Multiphysics Simulation And Performance Optimization Of Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC) is a clean and efficient chemical to electrical transfer device.The intermediate temperature SOFC is considered the most promising fuel cell for commercialization.In addition to high efficiency of energy transfer,SOFC has a great advantage of direct use of hydrocarbon derived from fossil resources and biomass as fuel.For commercialization of SOFCs,technological advances are required in improving the cell and stack performance and reducing the manufacture cost.Experimental works are expensive and time consuming.To improve the fuel cell performance,experiments can only choose a few targeted parameters to vary in a discretized manner.But in theoretical works,many intreasting parameters can be optimized easily and flexibly,and optimal engineering design can be quickly yielded. With the progress in the finite element method and the increased computational capabilities,multiphysics modeling can be performed for the complex multiphysics system of SOFC.The modeling is capable of providing more detailed information in the fuel cell and realizes faster design optimization than the experimental work.The dissertation describes the 2D and 3D modeling of SOFCs.The results include the optimal geometry parameters for stack cell performance and detailed information regarding the electric potential,current density and species molar fraction distributions in SOFCs.There are six chapters in the dissertation.The first chapter introduces the research object:SOFC.The second chapter describes the modeling method and tools used in the research.From chapter 3 to chapter 5,three modeling works are described. Chapter 3 is about 2D stack cell simulation of typical SOFCs.Chapter 4 is about 2D single cell and stack cell simulation of multilayer SOFCs.Chapter 5 is about 3D stack cell simulation of typical SOFCs.The last chapter summarizes the results.More details are listed below.In the first chapter,we give an overview of the research background of SOFCs, their common designs and materials,followed with a brief discussion of the planar design.At the end of the chapter,the thermdynamics and electrochemical principles of SOFC are given and the energy efficiency of the SOFC technology is evaluated.In the second chapter,we briefly review the modeling and simulation works on SOFCs.Then we discuss the voltage losses in SOFC cells and describe the settings for potential balance in our modeling works.The theoretical models for mass transports are described in detail.At the end of the chapter,the finite element multiphysics modeling tool,COMSOL,is introduced.In chapter 3,a 2D model for the planar SOFC stack is described.The model. considered the electric contact resistance between the electrode and interconnect rib, the gas transport in the electrodes,electronic and ionic conductions in the membrane-electrode assembly and the electrochemical reactions at the three phase boundaries.The model is capable of describing in detail the rib effect on the gas transport and the current distribution in the fuel cell.Based on the interplay of the concentration and ohmic polarizations,the optimal rib widths for different pitch sizes and different area specific contact resistance are calculated.The cathode thickness is also optimized.In chapter 4,the newly developed multilayer electrode SOFC is modeled.The multi-physical fields are described and discussed in detail.Based on the stack cell modeling,several design parameters are optimized for the stack cell performance.In chapter 5,a 3D model considering electric and ionic conduction,mass transport and electrochemical reactions is described.The modeling area includes a half circling unit along the channel.Based on rigorous mathematical analysis of ionic conducting equation,a method is developed to scale the electrolyte thickness by 10 folds with corresponding change in the electrolyte conductivity to moderate the thin film effect in the meshing step and decrease the number of degrees of freedom.The mathematical equivalent method can efficiently decrease the mesh node numbers and enhance the efficiency of numerical simulation.The model is used to optimize the rib width of co-flow,counter flow and cross flow SOFC stacks and the results are compared with the 2D results in chapter 3.The last chapter provides a brief summary of the research results in the dissertation.