Multiphysics Fully Coupled Modelling For Solid Oxide Fuel Cell Stack And Theoretical Simulation

Solid oxide fuel cell (SOFC) is a kind of clean energy conversions system which conerts the chemical energy of fuel gas into electricity and heat directly through the electrochemistry. SOFCs has a broad application prospect of industrialization, and can be used in transportation, distributed power station. Stack is the core unit of SOFCs worked as the power generator. It is very important to improve the energy efficiency and the service life of the stack for the SOFC industrialization. The working characteristics of industrial stack is very complex, flow, species and mass transfer, charge transport, electrochemical reaction, chemical reaction and heat transport are coupled with each other intensively. This complex coupling determines the performance of the stack. Very different from the button cell in the laboratory, the distribution of flow, electrochemical reaction and temperature within the stack are very complex, which make the performance of the stack is difficult to be accurately controlled. Using the traditional experimental method to control and optimize the industrial large-scale stack, is not only time consuming and expensive, but also difficult to accurately grasp the effect of the coupling on the performance. With the development of computer and software technology, SOFC multi physics coupling simulation has shown the ability of analyzing the performance. It can be used as the substitute or supplement of the experimental method for optimizing, controlling, designing of SOFC core components.This Ph.D dissertation focuses on the creation of the multi-physical fully coupled stack model and performance analysis. A briefly introduction of each chapter is given as follows.In the first chapter, we firstly introduced the background, the structures and working characteristics of SOFC. We also illustrated the application prospect of the planar type SOFC, as well as the technical problems that need to be overcome. Then, the development of the multi-physical fully coupled simulation was introduced in detailed, including the improvement of single cell modelling and the bottleneck of the true stack simulation.In the second chapter, we firstly explained the necessity of the fully coupled simulation of the industrial SOFC stack. Then, we solved two crucial problems which restricted the realization of the true stack modelling. These improvements were (1) reforming the multi-physics algorithm to reduce the computational resource and (2) creating the integrated multi-physics modules of SOFC stack modelling. We successfully established the fully coupled simulation tool for the large scale stack by embedding the analytical algorithm-’cathode-rib algorithm’and the inhouse subroutine-multi-physics module into CFD software Fluent. This module can reduce the computational resources for more than one order compared with the traditional CFD tool, and greatly enhanced the computational stability and convergence speed.In the third chapter, we put forward that the fully coupled simulation tool of the SOFC stack must meet the following requirements:(1) it must be able to calculate the small system model such as single cell or repeating unit of SOFC, just as the SOFC modules in Comsol or Fluent, (2) the accuracy must be verified, (3) users can be allowed to optimize the computional resources further, so as to achieve the higher efficiency of the three-dimensional fully coupled calculation, (4) to reduce the difficulty of debugging during the calculation. A series of calculations were carried out to (1) verify the accuracy of the stack modelling, (2) to optimize the grid settings, and (3) to develop a computational strategy with high stability.In the fourth chapter, we reported a 30-cell planar stack numerical model, which was the first true production stack model established with the full structures and solved by the fully coupled calculation in the world. With the help of the fully coupled simulation, we revealed the complex characteristic of the steady operational working condition within the production stack, and demonstrated the powerful computing power of this modelling tool. The simulation results were compared with the traditional simplified algorithms, which showed the necessity of developing a high precision fully coupled stack model.In order to optimize the flow assignment within the production stack, we created an analytical method-’attenuation equations’ to predict the flow uniformity, in the fifth chapter. With the help of the attenuation equations, we found that the working characteristic of SOFC stack could induce the attenuation behavior of the fuel gas uniformity very easily. A more in-depth analysis indicated that there was a leverage factor playing the role of amplifying or shrinking the attenuation behavior. By optimizing the leverage factor, we can efficiently optimize the flow uniformity and get the uniform fuel flow in the steady state. Subsequently, we established a series of 30-cell stack models to verify the theoretical prediction of the attenuation behavior. These calculations covered the simulations ranging from the simple flow simulations to the fully coupled simulations, to ensure the completeness of the verification. More important, we got the new criterion and exact direction for designing the stack channels to realize the high uniformity of fuel gas assignment, according to the analysis of the attenuation equation. Finally, we established a new 30-cell stack model based on the design of the analytical prediction, and its excellent high uniformity in steady state is confirmed by the fully coupled modelling. The method which combined the attenuation equations and simulations, showed the great potential and high efficiency in optimizing the flow uniformity within planar production SOFC stack.In the sixth chapter, a brief summary of the research is presented.