Dynamic modeling and control of planar SOFC power systems.

Abstract

This dissertation addresses the modeling and control of planar Solid Oxide Fuel Cell (SOFC) power systems, aimed at developing analysis tools and control solutions to enable this promising technology for mobile applications. The main focus of the research is to explore the dynamic characteristics of the SOFC system and to develop control strategies that can ensure efficient steady state and fast and safe transient operations. A system model, comprising a co-flow planar SOFC stack and a CPOX (Catalytic Partial Oxidation) reformer, is developed to capture both steady state and transient behavior of the system as well as the detailed spatial distribution of variables in the SOFC. This model serves as a numerical tool for system analysis and optimization, and as a virtual plant for control system performance evaluation. Due to its high order and complexity, however, it is not amenable to model-based control design. Substantial effort is therefore devoted to model simplification. A reduced-order model is derived by first minimizing the number of temperature layers assumed in the SOFC and then imposing quasi-static relations to approximate the bulk flow dynamics. An efficient iterative algorithm is designed to solve the coupled mass balance and current distribution. These efforts lead to a low-order model with 20 states (reduced from 196) and about 80% reduction in the computation time. Given the importance of the temperature distribution in the SOFC on system operation safety and the practical difficulty of directly measuring this temperature distribution profile, an observer using a realistic sensor configuration is proposed to provide the temperature distribution information for both control and monitoring. From system performance analysis, transient issues, including slow power response and large temperature and temperature gradient in the SOFC, are identified in the open-loop system response to load changes. Feedback controllers are designed to improve the load following and thermal management performance of the SOFC system. Different control strategies are also explored and the associated closed-loop performance are analyzed. The modeling and control analysis results provide the foundation for developing a complete power and thermal management solution for planar SOFC systems.