Simulations on Flow Non-uniformity in Parallel Channel of Solid Oxide Electrolysis Cells

Introduction The importance of environmental protection leads to the development on new energy hydrogen production technology. High-temperature solid oxide hydrogen production technology is widely investigated because of its non-polluting production process and close to 100% hydrogen production efficiency. However, the solid oxide electrolytic cell (SOEC) cannot be commercialized because of its short effective service life, in which the electrode degradation effect caused by the uneven distribution of reactant water vapor is one of the important reasons for the short life span of the SOEC. The flow inhomogeneity of the conventional parallel flow channel significantly affects the reaction efficiency and service life of solid oxide electrolytic cells, and the flow uniformity is dependent on the geometry of the SOEC flow channel. For the flow characteristics of the conventional Z-type solid oxide electrolytic cell, the molar fraction of water vapor gradually decreases from the two ends to the middle region, which affects the performance of SOEC. To improve the uniformity of water vapor distribution from the structure, a new channel structure was designed to change the width of the channel in accordance with a certain cross-sectional area ratio. The cross-sectional area of the channel from the two ends to the middle could be gradually increased to enhance a supply of water vapor in the middle region, so as to make the overall distribution of water vapor in the SOEC more uniform, eliminate the electrode degradation effect and prolong the service life of the solid oxide electrolytic cell. Methods The total cross-sectional area of the entire flow channel remained unchanged. Based on the characteristic of the water vapor mole fraction of the conventional Z-type flow channel that decreases regularly from the two sides to the middle, a control variable method was adopted according to a certain ratio of the width of the flow channel to calculate the size of the cross-sectional area of each channel. Different water vapor mole fractions of the different flow channels were used. A three-dimensional model of a Z-type flat plate solid oxide electrolysis cell with uniform and non-uniform channel widths was proposed. The continuity equation, momentum equations, energy equation, species transport equation coupled with the Nernst equation and the BV equation were numerically solved by a software named COMSOL to simulate the water vapor molar fraction, local current density, pressure drop, temperature and heat production power in the flow channel. The flow field characteristics of the conventional Z-type flow channel and the improved channel structure were obtained via simulating the process of solid oxide electrolysis of water. Results and discussion The non-uniformity coefficient at electric current densities from 1 000 A/m2 to 8 000 A/m2 at different channel width ratios (i.e.,1.000 0, 0.957 0, 0.920 0, 0.887 6, and 0.858 6) are simulated. The lowest inhomogeneity coefficients of the solid oxide electrolytic cell are obtained at all current densities when the channel width ratio is 0.920 0. Also, the non-uniformity increases with the increase of current density. When the current density is 8 000 A/m2, the non-uniformity coefficient is 0.152 for the traditional channel, but the non-uniformity coefficient for the optimized channel is 0.030 4 at a channel width ratio of 0.920 0, indicating that the distribution of water vapor mole fraction in the solid oxide electrolysis cell is the most uniform, with the difference in water vapor molar fractions between the middle and two end channels is only 10.2%. The hydrogen production rate is increased by 7%, compared to the traditional channel. Meanwhile, as the water vapor distribution becomes more uniform, the temperature difference of the solid oxide electrolytic cell decreases from 105.9 K to 97.2 K. The temperature uniformity is also significantly improved, thus reducing the electrode degradation effect and the risk of thermal stress concentration due to the electrochemical heat. Conclusions Changing the width ratio of Z-type channel could affect the distribution of water vapor. In the absence of water vapor in the middle region of the Z-channel, the flow field uniformity of the Z-channel optimized at a width ratio of 0.920 0 could be greatly improved. The main reason was that the cross-sectional area of the flow channel in the middle region was proportionally expanded, so that more water vapor was provided to the part lacking water vapor in the middle, and the electrolytic capacity of the electrolytic cell could be developed. The effect of electrode degradation was eliminated. This study indicated that the flow field inhomogeneity of the parallel flow channel itself could be mitigated via changing the geometry of the solid oxide electrolytic cell, which was more advantageous in terms of reprocessing cost and effectiveness rather than adding a flow distributor or using metal foam filling to improve the flow field distribution uniformity. © 2024 Chinese Ceramic Society. All rights reserved.