Evaluation Method For Engine Waste Heat Recovery System And Optimization Research Of Dual-loop Rankine Cycle

According to the heat balance of internal combustion engine(ICE),it is found that almost 60%-70% of the fuel combustion energy has been wasted by the engine exhaust and jacket cooling water.Therefore,ICE's thermal efficiency will be greatly improved if the waste heat is recovered.Recently,extensive research on engine's waste heat recovery has been developed at home and abroad,based on organic Rankine cycle(ORC)waste heat recovery technology from ICE shows great potential in future application.Considering the temperature of the exhaust from engine is relative high,which is easy to make organic fluids decompose when exchanging heat directly,then cycle needs the organic fluids with high decompose temperature,but the single loop ORC's efficiency is limited;the temperature of the jacket cooling water is low,which is in great temperature difference with the exhaust.Heat sources with great temperature difference have poor matching and the heat utilization for double heat sources is limited,some researchers designed a dual-loop Rankine cycle(DORC).While comparing to the single-loop organic Rankine cycle(SORC),the DORC has complex structure and more components.So the traditional evalution method using single thermodynamic index can hardly give complex thermodynamic cycle system a comprehensive evaluation.In order to evaluate and analyze the complex system's characters,there need a scientific and reasonable evaluation method.Based on the evaluation method for the analysis of the DORC and SORC's characters,then the optimization for DORC system.Aiming at the imperfection of ICE waste heat ORC recovery system's evaluation and optimal selection,three-level attribute factors including safety environmental protection and reliability,ther-modynamic properties,technology economy,energy conservation and emissions reduction benefits were under comprehensive consideration.This paper presents a three-level evaluation method combined with multi-level non-structural fuzzy decision,including evaluation index model building,system parameters optimization,evaluation and decision making.This comprehensive evaluation method has general applicability for medium and low temperature waste heat recovery thermodynamic cycle system.This paper established the basic organic Rankine cycle(BORC),regenerative organic Rankine cycle(RORC)and DORC model for desiel engine's exhaust waste recovery in Matlab platform.Optimized analysis on the three system was taken based the above method,results show that DORC has a better thermodynamic property compared with BORC and RORC,but when considering the technical economic evaluation indexes the DORC system is weak.Then different cycle structures(single or double heat source)and various working fluids for DORC system schemes were presented,Pareto compromise solutions were obtained through double objective optimization based on genetic algorithm.Judging from the evaluation results,the DORC system with the double heat sources(exhaust and jacket cooling water)performs the best when toluene-R123 are adopted as the working fluids,but schemes' s performance ranking order changes when considering different level evaluation factors.Taking a 6 cylinders four stroke rated power as 241 kW diesel engine exhaust as target heat source,a steam Rankine cycle-organic Rankine cycle(RC-ORC)dual-loop test bench was established.Experiment research on RC-ORC system's response under different diesel working conditions was carried out.When the diesel at torque 1152 N·m and at speed 1800r/min,up to 101 kW of waste heat recovered form the exhaust,the high-temperature loop's potential output power is 8.4kW and the low-temperature loop's potential output power is between 3.7kW and 6.35 kW.Experiment results indicated that higher evaporating pressure can obviously improve the performance of RC-ORC system while the higher superheat can decrease the potential output power.