Computational Modeling of Thermal and Electrical Fields of a High Power Density Solid Oxide Fuel Cell

Abstract

The electrical performance of solid oxide fuel cells (SOFC) has been traditionally characterized using isothermal cell tests and button cell tests. However, the evaluation of performance, operation, and structural integrity of cells in a typical SOFC stack are not only less amenable to confirmation through testing but are also significantly expensive than computational simulations. Computational models are invaluable in extending the measured isothermal cell test characteristics to predict both electrical performance and mechanical behavior of SOFCs in a stack under different operating conditions. The present investigation is part of an ongoing program of numerical developments and investigations to model the cell thermal and electrical characteristics in a stack environment. The ultimate objective is the development of an optimized cell geometry based on performance, structural integrity, and manufacturability. The flattened tubular high power density (HPD) cell, featuring five air channels fed by air feed tubes, was investigated. A CFD model of the HPD cell was developed using the commercial CFD software Fluent 6.2. A Fluent based SOFC model was used to simulate the electrochemical effects. The cathode, the anode, and the interconnection layers of the cell were resolved in the model and all modes of heat transfer, conduction, convection, and radiation were included. The results of the CFD model at isothermal conditions are presented and compared with experimentally measured isothermal cell V-J’s at 1000°C, 900°C, and 800°C. The model results agree well with the experimental data for cell temperatures of 1000°C and 900°C, after some tuning of exchange current density and tortuosity values. The agreement with the 800°C data however is not as good. The CFD model was then configured and analyzed with operating conditions typically encountered with a stack design that is currently under development. The resulting thermal, electrical, and flow fields are presented herein and discussed. It was found that the Fluent based SOFC model is a robust and effective tool for analyzing the complex and highly interactive three-dimensional electrical, thermal, and fluid flow fields, generally associated with the HPD cells. The computational time with the Fluent based model is however large in comparison with lumped-parameter approaches, mainly due to slow radiation convergence. Nevertheless, the comprehensive current density and thermal fields generated with the Fluent based model are necessary to enable a better prediction of thermal stresses within the cell, thereby permitting a more robust cell and module design.