Dynamic Model for Understanding Spatial Temperature and Species Distributions in Internal-Reforming Solid Oxide Fuel Cells

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Abstract
Direct internal reformation of methane in solid oxide fuel cells (SOFCs) leads to two major performance and longevity challenges: thermal stresses in the cell due to large temperature gradients and coke formation on the anode. A simplified quasi-two-dimensional direct internal reformation SOFC (DIR-SOFC) dynamic model was developed for investigation of the effects of various parameters and assumptions on the temperature gradients across the cell. The model consists of 64 nodes, each containing four control volumes: the positive electrode, electrolyte, negative electrode (PEN), interconnect, anode gas, and cathode gas. Within each node the corresponding conservation and chemical and electrochemical reaction rate equations are solved. The model simulates the counter-flow configuration since previous research (Achenbach, 1994, “Three-Dimensional and Time-Dependent Simulation of a Planar Solid Oxide Fuel Cell Stack,” J. Power Sources, 49 (1), p. 333) has shown this configuration to yield the smallest temperature differentials for DIR-SOFCs. Steady state simulations revealed several results where the temperature difference across the cell was considerably affected by operating conditions and cell design parameters. Increasing the performance of the cell through modifications to the electrochemical model to simulate modern cell performance produced significant changes in the cell temperature differential. Improved cell performance led to a maximum increase in the temperature differential across the cell of 31 K. An increase in the interconnect thickness from 3.5 to 4.5 mm was shown to reduce the PEN temperature difference about 50 K. Variation of other physical parameters such as the thermal conductivity of the interconnect and the rib width also showed significant effects on the temperature distribution. The sensitivity of temperature distribution to heat losses was also studied, showing a considerable effect near the fuel and air inlets. Increased heat transfer from the cell edges resulted in severe temperature gradients approaching 160 K/cm. The dynamic capability of the spatially resolved dynamic model was also demonstrated for a 45% power increase perturbation while maintaining constant fuel and air utilizations.

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