【作者】 孙洁；

【导师】 俞自涛； 郑梦莲；

【作者基本信息】 浙江大学 ， 工程热物理， 2021， 博士

【摘要】 作为储能技术的一种,液流电池具有功率和能量可分别调控、充放电响应迅速、寿命较长等优势,这使其在新能源并网、电网削峰填谷等方面具有良好的应用前景。在液流电池系统中,电解液在储罐和电堆之间循环流动,从而为液流电池的充放电提供反应物。液流电池中反应离子的传质过程主要包括电解液在流道结构中的流动、电解液在多孔电极中的流动以及反应离子的扩散和迁移。其中,流道结构会对电极中电解液流速分布和反应离子浓度分布产生重要影响,进而影响液流电池的电化学性能和系统能量效率。本文通过模拟和实验相结合的方法研究流道结构对液流电池电极中离子传质过程的作用机制并在此基础上提出流道结构的改进设计。传统的流道设计通常延续采用燃料电池中的平行流道、交叉型流道和蛇型流道。前人大量的实验和模拟结果均表明,与无流道情况相比,交叉型和蛇型流道结构均能够有效降低电解液流经多孔电极时的压降,并且提升电极中电解液流速分布和反应离子浓度分布的均匀性。然而,以下问题仍有待深入研究:(1)研究发现,多层碳纸、碳布或者较厚的碳毡作为液流电池的电极能够有效增加氧化还原反应活性位点数量,但是厚度方向上反应离子浓度分布不均匀性增加。如何通过改进流道结构设计提升厚度方向上反应离子浓度分布均匀性是进一步提升液流电池性能的关键问题之一;(2)对采用蛇型流道结构液流电池电极中电解液渗入量进行研究时,发现相邻流道间压力分布的显著不均匀造成电解液在电极内的分布同样不均匀,如何提升蛇型流道下电解液渗入电极的分布均匀性也是本文的研究问题之一;(3)现有流道结构设计研究多局限于实验室尺寸的电池,尚缺少针对从实验室尺寸液流电池到实际应用尺寸液流电池中不同流道结构扩展方式的研究;(4)上述研究多基于实验和有限元仿真模拟研究,尚缺少一定流道结构下反应离子在电极中的浓度分布解析模型。为了解决以上问题,本文的主要研究内容和结果如下:为了解决采用传统流道结构液流电池中存在的多孔电极厚度方向传质不均的问题,本研究提出了新型的三维交错蛇型流道结构设计,把流道结构拆分为两部分并分别加工在多孔电极厚度方向上的两侧,即双极板侧和多孔电极上靠近离子选择性透过膜的一侧,将流道结构的渗入渗出口在多孔电极平面方向上的2维分布扩展到在电极平面和厚度方向上的3维分布。实验研究表明,在一定的运行条件下,新的流道结构设计可以明显提高液流电池的极限电流密度(约60%)、充放电容量(约40%)以及系统能量效率(约4%)。基于液流电池单电池三维多物理场耦合模型的模拟研究表明,新型流道结构设计可以有效改善多孔电极厚度方向的对流传质过程,提高厚度方向反应离子浓度分布均匀性。为了解决采用传统蛇型流道结构液流电池中相邻流道间电解液渗入分布不均问题,本研究提出了相邻支流间变肋宽设计(倾斜型变肋宽、阶梯型变肋宽、半阶梯型变肋宽),通过改变相邻支流之间肋宽分布提高相邻支流对应位置间流阻分布与压差分布的匹配性。实验结果表明,在一定的运行条件下,变肋宽蛇型流道可以明显提高液流电池的极限电流密度(约50%)、充放电容量(约25%)以及系统能量效率(约4%)。模拟研究表明,变肋宽设计可以显著提高多孔电极平面方向上电解液渗入分布均匀性,进而提高多孔电极平面方向反应离子浓度分布均匀性。为了促进流道结构在大尺寸液流电池中的广泛应用,本文还研究了从实验室尺寸液流电池到实际应用尺寸液流电池中流道结构的不同扩展方式(几何相似、横截面和肋宽恒定、压降恒定、分流)对多孔电极中反应离子传质过程的影响。研究表明,不同的流道扩展方式下,多孔电极中电解液流速分布和反应离子浓度分布以及电池进出口压降存在明显区别。其中,分流设计下大尺寸液流电池多孔电极中反应离子传质均匀性最高,电压效率与小尺寸液流电池基本一致。但是,该设计存在电池进出口压降较大的问题,使液流电池系统能量效率明显降低(3%-5%)。因此,本研究进一步提出了增加入口主流道数量及其横截面积的改进设计方案以降低分流设计下电池进出口压降。最后,为了深入探究在一定的流道结构下多孔电极中反应离子浓度分布规律,本文基于多孔电极中反应离子传质和守恒方程推导了一维流动下电解液流动沿程反应离子浓度解析解并进行无量纲化处理。在此基础上进一步获得了不同流道结构对应的典型流动路径下(U型流动路径和L型流动路径)反应离子浓度分布解析解,进而探究了电极和流道尺寸以及运行条件对电极中反应离子浓度分布和浓差过电位的影响。研究表明,在一定的电解液渗入量下,电极厚度和流道宽度会通过电解液流速影响多孔电极当地Pe数进而影响电极中的反应离子浓度,而电解液入口流量和电池电流密度的改变则会影响多孔电极中反应离子消耗量与供应量之比(无量纲参数Iq)进而改变多孔电极中反应离子浓度分布规律。更多还原

【Abstract】 With their decoupled energy and power capacities,quick response,and long lifecycle,redox flow batteries(RFBs)are expected to play an important future role in energy systems such as renewable energy integration,and grid peak load shifting.In RFB systems,as the electrolytes are stored outside of the stacks,the mass transfer of reactive ions from the tanks to the carbon fiber surfaces of the porous electrodes,is operationally important.The mass transfer process includes the electrolyte flow in the flow field,the electrolyte flow in the porous electrode,and the ion diffusion and migration in porous electrode.When the electrolyte flows from the inlet to the cell outlet,the flow field engraved on the bipolar plate impacts the electrolyte velocity distribution along with the ion concentration distribution in the porous electrode,therefore affecting the RFB system energy efficiency.Based on experimental and simulative approaches,this paper investigates the effects of flow fields on ion mass transfer in porous electrodes and offers improved RFB flow field design.RFB flow fields follow fuel cell designs such as parallel,interdigitated,and serpentine flow fields.Previous studies have proved the effectiveness of interdigitated and serpentine flow fields on improving ion distribution in porous electrodes and reducing the electrolyte pressure drop in RFBs.However,several problems still exist:(i)increased electrode thickness can increase the numbers of reaction sites in the porous electrode,but poor mass transfer in the through-plane direction limits the flow cell performance;(ii)with flow cells using a conventional serpentine flow field,the electrolyte penetration distribution between the two adjacent channels is not uniform,which causes ion starvation in the local porous electrode;(iii)existing flow field research is mostly based on lab-scale flow cells,and research on application-scale designs is lacking;(iv)most flow field research is based on experimental and numerical methods,and related analytical model is scarce.To try to resolve these problems,the contents of this paper are as follows:To improve ion mass transfer in the electrode through-plane direction,a three-dimensional detached serpentine flow field was realized.The flow field included two serpentine parts,one engraved on the bipolar plate,and the other engraved on the near-membrane side of the porous electrode,which extends the flow field from a 2-D to 3-D distribution.Experimental results show that this novel flow field design could significantly improve the limiting current density(60%),charge-discharge capacity(40%),and energy efficiency(4%)of the flow cell.Simulation results based on a 3-D multi-physical numerical model of the flow cell show that the new flow field design could significantly enhance electrolyte convection in the electrode through-plane direction,which is beneficial to ion concentration distribution uniformity.To modify the electrolyte penetration distribution in the flow cell using conventional serpentine flow fields,rib width changing serpentine flow fields(sloped serpentine,partially sloped serpentine,stepwise serpentine)are proposed.Through changing the rib width distribution between the adjacent channels of the serpentine flow field,the under-the-rib flow resistance is modified to match the local pressure difference distribution.Experimental results show that the new flow field design could improve the limiting current density(50%),charge-discharge capacity(25%),and energy efficiency(4%)of the flow cell.Furthermore,the simulation results show that the new design could enhance electrolyte penetration uniformity in the in-plane direction of the porous electrode,and therefore improve electrolyte velocity and ion concentration distribution.To facilitate the application of flow field design in large-scale flow cells,different scale-up approaches(geometric similarity design,constant cross-section and rib width design,constant pressure drop design,split flow design)are proposed and discussed.The simulation results show that the electrolyte velocity,ion concentration distribution,and pressure drop in differently designed flow cells vary.The split flow design performed best at improving the mass transfer uniformity in the porous electrode.However,the pressure drop in this design was significant,which reduces the system energy efficiency by 3%-5%.Therefore,a split flow design with two inlets and a broader channel width was proposed to further decrease the flow cell pressure.Finally,to investigate ion concentration distribution in the flow cell porous electrode,a 1-D analytical model was derived,and non-dimensionalized,based on the Nernst-Planck equation and mass transfer conservation.Furthermore,analytical models with different flow path types(U-shaped and L-shaped)were also established.Based on this,the effects of electrode thickness,channel width,electrolyte flow rate,and applied current density on ion concentration distribution were studied.The results show that increased electrode thickness and channel width decreases the porous electrode local Pe number,and therefore decreases ion concentration.In addition,a decreased electrolyte flow rate and increased applied current density raises the ratio of the amount of ion consumption to the amount of supply(the Iq number),further reducing porous electrode ion concentration.更多还原