Study On Operating Parameters’ Optimization Of S-CO₂ Recompression Brayton Cycle Power Generation System For Marine Flue Gas Waste Heat

In recent years,the energy saving and emissions reduction has become the consensus in the global shipping industry.Marine engine waste heat power generation is an effective way to reduce the fuel consumption and greenhouse gas emissions emitted by the marine engine,which is of great significance to the energy saving and emissions reduction of shipping industry.This doctoral thesis proposes high-efficiency S-CO₂ recompression Brayton cycles which can match the temperature range of the exhaust gas waste heat as the bottom cycle to recover the waste heat from an ocean-going 9000TEU container ship main engine,and the corresponding theoretical and experimental researches are carried out for the steady-state operation principles as well as the system optimization design of the flue gas waste heat S-CO₂recompression Brayton cycle power generation system.The research contents focus on the establishment and modification of the mathematical models of the main components,including compressors,turbines and heat exchangers;system parametric study and key parameters decision;construction of suitable system layouts to match the characteristics of the main engine exhaust gases to take full use of the waste heat;multi-objective global optimization and performance comparison based on optimal allocation of the total heat transfer coefficient UA values of high and low temperature recuperators,where the net output power of the system W_net,thermal efficiencyη_th,exergy efficiencyη_ex,heat exchanger heat transfer area per unit power（APR）and levelized cost of energy（LCOE）are as the objective functions;performance and benefit analysis of S-CO₂recompression Brayton cycle waste heat power generation system under typical load conditions.The main research results are concluded as follows:1)A modeling method based on the combination of semi-empirical models and actual working characteristics of main components to establish mathematical models includes turbines,compressors,heat exchangers and the entire system was proposed,ensuring that the established mathematical models have a high calculation accuracy and reliability.2)The Principal Component Analysis was employed to evaluate and decide the key thermodynmiac parameters with significant effects on the sysem comprehensive performance,which could be used as key factors for system performance optimization or control strategies formulation.3)Three modified recompression Brayton cycles were presented on the basis of the RC cycle,including dual TAC cycle,intercooling RC cycle and intercooling dual TAC cycle,aiming to overcome the poor performance of RC cycle waste heat power generation system under low main engine load conditions.The results showed that the thermodynamic performance of the three modified cycles are improved and can better match the main engine working conditions to improve the utilization efficiency of the exhaust gases waste heat by comparing the performance of the RC cycle and modified cycles.Meanwhile,it has decided the optimal cycle layout and the operating parameters under different marine engine loads.4)Based on the optimal allocation of heat transfer coefficient values（UA）of high and low temperature recuperators,the comprehensive performance evaluation models that taken the thermodynamic performance,system size and economy into account were established to conduct the optimization design and performance analysis of the hundred kilowatts-class marine flue gas waste heat S-CO₂recompression Brayton cycle power generation system.Compared to the system performance before optimization,the optimal UA values of the low-temperature and high-temperature regenerators can be reduced by about 50%,which significantly improves the compact structure of the system and reduces the cost.The dual TAC cycle can be selected as the optimal one to recover the marine engine flue gas waste heat of the 9000TEU container ship.According to the Pareto front solutions between the objective functions obtained by NSGA-II based multi-objective optimizations,the optimization design guidelines for hundred kilowatts-class marine flue gas waste heat S-CO₂recompression Brayton cycle waste heat power generation system can be formed.5)Based on the determined optimal cycle layout and the typical main engine load conditions,the thermal coupling analysis of the thermodynamic parameters were further performed to figure out the optimal adjusting parameters of the system.And,the off-design analysis was conducted for the dual TAC cycle waste heat power generation system.Based on the off-design analysis results,the benefits of the main engine system integrated with the dual TAC cycle waste heat power generation system were evaluated through the part-load performance analysis.