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.
Highlights
Solid oxide fuel cells show promise as a future stationary power generation device
The interconnect and thermal conductivity both have a marked effect on the cell temperature differential. This is expected considering that as the IC thickness or thermal conductivity is increased the resistance to heat transfer is decreased thereby allowing heat generated in a hot part of the cell to be transferred to a cool part of the cell more alleviating large temperature gradients throughout the cell as evidenced by the decrease in the total cross cell temperature difference. These results suggest that DIR-SOFCs should be constructed using metallic interconnects that have a considerable thickness such that severe thermal gradients are avoided
Continued improvements in the performance of SOFCs will have an effect on the temperature gradients within the PEN
Summary
Solid oxide fuel cells show promise as a future stationary power generation device. Given the recent SECA improvements [1] this technology’s proximity to commercial emergence grows closer at an ever increasing rate. The use of high steam to carbon ratios dilutes the fuel in the anode channel thereby reducing performance. Despite these drawbacks to operating at high temperatures and steam to carbon ratios, this paper will consider these conditions so as to avoid the issue of coking and focus on the second impediment, which is the presence of high thermal stresses due to the excessive thermal gradients across the PEN resulting from the imbalance of the endothermic reformation and exothermic electrochemical reactions. (7) Coking is negligible due to the high steam to carbon ratio used in the following analyses. The first term in the preceding equation is the Nernst voltage calculated using the following equation:
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