On efficiency of both single fuel cells and stacks II

The model developed in the first part of this work is used to predict the cell potentials and the irreversible Gibbs free energy of a stack of 15 cells. The model starts from a phenomenological equation of a polarization curve with the extent of reaction as the independent variable. Two extreme kinds of flow of reagents, defined as Chain and Separate Flows respectively, are considered. The cell potentials are obtained by a combination of the potential of the two extreme cases of flow. The stack cell potentials and the efficiencies, estimated by the model, reproduce the general characteristics obtained by the experiments.     

固体氧化物燃料电池的变工况特性

以大面积电池和千瓦级电堆为研究对象,在确定的燃料成分、流量、和工作温度下,系统研究了电流阶梯变化、电流脉冲变化、电堆热启停以及冷热循环(冷启停)等工况下电堆的输出性能。结果表明:在小电流区域,电堆的电压和功率能够快速跟踪电流变化;在大电流区域,电池的电压出现波动和弛豫,电堆的功率也出现弛豫。热启停实验结果表明,SOFC电堆对电流的on-off变化具有足够的耐受性,一定数量的热启停不会导致电堆性能的明显衰减。而冷热循环会导致应力释放,引起接触电阻变化,从而使电堆性能衰减,5次以上热循环可使应力释放趋于缓和。     

Low-cost light weight high power density PEM fuel cell stack

A new approach to PEM fuel cell stack fabrication has been demonstrated. This approach is based on the use of light weight metal conductive elements together with nonconductive elements fabricated from engineering thermoplastics to yield a lightweight stack that can be manufactured from flat sheets of stock materials using inexpensive mass production techniques. The stack described here has a power output of over 520 W from a volume of 616 mL and a mass of 539 g for a power density of 967 W/kg and 846 W/L while operating at a pressure of 103 kPa with a 53% conversion efficiency. It is projected that with further refinement, PEM fuel cell stacks with power densities over 1,500 W/kg and 1,200 W/L can be produced.     

应用于大尺寸平板型固体燃料电池电堆的Al2O3-SiO2-CaO基密封材料软化特性及其验证

设计研制了Al₂O₃-SiO₂-CaO基密封材料,对其高温晶化与软化、热性能、界面黏结特性开展了原位观察,并进行了电堆实际应用验证。结果表明:在不高于1 100℃时该密封材料均为非晶态,850℃开始软化,900~1 000℃出现球化。热重分析表明密封材料在0~960℃的质量损失较小,约为0.06%;密封材料与连接板、电池界面黏结紧密,利于固体氧化物燃料电池(SOFC)电堆密封应用。采用研制的密封材料组装了2个5单元SOFC短堆,分别进行了热循环与稳定性研究。结果表明:2个5单元电堆的开路电压达到6.0 V,平均开路电压1.2 V,电堆1热循环前后在35 A(0.56 A/cm²)条件下输出功率为运行无衰减,电堆2在27 A(电流密度0.43 A/cm²)进行恒流放电,运行300 h较为稳定。     

Experimental and numerical studies of portable PEMFC stack

The objective of this work is to establish the design principles of a proton exchange membrane (PEM) fuel cell (FC) stack for portable applications. A combination of experiments and numerical simulations were carried out and the results analyzed to enhance understanding of the behavior of this portable PEMFC stack. A three-dimensional (3D) computational fluid dynamics (CFD)-based methodology was used to predict such as the current and temperature distributions of this portable PEMFC stack. The results show how the baseline operation and original design of this stack impact the local temperature, water content, water transport, and kinetic variables inside the individual cells. The outcome of this work will pursue the development of universal heuristics and dimensionless numbers correlated to portable PEMFC stack design.     

General relations for power generated and lost in a fuel cell stack

We derive the general electrodynamic relations for useful power generated by individual cell in a fuel cell stack and for power loss in a bipolar plate. The practical calculations with these relations are illustrated with the specially constructed one-dimensional case. The results are valid for stack of the cells of any type.     

5kW氢-空质子交换膜燃料电池堆的模型研究

采用机理模型和经验模型相结合的建模方法建立了一个5kw质子交换膜燃料电池堆实际装置的电化学模型。利用电池堆与单电池之间的内在关系,首先给出了单电池的数学描述,进而建立燃料电池堆的数学模型,其中包括热力学平衡电势、活化极化电势和欧姆极化电势等表达式,以及单电池内阻的经验公式。由于难以得到机理方程中的某些关键参数,因此采用实验设计,获得燃料电池堆的实验数据,运用线性回归的数学方法获得机理模型中活化极化电势方程中的相关参数,通过模拟研究和模型验证,所建立的模型可以较准确的描述燃料电池的极化曲线,预估出燃料电池的输出电压。     

An investigation of scale-up on the response of the direct methanol fuel cell under variable load conditions

The electrical response of the direct methanol fuel cell, based on solid polymer electrolyte, to variable load is reported. The dynamic power response of the direct methanol fuel cell is of importance particularly when the cell is used for transportation applications. The study reports the dynamic characteristics of a small-scale cell (active area 9 cm²), a large-scale cell (active area 272 cm²), and a three-cell stack. The effect of operating conditions (i.e., flow rate, cathode pressure and solution concentration) on the voltage response is described and the effect of a change of scale is discussed.     

Analysis of PEM fuel cell stacks using an empirical current–voltage equation

An empirical equation was developed to describe the electrode processes (activation, ohmic and mass-transfer) of PEMFC stacks over the entire current range. The potential–current and power–current curves of a strip PEMFC stack were fitted with the empirical equation under a variety of experimental humidity, temperature and stack length conditions. The concept of mass transfer impedance was defined mathematically in the present research. For the strip PEMFC stack, mass transfer impedance was only important at high currents. With decreasing humidity the mass transfer impedance increased considerably. With increasing temperature or stack cell number the mass transfer impedance increased only slightly.     

固体氧化物燃料电池堆的稳定工况特性

以大面积电池和千瓦级电堆为对象,研究了温度、燃料成分、流量等对阳极支撑型电堆性能的影响。结果表明:温度的影响最大,复数阻抗谱中高频弧对应的活化能最高;欧姆阻抗的活化能较低,表明其不全是离子电导的电阻,还包括双极板的表面电阻和可能的接触电阻。利用干氢气燃料测试时,在开路电压附近表现出较大的活化极化,且其活化能很小,表明该活化极化的速率控制步骤并非是电荷转移过程,而是对应某种表面扩散过程。模拟重整气燃料测试过程中活化极化不明显,但开路电压较低,性能比氢气燃料差。随着电堆工作电流的增加,燃料尾气的温度增加,表现出明显的热效应。     

Mitigating distortions during debinding of a monolithic solid oxide fuel cell stack using a multiscale,multiphysics model

Improving the power density of solid oxide fuel cell stacks would significantly enhance this technology for transportation. Using a monolithic structure to downsize the stack dimension offers a key to elevate the power density of solid oxide fuel cell stacks. This innovative design is promising but manufacturing is a challenge. The monolith is co-sintered in one firing step, and the gas channels are formed by burning off sacrificial organic materials. Structure distortion or fracture was observed in post-mortem investigations. In this work a multiscale, multiphysics modelling approach is proposed to describe and resolve this challenge in the debinding process occurring in a monolithic stack, i.e. the burning of organics and transportation of gases through the gradually opening microstructure, as well as the pressure build-up in the microstructure due to gas development. Simulation results show that a prominent pressure peak is experienced in the stack when a plasticiser (polyethylene glycol) and a pore-former (polymethyl methacrylate) are decomposed simultaneously. To reduce the high pressures, we investigate two possible strategies: (i) changing the mixture of organic additives; (ii) modifying the debinding temperature profile. Three tapes with different pore-formers are prepared, and the generated pressures during debinding of the three stacks are compared. The corresponding stack shapes after debinding are recorded. Numerical investigations show a good agreement with the post-mortem observations. By changing the composition of organics the distortion or fracturing of the stack can be avoided. Furthermore, to facilitate stack manufacturing, the high pressures can also be reduced by adjusting the heating rates and dwell temperatures of debinding. By using the new temperature profile suggested by the simulation study, the duration of debinding can also be reduced.     

Development and operation of a 150 W air-feed direct methanol fuel cell stack

A five-cell 150 W air-feed direct methanol fuel cell (DMFC) stack was demonstrated. The DMFC cells employed Nafion 117^® as a solid polymer electrolyte membrane and high surface area carbon supported Pt-Ru and Pt catalysts for methanol electrooxidation and oxygen reduction, respectively. Stainless steel-based stack housing and bipolar plates were utilized. Electrodes with a 225 cm² geometrical area were manufactured by a doctor-blade technique. An average power density of about 140 mW cm^–2 was obtained at 110 °C in the presence of 1 M methanol and 3 atm air feed. A small area graphite single cell (5 cm²) based on the same membrane electrode assembly (MEA) gave a power density of 180 mW cm^–2 under similar operating conditions. This difference is ascribed to the larger internal resistance of the stack and to non-homogeneous reactant distribution. A small loss of performance was observed at high current densities after one month of discontinuous stack operation.     

PEMFC电堆动态工况响应性能试验

质子交换膜燃料电池（PEMFC）电堆动态响应特性对PEMFC电堆的耐久性和可靠性具有很大影响。本文试验考察了PEMFC电堆在动态工况下的输出性能、单电池电压均衡性变化和动态响应特性。结果表明,在整个动态运行工况下,电堆运行良好,进出口冷却液温差小于5℃。电流阶跃变化时电堆电压均衡性出现突增变化,同时随着电流的增大,稳态时电堆均衡性变差。在超负荷（200A）运行工况下,电堆各单电池之间输出差异变大,均衡性持续变差,电堆中间和前端单电池电压明显降低。此外,在整个动态响应过程中电流阶跃上升时的电压最大下冲值比电流阶跃下降时的电压最大上调量大,但输出电压能在10s内达到相对稳定的状态（电压波动率＜0.02）。通过该研究,以期为实际车载电堆运行和控制优化提供参考。     

Steady-state operation of a compressor for a proton exchange membrane fuel cell system

The performance of a system consisting of a proton exchange membrane (PEM) fuel cell coupled to a centrifugal air compressor is simulated. Two modes of operation of the system are investigated: one in which the speed of the compressor is constant, and the other in which the compressor speed is varied with the electric load on the fuel cell stack. The operating characteristics of the compressor and the PEM fuel cell stack and their influence on the system efficiency are analyzed for a step change in the stack current. The effects of the fuel cell stack back-pressure and the electric load on the compressor power consumption and the system efficiency are studied. It is found that the system efficiency is higher when the fuel cell stack is operated at a constant oxygen gas stoichiometry by varying the compressor speed instead of at a constant compressor speed. The system model can be used to determine the rotation speed of the compressor for various electric loads.     

质子交换膜燃料电池冷启动堆栈温度预测模型

为了快速准确地预测出质子交换膜燃料电池(proton exchange membrane fuel cell,PEMFC)在冷启动过程中的启动时长及启动方法的应用效果,提出了以堆栈温度和温度增量分别作为BP(back propagation)神经网络预测目标的堆栈温度实时预测模型,分别为模型T和模型K,并采用四个不同的预测精度评估标准来评估预测结果的准确性。基于文献中三种冷启动工况实验数据对预测模型进行验证,结果表明,模型K的平均相对误差在三种工况下均低于模型T,分别为0.4553、0.9537和1.0844。模型T在早期预测阶段缺乏训练样本,预测结果的堆栈温度变化趋势为零,因而模型K在早期预测阶段具有更大优势。堆栈温度变化趋势预测方法能够为用户当前的PEMFC冷启动实现效果提供参考。     

A study on the start-up and performance of a kW-class molten carbonate fuel cell (MCFC) stack

A kW-class internal-manifolded molten carbonate fuel cell (MCFC) stack (52 cells) was assembled with inorganic adhesive under a suitable stacking pressure. The organic compounds in the matrices were burnt out under the conditions of slow and uniform elevation of temperature and big flow of oxygen gas in the stack. The stacking pressure dropped with elevating temperature. The output power of the stack at 150 mA cm⁻² was 1025.5 W when the reactant gas pressure and utilization were 0.5 MPa and 20%, respectively. The thermal-electrical efficiency of the stack was enhanced by increasing the pressure of the reactant. However, it was contrarily decreased when current density was increased.     

质子交换膜燃料电池电堆的动态热模型及其温度控制

A dynamic thermal transfer model of a proton exchange membrane fuel cell (PEMFC) stack is developed based on energy conservation in order to reach better temperature control of PEMFC stack. Considering its uncertain parameters and disturbance, we propose a robust adaptive controller based on backstepping algorithm of Lyaponov function. Numerical simulations indicate the validity of the proposed controller.     

A new technique for two-dimensional current distribution measurements in electrochemical cells

A new technique has been developed using a magnetic loop array to measure current distribution in electrochemical cells. The main advantage of this approach is the combination of high spatial and time resolution and stack integration with an easily handled measurement carried out independently of the cell operation. A polymer electrolyte fuel cell (PEFC) of technically relevant dimensions (about 600 cm² electrode area) with 5 × 8 current sensors has been constructed and operated, thus confirming the feasibility of the measurement technique.     

带有蛇形流场的微生物燃料电池串联堆性能特性

以四个成功启动的带有蛇形流场的单电池构造了微生物燃料电池串联堆(MFCS-S),测试了MFCS-S性能,探讨了其性能提升的限制因素,研究了增加反极电池阴、阳极电解液流量,采取混联的方式运行,移除反极电池和反接反极电池对电堆性能的影响。实验结果表明:MFCS-S在输出电压为2.11 V时获得最大功率密度(2226 mW·m^-2);在一定电流条件下,性能较差的单电池发生电压反极,这是限制MFCS-S性能提高的主要原因;增加反极电池阴、阳极流量虽然不能较大幅度地改善单电池反极,但是却能大幅度提高电堆功率密度;采用混联方式运行不但可以有效避免电池反极,而且可以大幅度提高电堆功率密度;移除反极电池并不能有效地避免电池的反极,反接反极电池反而进一步加剧反极。     

内部重整固体氧化物燃料电池发电系统的非线性预测控制

固体氧化物燃料电池（SOFC）发电系统运行除了电堆本体外还需要包含诸多其他辅助组件以期获得系统输出的最大效率,为了使SOFC电堆能够对纯氢以外的燃料具有更好的适用性,加入了燃料内部重整装置和燃烧室两个重要辅助组件。文中在对系统展开建模的基础上提出了采用非线性模型预测控制策略,能够更有效地使输出燃料气体的组分、温度、压力、浓度和流率满足燃料电池堆正常运行的需要,通过仿真分别论证了线性模型预测控制和非线性模型预测控制两种不同控制方案的有效性和适用性。