Research Of Multi-mode Ship Waste Heat Utilization Cogeneration System With Carbon Dioxide As Working Fluid

Fossil energy is the basis for the continuous development of human society.However,it has a significant impact on the ecological environment,due to its non-renewable nature and continuous use.Thereby,people have to devoted great efforts to look for clean and sustainable energy or new ways to energy saving and emission reduction.Marine diesel engine consumes a large amount of fuel during operation.Only 40-50% of the energy can be used effectively,while the rest contained in exhaust gas was wasted to the atmosphere.The waste heat utilization device can effectively utilize this wasted energy from the main engine and improve the fuel efficiency.In addition,due to the special working environment onboard the ship,the demand for cooling,heating and electric energy is not only huge in quantity,but also varies greatly with the change in sailing areas and seasons.It poses a challenge to the design of supply system for cooling and heating onboard the ship.In this thesis,a multi-mode cogeneration system driven by main engine exhaust waste heat is proposed for the purpose of improving the fuel efficiency of the main engine.Moreover,the quantity and types of different energy(cooling,heating and electricity)requirements can be effectively adjusted according to environmental or demand changes.The research work in this thesis is summarized as below:(1)The mathematical model for the proposed waste heat utilization system is established and verified.The thermodynamic cycle suitable for the combined system driven by main engine exhaust heat is selected.The performance of steam Rankine cycle,organic Rankine cycle and transcritical carbon dioxide power cycle is compared considering the thermodynamic performance and the actual conditions of marine engine room.The results show that the thermal efficiency of the transcritical Rankine cycle with carbon dioxide as working fluid reaches the highest efficiency of 20.86% under the same condition of the heat source.As a working fluid,carbon dioxide is non-toxic,eco-friendly and has good matching with the temperature of main engine exhaust gas,so it is suitable for power generation and heating by using exhaust waste heat of marine main engine.In addition,the performance of vapor compression refrigeration,absorption refrigeration and ejector refrigeration are compared.The results show that the ejector refrigeration with carbon dioxide as working fluid consumes less energy than the vapor compression refrigeration and has higher COP,so it can be used in the refrigeration powered by marine wasted heat.(2)The thermodynamic scheme of the cogeneration system combined carbon dioxide transcritical Rankine cycle and ejector refrigeration cycle are presented,and the thermodynamic performance analysis is carried out.The "Bohaizhenzhu" ro-ro passenger ship is taken as the research object,the effects of several key operation parameters,including split ratio,condensation temperature,pump outlet pressure and inlet pressure of ejector on the performance of waste heat recovery system are analyzed.The results show that the cooling capacity and the net output power of the system are 997.2k W and 392.3k W,respectively,under the design condition.In the CHP mode,the high-temperature(90 ℃)and lowtemperature(45℃)heating are 1098.8k W and 257.2k W respectively,and the net output work is 897 k W.The application of parallel turbine and regulating valves enable the flexible operation mode and adjustable cooling and heating output over a wide range.The split ratio has a great influence on the overall system performance such as the output ratio of production.The condensation temperature has a big influence on the net output work.The net output work decreases obviously with the condensation temperature increasing.The influence of the pump outlet pressure is mainly reflected in the net power and heat output.Ejector inlet pressure mainly affects the performance of the ejector and further affects the refrigeration cycle.(3)The thermal-economic mathematical model of the cogeneration system is established.Exergy,exergoeconomic and environmental analysis as well as multi-objective optimization is carried out.Exergoeconomic results show that the regenerator and condenser have poor exergoeconomic performance.In addition,split ratio and pump outlet pressure have the most obvious influence on the economic performance.The smaller split ratio and the higher pump outlet pressure results in better exergy performance.The environmental analysis based on the ship efficiency index shows that the ship efficiency index increases by 14% after the use of the waste heat cogeneration system.The optimization results show that when the maximum net output work and the minimum unit total cost are taken as the optimization objectives,the net output power and unit total cost are 575.7k W and 116.10￥/GJ,respectively.In the CHP mode,with the maximum heat output and the minimum unit total cost of the product as the optimization objectives,the high temperature heating output,net output power and unit total cost are 2580.8k W,1146.3k W and 79.48￥/GJ,respectively.