.15 K,393.15 K,423.15 K and 473.15 K respectively.The results indicated that under the given heat source temperature,the features of the working fluid with high average temperature of heat addition would change due to the difference of heat source final temperature(The outlet temperature of evaporator's heat source side).In order to select the working fluid with high average temperature of heat addition,the transition temperature was introduced to define the relationship between the features of the working fluid with high average temperature of heat addition and heat source final temperature.The results showed that,for the features of the working fluid with high average temperature of heat addition,when below the heat source final temperature corresponding to the transition temperature,the critical temperature and imaginary temperature difference were both lower,while when above the heat source final temperature corresponding to the transition temperature,the critical temperature and imaginary temperature difference were both higher.For different working fluids,when both of the critical temperature and the imaginary temperature difference were very close,the higher the isothermal heat addition coefficient was,the higher average temperature of heat addition was.The isentropic degree coefficient reflected the increasing amplitude of average temperature of heat rejection with an increase in evaporation temperature,and was the key parameter affecting the average temperature of heat rejection.The feature of working fluid with low average temperature of heat rejection was a large isentropic degree coefficient.Based on the difference between the physical properties of organic working fluid and ideal gas and considering the incidence angle and acceleration factor on the constraints of turbine design parameters,the design process of screening method was improved by adding the constraints on the structure of rotor and the relative velocity at rotor blade outlet,the model of radial inflow turbine with organic working fluid was established.The relationship between cyclic pressure ratio and the turbine isentropic efficiency was studied by combining theory and experiment.The results showed that the change rule for the cyclic pressure ratio and the isentropic efficiency of turbine was opposite,and the rotor expansion ratio was the main physical parameter affecting the thermodynamic performance of turbine.In general,the smaller the rotor expansion ratio was,the higher the turbine isentropic efficiency was.In the design,the rotor expansion ratio should be avoided to ensure that the turbine had a high thermal performance.In the off-design performance experiment for the radial inflow turbine,rotor expansion ratio decreased from 3.5541 to 2.2319,acceleration factor decreased from 3.6851 to 2.3529,and the isentropic efficiency of turbine increased from 46.56%to 71.45%with cyclic pressure ratio decreasing from 6.4817 to 4.7335.The parameter changes in the experiment were consistent with the changes in different design conditions.The exergy method was used to analyze the thermodynamic performance of ORC systems.Under the given heat source temperature,exergy loss of evaporator.radial inflow turbine and condenser and its influence on the exergy efficiency of ORC system were analyzed.The results showed that the sum of exergy losses of evaporator and condenser was dominant in the total losses of the system;The difference in both of the average temperature of heat addition and heat rejection of the cycle with different working fluids was the main factor that resulted in different exergy efficiency of ORC system,while the impact of turbine exergy loss was limited.On this basis,the effect of the cycle characteristic parameters on exergy efficiency of ORC system was studied,and the classification for working fluid with optimal thermodynamic performance was established.The research indicated that the effect of the critical temperature on ORC's thermodynamic performance was mainly reflected in the heat addition process,so the method of working fluid selection only by critical temperature had limitations.The classification established in this paper had the characteristics of clear applicable boundary and strong pertinence.Based on exergy efficiency and power cost,the comprehensive objective function was constructed,and the parameters of ORC system were optimized under the fixed and variable heat source parameters.Based on the average temperature difference of heat addition and heat rejection,the relationship between the cycle characteristic parameters and the comprehensive economy of system was analyzed,and the features of the optimal working fluid were studied.The results showed that improving the thermodynamic performance of the system inevitably weakened its economy.The features of the optimal working fluid were that the transition temperature was higher than Tsmax and the isentropic degree coefficient was large For the working fluid which the critical temperature was obviously lower than the heat source temperature,the comprehensive economy of the system was poor due to the excessive average temperature difference of heat addition.R1234yf under 358.15 K heat source,R227ea under 393.15 K heat source and R236fa under 423.15 K heat source were all optimal working fluids with high thermodynamic performance of the system and low system cost;R1233zd under 473.15 K heat source was the optimal working fluid with high system thermodynamic performance and high system cost. |
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