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The Research Of Modeling And Control Method For Intermediate Temperature Solid Oxide Fuel Cell System

Posted on:2010-07-30Degree:DoctorType:Dissertation
Country:ChinaCandidate:J YangFull Text:PDF
GTID:1102360302971062Subject:Materials science
Abstract/Summary:PDF Full Text Request
Solid Oxide Fuel Cell (SOFC) is a device of chemical energy conversion electrical energy with high efficient, clean, quiet and reliable. SOFC have many advantages, such as fuel source flexibility, combined heat and power, high performance, superior poison tolerance, high stability of solid state components, high power density, environmental friendly, etc. It is considered as one of the most promising and having commercial value for the fuel cell technology.The high temperature environment also brings great challenge regardless of the fact that the current operating temperature for SOFC has dropped to about 750℃. The high temperature and temperature change will affect the stack mechanical properties which lead to thermal fatigue, delaminating or even break. Besides, the temperature also impact to the electrical properties of SOFC, such as voltage, current density distribution and electric power, etc, therefore, the management and control of temperature is especially important for SOFC. At the same time, the study for SOFC at home and abroad gradually develops from materials, structures and models to the stack and system technology. The high efficient control system could operate the stack so reliable and secure that it brings the gas temperature and pressure stability, high performance of the output, and long life for the stack on load fluctuating. Furthermore, an external power load tracking could be researched and analyzed when the stack technology has advanced a certain extent. In this thesis, the intermediate temperature planar SOFC stack and system are considered as the research object. This thesis explores the control strategy of the thermal management of SOFC stack and the method of external power load tracking in order to achieve optimum temperature control and optimization of energy supply.The thesis firstly introduces the development of SOFC, in particular the current research status of the stack and system. Then it discusses the basic theory, research methods and algorithm used in modeling process. After analyzing the structure and composition of the various subsystems of model in detail, a simulation model of the SOFC stack is established by the software-Matlab. An off-line improved Takagi-Sugeno (T-S) fuzzy model is designed and implemented for replacing the physics model on the basis of modeling the thermal management system of SOFC stack, and the simulation results validate that the algorithm has high approximation. Moreover, an algorithm based on the on-line modeling of optimizing T-S fuzzy model and discrete optimization of nonlinear Model Predictive Control (MPC) could finish the real-time control of the stack temperature, and the simulation results show good performance. Finally, a method for dynamic power load tracking is discussed using a reference model law. The conclusions of the thesis are listed as below.(1) A model of kW class SOFC system is built. The SOFC system has five parts, including SOFC stack, thermal management subsystem (including stack thermal management and external thermal management), fuel processing subsystem, power electronics subsystem and control subsystem. The modeling process of SOFC stack is analyzed, and the SOFC stack consists of electrical characteristics subsystem, the cathode flow subsystem, the anode flow subsystem, and the stack thermal management subsystem. This paper discusses the structure and realization of four subsystems in Matlab program according to the basic principles and methods of modeling. Furthermore, the component and working principle of external thermal management system are described.(2) The thermal management subsystem of SOFC stack is simulated by an improved T-S fuzzy model. Enthalpy balance equation for temperature control is set up on the basis of mass conservation law and energy conservation law, and they can fulfill the calculation of enthalpy in and enthalpy out so that the physics model for temperature control is constructed. Actually, the model has two inputs, the air flow and hydrogen flow, and one output, the stack temperature. Then, a model built by an improved T-S fuzzy model is trained and identified through the data which the physical model produces. Finally, the result show that the temperature response value of the improved T-S fuzzy model is very close to the value of the physical model generated.(3) The Model Predictive Control (MPC) modeling is made for on-line and real-time the temperature control of SOFC stack. Firstly, a MPC frame is established. Thus, the optimized T-S fuzzy model which is constructed is simulated for a real SOFC stack, and its computation step and formula are introduced. The way of discrete optimization of control variable is explained and is applied to the temperature control of SOFC stack. Finally, the simulation result show that the temperature tracking curve between the MPC output and the actual output of the physical model approaches extremely.(4) The thesis discusses the method of external power load dynamic tracking. According to the change of external load starting, loading, unloading, and braking at different times, a reference model method is provided to implement the external power load dynamic tracking. Hence, a model of load dynamic tracking is set up, and the reference model is the model of SOFC electric characteristic. The relations between an external power load and the corresponding hydrogen flow are established, and the function of the external load tracking can realize.
Keywords/Search Tags:Solid oxide fuel cells (SOFC), System modeling, Takagi-Sugeno (T-S) fuzzy model, Model predictive control (MPC), Load tracking
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