Comprehensive Evaluation And Optimization Of Different Waste Heat Recovery Systems Based On Marine Diesel Engine Exhaust Gas Drives

In order to save energy and reduce pollutant emissions,this paper takes a V-type 16-cylinder marine diesel engine as the research object,and three waste heat recovery schemes,the steam Rankine-organic Rankine cycle（SRC-ORC）,Brayton-organic Rankine cycle（SBC-ORC）,and the supercritical organic Rankine-organic Rankine cycle（SORC-ORC）,are proposed based on the exhaust gas waste heat.The turbine inlet temperature and turbine inlet pressure of the high temperature stage cycle and the evaporation pressure of the ORC are used as optimization parameters,and the maximum output power(W_net),the minimum heat transfer area required per unit of output power（APR）,the minimum system equivalent carbon emission（SCE）and the minimum system risk(R_risk)are used as objectives for the comprehensive analysis and optimization based on NSGA-II-TOPSIS.And then the optimal scheme is determined by comparing the grey correlation method-entropy method,and the analytic hierarchy process（AHP）-sum product method,respectively.Finally,a new integrated analysis method（AHP-sum product method-NSGA-II）is proposed for the comprehensive analysis of thermodynamics,economics,environmental impact and system risk of the optimal scheme,and the optimization of the working fluid and operation mode of the combined system,which not only compares the comprehensive performance of pure working fluid and zeotropic mixtures,but also proposes a multi-mode recovery scheme for different operating conditions of diesel engines to improve the performance of the combined system.The exhaust gas waste heat analysis of the ship’s diesel engine shows that the exhaust gas of this diesel engine is not only of considerable energy but also of high quality.The exhaust gas has a maximum energy and an exergy of 2440k W and 863 k W,respectively.The exhaust gas energy of the diesel engine accounts for 31.4～54.6%of the total energy generated by the fuel.The results of the optimization analysis of each combined system based on NSGA-II-TOPSIS show that the SORC-ORC has a maximum W_netof 612.4k W.but the APR,SCE and R_riskof the SRC-ORC are optimal with 1.34 m²/k W,76.9 tons,and 1.54×10^-4injured/year,respectively.While the system of SBC-ORC has sub-optimal performance.The SRC-ORC,SBC-ORC and SORC-ORC are evaluated by the grey correlation method-entropy method,and their correlation degrees can be obtained as 0.98,0.41 and 0.53,respectively（the higher correlation degree indicates the better comprehensive performance）.Meanwhile,the comprehensive evaluation indexes F obtained by the AHP-sum product method are 0.005,1.3 and 0.21,respectively（the smaller the comprehensive evaluation index F indicates the better comprehensive performance）.Based on the results of the above comprehensive evaluation,the optimal scheme can be concluded as SRC-ORC.To facilitate the comprehensive evaluation of combined systems,a new comprehensive analysis method is proposed:AHP-sum product method-NSGA-II.In this method,the importance of the objective functions obtained by F1 is in the following order:system risk>environmental impact>economics>thermodynamic performance.By performing system optimization under the guidance of the integrated evaluation index F1,the results show that the R_riskobtained by F1 is 4.82%lower compared with NSGA-Ⅱ-TOPSIS.This indicates that F1 focuses more on the performance index of system risk and shows that the AHP-sum-product method-NSGA-II can provide more reference solutions for practical engineering than the traditional NSGA-II-TOPSIS method.Therefore,this method is very beneficial in the comprehensive analysis and optimization of combined system for marine diesel engines.The optimization of the working fluid indicates that using R600/R1233zd（where the mass fraction of R600 is 30%）instead of R600 can lead to further improvements in system performance.Specifically,this modification increases W_netby 0.30%,energy efficiency(η_th)by 0.90%,exergy efficiency(η_ex)by0.77%,SCE by 0.33%,while decreasing APR by 0.15%,the levelized cost of electricity（LCOE）by 0.24%,and the payback period（PBP）by 0.21%.Furthermore,the energy efficiency design index（EEDI）increases by 0.05%,and the R_riskis reduced by 31.44%.This suggests that the use of zeotropic mixtures improves system thermodynamic performance and reduces system economics and system risk,but the use of pure working fluids has a more favorable environmental impact.In terms of system mode optimization,the use of a single mode（SRC or ORC mode）can increase diesel efficiency by 5.67%and 4.63%compared to a single diesel engine,while the use of a multi-mode combination can increase by 6.41%.This shows that the multi-mode combination can better match the variable operating conditions of the diesel engine to improve the system performance compared to the conventional single mode.Therefore,in the design and optimization of combined systems for marine diesel engines,it is very important to select the suitable working conditions and mode combinations,which can significantly improve the system performance and efficiency.