Dynamic modeling and analysis of multiple SOFC system configurations

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Slippey, Andrew J.
Dynamic model; Fuel cell; Fuel utilization; POX reformer; Solid oxide
thesis / dissertation description
Solid oxide fuel cells (SOFC) are a variety of high temperature fuel cells with particular advantages such as fuel flexibility, internal fuel reforming capability, and combined heat and power (CHP) applications. To aid in the advancement of this technology, this work develops dynamic, computer-based, mathematical models of two SOFC configurations employing different SOFC and reformer technologies. Starting from an existing recirculation-based tubular SOFC system with a steam reformer, new component models are developed for a planar SOFC stack and a partial oxidation (POX) reformer. Both the new and existing component models were updated and improved by including new pressure dynamics and current distribution schemes. A structured method for model development and management through hierarchical libraries developed herein allows easy modification of the models on multiple levels for simulation of various SOFC system configurations. The pertinent physical phenomena are captured, including temperature, pressure, chemical, and electrochemical dynamics. Analysis of the simulation results provides insights into the varied time scales and lays the ground work for future development of hybrid control schemes. Simulation also shows the interconnection of individual physical phenomena, giving a complex and rich dynamical behavior to SOFC systems. Model-based analysis of the two configurations reveals multiple common behaviors of SOFC systems, valid across configurational variations. Of particular interest for control is the performance parameter, fuel utilization. A generalized approach for generating closed-form expressions for fuel utilization is developed to accurately predict steady-state conditions as a function of input conditions. The closed-form solutions obtained by this approach for different configurations demonstrate fuel utilization as an invariance property that can be exploited in feedback control of SOFC systems where knowledge of the system and sensing capabilities are limited.