Simulation And Optimization Of Solid Oxide Fuel Cell Cogeneration System

The world energy crisis is becoming increasingly severe and environmental pollution is becoming more and more serious.People are eagerly looking for efficient and environmentally friendly energy utilization methods.Solid Oxide Fuel Cell（SOFC）has been widely paid attention in the world for its high efficiency,environmental protection and strong adaptability,and is known as one of the most promising power generation methods in the 21st century.In view of the high exhaust temperature of SOFC,SOFC and gas turbine are coupled into SOFC/GT system,and the waste heat of gas turbine exhaust is recovered by the low and medium temperature waste heat utilization system.Because of its wide waste heat utilization range,better power generation performance,carbon dioxide has a wide range of mature applications in the fields of refrigeration,heat pumps,and nuclear power.Therefore,CO₂ cycle is used as the waste heat utilization cycle in the paper,and LNG cold energy utilization system is introduced.The SOFC/GT/TRCC/ORC system and SOFC/MGT/TRCC/ORC system are established respectively,the thermodynamic performance of the two systems are studied,and the influence of key parameters on performance and cost is analyzed,and the optimal operating conditions are finally found.At the same time,the part load performance of the SOFC/MGT/TRCC/ORC system is analyzed.The main research contents are as follows:Firstly,the mathematical model of SOFC is established,and the accuracy of SOFC model are verified by comparing the reference model data.And on the basis of the SOFC,the thermodynamic and economic model of SOFC/GT/TRCC/ORC system is established.The results show that SOFC,combustor,and the bottom cycle condensers are the components with the largest loss,and the exergy loss can be reduced by optimizing the structure of these components and operating parameters of the system,so as to improve the overall energy utilization efficiency.The electrical efficiency,exergy efficiency and thermal efficiency of the system are 68.04%,65.65%and 68.72%respectively,and the total system cost is 90.97USD/h.Increasing the SOFC inlet temperature can improve the thermodynamic performance of the system,but it will increase the total cost.Although the increase of current density can greatly increase the output work of the system,it will decrease the efficiencies of the system and increase the total cost of the system.The ratio of steam-to-carbon ratio has little effect on the thermodynamic performance and economic cost of the system.The multi-objective optimization of the system was carried out by genetic genetic algorithm,and the optimal design point of the system was found by the TOPSIS decision algorithm based on entropy weight.Under the optimal design condition,the electrical efficiency,exergic efficiency and thermal efficiency of the system are 69.93%,67.51%and 71.11%respectively,and the total cost of the system is 55.99USD/h.Secondly,considering that the parameter change range of the conventional SOFC/GT system is relatively narrow during optimization analysis,the SOFC top cycle process is optimized,and the multi-stage air preheating and anode exhaust gas reflux are used to expand the working range of the system parameters.On this basis,the SOFC/MGT/TRCC/ORC system based on LNG cold energy recovery is established.Exergy analysis is conducted to find the largest exergy damaged parts of the system:SOFC,combustion chamber and two bottom cycle condensers,which indicates the direction for further optimization of the system.The influences of the key parameters,such as anode split ratio,prereforming rate,SOFC inlet temperature,current density and steam-to-carbon ratio on the thermodynamic performance and economic cost of the system are analyzed.The results show that the thermal efficiency and exergic efficiency of the system are 68.9%and 66.29%respectively,and the total cost of the system is 9.92USD/h.The increase of anode split ratio will decrease the thermodynamic performance of the system,but it is beneficial to the economic cost when the split ratio is less than 0.5.An increase in the prereforming rate will reduce the efficiencies of the system and reduce the output work,but it will reduce the economic cost.SOFC inlet temperature increase will improve the efficiencies of the system,and the system cost will decrease as the temperature rises after 520℃.The increase of current density reduces the thermodynamic performance of the system,but reduces the cost of the system.The increase of steam-to-carbon ratio reduces the thermodynamic performance of the system and increases the cost of the system at the same time,but both changes are small.Under the optimal design condition,the thermal efficiency,exergic efficiency and total cost of the system were 63.57%,61.16%and 8.17USD/h,respectively.Finally,the part load analysis of SOFC/MGT/TRCC/ORC system is carried out based on the part load performance map of gas turbine system.The results show that the output power of the system increases with the rise of gas turbine load,as well as the thermal efficiency and exergy efficiency.of the system.However,the cost of fuel and CO₂ emission of the system also keep rising.When the actual load of GT system increases from 50% to 110%of the rated load,fuel cost and CO₂ emission cost increase by 30.22%;system cooling capacity increased by 17.83%;the power load of the system increased by 32.90%;the actual load of the system increased from 78.11%of the rated load to 103.81%;the thermal efficiency and exergic efficiency of the system increased by 2.05%and 2.04%respectively.