Influence of Convective Heat Transfer by Air Flow on Local Current/Temperatures along Microtubular Solid Oxide Fuel Cells In-situ Identified by Electrodesegmentation Method for Co- and Counter-flow Configurations
Major advantages of solid oxide fuel cells (SOFC), such as employing non-precious metals as catalyst, tolerating various fuels, and releasing high quality waste heat, are virtues of high operation temperature (873-1273 K). On the other hand, thermo-mechanical stresses, as the primary problem of the SOFC systems develop due mainly to the heat treatment and operation at high temperatures. For both planar and tubular designs, a number of numerical studies addressing the thermo-mechanical stresses have been published; the thermal expansion coefficient mismatch, residual stresses, and the spatial surface temperature variations during the operation, have been stated to be the primary reasons [1-3]. To estimate the stresses stemming particularly from the spatial temperature variations, researches have been relying on the local temperatures computed numerically with thermo-electrochemical models [1,2,4,5,6]. However, as repeatedly stated, due to the difficulty in conducting experiments at high temperatures, the models were hardly validated in terms of surface temperatures. We thus measured the local current and temperatures along a microtubular SOFC with the electrode-segmentation method. Given the surplus heat generated during the cell operation is commonly removed by excess air flow method, and this method likely affects the temperature distribution along the cells, we conducted experiments for co- and counter-flow configurations under various air flow rates.
The tubular anode substrate of the cell was made of NiO/YSZ (65:35 wt %). Upon coating with the 8YSZ electrolyte colloid by dip-coating, we sintered the tube at 1693 K for two hours. With a special mask designed for the segmentation of the cell, we brush-coated the La0.7Sr0.3MnO3/YSZ cathode slurry (10:3 wt %) on the electrolyte surface by a cotton swab and subsequently sintered at 1323 K for two hours. Eventually, we connected the silver wires and thermocouples to the regarding segments for local current/voltage, and temperature measurements.
As depicted in Fig. 1, we fed fuel and air to the anode and cathode, respectively, in the co-flow configuration. For counter-flow configuration, we exchanged the fuel inlet and outlet, retaining inlet and outlet of the air flow. Note that we keep the segment labels as on in Fig. 1 throughout the discussion, despite the switch of the flow configuration. Though the inlet air temperature was 298 K, the fuel flow was pre-heated before entering the cell. Prior to fuel supply, we sustained the cathode surface temperatures of the segments at 1073 K by an electric furnace.
Results and Discussion
The temperature distribution profile along the cell alters with the gas flow configuration. With the rising air flow rate at 0.6 V in the co-flow configuration, the midstream and downstream segment temperatures arise, while the upstream segment temperature decreases with a relatively steeper slope. This entails a larger maximum temperature difference along the cell. We observe the same trend in the local currents, however, the slope of the current variations in all the segments are rather small and resemble each other.
With the increasing air flow rate, at 0.6V in the counter-flow configuration, all the segment temperatures drop down, where the slope of the upstream segment is relatively higher than the other segments. As a result, the maximum temperature along the cell becomes larger. The midstream and downstream segment currents increase with rather small slopes, whereas the upstream segment current remains nearly constant.
The in-situ identified current/temperature distribution profiles for co- and counter –flow configurations differ from each other considerably. With the increasing air flow rate, the maximum temperature difference along the cell grows in both flow configurations; where the total current output variation is rather small. The counter-flow configuration exhibits larger maximum temperature difference along the cell comparing to the co-flow case. With these findings, we can deduce larger thermo-mechanical stresses at high air flow rates in the counter-flow configuration; which we will explore numerically on the basis of local temperatures in-situ measured here.
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