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.     

Modeling two-phase flow in PEM fuel cell channels

This paper is concerned with the simultaneous flow of liquid water and gaseous reactants in mini-channels of a proton exchange membrane (PEM) fuel cell. Envisaging the mini-channels as structured and ordered porous media, we develop a continuum model of two-phase channel flow based on two-phase Darcy's law and the M² formalism, which allow estimate of the parameters key to fuel cell operation such as overall pressure drop and liquid saturation profiles along the axial flow direction. Analytical solutions of liquid water saturation and species concentrations along the channel are derived to explore the dependences of these physical variables vital to cell performance on operating parameters such as flow stoichiometric ratio and relative humility. The two-phase channel model is further implemented for three-dimensional numerical simulations of two-phase, multi-component transport in a single fuel-cell channel. Three issues critical to optimizing channel design and mitigating channel flooding in PEM fuel cells are fully discussed: liquid water buildup towards the fuel cell outlet, saturation spike in the vicinity of flow cross-sectional heterogeneity, and two-phase pressure drop. Both the two-phase model and analytical solutions presented in this paper may be applicable to more general two-phase flow phenomena through mini- and micro-channels.     

Impact of channel geometry on two-phase flow in fuel cell microchannels

An important function of the gas delivery channels in PEM fuel cells is the evacuation of water at the cathode. The resulting two-phase flow impedes reactant transport and causes parasitic losses. There is a need for research on two-phase flow in channels in which the phase fraction varies along the flow direction as in operating fuel cells. This work studies two-phase flow in 60 cm long channels with distributed water injection through a porous GDL wall to examine the physics of flows relevant to fuel cells. Flow regime maps based on local gas and liquid flow rates are constructed for experimental conditions corresponding to current densities between 0.5 and 2 A cm⁻² and stoichiometric coefficients from 1 to 4. Flow structures transition along the length of the channel. Stratified flow occurs at high liquid flow rates, while intermittent slug flow occurs at low liquid flow rates. The prevalence of stratified flow in these serpentine channels is discussed in relation to water removal mechanisms in the cathode channels of PEM fuel cells. Corners facilitate formation of liquid films in the channel, but may reduce the water-evacuation capability. This analysis informs design guidelines for gas delivery microchannels for fuel cells.     

Dry gas operation of proton exchange membrane fuel cells with parallel channels: Non-porous versus porous plates

We present a study of proton exchange membrane (PEM) fuel cells with parallel channel flow fields for the cathode, dry inlet gases, and ambient pressure at the outlets. The study compares the performance of two designs: a standard, non-porous graphite cathode plate design and a porous hydrophilic carbon plate version. The experimental study of the non-porous plate is a control case and highlights the significant challenges of operation with dry gases and non-porous, parallel channel cathodes. These challenges include significant transients in power density and severe performance loss due to flooding and electrolyte dry-out. Our experimental study shows that the porous plate yields significant improvements in performance and robustness of operation. We hypothesize that the porous plate distributes water throughout the cell area by capillary action; including pumping water upstream to normally dry inlet regions. The porous plate reduces membrane resistance and air pressure drop. Further, IR-free polarization curves confirm operation free of flooding. With an air stoichiometric ratio of 1.3, we obtain a maximum power density of 0.40 W cm⁻², which is 3.5 times greater than that achieved with the non-porous plate at the same operating condition.     

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.     

Determination of the optimal active area for proton exchange membrane fuel cells with parallel,interdigitated or serpentine designs

Effects of active area size on steady-state characteristics of a working PEM fuel cell, including local current densities, local oxygen transport rates, and liquid water transport were studied by applying a three-dimensional, two-phase PEM fuel cell model. The PEM fuel cells were with parallel, interdigitated, and serpentine flow channel design. At high operating voltages, the size effects on cell performance are not noticeable owing to the occurrence of oxygen supply limit. The electrochemical reaction rates are high at low operating voltages, producing large quantity of water, whose removal capability is significantly affected by flow channel design. The cells with long parallel flow field experience easy water accumulation, thereby presenting low oxygen transport rate and low current density. The cells with interdigitated and serpentine flow fields generate forced convection stream to improve reactant transport and liquid water removal, thereby leading to enhanced cell performance and different size effect from the parallel flow cells. Increase in active area significantly improves performance for serpentine cells, but only has limited effect on that of interdigitated cells. Size effects of pressure drop over the PEM cells were also discussed.     

Experimental research on water management in proton exchange membrane fuel cells

A simulated cathode flow channel experiment system was set up based on the gas flow rate and water flow rate in the PEM fuel cell. With the assistance of the visualization system, high-sensitivity double parallel conductance probes flow regime inspecting technique was adopted successfully in the experiment system to inspect the flow regime of the gas–liquid two-phase flow in the PEM fuel cell. The research results show that the double parallel conductance probes inspecting system and the flow regime image system for the gas–liquid two-phase flow in the PEM fuel cell simulated channel both can judge the slug flow and annular flow in it, and the double parallel conductance probes flow regime inspecting system can divide the annular flow into three subtypes. The main probes inspecting system and the assistant image system validate reciprocally, which enhances the experimental veracity. The typical flow regimes of the PEM fuel cell simulated channel include slug flow, annular flow with big water film wave, annular flow with small water film wave and annular flow without water film wave. With the increase of the liquid superficial velocity, the frequencies of liquid slug and wave of liquid film increase. The flow regime map in the flow channel of the PEM fuel cell was developed. The flow regime of the gas–liquid two-phase flow in a PEM fuel cell in different operating conditions can be forecasted with this map. With the PEM fuel cell operating condition in this study, the flow regimes of gas–liquid two-phase flow for different cases are all annular flow with small water film wave, and the liquid film waves more with bigger current density. With the location closer to the channel outlet, the liquid film waves are more for the same current density.     

Transient response of PEM fuel cells with parallel and interdigitated flow field designs

Transient characteristics of proton exchange membrane (PEM) fuel cells with parallel and interdigitated flow fields upon changes in voltage load were investigated by applying a three-dimensional, two-phase model. Effects of channel to rib width ratios and cathode inlet flow rates on the transient response of PEM fuel cell were examined in detail. Current overshoot and undershoot occur because the time scale for the voltage change is much shorter than for the oxygen concentration changes. Therefore, the oxygen concentrations on the cathode diffusion layer-catalyst layer interface immediately after the voltage changes are essentially the same as before the voltage changes, which results in higher reaction rates causing overshoots when the voltage decreases or lower reaction rates causing undershoots when the voltage increases. The predictions also show that as the voltage decrease rate is reduced, the overshoot peak weakens and the response time shortens. Since the interdigitated flow field has higher oxygen concentrations on the cathode diffusion layer-catalyst layer interface due to the forced convection, the overshoot peaks and the undershoot valleys are all greater than for the parallel flow field. For both flow fields, larger channel to rib width ratios cause larger overshoots, smaller undershoots and longer response times.     

Measurement and modeling of liquid film thickness evolution in stratified two-phase microchannel flows

Polymer electrolyte membrane (PEM) fuel cells incorporating microchannels (D < 500 μm) can benefit from improved fuel delivery and convective cooling. However, this requires a better understanding of two-phase microchannel transport phenomena, particularly liquid–gas interactions and liquid clogging in cathode air-delivery channels. This paper develops optical fluorescence imaging of water films in hydrophilic channels with varying air velocity and water injection rate. Micromachined silicon test structures with optical access and distributed water injection simulate the cathode channels of a PEM fuel cell. Film thickness data vary strongly with air velocity and are consistent with stratified flow modeling. This work facilitates the study of regime transitions in two-phase microchannel flows and the effects of flow regimes on heat and mass transfer and axial pressure gradients.     

Time delay control for fuel cells with bidirectional DC/DC converter and battery

Transient behavior is a key property in the vehicular application of proton exchange membrane (PEM) fuel cells. A better control technology is constructed to increase the transient performance of PEM fuel cells. A steady-state isothermal analytical fuel cell model is constructed to analyze mass transfer and water transport in the membrane. To prevent the starvation of air in the PEM fuel cell, time delay control is used to regulate the optimum stoichiometric amount of oxygen, although dynamic fluctuations exist in the PEM fuel cell power. A bidirectional DC/DC converter connects the battery to the DC link to manage the power distribution between the fuel cell and the battery. Dynamic evolution control (DEC) allows for adequate pulse-width modulation (PWM) control of the bidirectional DC/DC converter with fast response. Matlab/Simulink/Simpower simulation is performed to validate the proposed methodology, increase the transient performance of the PEM fuel cell system and satisfy the requirement of energy management.     

Three‐dimensional optimisation of a fuel gas channel of a proton exchange membrane fuel cell for maximum current density

Proton exchange membrane (PEM) fuel cells operated with hydrogen and air offer promising alternative to conventional fossil fuel sources for transport and stationary applications because of its high efficiency, low‐temperature operation, high power density, fast start‐up and potable power for mobile application. Power levels derivable from this class of fuel cell depend on the operating parameters. In this study, a three‐dimensional numerical optimisation of the effect of operating and design parameters of PEM fuel cell performance was developed. The model computational domain includes an anode flow channel, membrane electrode assembly and a cathode flow channel. The continuity, momentum, energy and species conservation equations describing the flow and species transport of the gas mixture in the coupled gas channels and the electrodes were numerically solved using a computational fluid dynamics code. The effects of several key parameters, including channel geometries (width and depth), flow orientation and gas diffusion layer (GDL) porosity on performance and species distribution in a typical fuel cell system have been studied. Numerical results of the effect of flow rate and GDL porosity on the flow channel optimal configurations for PEM fuel cell are reported. Simulations were carried out ranging from 0.6 to 1.6 mm for channel width, 0.5 to 3.0 mm for channel depth and 0.1 to 0.7 for the GDL porosity. Results were evaluated at 0.3 V operating cell voltage of the PEM fuel cell. The optimisation results show that the optimum dimension values for channel depth and channel width are 2.0 and 1.2 mm, respectively. In addition, the results indicate that effective design of fuel gas channel in combination with the reactant species flow rate and GDL porosity enhances the performance of the fuel cell. The numerical results computed agree well with experimental data in the literature. Consequently, the results obtained provide useful information for improving the design of fuel cells. Copyright © 2011 John Wiley & Sons, Ltd.     

Visualization and measurements of periodic boiling in silicon microchannels

A simultaneous visualization and measurement investigation has been carried out on flow boiling of water in parallel silicon microchannels of trapezoidal cross-section. Two sets of parallel microchannels, having hydraulic diameters of 158.8 and 82.8 μm, respectively, were used. The visualization study shows that once boiling heat transfer is established, two-phase flow and single-phase liquid flow appear alternatively with time in the microchannels. Large-amplitude/long-period fluctuations with time in wall temperatures, fluid temperatures, fluid pressures, and fluid mass flux, are measured for the first time during flow boiling in the microchannels. The fluctuation periods are found to be dependent on channel size, heat flux, and mass flux. The mechanism of the periodic boiling fluctuations in this experiment as well as their comparisons with other boiling fluctuations phenomena reported previously, are also discussed. The experimental results confirm that large-amplitude/long-period boiling fluctuations can be sustained when the fluctuations of pressure drop and mass flux have phase differences.With the aid of a microscope and high-speed video recording system, bubbly flow, slug flow, churn flow, and other peculiar flow patterns, are observed during two-phase flow periods in the microchannels.     

Numerical study of a new cathode flow-field design with a sub-channel for a parallel flow-field polymer electrolyte membrane fuel cell

The cathode flow-field design of a polymer electrolyte membrane (PEM) fuel cell is crucial to its performance, because it determines the distribution of reactants and the removal of liquid water from the fuel cell. In this study, the cathode flow-field of a parallel flow-field PEM fuel cell was optimized using a sub-channel. The main-channel was fed with moist air, whereas the sub-channel was fed with dry air. The influences of the sub-channel flow rate (SFR, the amount of air from the sub-channel inlet as a percentage of the total cathode flow rate) and the inlet positions (SIP, where the sub-channel inlets were placed along the cathode channel) on fuel cell performance were numerically evaluated using a three-dimensional, two-phase fuel cell model. The results indicated that the SFR and SIP had significant impacts on the distribution of the feed air, removal of liquid water, and fuel cell performance. It was found that when the SIP was located at about 30% along the length of the channel from main-channel inlet and the SFR was about 70%, the PEM fuel cell exhibited much better performance than seen with a conventional design.     

Comparison of PEMFC performance with parallel serpentine and parallel serpentine-baffled flow fields under various operating and geometrical conditions; a parametric study

Optimal flow channel design of a fuel cell is crucial to further improve the performance of polymer electrolyte membrane fuel cell (PEMFC). In this work, a comprehensive parametric study was conducted to analyze the performance of a PEMFC with conventional parallel serpentine flow fields (PSFF) and parallel serpentine-baffled flow fields (PSBFF). A three-dimensional two-phase computational fluid dynamics model was used to numerically simulate the fuel cell performance. The effects of operating parameters such as pressure, temperature, and stoichiometric ratio, as well as the geometric parameters of channel height to channel width ratio and rib width to channel width ratio for both flow fields on fuel cell performance were investigated. The results show that as pressure, temperature, and stoichiometric ratio increase, cell performance increases for both flow fields, with a more substantial rate of improvement for the PSBFF design. A 16.1% improvement in cell performance at an operating pressure of 1 atm, a 19.9% improvement at a cell temperature of 70 °C, and a 16.1% improvement at a stoichiometric ratio of 2 were obtained when PSBFF was used instead of PSFF. By increasing the channel height and rib width, the cell performance for PSBFF remains almost constant due to the improved forced convection of the gas mixture and the reduction in concentration loss, while the cell performance for PSFF decreases significantly. At the largest channel height to channel width ratio of 1.5 and rib width to channel width ratio of 1.315 studied in this work, an improvement in cell performance of 53.3% and 58.5%, respectively, was achieved when PSBFF was used instead of PSFF. In addition, PSBFF was more effective in removing water from the porous zones than PSFF under all conditions.     

Channel aspect ratio effect for serpentine proton exchange membrane fuel cell: Role of sub-rib convection

A complete three-dimensional, two-phase, non-isothermal model for proton exchange membrane (PEM) fuel cells was used to investigate the effect of the sub-rib convection on the performances for the single and triple serpentine flow fields at various channel aspect ratios and different thermal constraints. The occurrence of sub-rib convection, which is affected by the serpentine flow field, significantly influences the cell performance if the oxygen supply or membrane moisture content was limited. For single serpentine flow field in which sub-rib convection presents under all ribs, changing channel aspect ratio has minimal effects on cell performance since the oxygen supply is sufficient. For triple serpentine flow field or for serpentine cell with poor external heat loss, owing to limited sub-rib convection or to low membrane moisture content, decrease in channel aspect ratio significantly enhances cell performance. Blocking up the sub-rib convection markedly reduces cell performance. Flow field design for PEM fuel cell should take into consideration the effects of sub-rib convection flow on cell performance.     

Analysis and optimization of high temperature proton exchange membrane (HT-PEM) fuel cell based on surrogate model

The stoichiometric ratio and flow channel geometry play a vital role in the performance of high temperature proton exchange membrane (HT-PEM) fuel cells. Because of the high cost of experiments or simulations, most analyses and optimization of the stoichiometric ratio and flow channel geometry are limited to several points in the entire design domain. In this study, an analysis and optimization method for HT-PEM fuel cells based on the surrogate model was proposed. Surrogate models were constructed using some of the available budgets of samples to analyze and optimize the entire design domain. With this method, it was indicated that the effect of the cathode stoichiometric ratio is more significant to the cell performance than the anode stoichiometric ratio and there are significant nonlinear interactions among the flow channel geometry parameters. At the fixed operating voltage, the flow channel geometry with the maximum current density and that with the maximum real power were obtained. Compared with the base design, the designs obtained by the surrogate model improve the current density and real power by 10.54% and 3.93%, respectively. Thus, this analysis and optimization method is demonstrated to be helpful and deserves attention in future research.     

Water management studies in PEM fuel cells,Part II: Ex situ investigation of flow maldistribution,pressure drop and two-phase flow pattern in gas channels

Two-phase flow of water and reactant gases in the gas distribution channels of proton exchange membrane fuel cells (PEMFCs) plays a critical role in proper water management. In this work, the two-phase flow in PEMFC cathode parallel channels is studied over a wide range of superficial air velocity (air stoichiometry) and superficial water velocity in a specially designed ex situ experimental setup, which enables the measurement of instantaneous flow rates in individual gas channels and simultaneous visualization of the water flow structure. It is found that the two-phase flow at low superficial air velocities (air stoichiometry below 5) is dominated by slugs or semi-slugs, leading to severe flow maldistribution and large fluctuations in the pressure drop. Slug residence time, measured from the video observation and the instantaneous flow rate data, is found to be a new parameter to describe the slug flow. At higher air velocities, a water film is formed on the channel walls if they are hydrophilic. The pressure drop for the film flow is characterized by smaller but frequent fluctuations, which are found to result from the water buildup at the channel-exit manifold interface. As the superficial air velocity increases further, mist flow is obtained where little water buildup is observed. The water buildup in the gas channels at the two-phase flow is well described by the two-phase friction multiplier, defined as the ratio of the two-phase pressure drop to the single gas phase pressure drop. It is found that the two-phase friction multiplier increases with increasing water flow rate. A flow pattern map is developed using superficial water and air velocities with clearly defined transition regions.     

Prediction performance of PEM fuel cells by gene expression programming

In the present study, gene expression programming has been utilized to evaluate the output voltage of different PEM fuel cells as the performance symbol of these structures. A total number of 843 data were collected from the literature, randomly divided into 682 and 161 sets, and then trained and tested, respectively by different models. The used data as input parameters were consisted of current density, fuel cell temperature, anode humidification temperature, cathode humidification temperature, operating pressures, fuel cell type, O₂ flow rate, air flow rate and active surface area of the PEM fuel cells. According to these input parameters, in the gene expression programming models, the voltage of each PEM fuel cell in different conditions was predicted. The training and testing results in the gene expression programming model have shown an acceptable potential for predicting voltage values of the PEM fuel cells in the considered range.     

Modeling and control of air stream and hydrogen flow with recirculation in a PEM fuel cell system—II. Linear and adaptive nonlinear control

In this part of the paper, linear and nonlinear multivariable controllers are designed for the air stream and hydrogen flow with recirculation in a proton exchange membrane (PEM) fuel cell system. The focus of the model is to obtain the desired transient performance of air stoichiometric ratio, cathode inlet pressure, and pressure difference between the anode and the cathode. Based on linearization of the nonlinear dynamic model in the first part of this paper, the coupling between control inputs and performance is analyzed first. The phase relationship between the stack voltage and water transport in frequency domain is meaningful to the future humidity estimation and active purge operation. Then, linear quadratic Gaussian (LQG) algorithm based on observer feedback is used for set-point tracking, and a model-predictive controller (MPC) with an on-line neural network identifier is also designed to improve robustness. Compared with decentralized PI controllers, the multivariable controllers improve the transient response and shows better disturbance rejection capability.     

阴极流道分流进气对燃料电池性能影响的实验研究

针对高工作电流密度下,燃料电池内局部水淹导致的传质损失问题,本研究提出了一种阴极流道多进口分流进气方式。实验研究了三种典型分流口位置及分流进量对电池性能的影响。研究发现随着分流口远离阴极主进气口,电池性能呈现先上升后下降的趋势,且当分流口靠近主进气口时,增加分流量有助于电池性能提升,但分流量的增加对电池性能的提升存在一个极限值;因此,在对电池进行分流进气优化时需综合考虑分流口位置和分流量的影响。当分流口为SIP-30%且分流量为按化学当量比ξc = 0.75取值时,分流进气方式相比传统进气方式,电池的最大功率密度高出17.8%。