Dynamic Modeling And Control Of Planar Solid Oxide Fuel Cell Systems

Due to the advantages of quiet operation, environmental friendly and high efficiency, SOFC (Solid oxide fuel cell) becomes the most promising power technology in21century. Although the SOFC stack achieves a good performance in laboratory conditions, the stand-alone SOFC system still faces the challenges of long-life time, high efficiency and load following in the process of the large-scale implementation. In order to achieve an stable and long-life operation for a stand-alone solid oxide fuel cell system, the temperature constraints, load following and high efficiency are studied in this dissertation from the aspects of thermo-electrical modeling, steady-state analysis, observer design and controller development. Based on an engineering project, a steam reformer and a5kW scale pure hydrogen planar SOFC stand-alone system is studied in this dissertation.Firstly, a high-fidelity physical model of a improved SOFC system comprising a co-flow SOFC stack, a tail-burner, two heat-exchangers, a blower and a bypass valve, is developed to capture both steady state and transient behavior of the system as well as the temperature distributions in SOFC along the direction of gas flow. In order to ensure the accuracy of the system model, the electrical characteristics of stack are validated by plenty experimental data from two SOFC stacks (22and24cells) assembled in lab. The results confirm the effectiveness of the work in this dissertation.Based on this model, the steady state performance of the system is analyzed and optimized, which gives insights into the sensitivity of input variables to system temperature constraints, load following and system efficiency. And then, the system open-loop response between two optimal operation points are investigated. Moreover, the simulation results show that the bypass valve in SOFC system can be used to improve the system efficiency and manage spatial temperature distribution both. As the spatial temperature distribution profile is critical for the safe operation of SOFC system, and cannot be obtained by practical directly measuring at an acceptable cost, an accurate and fast linear observer is developed according to a novel sensor configuration estimation method to estimate the spatial temperature distribution profile in SOFC. Finally, two control strategies named "T-S fuzzy model based constrained generalized predictive control (TS-CGPC)" and "optimal operation points based temperature constraints feed forward and load following feedback control (OOPFF-FB)" are implemented to steam reformer and the5kW SOFC system separately. Due to the steam reformer model can be modified by experimental data from the steam reformer prototype, a "T-S CGPC" controller is developed for practical implementation. In order to achieve fast calculation, the CARIMA model in GPC is revised with the consequent parameters of online T-S fuzzy model and the input constraints is handled by the Lagrange multiplier technique. The simulation results shows the reforming process can be well managed by TS-CGPC controller, which is better than PID controller. However, the SOFC system is under assembly process, and the SOFC system model cannot be modified. The system order and time-delay coefficients are not available for the development of TS-CGPC controller for SOFC system. Therefore, based on the steady-state analysis and temperature observer, the control strategy of "OOPFF-FB" is proposed for the cooperative control of temperature, power, efficiency in SOFC system. The simulation results demonstrate that the temperature variation of SOFC is effectively restrained during fast load following, and the system efficiency is guaranteed by the optimal operation point. In a word, the SOFC system achieves high system efficiency and fast load following capability without temperature constraints violation.The modeling and control analysis results in this work can be extended to any SOFC systems with different configurations and other similar nonlinear system, and provides a valuable solution for the control of SOFC system to achieve high efficiency, longlife time and fast load following capability.