固体氧化物电解池平行流道流动不均匀性优化模拟

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

氢能是一种具有高能量密度、无污染的可再生能源。当前,固体氧化物电解水制氢技术(SOEC)对于流场分布的关注较少,而平行流道的流动不均匀性会显著影响SOEC的反应效率和使用寿命。本研究结合燃料电池中对反应物流动情况的研究建立了电化学场、传热场、流场的多物理场耦合仿真模型研究反应物在电解中的分布情况,按照不同的流道宽度比改变平行流道的流道宽度,设计了一种新型Z型平行流场。结果表明:与传统Z型流场相比,8000 A/m^(2)电流密度下水蒸气摩尔分数不均匀系数由0.1520下降至0.0304,电解质局部电流密度差值由6000 A/m^(2)下降至2000 A/m^(2),温差由105.9 K下降至97.2 K,均匀性有明显的提升。各个电流密度下,在出口处的产氢速率上升7%左右,在相同时间内可以产生更多的氢气,提高了制氢的效率。

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 1000 A/m^(2)to 8000 A/m^(2)at different channel width ratios(i.e.,1.0000,0.9570,0.9200,0.8876,and 0.8586)are simulated.The lowest inhomogeneity coefficients of the solid oxide electrolytic cell are obtained at all current densities when the channel width ratio is 0.9200.Also,the non-uniformity increases with the increase of current density.When the current density is 8000 A/m^(2),the non-uniformity coefficient is 0.152 for the traditional channel,but the non-uniformity coefficient for the optimized channel is 0.0304 at a channel width ratio of 0.9200,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 0.9200 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.