Design And Analysis Optimization Of Energy Utilization System For LNG Powered Ships Based On Advanced Exergy Analysis

The LNG stored in LNG powered ships is at a low temperature of-162 ℃ and needs to be vaporized before being supplied to the main engine for combustion.During the vaporization process,approximately 830 k J/kg of cold energy is released.At the same time,a large amount of waste heat from the engine is also released into the environment,of which about 50% of the energy is wasted.Therefore,in order to improve energy utilization efficiency and ship operation efficiency,this study aims to design a reasonable and efficient energy utilization system for LNG powered ships,fully tap into the potential of LNG cold energy and engine waste heat recovery,and for the first time use advanced exergy analysis methods to analyze and optimize the thermal performance of the energy utilization system for LNG powered ships.Based on this,this article mainly conducts research on the following aspects.(1)Selecting a 215000 ton VLCC-LNG power ship as the research object,considering the practical application background of the ship,and combining the principle of "temperature matching and cascade utilization",two integrated utilization schemes for LNG cold energy and intermediate temperature flue gas waste heat of the ship are proposed.Scheme 1 is a two-stage turbocharging,three-level horizontally nested Rankine cycle full power generation system;Scheme 2 is a two-stage turbocharging and three-level longitudinally nested Rankine cycle full power generation system.Both schemes can fully utilize LNG cold energy and medium temperature flue gas waste heat for power generation,and the system structure is compact and simple.(2)Simulate the two proposed energy utilization systems using HYSYS,perform thermodynamic analysis on the simulation results using advanced exergy analysis methods,and compare the analysis results with traditional exergy analysis methods.Due to different standards used,there is a significant difference in the analysis results of the two exergy analysis methods.In Scheme 1 system,the traditional exergy analysis method considers heat exchanger 1 with a relative exergy destruction of 24.40% to be the main factor affecting the overall system performance;The advanced exergy analysis method indicates that the avoidable internal relative exergy destruction(8.35%)of expander 3 is the highest,and priority should be given to improving the overall performance of the system.In Scheme2 system,the traditional exergy analysis method considers expander 4 with a relative exergy destruction of 15.31% as the second important factor affecting the overall system performance;The advanced exergy analysis method indicates that expander 2 has the second highest avoidable endogenous relative exergy destruction(2.50%),and has the second priority in improving the overall performance of the system.In addition,the two exergy analysis methods provide different priority strategies and establish corresponding priority orders for system improvement.(3)Advanced exergy analysis methods can further consider the complex interactions and real improvement potential in the system,thereby gaining a more comprehensive understanding of system performance and potential issues.The proportion of avoidable exergy destruction in the entire system in Scheme 1 is 48.65%,indicating that the system has 48.65% room for improvement;The proportion of internal exergy destruction is 56.43%,indicating that the interaction between various components of the system is moderate,and there is a certain degree of independence between each component.The internal efficiency of system components and the interrelationships between system components have almost equal effectiveness in improving system performance.The proportion of avoidable exergy destruction in the entire system in Scheme 2 is 40.46%,indicating that the system has an improvement potential of 40.46%;The proportion of internal exergy destruction is 58.72%,indicating that there is a certain degree of independence among the various components of the system,as well as mutual constraints.(4)In order to reduce avoidable exergy destructions,the optimization suggestions proposed by advanced exergy analysis for the two systems are optimized by working fluid optimization and global parameter optimization based on genetic algorithm for Scheme 1and Scheme 2.The optimization of working fluids has determined two schemes to use the same working fluid in their corresponding level of power generation cycle;Parameter optimization is achieved by combining MATLAB and HYSYS,and genetic algorithms are used to adjust and optimize key decision variables.After optimization,the exergy efficiency of the two systems has been increased to 54.63% and 48.32%,respectively.(5)Conduct system cost and economic benefit analysis on the optimized two systems in order to efficiently evaluate risks and determine system value.The total annual net income of Scheme 1 system is 1.6358 million yuan,and the fund recovery cycle is 3.74years;The total annual net income of Scheme 2 system is 3.8336 million yuan,and the fund recovery cycle is 5.49 years.Based on the economic analysis and comparison of the two systems,it is believed that if the ship has a higher demand for waste heat recovery and power generation,Scheme 2 system is the best choice for the shipowner.