Exergy Analysis And Working Fluid Selection Of Dual-loop Organic Rankine Cycle For Waste Heat Recovery Of An Internal Combustion Engine

At present,only about 30%of the combustion fuel heat of internal combustion engine（ICE）is converted into effective work,and most of the energy is taken away by cooling water and engine exhaust,which results in energy waste problems and environmental pollution.Compared with the traditional single-stage organic Rankine cycle,the double organic Rankine cycle（DORC）can recover waste heat from high-temperature exhaust and low-temperature coolant at the same time,and has higher thermal performance.However,the DORC system is more complex,and its performance still needs to be further improved.Therefore,in this paper,the DORC system of the inline 6-cylinder 4-stroke turbocharged diesel engine is taken as the research object,and the system is analyzed based on the second law of thermodynamics to discuss the key components that affect the performance of the system;the multi-objective optimization is used to select the suitable working fluid,then the screening criteria of the working fluids is proposed to provide theoretical guidance for further improving the comprehensive performance of the DORC system.The main work of the paper is:（1）The performance of the DORC system is analyzed by the exergy analysis.Based on the traditional exergy analysis and advanced exergy analysis,the thermodynamic model of DORC system is established to analyze the distribution characteristics of the system’s exergy destruction;the sensitivity analysis is used to further study for the effect of system operating parameters on endogenous-avoidable exergy destruction.The results show that the exergy destruction of the high-temperature evaporator,low-temperature turbine,the first low-temperature evaporator,and high-temperature condenser is large,which is a part of the system that needs to be improved;however,the avoidable exergy destruction of the low-temperature turbine,high-temperature evaporator,and high-temperature turbine is large,with greater potential for improvement;reducing the temperature pinch point of the evaporator and improving the efficiency of the turbine can reduce the avoidable exergy destruction.（2）The multi-objective optimization model of DORC system is established.The influence of system operation parameters on the objective function is analyzed to determine the thermal performance index and economic performance index.The multi-objective optimization model of the DORC system is established,the non-dominated sorting genetic algorithm（NSGA-Ⅱ）is applied to carry out multi-objective optimization with R245fa/R134a as the candidate working fluid in the study.The results show that under optimal operating conditions,the system efficiency and PBP are 25.82%and 2.69 years,respectively.The corresponding high-temperature condensation pressure P₃ is 695k Pa,low-temperature evaporation pressure P₈ is1890k Pa,and the low-temperature condensation pressure P₉ is 839k Pa.（3）The selection of working fluid pairs in the DORC system is established.With the efficiency and the PBP as the objective functions,using the non-dominated sorting genetic algorithm（NSGA-Ⅱ）,the performance of 24 candidate working pairs in the DORC system is optimized by multi-objective;the comprehensive performance model of the system is established by using the grey correlation method,and the thermoeconomics screening criterion of the working fluid pairs in the DORC system is determined.The results show that the critical temperature can be used as a simple screening criterion based on thermoeconomics for the working fluid pairs in the DORC system.When the critical temperature of working fluid in the high-temperature cycle is580-600K,and the critical temperature of working fluid in the low-temperature cycle is 380-415K,which is the ideal working fluid pair;among the 24 candidate working fluid pairs,the best working fluid pair is toluene/R124,whose efficiency and PBP are39.16%and 1.26 years respectively.The research results of this paper will provide theoretical guidance for improving the performance of the components in the DORC system,promote its efficient use of internal combustion engine waste heat,and also provide a new idea for the selection of working fluids pairs.