The Research Of Dynamic Analysis And Control Methods Of The Solid Oxide Fuel Cells

The study of the various energy sources has brought the fuel cells, which are clean, pro-environment and efficient, to the forefront of the renewable energy research, when the non-renewable resources have dropped sharply and environment degraded. Of them, the Solid Oxide Fuel Cell (SOFC) which possesses the quality of resourcefulness, high convertibility, wide range and friendly environment has become one of the most potential energy resources for commercial development. It needs800-1000℃even higher temperature in the enclosed environment for the work properly. It causes difficulties for the studies on material processing, structural design and other aspects of the process control because of its higher temperature and faster speed in the chemical reaction process. In its enclosed working environment, part of the unmeasurable process parameters directly affects the analysis and study of its electrical characteristics, thermal management, and stability. At present, the low-cost, high-efficiency methods with numerical simulation analysis supplemented by experimental testing and parameter fitting have become the main means of speeding up the development of SOFC and the power generation.In order to understand the steady-state characters of SOFC in the microscopic and the distribution of the key parameters, this thesis adopts multiple physical field coupling models to carry out the numerical simulation for the planar structure of SOFC. Based on this, it builds the dynamic modeling of the stack, and simplifies the model using time scales. In the process of control method research, first, it simplifies the model and establishes the nonlinear model for controlling the temperature; second, it puts forward the predictive control strategy of SOFC power system which contains internal fuel reforming and exhaust gas recycling. The main work of the paper is presented in the following:1. Based on the analysis of the principle and characteristics of SOFC electrochemistry, thermodynamics theory, multi-physics coupling model is established for SOFC single cell and its steady state is analyzed. In the establishment, the various factors of affected internal battery of heat and mass transfer are fully considered. The model contains series of conservation laws of fluid mechanics, thermodynamics, electrochemistry which adopts the method of setting boundary coupling condition in different physical domains, the scales. This method solves the problems of the difference model attributed and unmatched magnitude. Based on this, the work has built the three-dimensional simulation model of the single battery under the steady-state operating point, describing the distribution of oxidizing agent quality, gas flow rate, gas pressure and electric potential under the multiple steady-state operating points inside the single cell using3D images so as to master the working point of the steady state performance.2. On the working of the numerical simulation, the stack’s lumped model is studied on the basis of analysis of the results obtained from the experiment. First, the dynamic behavior of the internal stack attributed to resistive, capacitive, inductive elements in the circuit, which causes the impact on it. Of them, the focus is on the identification and equivalent of the element of capacitance in the modeling. Then the time scale analysis method is introduced, the expression of the dynamic behavior is classified as constant value or as a function, so that the resulting model can adequately characterize the cell stack within these potential resistive, capacitive and inductive element in the work process parameter variations. All of these provide the accurate, visible, and controllable mathematical model for the following work of control method of the research.3. It first analyzes the relationships of the changes influenced by the internal temperature of the stack parameters directly or indirectly, and then combined the lumped model with NARIMAX model the system according to the system control design, which obtains the identification model containing the stack temperature and output voltage. It also uses conventional methods and improved one, respectively, to control inlet gas flow rate, composition and temperature in real-time. In the improvement, the Levenberg-Marquardt algorithm is applied as a parameter prediction. By comparison of the two control effect, the proposed algorithm reduces the computation process into the possibility of local minima and make system changes caused by the load current disturbance has rapid and effective inhibition.4. In order to make the SOFC pile have better applicability for composite fuel, the two important auxiliary parts-fuel reforming and exhaust gas recirculation-are added to the established pile model, expending the SIMULINK simulation model to use CH4, CO, H2, N2etc mixed gases nonlinear model and express in nonlinear state space equation method. For the extended system, the nonlinear predictive control system of the self-adaptive UKF parameters and robust model are used respectively, then put forward the optimization objectives and constraints of control and robust model directly from the point of energy maximization, loss minimization. All of these result better effectiveness. The control method performs steadily when the target value conflicts with the boundary constraints in SOFC system and quickly get the suboptimal control closest to the target control input of the optimal value.This research work has its practical guidance and application significance to develop the SOFC pile and the integrated system in material preparation, structure design, performance analysis, and the control strategy, has laid the solid foundation for the further establishment of SOFC-GT system in the control design and system fault diagnosis.