Optimization of Parallel and Serpentine Configurations for Polymer Electrolyte Membrane Fuel Cells

A network‐based optimization model was developed to optimize the channel dimensions of flow fields in order to achieve a uniform flow distribution and improve the performance of polymer electrolyte membrane (PEM) fuel cells. Different flow field configurations, including parallel, parallel‐in‐series, and serpentine, were investigated using the present optimization model. Two cases, with and without considering reactant consumption, were compared to show the effect of including reactant consumption on the flow field designs. The results demonstrated that the optimized designs significantly improved the flow velocity distribution in all the configurations investigated. The optimized designs with consideration of reactant consumption exhibited more uniform flow velocity distribution when the entire fuel cell unit was considered. Additionally, the performances of PEM fuel cells for the conventional and optimized flow field designs were studied with a three‐dimensional, two‐phase fuel cell simulation model, and the computational results showed that the optimized designs with considering reactant consumption produced the highest maximum power density for each configuration investigated. These results show that the network‐based model is capable of optimizing various flow field configurations with flexibility and indicate the importance of considering reactant consumption in the optimization model.     

Flow‐Field Designs of Bipolar Plates in PEM Fuel Cells: Theory and Applications

A generalized model developed by Wang was modified for flow field designs of the most common layout configurations with U‐type arrangement, including single serpentine, multiple serpentine, straight parallel, and interdigitated configurations. A direct and quantitative relationship was established among flow distribution, pressure drop, configurations, structures, and flow conditions. The model was used for a direct, systematic, and quantitative comparison of flow distributions and pressure drops among the most common layout configurations of interest. The straight parallel configuration had the lowest pressure drops but suffered the most possibility of the uneven flow distribution across the channels. The single serpentine had the best flow distribution but had the highest pressure drops. The flow distribution and the pressure drop in the multiple serpentine was between the straight parallel and the single serpentine. Finally, we suggested basic criteria of the flow field designs of bipolar plates for the industrial applications. This provides a practical guideline to evaluate how far a fuel cell is from design operating conditions, and measures how to improve flow distribution and pressure drop.     

质子交换膜燃料电池流场板研究进展

流场板是质子交换膜燃料电池的核心部件之一，其结构直接影响着反应气体的利用效率以及燃料电池的排水及散热性能。综述了近十余年来质子交换膜燃料电池流场板的设计与研究进展。研究者们基于平行流场、蛇形流场、交指流场、点状流场，从流道尺寸、流道截面、进口分配段、流道布置等方面开展结构设计和优化，不同程度提高了燃料电池水热管理以及电性能。此外，各种形式的组合流场可综合不同流场优点，多级分形仿生流场优化了反应物、压力与电流密度分布，三维精细化流场通过改善供气方式降低了浓差极化。     

液流电池流场结构设计与优化研究进展

液流电池结构设计与优化研究是改善电池内部电解液流动性能、提高电堆功率密度和可靠性的重要途径之一。在石墨板上设计并行、交指和蛇形等流道是液流电池使用的传统流道结构,其缺点为流道种类单一、石墨板成本高及机械性能差。为了克服上述缺点,波纹状并行、分离式蛇形、螺旋形等新型流道,在电极上构建流道、引入独立的流道部件、环形与梯形等异形结构等先后被提出。本文从双极板、电极上的结构设计和异形结构设计与优化三方面系统综述了近年来液流电池结构设计与优化研究进展,阐释流场结构等对电池性能的影响机制及其与电池运行、装配间的适配规律,并提出进一步改善电池性能并适合普及应用的流场结构形式。     

Experimental and Numerical Study of Serpentine Flow Fields for Improving Direct Methanol Fuel Cell Performance

Methanol crossover is an important issue as it affects direct methanol fuel cell (DMFC) performance. But it may be controlled by selecting a proper flow field design. Experiments were carried out to investigate the effect of single, double and triple serpentine flow field configurations on a DMFC with a 25 cm² membrane electrode assembly (MEA) with a constant open ratio. A three dimensional model was also developed for the anode of the DMFC to predict methanol concentration and cell current density distributions. Experimental and model results show that at lower methanol concentrations (0.25–0.5M), single serpentine flow field (SSFF) provides high peak power density, while a double serpentine flow field (DSFF) gives high peak power density at a high methanol concentration (1–2M). Single and double serpentine flow fields exhibit the same peak power density (33 mW cm⁻²) at 1M. But the cell efficiency of double serpentine flow field is 12.5% which is 3.5% point greater than single serpentine flow field. This is attributed to reduced mixed potential. triple serpentine flow field (TSFF) shows the lowest peak power density and cell efficiency, which is attributed to high mass transfer resistance.     

Numerical and Experimental Verification of the Polymer Electrolyte Fuel Cell Performances Enhanced by Under‐Rib Convection

Understanding the current density distributions in polymer electrolyte fuel cells (PEFCs) is crucial for designing cell components, such as the flow field of bipolar plates. A new serpentine flow field equipped with sub‐channels and by‐passes (SFFSB) was numerically and experimentally confirmed to enhance the reactant transport rates and liquid removal efficiency compared with a conventional advanced serpentine flow field (CASFF). Consequently, the maximum current and the power densities of the SFFSB were increased due to the promotion of under‐rib convection. In this study, current density distributions are measured under transient conditions to verify the PEFC performances enhanced by under‐rib convection. The current density distributions of the SFFSB are compared with those of the CASFF. The results show that the SFFSB has a higher local current density and a more uniform distribution than the CASFF, therefore, the PEFC performances with the new flow field of SFFSB is enhanced by the better current density distributions.     

Thermal and hydrodynamic characteristics of constructal tree‐shaped minichannel heat sink

A three‐dimensional thermal and hydrodynamic model for constructal tree‐shaped minichannel heat sink is developed. The heat and fluid flow in the constructal heat sink with an inlet hydraulic diameter of 4 mm are numerically analyzed, taking into consideration conjugate heat transfer in the channel walls. The pressure drop, temperature uniformity, and coefficient of performance (COP) of the constructal tree‐shaped heat sink are evaluated and compared with those of the corresponding traditional serpentine flow pattern. The results indicate that the constructal tree‐shaped minichannel heat sinks have considerable advantages over the traditional serpentine flow patterns in both heat transfer and pressure drop. The strong and weak heat flow can be effectively allocated in tree‐shaped flow structures; hence, the inherent advantage of uniform temperature on the heating surface in the constructal tree‐shaped heat sink is demonstrated. And in tree‐shaped flow structures, the local pressure loss due to confluence flow is found to be larger than that due to diffluence flow. In addition, an aluminum constructal tree‐shaped minichannel heat sink is fabricated to conduct the verification experiment. The experimentally measured temperature distribution and pressure drop are in agreement with the numerical simulation, which verifies that the present model is reasonable. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

全钒液流电池电解液分布的数值模拟

为了提高全钒液流电池双极板流道电解液分布均匀性,考察流体流动行为,本文基于计算流体力学,在传统平直并联流道基础上通过增加倾斜挡板和入口流堰,改进流道结构;同时探究钒电池用电解液在分段式多通道蛇形流道内流体水力学特征。数值模拟结果表明：分段式多通道蛇形流道既可以保持传统蛇形流道流体均匀分配的性能,又能有效降低流阻,减少泵耗;合适的电解液流速及其均匀分布可以优化电解液活性物质浓度分布,提高电解液稳定性,增大钒电池能量效率。     

Enhancing the Efficiency of SOFC‐Based Auxiliary Power Units by Intermediate Methanation

Fuel‐cell‐based auxiliary power units benefit from the high power density and fuel flexibility of solid oxide fuel cells (SOFCs), facilitating straightforward onboard fuel processing of diesel or jet fuel. The preferred method of producing the fuel gas is autothermal reforming, which to date has shown the best practical applicability. However, the resulting reformate is poor in methane, so that cell cooling is not supported by internal methane steam reforming. Accordingly, large flow rates of excess air are required to cool the stack. Hence, the power demand of the cathode air blower significantly limits the net electrical power output of the system and large cathode flow channels are required. The present work examines attempts to further increase the system efficiency in middle‐distillate‐fueled SOFC systems by decreasing the cathode air flow rates. The proposed concept is generally based on inducing endothermic methane steam reforming (MSR) inside the cells by augmenting the methane content in an upstream methanation step. Methanation, however, can only yield significant methane production rates if the reaction temperature is limited. Therefore, four process layouts are presented that include different cooling measures. Based on these setups, the general feasibility and the benefit of intermediate methanation are demonstrated.     

CFD simulation for steam distribution in header and tube assemblies

The study of the flow characteristics in manifolds (dividing, combining, parallel or Z-manifold and reverse or U-manifold) is a classic subject of engineering fluid dynamics and hydrodynamics. These manifolds are widely used in chemical processes, electronic cooling equipment, solar collectors, spargers, microchannels, fuel cells, heat exchangers and refrigerant distribution in multi-split type of air conditioner, etc. In the literature extensive work has been done for finding out flow distribution in plate-fin heat exchanger, microchannels and spargers. Present work focuses on the flow and pressure distribution in piping networks, which has gained importance in many areas such as air distribution in diffuser system of aerobic biological treatment, steam distribution in passive decay heat removal systems, etc.     

Three Dimensional Model of a High Temperature PEMFC. Study of the Flow Field Effect on Performance

A three‐dimensional isothermal model of a high temperature polymer membrane fuel cell equipped with polybenzimidazole membrane is described. All major transport phenomena were taken into account except the species crossover through the membrane. The cathode catalyst layer was treated as spherical catalyst agglomerates with porous inter‐agglomerate spaces. The inter‐agglomerate spaces were filled with a mixture of electrolyte (hot phosphoric acid) and polytetrafluoroethylene (PTFE). This approach proved to be an essential requirement for accurate simulation. In this particular paper, the influence of different flow field designs and dimensions on performance was intensely study. Traditional configurations were tested (straight, serpentine, pin‐in, and interdigitated), and new designs were proposed. With these new designs, we tried to maximize performance by providing homogeneous reactants distribution over the active area keeping low‐pressure drop and relatively high velocity. The dimension and position of the inlet and outlet manifolds were also analyzed. From the obtained results a massive influence of the manifolds position and dimension on performance was observed. This fact can provide important guidelines for future bipolar plates optimization.     

Current Advances in Polymer Electrolyte Fuel Cells Based on the Promotional Role of Under‐rib Convection

Literature data on the promotional role of under‐rib convection for polymer electrolyte fuel cells (PEFCs) fueled by hydrogen and methanol are structured and analyzed, thus providing a guide to improving fuel cell performance through the optimization of flow field interaction. Data are presented for both physical and electrochemical performance showing reactant mass transport, electrochemical reaction, water behavior, and power density enhanced by under‐rib convection. Performance improvement studies ranging from single cell to stack are presented for measuring the performance of real operating conditions and large‐scale setups. The flow field optimization techniques by under‐rib convection are derived from the collected data over a wide range of experiments and modeling studies with a variety of components including both single cell and stack arrangements. Numerical models for PEFCs are presented with an emphasis on mass transfer and electrochemical reaction inside the fuel cell. The models are primarily used here as a tool in the parametric analysis of significant design features and to permit the design of the experiment. Enhanced flow field design that utilizes the promotional role of under‐rib convection can contribute to commercializing PEFCs.     

Optimization of the fuel rod's arrangement cooled by turbulent nanofluids flow in pressurized water reactor (PWR)

In this paper, response surface methodology(RSM) based on central composite design(CCD) is applied to obtain an optimization design for the fuel rod's diameter and distance cooled by turbulent Al_2O_3–water nanofluid for a typical pressurized water reactor(PWR). Fuel rods and nanofluid flow between them are simulated 3D using computational fluid dynamics(CFD) by ANSYS-FLUNET package software. The RNG k–ε model is used to simulate turbulent nanofluid flow between the rods. The effect of different nanoparticles concentration is also investigated on the Nusselt number from heat transfer efficiency view point. Results reveal that when distance parameter(a) is in the minimum level and diameter parameter(r) is in the maximum possible level, cooling the rods will be better due to higher Nusselt number in this situation. Also, using the different nanoparticles on the cooling process confirms that Al_2O_3 averagely 17% and TiO_2 10% improve the Nusselt numbers.     

PEMFC流道截面对气体浓度在催化层分布影响的数值分析

以蛇形流场质子交换膜燃料电池阴极为研究对象,取其中一部分建立三维、稳态的数学计算模型,利用CFD（计算流体动力学）方法研究了质子交换膜燃料电池阴极内的流动和传质过程,得到了阴极内氧气和水蒸气的质量分数的分布情况,探讨了流道宽度和深度对气体在催化层空间分布的影响,为燃料电池流场的设计和改进提供了参考依据。     

基于框架设计的液流电池流场优化模拟研究

液流电池通常采用对角平推流流场,会形成电解液滞留区,造成电池局部浓差极化大,影响综合性能。鉴于此,提出了一种基于框架设计的流场优化方法,通过设计电极框架,可以得到“蛇形流道”和“平行流道”两类流场。以全钒液流电池为例,通过数学建模,研究了不同流场结构和参数对于多孔电极内电解液流动特性、电化学反应和温度变化特性的影响规律。计算结果与实验结果一致性良好,结果表明：电解液在“平行流场”内的流动均匀性比在“蛇形流场”内好,且不存在滞留区,同时在“平行流场”内浓差极化也较“蛇形流场”低;此外,对于同样的电极面积,在电极内部的“平行流道”越多,电解液的流速分布越均匀,反应特性越好。     

Characterization of gas flow configurations for phosphoric acid fuel cells

The cell performance and polarization distribution in the horizontal plane of a phosphoric acid fuel cell (PAFC) were studied for thirteen different types of gas flow. In all cases, the potentials of the anode and cathode in the fuel outlet area shifted to positive directions. At 90% utilization of fuel, the potential shift and the cell voltage changed significantly from one type to another. Experimental results show that the cell voltage increased, and the potential shift decreased in the following order; CROSS-FLOW < RETURN-FLOW = CO-FLOW < COUNTER-FLOW < type E2. The extent of the change between various types depended on the following three conditions. (1) The fuel gas has an opportunity to be used twice in the horizontal plane of a cell. (2) The fuel gas flow is in parallel with the air flow. (3) The fuel gas flow is opposite to the air flow. These are requirements for obtaining good and stable performance of a fuel cell. The CROSS-FLOW case has none of these three conditions, the RETURN-FLOW case has the first condition, the CO-FLOW case has the second, the COUNTER-FLOW case has the second and the third conditions, and the best gas flow type (type E2) has all.     

Optimization of Methanol Yield from a Lurgi Reactor

Methanol is an important chemical with the potential to become an alternative fuel. An optimization study was performed for a Lurgi methanol synthesis reactor using the commercial process simulator Aspen Plus. The optimization routine is coupled with a steady‐state model of the methanol synthesis reactor. Syngas inlet temperature, steam drum pressure, and cooling water volumetric flow rate were optimized so that methanol production in the reactor outlet was maximized. The methanol yield increased by 7.04 %.     

Use of Grooved Microchannels to Improve the Performance of Membrane‐Less Fuel Cells

In this work, the fluid dynamics within a membrane‐less microchannel fuel cell is analyzed computationally. The membrane‐less design is a result of the laminar nature of the fluid flow at small Reynolds numbers, restricting the fuel and oxidant to the vicinity of the corresponding electrodes, without the need of a proton exchange membrane (PEM). However, the performance of such cells is limited by the slow diffusive mass transport near the electrodes, with a large fraction of the reactants leaving the channel without coming in contact with the catalytic surfaces, and thus not being used. We mitigate this problem through the introduction of channel surface modification consisting of angled grooves designed to create convective flows that direct the reactants toward the active surfaces. The grooved structures are optimized for maximum fuel utilization. Operation of this type of cells at Péclet numbers close to 2,500 leads to a performance doubling compared with unmodified cells. Moreover, this increase in efficiency is accompanied by a more uniform distribution of the current across the electrodes, reducing the possibility of hot spots being developed.     

Comparison of multi-inlet and serpentine channel design on water production of PEMFCs

Two flow field designs, a new multi-inlet design and a conventional serpentine design, for a PEMFC together with relative humidity (RH) and porosity of the gas diffusion layer (GDL) are studied in relation to net water production using a 3-dimensional computational fluid dynamics simulation. The results show that (1) with increasing GDL porosity, discharged water in the serpentine design slightly increases, because accumulated water decreases, whereas discharged water in the multi-inlet design decreases due to a reduction of generated water; (2) although fuel cell power performance improves as RH increases, net water production decreases in both designs, because more water is accumulated; and (3) comparatively higher power and net water production are observed with the multi-inlet design, owing to uniform distributions of reactant gas and water. It is determined that, for net water production without compromising power production, input water should be decreased and, for higher cell performance, flow field design like multi-inlet design should be developed.     

Three‐Dimensional Computational Fluid Dynamics Modeling of a Planar Solid Oxide Fuel Cell

A three‐dimensional computational fluid dynamics model was developed to study the performance of a planar solid oxide fuel cell (SOFC). The governing equations were solved with the finite volume method. The model was validated by comparing the simulation results with data from literature. Parametric simulations were performed to investigate the coupled heat/mass transfer and electrochemical reactions in a planar SOFC. Different from previous two‐dimensional studies the present three‐dimensional analyses revealed that the current density was higher at the center along the flow channel while lower under the interconnect ribs, due to slower diffusion of gas species under the ribs. The effects of inlet gas flow rate and electrode porosity on SOFC performance were examined as well. The analyses provide a better understanding of the working mechanisms of SOFCs. The model can serve as a useful tool for SOFC design optimization.