Liquid water transport in straight micro-parallel-channels with manifolds for PEM fuel cell cathode

Water management in a proton exchange membrane (PEM) fuel cell stack has been a challenging issue on the road to commercialization. This paper presents a numerical investigation of air–water flow in micro-parallel-channels with PEM fuel cell stack inlet and outlet manifolds for the cathode, using a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different air–water flow behaviours inside the straight micro-parallel-channels with inlet and outlet manifolds were simulated and discussed. The results showed that excessive and unevenly distributed water in different single PEM fuel cells could cause blockage of airflow or uneven distribution of air along the different flow channels. It is found that for a design with straight-channels, water in the outflow manifold could be easily blocked by air/water streams from the gas flow channels; the airflow could be severely blocked even if there was only a small amount of water in the gas flow channels. Some important suggestions were made to achieve a better design.     

Numerical investigation of pressure drop characteristics of gas channels and U and Z type manifolds of a PEM fuel cell stack

Fluid flow manifold plays a significant role in the performance of a fuel cell stack because it affects the pressure drop, reactants distribution uniformity and flow losses, significantly. In this study, the flow distribution and the pressure drop in the gas channels including the inlet and outlet manifolds, with U- and Z-type arrangements, of a 10-cell PEM fuel cell stack are analyzed at anode and cathode sides and the effects of inlet reactant stoichiometry and manifold hydraulic diameter on the pressure drop are investigated. Furthermore, the effect of relative humidity of oxidants on the pressure drop of cathode are investigated. The results indicate that increase of the manifold hydraulic diameter leads to decrease of the pressure drop and a more uniform flow distribution at the cathode side when air is used as oxidant while utilization of humidified oxidant results in increase of pressure drop. It is demonstrated that for the inlet stoichiometry of 2 and U type manifold arrangement when the relative humidity increases from 25% to 75%, the pressure drop increases by 60.12% and 116.14% for oxygen and air, respectively. It is concluded that there is not a significant difference in pressure drop of U- and Z-type arrangements when oxygen is used as oxidant. When air is used as oxidant, the effect of manifold type arrangement is more significant than other cases, and increase of the stoichiometry ratio from 1.25 to 2.5 leads to increase of pressure drop by 527.3%.     

Numerical investigation of the impact of two-phase flow maldistribution on PEM fuel cell performance

Flow maldistribution usually happens in PEM fuel cells when using common inlet and exit headers to supply reactant gases to multiple channels. As a result, some channels are flooded with more water and have less air flow while other channels are filled with less water but have excessive air flow. To investigate the impact of two-phase flow maldistribution on PEM fuel cell performance, a Volume of Fluid (VOF) model coupled with a 1D MEA model was employed to simulate two parallel channels. The slug flow pattern is mainly observed in the flow channels under different flow maldistribution conditions, and it significantly increases the gas diffusion layer (GDL) surface water coverage over the whole range of simulated current densities, which directly leads to poor fuel cell performance. Therefore, it is recommended that liquid and gas flow maldistribution in parallel channels should be avoided if possible over the whole range of operation. Increasing the gas stoichiometric flow ratio is not an effective method to mitigate the gas flow maldistribution, but adding a gas inlet resistance to the flow channel is effective in mitigating maldistribution. With a carefully selected value of the flow resistance coefficient, both the fuel cell performance and the gas flow distribution can be significantly improved without causing too much extra pressure drop.     

Numerical modeling of manifold design and flow uniformity analysis of an external manifold solid oxide fuel cell stack

Computational fluid dynamics (CFD) technique and experimental measurement are combined to investigate the effects of several geometric parameters on flow uniformity and pressure distribution in an external manifold solid oxide fuel cell (SOFC) stack. The model of numerical simulation is composed of channels, tubes and manifolds based on a realistic 20-cell stack. Analysis results show that gas resistance in the channel can improve the flow uniformity. However, channel resistance only has a limited effect under high mass flow rate. With the increase of inlet tube diameter, the flow uniformity improves gradually but this has little impact on pressure drop. On contrary, the larger diameter of outlet tube reduces the pressure drop effectively with minor improvement on flow uniformity. The dimensions of the flared inlet tube and the round perforated sheet in the manifold are designed to optimize both flow uniformity and pressure drop. Practical experimental stack is established and the velocity in the outlet of the channel is measured. The trends of the experimental measurements are corresponding well with the numerical results. The investigation emphasizes the importance of geometric parameters to gas flow and provides optimized strategies for external manifold SOFC stack.     

Dominant frequency of pressure drop signal as a novel diagnostic tool for the water removal in proton exchange membrane fuel cell flow channel

In this work, a transparent assembly was self-designed and manufactured to perform ex situ experimental study on the liquid water removal characteristics in PEM fuel cell parallel flow channels. It was found that the dominant frequency of the pressure drop across the flow channels may be utilized as an effective diagnostic tool for water removal. Peaks higher than 1 Hz in dominant frequency profile indicated water droplet removals at the outlet, whereas relatively lower peaks (between 0.3 and 0.8 Hz) corresponded to water stream removals. The pressure drop signal, although correlated with the water removal at the outlet, was readily influenced by the two phase flow transport in channel, particularly at high air flow rates. The real-time visualization images were presented to show a typical water droplet removal process. The findings suggest that dominant frequency of pressure drop signal may substitute pressure drop as a more effective and reliable diagnostic tool for water removal in PEM fuel cell flow channels.     

Spatially resolved optical measurements of water partial pressure and temperature in a PEM fuel cell under dynamic operating conditions

In situ non-intrusive measurements of water vapor partial pressure and temperature were performed simultaneously along two gas channels on the cathode side of a PEM fuel cell using tunable diode laser absorption spectroscopy. This measurement technique developed by us was utilized earlier to make measurements in a single bipolar plate channel of a prototype PEM fuel cell. The current study examines the variation of water partial pressure and temperature near the flow inlet and outlet during operation under both steady state and time-varying load conditions. For steady-state operation, an increase in the water vapor partial pressure was observed with increasing current density due to electrochemical production of water. As expected, the measurement channel near the inlet of the flow path showed a lower water vapor partial pressure than the outlet under identical load conditions; however, the quantitative distribution of water content across the cell is important to understanding operational behavior of a PEM fuel cell. These quantitative water concentration differences between two measurement channels are reported with variations in cell load and temperature. Temperature in the gas phase remained constant due to thermal equilibrium of the fuel cell. For time varying operation, no phase lag was observed between the load and the water vapor partial pressure. The outlet measurement channel showed higher partial pressure than the inlet with larger differences for increasing cell load. The transient data matched the steady-state measurements at the same conditions. A temperature rise from the controlled value was observed at high current densities for the unsteady operation; thus, the temperature did not equilibrate on the same time scale as the water partial pressure.     

Performance analysis of a proton exchange membrane fuel cell using tree-shaped designs for flow distribution

The performance of a proton exchange membrane (PEM) fuel cell is directly associated to the flow channels design embedded in the bipolar plates. The flow field within a fuel cell must provide efficient mass transport with a reduced pressure drop through the flow channels in order to obtain a uniform current distribution and a high power density. In this investigation, three-dimensional fuel cell models are analyzed using computational fluid dynamics (CFD). The proposed flow fields are radially designed tree-shaped geometries that connect the center flow inlet to the perimeter of the fuel cell plate. Three flow geometries having different levels of bifurcation were investigated as flow channels for PEM fuel cells. The performance of the fuel cells is reported in polarization and power curves, and compared with that of fuel cells using conventional flow patterns such as serpentine and parallel channels. Results from the flow analysis indicate that tree-shaped flow patterns can provide a relatively low pressure drop as well as a uniform flow distribution. It was found that as the number of bifurcation levels increases, a larger active area can be utilized in order to generate higher power and current densities from the fuel cell with a negligible increase in pumping power.     

Numerical study of water management in the air flow channel of a PEM fuel cell cathode

The water management in the air flow channel of a proton exchange membrane (PEM) fuel cell cathode is numerically investigated using the FLUENT software package. By enabling the volume of fraction (VOF) model, the air–water two-phase flow can be simulated under different operating conditions. The effects of channel surface hydrophilicity, channel geometry, and air inlet velocity on water behavior, water content inside the channel, and two-phase pressure drop are discussed in detail. The results of the quasi-steady-state simulations show that: (1) the hydrophilicity of reactant flow channel surface is critical for water management in order to facilitate water transport along channel surfaces or edges; (2) hydrophilic surfaces also increase pressure drop due to liquid water spreading; (3) a sharp corner channel design could benefit water management because it facilitates water accumulation and provides paths for water transport along channel surface opposite to gas diffusion layer; (4) the two-phase pressure drop inside the air flow channel increases almost linearly with increasing air inlet velocity.     

Application of Open Pore Cellular Foam for air breathing PEM fuel cell

Open Pore Cellular Foam (OPCF) has received increased attention for use in Proton Exchange Membrane (PEM) fuel cells as a flow plate due to some advantages offered by the material, including better gas flow, lower pressure drop and low electrical resistance.In the present study, a novel design for an air-breathing PEM (ABPEM) fuel cell, which allows air convection from the surrounding atmosphere, using OPCF as a flow distributor has been developed. The developed fuel cell has been compared with one that uses a normal serpentine flow plate, demonstrating better performance.A comparative analysis of the performance of an ABPEM and pressurised air PEM (PAPEM) fuel cell is conducted and poor water management behaviour was observed for the ABPEM design.Thereafter, a PTFE coating has been applied to the OPCF with contact angle and electrochemical polarisation tests conducted to assess the capability of the coating to enhance the hydrophobicity and corrosion protection of metallic OPCF in the PEM fuel cell environment. The results showed that the ABPEM fuel cell with PTFE coated OPCF had a better performance than that with uncoated OPCF.Finally, OPCF was employed to build an ABPEM fuel cell stack where the performance, advantages and limitations of this stack are discussed in this paper.     

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.     

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.     

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.     

Cross-leakage flow between adjacent flow channels in PEM fuel cells

In polymer electrolyte membrane (PEM) fuel cells, serpentine flow channels are used conventionally for effective water removal. The reactant flows along the flow channel with pressure decrease due to the frictional and minor losses as well as the reactant depletion because of electrochemical reactions in the cells. Because of the short distance between the adjacent flow channels, often in the order of 1 mm or smaller, the pressure gradient between the adjacent flow channels is very large, driving part of reactant to flow through the porous electrode backing layer (or the so-called gas diffusion layer)—this cross-leakage flow between adjacent flow channels in PEM fuel cells has been largely ignored in previous studies. In this study, the effect of cross-flow in an electrode backing layer has been investigated numerically by considering bipolar plates with single-channel serpentine flow field for both the anode and cathode side. It is found that a significant amount of reactant gas flows through the porous electrode structure, due to the pressure difference, and enters the next flow channel, in addition to a portion entering the catalyst layer for reaction. Therefore, mixing occurs between the relatively high concentration reactant stream following the flow channel and the relatively low reactant concentration stream going through the electrode. It is observed that the cross-leakage flow influences the reactant concentration at the interface between the electrode and the catalyst layer, hence the distribution of reaction rate or current density generated. In practice, this cross-leakage flow in the cathode helps drive the liquid water out of the electrode structure for effective water management, partially responsible for the good PEM fuel cell performance using the serpentine flow channels.     

The effect of anode bed geometry on the hydraulic behaviour of PEM fuel cells

The influence of the anode bed geometry on the hydraulic behaviour of PEM fuel cells is assessed. Three basic geometrical patterns are studied, namely the interdigitated, the parallel and the serpentine ones, in their original, as well as in modified forms of them, in which their angles have been smoothed. Issues concerning the anode flow field of a fuel cell, the influence of the Reynolds number on pressure drop, the mass flowrate distribution along the anode bed channels and the residence time of the fluid inside the fuel cell are investigated. All different geometries are studied by means of 3D numerical flow simulations. The results indicate that the pressure drop, flowrate nonuniformity in bed channels and residence time increase as the flow Reynolds number increases. The effect of geometry smoothing on the results is also assessed.     

Gas–liquid two-phase flow patterns in parallel channels for fuel cells

Two-phase flow in horizontal parallel channels has been experimentally investigated under fuel cell related operating conditions. Pronounced hysteresis is observed in the pressure drop versus flow characteristic curve when starting from either flooded or dry conditions. When gas is introduced into channels initially filled with water (flooded initial condition), both gas and liquid tend to flow predominantly in one channel at low gas or liquid flow velocities. As the gas flow velocity increases, even distribution of gas and liquid flow in both channels is observed, accompanied with a sudden decrease in the pressure drop. On the other hand, even gas and liquid flow distribution between both channels is found at comparatively lower gas flow velocities when starting with dry-gas flow conditions with liquid introduced into channels filled with gas (stratified flow regime). The flow regimes of this system are visualized in plots of the pressure drop against gas and liquid flow velocities. However, this phenomenon tends to vanish at high gas and liquid flow velocities, suggesting that high gas and liquid flow velocities are required to ensure even flow distribution in parallel channels. The hysteresis points appear at the same level of the pressure drop, reflecting intrinsic characteristics of the parallel channels used in this study. These results have important implications for PEM fuel cell operational strategies. In order to avoid reactant mal-distribution in parallel flow channels in the flow field in the two-phase flow regime, fuel cells should be operated at sufficiently high gas flow velocities.     

Innovative cathode flow-field design for passive air-cooled polymer electrolyte membrane (PEM) fuel cell stacks

A novel cathode flow-field design suitable for a passive air-cooled polymer electrolyte membrane (PEM) fuel cell stack is proposed to enhance the water-retaining capability under excess dry air supply conditions. The innovative cathode flow-field is designed to supply more air to the cooling channels and further enables deceleration of the reactant air in the gas channels and acceleration of the coolant air in the cooling channels simultaneously along the air flow path. Therefore, the design facilitates the waste heat removal through the cooling channels while the water removal by the reactant air is minimized. The conceptual cathode flow-field design is validated using a three-dimensional PEM fuel cell model. The detailed simulation results clearly demonstrate that the new cathode flow-field design exhibits superior water-retaining capability compared with a conventional cathode flow-field design (parallel flow channel configuration) under typical air-cooled fuel cell operating conditions. This study provides a new strategy to design cathode flow-fields to alleviate notorious membrane dehydration and unstable performance issues in a passive air-cooled PEM fuel cell stack.     

An analytical analysis on the cross flow in a PEM fuel cell with serpentine flow channel

A serpentine flow channel can be considered as neighboring channels connected in series, and is one of the most common and practical channel layouts for polymer electrolyte membrane (PEM) fuel cells, as it ensures the removal of liquid water produced in a cell with good performance and acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross directly to the neighboring channel via the porous gas diffusion layer (GDL) due to the high‐pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area resulting in a substantially lower amount of pressure drop in an actual PEM fuel cell compared with the case without cross flow. In this study, an analytical solution is obtained for the cross flow in a PEM fuel cell with a serpentine flow channel based on the assumption that the velocity of cross flow is linearly distributed in the GDL between two successive U‐turns. The analytical solution predicts the amount of pressure drop and the average volume flow rate in the flow channel and the GDL. The solution is validated over a wide range of the thickness and permeability of the GDL by comparing the results with experimental measurements and 3‐D numerical simulations in literature. Excellent agreement is obtained for the permeability less than 10^?9 m², which covers the typical permeability values of the GDLs in actual PEM fuel cells. The solution presents an accurate and efficient estimation for cross flow providing a useful tool for the design and optimization of PEM fuel cells with serpentine flow channels. Copyright © 2010 John Wiley & Sons, Ltd.     

A critical review of two-phase flow in gas flow channels of proton exchange membrane fuel cells

Water management in PEM fuel cells has received extensive attention due to its key role in fuel cell performance. The unavoidable water, from humidified gas streams and electrochemical reaction, leads to gas-liquid two-phase flow in the flow channels of fuel cells. The presence of two-phase flow increases the complexity in water management in PEM fuel cells, which remains a challenging hurdle in the commercialization of this technology. Unique water emergence from the gas diffusion layer, which is different from conventional gas-liquid two-phase flow where water is introduced from the inlet together with the gas, leads to different gas-liquid flow behaviors, including pressure drop, flow pattern, and liquid holdup along flow field channels. These parameters are critical in flow field design and fuel cell operation and therefore two-phase flow has received increasing attention in recent years. This review emphasizes gas-liquid two-phase flow in minichannels or microchannels related to PEM fuel cell applications. In situ and ex situ experimental setups have been utilized to visualize and quantify two-phase flow phenomena in terms of flow regime maps, flow maldistribution, and pressure drop measurements. Work should continue to make the results more relevant for operating PEM fuel cells. Numerical simulations have progressed greatly, but conditions relevant to the length scales and time scales experienced by an operating fuel cell have not been realized. Several mitigation strategies exist to deal with two-phase flow, but often at the expense of overall cell performance due to parasitic power losses. Thus, experimentation and simulation must continue to progress in order to develop a full understanding of two-phase flow phenomena so that meaningful mitigation strategies can be implemented.     

Investigation of bio-inspired flow channel designs for bipolar plates in proton exchange membrane fuel cells

Proton exchange membrane (PEM) fuel cell performance is directly related to the flow channel design on bipolar plates. Power gains can be found by varying the type, size, or arrangement of channels. The objective of this paper is to present two new flow channel patterns: a leaf design and a lung design. These bio-inspired designs combine the advantages of the existing serpentine and interdigitated patterns with inspiration from patterns found in nature. Both numerical simulation and experimental testing have been conducted to investigate the effects of two new flow channel patterns on fuel cell performance. From the numerical simulation, it was found that there is a lower pressure drop from the inlet to outlet in the leaf or lung design than the existing serpentine or interdigitated flow patterns. The flow diffusion to the gas diffusion layer was found be to more uniform for the new flow channel patterns. A 25 cm² fuel cell was assembled and tested for four different flow channels: leaf, lung, serpentine and interdigitated. The polarization curve has been obtained under different operating conditions. It was found that the fuel cell with either leaf or lung design performs better than the convectional flow channel design under the same operating conditions. Both the leaf and lung design show improvements over previous designs by up to 30% in peak power density.     

A numerical study on thermal analysis and cooling flow fields effect on PEMFC performance

Thermal management and water management are two important interconnected topics in the design and increase the efficiency of PEM fuel cells. Suitable cooling flow field design with proper performance is an important factor in increasing the lifetime of PEM fuel cell, because non-uniformity of temperature reduces the stability and durability of PEM fuel cell. Different cooling strategies are considered for removing of heat generation by PEM fuel cell, because the fuel cell temperature remains in a tolerable range and homogeneous as possible. In the first step, determine the value and location of heat sources in fuel cell, is important and appropriate cooling strategy can be applied. In this study, a PEM fuel cell with serpentine gas flow field is simulated with six different cooling flow fields simultaneously, e.g. conventional serpentine (Model 1), typical of MPSFFs (model 2), typical of serpentine (Model 3), parallel-serpentine (Model 4), conventional spiral (Model 5) and conventional parallel (Model 6). This simulation showes a correct perception of temperature distribution in PEMFC. The results indicate that Model 5 has a good temperature and performance based on the minimum and maximum temperature gradient, Index of uniform temperature (IUT), however has a more pressure drop. For second choice when the pressure drop is important, the Model 3 has a better performance than other models. Also, thermal analysis in these cases shows that ohmic, entropic and reaction heating included 20%, 35%, 45% of the total heat generation by PEMFC, approximately.