Research On Improving Performance Of Marine Power System Based On Supercritical CO₂ Brayton-Kalina Combined Cycle

With the continuous rise of international oil prices and the proportion of expenditure on ship fuel costs has gradually increased.Besides,the International Maritime Organization(IMO)has proposed the Energy Efficiency Design Index(EEDI)based on the needs of the healthy development of the ship shipping industry.Therefore,issues such as improving the ship fuel consumption rate and reducing EEDI have received widespread attention in the shipping industry.Ship waste heat recovery belongs to the secondary utilization of heat energy,which can not only improve the economic performance of the ship power system but also meet the increasingly strict EEDI requirements.In recent years,a new type of power generation technologies such as the supercritical carbon dioxide Brayton cycle and Kalina cycle have shown good application prospects in ship waste heat recovery.So,to make full use of the marine waste heat energy,the paper selects the 8S90ME-C10.2 low-speed marine diesel engine as the research object and analyzes the supercritical carbon dioxide Brayton cycle and Kalina cycle to recover the marine waste heat energy.Firstly,the paper uses thermodynamic analysis methods such as thermal balance,exergy balance,and coefficient of energy quality to study the distribution situation and quality characteristics of waste heat under different loads on the marine main engine.The results show that the exhaust gas behind the turbine and the bypass exhaust gas are essential components of the available waste heat source,which has a high utilization value and is suitable for use as a heat source for a ship’s waste heat recovery system.Secondly,according to the respective characteristics of the supercritical carbon dioxide Brayton cycle and Kalina cycle,the paper designs the Brayton-Kalina combined cycle system.A thermodynamic analysis model of the combined cycle system was established in MATLAB software.From the perspective of thermal performance and economic performance,the parameter analysis,single-objective optimization,and multi-objective optimization of the combined cycle system were performed.The influence of the combined cycle system on ship power system economic performance under different loads is calculated and analyzed.The results show that the combined cycle system after multi-objective optimization matching can recover 5.64% of the power of the main engine energy,the maximum thermal efficiency of the diesel engine can reach 52.24% and can reduce the average annual fuel consumption of ship power generation auxiliary equipment by 37.37%.Finally,the paper uses modular modeling ideas and lumped parameter methods to build a diesel engine average model and a combined cycle system dynamic simulation model on the MATLAB/Simulink platform.The dynamic characteristics of the combined cycle system under load decreasing,load increasing,and load continuous changing of diesel engines are analyzed.The results show that the main parameters of the combined cycle system are directly proportional to changes in the load of the main engine,and the main parameters of the cycle system have a significant hysteresis phenomenon with the change of the load of the main engine,indicating that thermal inertia has a more significant impact on the cycle system.The research results in this article can provide a reference for the deep utilization of the marine main engine waste heat by using the supercritical carbon dioxide Brayton cycle combined with the Kalina cycle and also lay the foundation for the control strategy of Brayton-Kalina combined cycle system.