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.     

Multi-pass serpentine flow-fields to enhance under-rib convection in polymer electrolyte membrane fuel cells: Design and geometrical characterization

The flow-field for reactant distribution is an important design factor that influences the performance of polymer electrolyte membrane fuel cells (PEMFCs). Under-rib convection between neighboring channels has been recognized to enhance the performance of PEMFCs with serpentine flow-fields. This study presents a simple design method to generate multi-pass serpentine flow-fields (MPSFFs) that can maximize under-rib convection in a given cell area. Geometrical characterization indicates that MPSFFs lead to significantly higher under-rib convection intensities and more uniform conditions, such as reactant concentrations, temperature, and liquid water saturation, compared with conventional serpentine flow-fields. The implications of the enhanced under-rib convection due to MPSFFs on the performance of PEMFCs are discussed.     

Local transport phenomena and cell performance of PEM fuel cells with various serpentine flow field designs

The flow field design in bipolar plates is very important for improving reactant utilization and liquid water removal in proton exchange membrane fuel cells (PEMFCs). A three-dimensional model was used to analyze the effect of the design parameters in the bipolar plates, including the number of flow channel bends, number of serpentine flow channels and the flow channel width ratio, on the cell performance of miniature PEMFCs with the serpentine flow field. The effect of the liquid water formation on the porosities of the porous layers was also taken into account in the model while the complex two-phase flow was neglected. The predictions show that (1) for the single serpentine flow field, the cell performance improves as the number of flow channel bends increases; (2) the single serpentine flow field has better performance than the double and triple serpentine flow fields; (3) the cell performance only improves slowly as the flow channel width increases. The effects of these design parameters on the cell performance were evaluated based on the local oxygen mass flow rates and liquid water distributions in the cells. Analysis of the pressure drops showed that for these miniature PEMFCs, the energy losses due to the pressure drops can be neglected because they are far less than the cell output power.     

Model based design and test of cooling plates for an air-cooled polymer electrolyte fuel cell stack

Design of an effective cooling system in polymer electrolyte membrane fuel cells (PEMFCs) is vital for the heat management and overall performance of stacks. Depending on the stack size and application, either air or water-cooling can be used to extract excess heat and maintain the desired temperature distribution throughout the stack.A computational model previously assembled by the authors has been used to design cooling plates for a typical air-cooled stack configuration. The aim of these designs was to minimise temperature differences between cells, and dissipate heat from the stack. Three different cooling plate designs are analysed both computationally and experimentally within stacks containing electrically heated pads in place of active MEAs.Good agreement was achieved between the model and experiment, and results showed that implementing a cooling plate is an effective way to balance temperature variation within a stack and minimise thermal issues. It was found that the temperature variation may be minimised by implementing plates with wider cooling channels. As a result, more air may be forced through the channels with less resistance, which minimises the power required by the air blower, and hence the parasitic load on the system.     

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.     

Three-dimensional numerical study on cell performance and transport phenomena of PEM fuel cells with conventional flow fields

In this paper, a three-dimensional numerical model of the proton exchange membrane fuel cells (PEMFCs) with conventional flow field designs (parallel flow field, Z-type flow field, and serpentine flow field) has been established to investigate the performance and transport phenomena in the PEMFCs. The influences of the flow field designs on the fuel utilization, the water removal, and the cell performance of the PEMFC are studied. The distributions of velocity, oxygen mass fraction, current density, liquid water, and pressure with the convention flow fields are presented. For the conventional flow fields, the cell performance can be enhanced by adding the corner number, increasing the flow channel length, and decreasing the flow channel number. The cell performance of the serpentine flow field is the best, followed by the Z-type flow field and then the parallel flow field.     

Numerical study on thermal performance of non-uniform flow channel designs for cooling plates of PEM fuel cells

Due to the limited cooling capacity of air, large-scale proton exchange membrane (PEM) fuel cell stacks are generally cooled by liquid cooling where liquid water is circulated through the flow channels of cooling plates. Effective cooling is essential for the stability, durability, and performance of PEM fuel cells. In this study, cooling plates with conventional straight channel and novel non-uniform flow channel designs are investigated and analyzed by using a three-dimensional model. The simulated results are presented in terms of pressure drop, average temperature, maximum temperature, temperature difference between the maximum temperature and minimum temperature, and the temperature uniformity index. In addition, the effects of heat flux and inlet Reynolds number on the cooling performance are studied. It is concluded that the cooling performance is significantly improved as the novel flow channel designs are applied.     

Thermal analysis of air-cooled PEM fuel cells

Air-cooled proton exchange membrane fuel cells (PEMFCs), having combined air cooling and oxidant supply channels, offer significantly reduced bill of materials and system complexity compared to conventional, water-cooled fuel cells. Thermal management of air-cooled fuel cells is however a major challenge. In the present study, a 3D numerical thermal model is presented to analyze the heat transfer and predict the temperature distribution in air-cooled PEMFCs. Conservation equations of mass, momentum, species, and energy are solved in the oxidant channel, while energy equation is solved in the entire domain, including the membrane electrode assembly (MEA) and bipolar plates. The model is validated with experiments and can reasonably predict the maximum temperature and main temperature gradients in the stack. Large temperature variations are found between the cool incoming air flow and the hot bipolar plates and MEA, and in contrast to water-cooled fuel cells, significant temperature gradients are detected in the flow direction. Furthermore, the air velocity and in-plane thermal conductivity of the plate are found to play an important role in the thermal performance of the stack.     

Effect of serpentine flow-field designs on performance of PEMFC stacks for micro-CHP systems

Serpentine flow-fields are widely used for polymer electrolyte membrane (PEM) fuel cells due to effective water removal. In this study, the effects of serpentine flow-field designs on the performance of a commercial-scale PEM fuel cell stack for micro-CHP (Combined Heat & Power) systems, which use reformed gas as fuel, are investigated by performing both computational fluid dynamics (CFD) simulations and experimental measurements. First, we design four different serpentine flow-fields in which the total channel area (defined as open channel area in this study) of a flow-field plate is altered without changing other design parameters such as the channel cross-sectional area and the channel length. Then, CFD simulations and experimental measurements are performed to assess the performance of each flow-field design. The CFD simulation results show that the current density distributions and average current densities are very insensitive to the open channel area. Thus, the information from the simulations is not sufficient to judge whether the open channel area affects the performance of a PEM fuel cell. On the other hand, the experimental measurements indicate that the performances of four fuel cell stacks, each with one of the four flow-field designs used in the simulations, are considerably different. Increasing the open channel area of a serpentine flow-field improves the performance of the PEM fuel cell up to a certain extent.     

HEAT TRANSFER IN 3-D SERPENTINE CHANNELS WITH RIGHT-ANGLE TURNS

Laminar flow and heat transfer in square serpentine channels with right-angle turns, which have applications in heat exchangers, were numerically studied. A finite volume code in FORTRAN was developed to solve this problem. For solving the flow field, a colocated-grid formulation was used, as opposed to the staggered-grid formulation, and the SIMPLE algorithm was used to link the velocity and pressure. The line-by-line method was used to solve the algebraic equations. The temperature field was solved for the uniform-wall-heat-flux boundary condition. The developed numerical code was validated by solving for fully developed flow and heat transfer in a square straight channel. The grid-independent solution was established for a reference case of serpentine channel with the highest Reynolds number. Periodically fully developed flow and heat transfer fields in serpentine channels were solved for different geometry parameters, for different Reynolds numbers, and for two different Prandtl numbers (for air and water, respectively). The enhancement of the heat transfer mechanism was explained by studying the plotted flow-field velocity vectors in different planes. The heat transfer performance of serpentine channels is better than that for straight channels for Pr = 7.0 and is worse than that for straight channels for Pr = 0.7.     

Liquid water transport in PEMFC cathode with symmetrical biomimetic flow field design based on Murray's law

Water management in the flow field as well as the flooding process in the gas diffusion and catalyst layers enormously influence proton exchange membrane fuel cells (PEMFCs) performance and reliability. Researchers have developed many different designs for flow channels that can be used to distribute fuel or oxidant in PEMFCs (proton exchange membrane fuel cells). Among these designs, novel biomimetic designs have captured special attentions from researchers due to their capability of distributing fluids effectively. This study presents an investigation of the liquid water transport within a porous layer and a symmetrical biomimetic flow field based on Murray's law. The volume of fluid (VOF) method is employed, and the dynamic contact angle (DCA) effects are also considered for better prediction of water distribution. The water transport process and water distribution inside the porous layer and flow field are obtained from the simulation results. Recommendations are given for this type of flow field design based on the behaviors of liquid water in the porous layer and flow field.     

An investigational back surface cooling approach with different designs of heat‐absorbing pipe for PV/T system

A solar photovoltaic panel accumulates substantial amount of heat, which deteriorates its performance. Thus, to minimize the panel temperature, 3 new designs (semioval serpentine, circular spiral, and circular spiral semiflattened) of absorbers for back surface cooling are introduced in this paper. Experimental investigations on the panel performances with and without the designed cooling systems are performed. A similar experiment with an existing serpentine design of absorbers is also conducted, and the results of all the experiments are compared. The circular spiral semiflattened design absorber has shown preeminent performance among all the absorbers in terms of the highest improvement in efficiency (4.32%), fill factor (19.80%), etc.     

Different flow fields,operation modes and designs for proton exchange membrane fuel cells with dead-ended anode

Water and nitrogen can accumulate in the anode channel in proton exchange membrane fuel cells (PEMFCs) with dead-ended anode (DEA) and can affect cell performance significantly. In this paper, the cell performance characteristics in DEA PEMFCs with three different anode flow fields under two operating modes are studied through measuring the cell voltages and local current densities. The effect of the anode exit reservoir is also studied for the three different anode flow fields. The experimental results show that the interdigitated flow field has the most stable cell performance under both constant pressure and pressure swing supply modes. Parallel and serpentine flow fields lead to very non-uniform local current distributions under constant pressure supply mode and experience severe fluctuations and spikes in local current densities under pressure swing supply mode. The results also show that anode pressure swing supply mode can achieve more stable cell performance than anode constant pressure supply mode for parallel and serpentine anode flow fields. The anode exit reservoir can significantly improve cell performance stability for parallel and serpentine flow fields, but has no significant effect on interdigitated flow fields. Besides, the results further show that PEMFCs with DEA can maintain very stable operation with anode serpentine flow field and an anode exit reservoir under pressure swing operation.     

Effects of cathode flow fields on direct methanol fuel cell-simulation study

Flow-field design of direct methanol fuel cell (DMFCs) plays an important role affecting the cell performance. Previous studies suggest that the combination of anode parallel flow field and cathode serpentine flow-field present the best and stable performance. Among these, cathode flow-field holds higher influence than that of anode. However, more detailed experiments needed to be done to find out the reasons. In this study, CFDRC half-cell models are adopted to simulate the flow phenomena within serpentine, parallel and grid flow field. We find that gas is well distributed within serpentine flow field while barren region are observed within parallel flow field. These factors contribute to the cell performance greatly. In addition, the durability test of DMFCs using parallel flow field is improved when the flow rate is increased or the current is uphold at inferior, so the barren region maintained at an acceptable level.     

Development of open-cathode polymer electrolyte fuel cells using printed circuit board flow-field plates: Flow geometry characterisation

Open-cathode air-breathing fuel cells have the advantage of reduced system complexity and simplified operation, as oxygen is taken directly from ambient air without the need for blowers/compressors. In this study, printed circuit boards (PCBs) are used as flow-field plates. The use of PCBs offers the potential for significant cost reduction due to their well-established manufacturing processing and low materials cost. This study investigates the effect of varying the cathode geometry (parallel and circular) and opening ratios (43%, 53% and 63%) on fuel cell performance using polarisation curves, electrochemical impedance spectroscopy (EIS) and thermal imaging. The results obtained indicate that circular openings afford lower Ohmic resistance than parallel flow-field designs, which helps improve contact between the gas diffusion layer and flow-field plate. However, flow-field plates with circular openings suffer from greater mass transport limitation effects. Likewise, greater opening ratios offer better mass transport but increased Ohmic resistance as a result of the reduced area of lands/ribs. The thermal imaging results reveal lower temperature in the middle of the fuel cell due to “bowing” of the printed circuit board flow field plates which reduces the local current density. A trade-off between these factors results in a design with a maximum area specific power density of 250 mW cm⁻².     

Thermal management issues in a PEMFC stack – A brief review of current status

Understanding the thermal effects is critical in optimizing the performance and durability of proton exchange membrane fuel cells (PEMFCs). A PEMFC produces a similar amount of waste heat to its electric power output and tolerates only a small deviation in temperature from its design point. The balance between the heat production and its removal determines the operating temperature of a PEMFC. These stringent thermal requirements present a significant heat transfer challenge. In this work, the fundamental heat transfer mechanisms at PEMFC component level (including polymer electrolyte, catalyst layers, gas diffusion media and bipolar plates) are briefly reviewed. The current status of PEMFC cooling technology is also reviewed and research needs are identified.     

Numerical analysis of modified parallel flow field designs for fuel cells

Bipolar plates engraved with flow fields are key components in proton exchange membrane fuel cells (PEMFCs). These flow fields are important because they isolate and enhance the diffusion of the reactant for the electrochemical reaction. The flow fields on these plates are pathways that both supply reactant and remove reaction products from the anode and cathode of a PEMFC. Fluid flow in these flow fields can greatly affect the performance and life span of the device. In this study, conventional and modified parallel flow field designs were analyzed using computational fluid dynamic modeling. The designs split flow into variant channel widths to facilitate even reactant distribution. Flow characteristics are presented, including the pressure and velocity variations in the flow channels across the flow field and comparison of the pressure-drop characteristics of different flow fields. The results show that multiple stages of flow distribution can achieve an evenly distributed pressure drop with an ideal distribution of reactant among channels.     

Evaluation of flow field design effects on proton exchange membrane fuel cell performance

Fluid distribution, conduction, and heat control are important phenomenon in the fuel cell fraternity, therefore it is crucial to develop a state-of-the-art bipolar plate (BP) to attain optimum cell performance. Metal foam (MF) and fine mesh have attracted a lot of attention in mitigating some of the challenges associated with straight, and serpentine channels. In this study, MF, 3D fine mesh, fine wire mesh (FWM) flow fields are compared with triple serpentine flow field to develop an optimum design for improved PEMFC performance. Two different foam designs are studied to attenuate the existing drawback associated with MF, mainly caused by high water retention. The 3D fine mesh is leading in performance under anodic and cathodic stoichiometry of 1 and 3 respectively. On increasing the anodic and cathodic stoichiometry to 1.2 and 3.5 respectively, the FWM took the lead. This is brought by the improved water drainage under high stoichiometry. Because FWM is already in mass production, although for other purposes, it is cost competitive over the other designs. The fine mesh and the MF have the potential to break down large water droplet making them easy to drain. They also showed symmetric fluid flow, compared to the serpentine design.     

Characterization of water management in metal foam flow-field based polymer electrolyte fuel cells using in-operando neutron radiography

Metal foam flow-fields have shown great potential in improving the uniformity of reactant distribution in polymer electrolyte fuel cells (PEFCs) by eliminating the ‘land/channel’ geometry of conventional designs. However, a detailed understanding of the water management in operational metal foam flow-field based PEFCs is limited. This study aims to provide the first clear evidence of how and where water is generated, accumulated and removed in the metal foam flow-field based PEFCs using in-operando neutron radiography, and correlate the water ‘maps’ with electrochemical performance and durability. Results show that the metal foam flow-field based PEFC has greater tolerance to dehydration at 1000 mA cm⁻², exhibiting a ~50% increase in voltage, ~127% increase in total water mass and ~38% decrease in high frequency resistance (HFR) than serpentine flow-field design. Additionally, the metal foam flow-field promotes more uniform water distribution where the standard deviation of the liquid water thickness distribution across the entire cell active area is almost half that of the serpentine. These superior characteristics of metal foam flow-field result in greater than twice the maximum power density over serpentine flow-field. Results suggest that optimizing fuel cell operating condition and foam microstructure would partly mitigate flooding in the metal foam flow-field based PEFC.     

Numerical evaluation of a PEM fuel cell with conventional flow fields adapted to tubular plates

In this research a 3D numerical study on a PEM fuel cell model with tubular plates is presented. The study is focused on the performance evaluation of three flow fields with cylindrical geometry (serpentine, interdigitated and straight channels) in a fuel cell. These designs are proposed not only with the aim to reduce the pressure losses that conventional designs exhibit with rectangular flow fields but also to improve the mass transport processes that take place in the fuel cell cathode. A commercial computational fluid dynamics (CFD) code was used to solve the numerical model. From the numerical solution of the fluid mechanics equations and the electrochemical model of Butler-Volmer different analysis of pressure losses, species concentration, current density, temperature and ionic conductivity were carried out. The results were obtained at the flow channels and the catalyst layers as well as in the gas diffusion layers and the membrane interfaces. Numerical results showed that cylindrical channel configurations reduced the pressure losses in the cell due to the gradual reduction of the angle at the flow path and the twist of the channel, thus facilitating the expulsion of liquid water from the gas diffusion layers and in turn promoting a high oxygen concentration at the triple phase boundary of the catalyst layers. Moreover, numerical results were compared to polarization curves and the literature data reported for similar designs. These results demonstrated that conventional flow field designs applied to conventional tubular plates have some advantages over the rectangular designs, such as uniform pressure and current density distributions among others, therefore they could be considered for fuel cell designs in portable applications.