Performance Analysis And Design Optimization Of Solid Oxide Cell Stack

Solid oxide fuel cell(SOFC)is an efficient energy conversion device which can directly convert chemical energy of fuels into electricity energy without combustion.It posses many advantages such as high efficiency,low pollution,no noise and fuel flexibility.Solid oxide electrolysis cell is an energy conversion and storage device which can convert electricity and heat to chemical energy,which can be stored in reaction product.SOFC and SOEC(SOC)mainly includes inter-connector,anode,cathode,electrolyte,frames and sealant.The materials system of SOEC is almost the same as SOFC,and the electrode reaction of SOEC is the reverse process of that of SOFC,so the development of SOFC and SOEC can learn from each other.However,the different electrode reaction processes also lead to the remarkable difference in geometry design and performance degradation mechanism,hence,the separate research is also necessary for SOFC and SOEC.The multiphysical processes inside SOC stack includes:mass transfer,species transfer,momentum transfer,energy transfer,charger transfer,chemical reaction and electrochemical reactions.All the processes couple with each other,making the physical field distribution considerably complex.Experiment is expensive and time-consuming,it is labored especially in exploring a large combination set of design and operating parameters space.Besides,due to the high temperature operation and strict sealing requirement,the detailed physical field distribution information inside SOC is difficult to measure.As the development of computer technology and commercial software,the precision and reliability of numerical simulation becomes higher.Simulation tool can be an efficient alternative in optimization design of structure,selection of operating parameter and prediction of high performance material.Even though the development of single SOC has gained great improvement,the performance degradation of assembled stack is still the problem demanding prompt solution.As a promising electricity production and energy storage technology,SOC stack technology faces three demanding challenges during commercialization process:performance improvement,lifetime lengthening and cost reduction.Stress is the source of decrease of stack stability and reliability.The main origin of stress includes:manufacturing process,reduction-re-oxidation cycles,mismatch of thermal expansion coefficient between components and temperature gradient.Hence,to reduce stress by optimization design is one of the primary research goal at present.However,research about electrocehmcial and mechanical performance of large-scale SOC stack at present is rare,the behavior research of large-scale stack by numerical computation is of great significance to SOC stack technology.This dissertation is to give insight into the internal physical fields distribution information of SOC stack with high geometry resolution by multiphysics coupling method.By parametric study,it investigates the main factors limiting electrochemcial performance in order to find out the optimization design scheme.Then,the temperature field obtained by multiphysics coupling method is used as thermal load to be imported into mechanical model to simulate the thermal stress and strain,which is used as base to conduct mechanical performance analysis and reveal the underlying mechanism,in order to provide guidance to experiment development.In chapter one,the basic information of SOC and research background are mainly introduced.Firstly,the constituting components,working principle of SOC,and the development history are detailedly described.Then,literature overview about the simulation of SOC are conducted and the innovation of this thesis is described.In chapter two,the multiphysics coupling model of SOC is introduced,it includes mass,species,momentum,energy conservation equation,chemical reaction,and electrochemical reactions.The gas diffusion model and heat transfer model have been clearly introduced.In chapter three,multiphysics model of SOFC stack is firstly introduced.The physical fields distribution of 30-cell H2 and CH4 SOFC stack are investigated,and the behavior characteristic and difference,underlying mechanism are comparatively analyzed.The influence of flow module,fuel utilization,air ratio and inlet temperature on SOFC physical fields distribution and output performance are studied.It reveals that air flux affects H2 and CH4 stack temperature with different mechanism.Besides,the fuel uniformity of CH4 stack is more sensitive to improper flow module design.There are also remarkable differences in flow module design,physical field distribution and performance output for stack fueled by the two fuels.In chapter four,the research progress of SOFC mechanical performance is firstly described.Then,mechanical model is constructed to compare the influence of temperature distribution on the mechanical performance of a SOFC stack with high geometry resolution.It finds out that uniform temperature can greatly improve mechanical performance by decreasing the stress,strain,failure probability of all the components.Whereas,the uneven temperature distribution can induce stress concentration easily.Besides,by comparative analysis,it finds out the problem induced or not induced by temperature gradient.In addition,by decouple the creep of all the components,the long-term interaction between different components can be distinguished.This chapter also focus on the research about SOFC stack under different methane steaming pre-reforming ratio,by considering the creep effect,the failure probability and creep rate evolution with time have been investigated and the working lifetime has been predicted.It reveals that partially CH4 steam reformation is beneficial to electrolyte and cathode,while the fully external reformation is beneficial to long-term reliability of anode and sealant.The partially CH4 steam reformation can keep all the component have relatively low failure probability and the whole stack has longest working lifetime.In chapter five,a comprehensive multiphysics model of SOEC has been developed.Based on planar SOEC inter-connector constructed by channel-rib fabrication,parameters sensitivity analysis has been conducted to distinguish major and secondary factors affecting SOEC performance and optimal rib width.Finally,it finds out three main factors affecting optimal rib width:pitch width,polarization loss,area specific resistance(ASR)between rib and electrode.An analytical expression to predict optimal rib width is obtained taking these three parameters as variable.Chapter six summarizes the thesis.