被动式直接甲醇燃料电池的瞬态质量传输的数值模拟研究

发布时间：2018-08-27 06:28

【摘要】：直接甲醇燃料电池(Direct Methanol Fuel Cell,DMFC)作为燃料电池中重要的组成部分,具有高的能量密度,燃料易于储存,产物无污染等多重优点,使其最有希望成为新型的便携式能源。特别是被动式DMFC减少了辅助设备热量的消耗,使设备简单化,逐渐成为人们的研究热点。然而在被动式DMFC运行中,由于其自发的供料,电池内部传质传热无法达到最优状态,影响其电池性能,因此对被动式DMFC进行数值模拟研究,不但可以更好地了解物质传输以及电化学反应的规律,优化电极结构和操作环境,更节约的时间和成本,对实现高性能的被动式DMFC有重大的意义。首先本文通过建立瞬态的多相流被动式甲醇燃料电池的模型,研究分析了不同电流密度、电压、微孔型涂层(micro-porous layer,MPL)和燃料进料条件对燃料电池整个放电过程(燃料从满到空的过程)的影响。结果表明,对于所有操作环境来说,在适当的电流密度和电压操作条件下,能够有效的限制甲醇渗透的通量保证能量的转化效率。阳极MPL在MPL和气体扩散层(gas diffusion layer,GDL)表面产生足够的流动阻力,减少了甲醇渗透速率,从而提高电池的燃料效率。另外增加燃料罐的体积可以有效的提高电池的能量密度,但是对燃料效率和能量效率影响较低。其次,考虑到在燃料电池的实际操作过程中,便携式DMFC会在不同的方向进行操作。基于瞬态多相流被动式DMFC模型,本文研究竖直和水平的操作方向对电池性能的影响。数值模拟的结果表明在低电流的条件下,DMFC在竖直方向操作时,甲醇渗透速率较低,所以其性能要比水平方向操作较好;而高电流时,结果相反,由于竖直方向操作在阴极会产生大量的水而无法排出,影响其性能。另外,疏水性微孔层(MPL)对不同方向的电池操作影响也不同。最后,本文又提出了采用纯甲醇蒸汽进料的方式,通过增加进料浓度极大的提高了电池的性能和寿命。通过建立被动式瞬态的甲醇蒸汽进料模型,研究不同工况下质量传输的现象,预蒸发膜和气体传输涂层(Vapor transport layer,VTL)对甲醇蒸汽起到生成和传输作用。结果表明增加电流密度、降低蒸发器的开孔率或者增加膜的厚度能够有效的减少甲醇渗透速率,提高电池的性能。阴极MPL对增加水的回流减少水流失有着重要的作用。本文通过优化操作条件和电池结构,进行瞬态模拟分析,更好的了解物质传输的特点,对被动式直接甲醇燃料电池的瞬态研究提供了良好的理论依据。
[Abstract]:Direct methanol fuel cell (Direct Methanol Fuel Cell,DMFC), as an important component of fuel cell, has many advantages, such as high energy density, easy storage of fuel, no pollution and so on, which makes it the most promising portable energy. In particular, passive DMFC reduces the heat consumption of auxiliary equipment, simplifies the device and gradually becomes a research hotspot. However, in the operation of passive DMFC, due to its spontaneous feeding, the internal mass transfer and heat transfer of the battery can not reach the optimal state, which affects the performance of the battery, so the numerical simulation of passive DMFC is carried out. It can not only better understand the law of material transport and electrochemical reaction, optimize the electrode structure and operating environment, save more time and cost, but also have great significance for the realization of high performance passive DMFC. In this paper, the transient multiphase flow passive methanol fuel cell model is established, and the different current density and voltage are studied and analyzed. Effects of microporous coating (micro-porous layer,MPL) and fuel feeding conditions on the whole discharge process of fuel cells (from full to empty). The results show that under the appropriate current density and voltage operating conditions, the flux of methanol permeation can be effectively limited to ensure the energy conversion efficiency for all operating environments. The flow resistance of anode MPL on the surface of MPL and gas diffusion layer (gas diffusion layer,GDL) is sufficient to reduce the methanol permeation rate and thus improve the fuel efficiency of the cell. In addition, increasing the volume of the fuel tank can effectively increase the energy density of the cell, but has little effect on the fuel efficiency and energy efficiency. Second, considering that the portable DMFC operates in different directions during the actual operation of the fuel cell. Based on the passive DMFC model of transient multiphase flow, the effect of vertical and horizontal operating direction on the performance of the battery is studied in this paper. The results of numerical simulation show that under the condition of low current, methanol permeation rate of DMFC is lower in vertical direction, so its performance is better than that in horizontal direction, but at high current, the result is opposite. Due to vertical operation in the cathode will produce a large amount of water and can not be discharged, affecting its performance. In addition, the hydrophobic microporous layer (MPL) has different effects on the operation of the cells in different directions. Finally, the method of pure methanol steam feeding is proposed, which can greatly improve the performance and life of the battery by increasing the feed concentration. A passive transient methanol vapor feed model was established to study the mass transport phenomena under different operating conditions. Pre-evaporation film and gas transfer coating (Vapor transport layer,VTL) were used to generate and transport methanol steam. The results show that increasing the current density, reducing the opening rate of evaporator or increasing the thickness of the membrane can effectively reduce the methanol permeation rate and improve the performance of the battery. Cathode MPL plays an important role in increasing water reflux and reducing water loss. By optimizing the operating conditions and the structure of the battery, the transient simulation analysis is carried out to better understand the characteristics of mass transfer, which provides a good theoretical basis for the transient study of passive direct methanol fuel cells.
【学位授予单位】：吉林大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM911.4

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