| In recent years,while developing new energy actively,countries have also invested huge resources in the efficient use of traditional energy.As a kind of traditional power plant widely used in industrial system,the internal combustion engine has not been effectively utilized nearly 60-70%of waste heat.How to recover the waste heat of internal combustion engine efficiently and improve the utilization ratio of fuel has become a hot issue.According to the characteristics of waste heat of internal combustion engine,a two-stage organic Rankine cycle recovery system with LNG(liquefied natural gas)as cold source is established in this paper.In order to evaluate the thermodynamic performance and thermal economy of the waste heat recovery system more comprehensively,this paper studies the system from the aspects of mathematical modeling,sensitivity analysis and system optimization(1)System modeling and working fluid optimization.The system is modeled according to the first law and the second law of thermodynamics.And the thermodynamic characteristics of the system,such as thermal efficiency,exergy efficiency and exergy loss are studied.According to the basic theory and calculation method of thermoeconomics,the thermal economy of the system is modeled and the important economic parameters such as total cost rate and investment cost rate are calculated.The results show that the condenser has the highest loss ratio,and the pump loss is almost negligible.At the same time,the evaporator,an important part of organic Rankine cycle,is designed and calculated.Because the organic working fluid as a cold source undergo phase transformation in the heat absorption process,its heat transfer coefficient and Reynolds number are different in different phase regions,so the length of the heat exchanger is calculated according to the three phases of the cold source.Under the condition of heat exchange,the size parameters of heat exchanger are calculated.In this paper,the organic working fluid in organic Rankine cycle is optimized,and the maximum efficiency and thermal efficiency are compared.Finally,the optimal working fluid pairs are n-butane and butane.(2)System sensitivity analysis.Based on the determination of the working fluid pair and the calculation model of the combined system,the thermodynamic and economic properties of the turbine outlet pressure,turbine inlet temperature,turbine inlet pressure and condenser outlet temperature are analyzed.The results show that the net output power of the system decreases with the increase of the turbine outlet pressure.In order to obtain the maximum output power,the outlet pressure of the turbine should be reduced as far as possible within the permissible range,the inlet temperature and inlet pressure of the turbine should be increased,and the outlet temperature of the condenser should be reduced.(3)Multi objective optimization of the system.Based on the calculation and analysis of the above-mentioned system,through the multi-objective genetic NSGA-II algorithm to optimize the system objective function,the Pareto frontier graph is finally obtained.As the non-inferior solution after optimization,each point on the graph needs to select the best solution that best meets the actual needs based on TOPSIS decision algorithm.The results show that the optimal solutions are 83kw and 373212$,respectively.When the objective function is the minimum external investment cost and the maximum output power;the optimal solutions are 0.19 and 83105 $,respectively.When the objective function is the minimum external investment cost and the maximum thermal efficiency;the optimal solutions are 0.12 and 69448 $,respectively,when the objective function is the minimum external investment cost and the maximum thermal efficiency.Through the comprehensive analysis of the waste heat recovery system,the feasibility of the system operation is verified.The research results can provide reference for the design and operation of organic Rankine cycle system. |
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