Numerical investigation of transport phenomena in high temperature proton exchange membrane fuel cells with different flow field designs

In this work, a three-dimensional, non-isothermal, steady-state model for high temperature proton exchange membrane fuel cells with phosphoric acid polybenzimidazole membrane has been developed using computational fluid dynamics. The importance of the gas flow field design on the transport characteristics and cell performance is revealed by solving the mass, momentum, species, energy, and charge conservation equations. The numerical results show that the best cell performance is provided by the fuel cell with serpentine flow channel flow field. However, the pressure drop is also very high due to the large length of the serpentine channel. In addition, the velocity, oxygen mass fraction, and temperature distributions are unevenly distributed over the entire active area of the fuel cell having straight channels with small manifolds, especially at low cell voltages when a large amount of oxygen is required. The cell performance and durability can be significantly affected by the uniformity of the reactants within the fuel cell. It is suggested that the flow field configurations must be optimized to obtain uniform distributions of the reactants, maximize the cell performance, and minimize the pressure drop penalty. The present results provide detailed information about transport characteristics within fuel cells and give guidelines for design and manufacturing of current collectors.     

Impact of cathode channel depth on performance of direct methanol fuel cells

The major functions of bipolar plates in fuel cell systems are to transport effectively the reactants and the products to and from the electrodes, and to collect efficiently the current that is generated in the cell. A suitable approach to enhance the performance of the bipolar plate with respect to mass transport is to optimize the channel dimension and shape. In this study, the impact of the cathode channel depth on the performance of direct methanol fuel cells is investigated. When the channel depth of the bipolar plate is decreased from 1.0 to 0.3 mm, the cell performance increases and also remains stable during continuous operation of the cell. The decreased channel depth leads to an increase in the linear velocity of the reactants and products at a given volumetric flow rate that, in turn, facilitates their mass transport. Furthermore, in smaller channels (shallower channels), the pressure drop is increased and this can lead to an increase in partial pressure of the oxygen, which has a positive impact on cell performance. The effect of cathode channel depth on the transport behaviour of reactants and products is studied by means of employing the transparent plates, which are designed to monitor visually the flow of reactants and products in the cathode channels. Additionally, the pressure drop and linear velocity in the cell is calculated by using a computational fluid dynamics (CFD) technique.     

Development of a novel radial cathode flow field for PEMFC

This paper presents an innovative radial flow field design for PEMFC cathode flow plates. This new design, which is in the form of a radial flow field, replaces the standard rectangular flow channels in exchange for a set of flow control rings. The control rings allow for better flow distribution and use of the active area. The radial field constructed of aluminum and plated with gold for superior surface and conductive properties. This material was selected based on the results obtained from the performance of the standard flow channels of serpentine and parallel designs constructed of hydrophilic gold and typical hydrophobic graphite materials. It is shown that the new flow field design performs significantly better compared to the current standard parallel channels in a dry-air-flow environment. The polarization curves for a dry flow, however, show excessive membrane drying with the radial design. Humidifying the air flow improves the membrane hydration, and in the meantime, the fuel cell with the innovative radial flow field produces higher current compared to other channel designs, even the serpentine flow field. The water removal and mass transport capacity of the radial flow field was proven to be better than parallel and serpentine designs. This performance increase was achieved while maintaining the pressure drop nearly half of the pressure drop measured in the serpentine flow field.     

Experimental investigation of the effect of channel length on performance and water accumulation in a PEMFC parallel flow field

Longer channels within serpentine flow fields are highly effective at removing liquid water slugs and have little water accumulation; however, the long flow path causes a large pressure drop across the cell. This results in both a significant concentration gradient between the inlet and outlet, and high pumping losses. Parallel flow fields have a shorter flow path and smaller pressure drop between the inlet and outlet. This low pressure drop and multiple routes for reactants in parallel channels allows for significant formation of liquid water slugs and water accumulation. To investigate these differences, a polymer electrolyte membrane fuel cell parallel flow field with the ability to modify the length of the channels was designed, fabricated, and tested. Polarization curves and the performance, water accumulation, and pressure drop were measured during 15 min of 0.5 A cm⁻² steady-state operation. An analysis of variance was performed to determine if the channel length had a significant effect on performance. It was found that the longer 25 cm channels had significantly higher and more stable performance than the shorter 5 cm channels with an 18% and an 87% higher maximum power density and maximum current density, respectively. Channel lengths which result in a pressure drop, across the flow field, slightly larger than that required to expel liquid water slugs were found to have minimal water accumulation and high performance, while requiring minimal parasitic pumping power.     

Network based optimization model for pin-type flow field of polymer electrolyte membrane fuel cell

The flow field of a bipolar plate distributes reactants for polymer electrolyte membrane (PEM) fuel cells and removes the produced water from the fuel cells. It greatly influences the performance of fuel cells, especially the concentration losses. Pin-type design, as one of the widely utilized flow field configurations, has the advantage of low pressure drop but the disadvantage of uneven flow distribution and stagnant areas. In this paper, a network-based optimization model was developed to optimize the pin-type flow field configuration. Two optimization cases, uniform flow distribution with and without considering reactant consumption along flow channels, were investigated and the corresponding optimized designs were developed. Three-dimensional numerical simulations of PEM fuel cell were conducted to validate the proposed optimization model and also to compare the performance of the fuel cells using the optimized designs and the regular pin-type design. The numerical results verified the designs generated with the present optimization model. Both the optimized designs exhibited substantially higher fuel cell performance compared to the regular pin-type configuration. Moreover, the optimized design with considering reactant consumption performed even better than the one without considering the consumption, emphasizing the significance of reactant consumption in the flow field optimization model.     

Analysis and optimization of flow distribution in parallel-channel configurations for proton exchange membrane fuel cells

Parallel channels have many advantages, such as low pressure drop and easy fabrication, but they may cause flow maldistribution which would result in low reaction efficiency. This study presents an analytical model to calculate the flow distribution of the parallel channels based on the assumption of the analogy between fluid flow and electrical network. The model, which ultimately releases from the solution of a set of nonlinear equations, is validated by comparing with the results obtained from three-dimensional computational fluid dynamics (CFD) simulations. Consequently, the model is used to optimize the geometric dimension of a parallel plate to obtain a uniform flow field distribution.     

Novel serpentine-baffle flow field design for proton exchange membrane fuel cells

An appropriate flow field in the bipolar plates of a fuel cell can effectively enhance the reactant transport rates and liquid water removal efficiency, improving cell performance. This paper proposes a novel serpentine-baffle flow field (SBFF) design to improve the cell performance compared to that for a conventional serpentine flow field (SFF). A three-dimensional model is used to analyze the reactant and product transport and the electrochemical reactions in the cell. The results show that at high operating voltages, the conventional design and the baffled design have the same performance, because the electrochemical rate is low and only a small amount of oxygen is consumed, so the oxygen transport rates for both designs are sufficient to maintain the reaction rates. However, at low operating voltages, the baffled design shows better performance than the conventional design. Analyses of the local transport phenomena in the cell indicate that the baffled design induces larger pressure differences between adjacent flow channels over the entire electrode surface than does the conventional design, enhancing under-rib convection through the electrode porous layer. The under-rib convection increases the mass transport rates of the reactants and products to and from the catalyst layer and reduces the amount of liquid water trapped in the porous electrode. The baffled design increases the limiting current density and improves the cell performance relative to conventional design.     

Pressure drop and flow distribution in multiple parallel-channel configurations used in proton-exchange membrane fuel cell stacks

Single U- and Z-type parallel-channel configurations for gas distributor plates in planar fuel cells reduce the pressure drop but give rise to the problem of severe flow maldistribution wherein some of the channels may be starved of the reactants. In this paper, previous analytical solutions obtained for single U- and Z-type flow configurations are extended to multiple U- and multiple Z-type flow configurations of interest to fuel cell applications. Algorithms to calculate flow distribution and pressure drop in multiple U- and Z-type flow configurations are developed. The results are validated by comparison with those obtained from three-dimensional computational fluid dynamics (CFD) simulations. It is found that there is a significant improvement in the flow distribution in some configurations without paying for extra pressure drop. The possibility of unmatched distribution on the cathode and the anodes sides is also highlighted. Careful design of the flow configuration is therefore necessary for optimum performance.     

Development of bipolar plates with different flow channel configurations for fuel cells

Bipolar plates include separate gas flow channels for anode and cathode electrodes of a fuel cell. These gases flow channels supply reactant gasses as well as remove products from the cathode side of the fuel cell. Fluid flow, heat and mass transport processes in these channels have significant effect on fuel cell performance, particularly to the mass transport losses. The design of the bipolar plates should minimize plate thickness for low volume and mass. Additionally, contact faces should provide a high degree of surface uniformity for low thermal and electrical contact resistances. Finally, the flow fields should provide for efficient heat and mass transport processes with reduced pressure drops. In this study, bipolar plates with different serpentine flow channel configurations are analyzed using computational fluid dynamics modeling. Flow characteristics including variation of pressure in the flow channel across the bipolar plate are presented. Pressure drop characteristics for different flow channel designs are compared. Results show that with increased number of parallel channels and smaller sizes, a more effective contact surface area along with decreased pressured drop can be achieved. Correlations of such entrance region coefficients will be useful for the PEM fuel cell simulation model to evaluate the affects of the bipolar plate design on mass transfer loss and hence on the total current and power density of the fuel cell.     

Pyramidal direct methanol fuel cells

The main concept of this article is to allow for the optimized tree-like design of the flow distributors to actually define the shape of the fuel cell, thereby eliminating problems associated with the mismatch between a predecided rectangular shape and a functionally preferred channel distribution system. The work focuses on direct methanol fuel cells (DMFC). A one-dimensional across-the-cell model is extended to a two-dimensional along-the-channel model and numerically solved to predict the polarization curves of a direct methanol fuel cell with tree network channels and of a cell with traditional serpentine channels. For both flow configurations, pressure drop and pumping power are estimated. Net power densities are computed in terms of constructal parameters and operating conditions. In contrast to the traditional rectangular shape of fuel cells, the resulting “pyramidal” or “double-staircase” shape is based on the functionality of the fluid distribution system. It is found that tree network channels can provide substantially improved electric and net power densities compared to the traditional non-bifurcating single serpentine channels, as a result of their intrinsic advantage with respect to both mass transfer and pressure drop. For six (12) branching levels and inlet channel diameters of 0.05, 0.04, and 0.03 cm, the tree network channels allow for 14% (21%), 17% (26%), and 30% (46%) higher net power densities, respectively.     

Design of thermal management subsystem for a 5 kW polymer electrolyte membrane fuel cell system

High-efficiency thermal management subsystem has a key role on the PEM fuel cell performance and durability. In this study, design of thermal management subsystem for a 5 kW PEM fuel cell system is investigated. A numerical model is presented to study the cooling flow field performance. The number of parallel channels in parallel serpentine flow field is selected as the design parameter of the flow field and its optimum value is obtained by compromising between the minimum pressure drop of coolant across the flow field and maximum temperature uniformity within the bipolar plate criteria. The optimum coolant flow rate is also determined by compromising between different criteria. Test results of a 5-cells short stack are presented to verify the numerical simulation results.     

Optimization of blocked flow field performance of proton exchange membrane fuel cell with auxiliary channels

The flow field optimization design is one of the important methods to improve the performance of proton exchange membrane fuel cell (PEMFC). In this study, a new structure with staggered blocks on the parallel flow channels of PEMFC and auxiliary flow channels under the ribs is proposed. Through numerical calculation method, the effect of blocks auxiliary flow field (BAFF) on pressure drop, reactant distribution and liquid water removal in the fuel cells are investigated. The results show that when the operating voltage is 0.5 V, the current density of BAFF is 21.74% higher than that of the straight parallel flow field (SPFF), and the power density reaches 0.65 W cm^?2. BAFF improves performance by equalizing the pressure drop across sub-channels, promoting the uniform distribution of reactant, and enhancing transport across the ribs. In addition, through parameter analysis, it is found that BAFF can discharge liquid water in time at the conditions of high humidification, high current density and low temperature, which ensures the output performance of the fuel cell and improves the durability of the fuel cell. This paper provides new ideas for the improvement of PEMFC flow field design, which is beneficial to the development of PEMFC with high current density.     

基于神经元的PEMFC仿生流道性能模拟研究

质子交换膜燃料电池的流道结构对反应气体的流动和压降等具有重要影响。受神经元结构启发,提出一种兼顾径向流道和仿生流道在压降和气体分布均匀性优点的新型仿生流道结构。通过COMSOL软件模拟研究该新型流道的分支数（2~9）对质子交换膜燃料电池的性能曲线、阴极氧浓度分布、水浓度分布及压降的影响。结果表明：增加流道分支数可提高质子交换膜燃料电池的输出性能,其中9分支流道的峰值功率密度最大,为0.32 W/cm²,相比于2分支流道增加了的146.15%;分支数的增加也会提高氧浓度分布的均匀性,阴极气体扩散层与催化层交界面处的平均氧浓度从0.44 mol/m³提高到1.42 mol/m³,氧气不均匀度从2.13降低至0.90;分支数的增加也明显改善了弧形流道内的水浓度分布。此外,随着流道分支数从2增加到9,流道压降从38.57 Pa递减至4.47 Pa,质子交换膜燃料电池的输出功率从0.40 W递增到1.56 W。     

An improved serpentine flow field with enhanced cross-flow for fuel cell applications

The paper describes a flow field design which is based on the improvement of the local cross-flow conditions in a split serpentine flow field. The layout of the flow field is such that the cross-flow is higher in the oxygen-depleted portion of the adjacent serpentine channel in a split serpentine flow field with more than one serpentine channels. The present arrangement offers the quadruple advantage of uniform reactant distribution over the entire cell active area; low overall pressure drop, thereby reducing the parasitic power losses; effective liquid water evacuation in the U-bends; and more oxygen replenishment in the oxygen-deficient portions of the serpentine channel. Computational fluid dynamics (CFD) and experimental analysis carried out on the proposed design with three serpentine channels confirm the benefits.     

A comparison on the dynamical performance of a proton exchange membrane fuel cell (PEMFC) with traditional serpentine and an open pore cellular foam material flow channel

Minimising the pressure drop in flow channels, ensuring high efficiency and utilisation of open pore cellular foam (OPCF) material in place of a traditional serpentine channel are investigated in this work. The paper establishes novel mathematical model that takes into account the effect of pressure drop in the flow channel and compares the dynamics of a porous flow channel with those of the traditional serpentine flow channel. The performance of a Polymer Electrolyte Membrane fuel cell with porous foam flow channel is analysed under static and transient conditions. The fuel cell mass transport equations are used in the model that also takes into account the effect of varying the current on the stack temperature. The membrane water content and IV-curves are analysed and simulation results are presented based on the mathematical models of the proposed system using the MATLAB®/Simulink® environments. The effect of varying pore diameter, porosity, and the flow velocity on pressure drop are also investigated using sensitivity analysis. Due to the lower pressure drop provided by the uniform distribution of reactants in OPCF channel, an improvement of approximately 55% is observed in current density when compared with that of the serpentine channel under the same operating conditions. The investigation further concluded that a higher pore diameter can have a lower drop in pressure provided the flow velocity of the reactant does not exceed 6 m/s.     

Fluid dynamics performance of different bipolar plates: Part I. Velocity and pressure fields

The flow distribution obtained with three different bipolar plate geometries has been studied, analyzing their fluid dynamic performance. Three plate topologies have been selected in some way representative of different design models commonly used. The configurations tested are a set of parallel diagonal channels, a branching cascade type, and a serpentine distribution of parallel channel blocks. The flow distribution across the plates has been first simulated numerically. To visualize the flow pattern at the plate channels plane, a laser-induced fluorescence (LIF) trace tracking technique has been applied. From experimental and numerical simulations it is concluded that the diagonal topology distributes the flow in a non-uniform way. On the other hand, an inadequately large pressure drop is established along the serpentine-parallel plate. In the cascade case, both velocity and pressure fields are very uniform, and it can be expected to produce a very homogeneous distribution of reactants over the catalyst layer.     

An innovative application of multiple impinging jet flow field in fuel cells

The flow field in a fuel cell is expected to distribute the reactants as uniformly as possible over the active plate, support the reasonable pressure drop across the channel and maximize the mass transfer through the catalyst layers. To simultaneously accomplish these requirements, an innovative multiple impingement jet flow field (MIJFF) is proposed in this study. A three-dimensional thermo-fluid simulation is used to evaluate the proposed idea and compare its performance against the commonly used parallel field flow (PFF). The domain of calculations includes a channel with multiple impingement jets linked to a porous gas diffusion layer under low Reynolds flow conditions. The results reveal that the suggested MIJFF design significantly increases the transport of the reactant gases through the catalyst layer. The penetration depth into the catalyst layer in the MIJFF arrangement is higher than that of the PFF setup and the use of the catalyst layer is optimized, which in turn can lead to a reduction of the activation drop. Compared to a PFF design under similar operating conditions, the mean Nusselt number is shown to increase by a factor of about 3.5 in the MIJFF setup. Furthermore, the temperature is more uniformly distributed in the MIJFF pattern which results in more effective distribution of the reactant gases over the active surface. The current study shows that under equal pressure drop conditions, the MIJFF exhibits a much higher performance than a PFF channel design. That is while the corresponding flow rate for the MIJFF is much lower than that of PFF.     

Experimental validation of two-phase pressure drop multiplier as a diagnostic tool for characterizing PEM fuel cell performance

Water management is a key area of interest in improving the performance of Proton Exchange Membrane fuel cells. Cell flooding and membrane dehydration are two extreme conditions arising from poor water management. Pressure drop has been recognized as a good diagnostic tool to determine the presence of liquid water in the reactant channels. Presence of liquid water in the channels increases the mass transport resistances and therefore reduces the cell performance, which is quantified by the cell voltage at a set current density. Since the two-phase pressure drop multiplier is uniquely related to the water content in the channel, it serves as a good diagnostic tool for directly predicting the cell performance. Experiments are carried out to establish the relationship between the pressure drop multiplier and cell voltage at different operating conditions. Cell temperature was varied from 30 °C to 80 °C and the inlet RH was varied from 0 to 95%. At the lower temperatures, a two-phase multiplier below 1.5 reduces flooding in the flow field. However, at the higher temperatures, a two-phase flow multiplier above 1.2 is preferred as it indicates the membrane remains hydrated for improved performance from the cell. The two-phase pressure drop multiplier has been successfully demonstrated as a diagnostic tool to predict cell flooding and membrane dehydration.     

Numerical simulation of effect of catalyst wire-mesh pressure drop characteristics on flow distribution in catalytic parallel plate steam reformer

Steam reforming of hydrocarbons using a catalytic plate-type-heat-exchanger (CPHE) reformer is an attractive method of producing hydrogen for a fuel cell-based micro combined-heat-and-power system. In this study the flow distribution in a CPHE reformer, which uses a coated wire-mesh catalyst, is considered to investigate the effect of catalyst wire-mesh pressure drop characteristics on flow distribution in the CPHE reformer. Flow distribution in a CPHE reformer is rarely uniform due to inlet and exhaust manifold design. Poorly-designed manifolds may lead to severe flow maldistribution, flow reversal in some of the CPHE reformer channels and increased overall pressure drop. Excessive flow maldistribution can significantly reduce the CPHE reformer performance. Detailed three-dimensional models are used to investigate the flow distribution at three different catalyst wire-mesh pressure drop coefficients and at five different flow rates. Experiments are performed on a single CPHE reformer channel to evaluate the pressure drop characteristics of the catalyst wire-mesh in the current CPHE reformer design. The results are used in the numerical model where the catalyst zone is simulated as domains with momentum source to account for the pressure drop. The numerical model is verified experimentally, numerical and experimental results are found to be in good agreement. The study shows that severe flow maldistribution exists in the current reformer stack. At nominal load some channels in the CPHE reformer receive up to four times the average mass flow, while other channels have reversed flow. Flow maldistribution and flow reversal can be improved significantly by increasing the pressure drop characteristics of the catalyst wire-mesh.     

Detailed analysis of polymer electrolyte membrane fuel cell with enhanced cross‐flow split serpentine flow field design

Most generally used flow channel designs in polymer electrolyte membrane fuel cells (PEMFCs) are serpentine flow designs as single channels or as multiple channels due to their advantages over parallel flow field designs. But these flow fields have inherent problems of high pressure drop, improper reactant distribution, and poor water management, especially near the U‐bends. The problem of inadequate water evacuation and improper reactant distribution become more severe and these designs become worse at higher current loads (low voltages). In the current work, a detailed performance study of enhanced cross‐flow split serpentine flow field (ECSSFF) design for PEMFC has been conducted using a three‐dimensional (3‐D) multiphase computational fluid dynamic (CFD) model. ECSSFF design is used for cathode part of the cell and parallel flow field on anode part of the cell. The performance of PEMFC with ECSSFF has been compared with the performance of triple serpentine flow design on cathode side by keeping all other parameters and anode side flow field design similar. The performance is evaluated in terms of their polarization curves. A parametric study is carried out by varying operating conditions, viz, cell temperature and inlet humidity on air and fuel side. The ECSSFF has shown superior performance over the triple serpentine design under all these conditions.