Multiphase Non-isothermal Model Research Articles

Enhancing water hydration in air-cooled proton exchange membrane fuel cell using a staggered tapered slotted flow field

Air-cooled proton exchange membrane fuel cell (AC-PEMFC) is widely considered as a promising power source for unmanned aerial vehicles (UAVs) due to its merits such as high energy density, short refueling time, and simple auxiliary system. However, the performance of AC-PEMFC is not satisfactory due to the poor membrane hydration caused by the large air supply for heat dissipation demand. This study proposes a staggered tapered slotted flow field (STSF) configuration to address this issue, which has higher contact area between the airflow and the bipolar plate by arranging tapered and slotted sections in the channels along the airflow direction, aiming to enhance the cooling effect and improve the membrane water hydration. Utilizing a three-dimensional (3D) multiphase non-isothermal model verified against experimental data, it was found that the STSF configuration reduces the internal temperature of the cell by about 14.2–28.3 K and increases the water content in the membrane by about 35.1–85.7 % compared with traditional straight channels. In addition, the STSF configuration can enhance mass transfer by inducing cross-flow, reducing concentration losses, which takes more effect for UAVs working at high altitude. Moreover, the slotted sections reduced the volume and weight of the bipolar plates, contributing to an additional power density improvement. Finally, the pressure drop within the flow channels and net power was compared. Due to the increased contact area between the cooling airflow and the bipolar plates, the STSF configuration inevitably results in a higher pressure drop within the channels, but the net power of PEMFC with STSF still increased under severe conditions by 0.080 W.

Effects of cathode gas channel design on air-cooled proton exchange membrane fuel cell performance

In the study, the impact of cathode gas channel (CGC) structure on the operation of air-cooled PEMFCs is investigated by a multiphase nonisothermal model. First, the influences of tapered channel configuration on the distribution of key variable distributions are studied. The distributions of these variables are found to be coupled with the reactive transport process within the catalyst layer (CL). The variation of local current density is found to be positively correlated with the dissolved water content. Although the tapered channel design can improve the current density, the weight of the bipolar plates (BPs) will be inevitably increased. Finally, the trapezoidal channel design is proposed aiming at improving the performance without increase of fuel cell weight. Several performance indexes are employed to comprehensively evaluate the different CGC designs, and the results demonstrate that the inverted trapezoidal CGC design, with which good thermal and water management are achieved, presents the best performance among the different CGC designs studied, leading to 18.9 % increase of the mass power density and 18.6 % increase of volume power density compared with the base case design.

Investigation of water effects on hydrogen dissolution, diffusion, and reaction in high temperature proton exchange membrane fuel cell

High-temperature proton exchange membrane fuel cell with phosphoric acid-doped polybenzimidazole membrane is typically utilized with a methanol steam reformer due to its high CO tolerance. However, methanol reformate commonly contains 20 % water, and its effect on the high temperature fuel cell is unclear. A comprehensive 3D multiphase non-isothermal model with an enhanced electrochemical kinetics model considering the water influence mechanism is developed. The water influence mechanism includes the proton conductivity, hydrogen dissolution on the pore-ionomer interface, hydrogen diffusion inside the ionomer, and hydrogen surface reaction on the platinum. The sensitivity analysis found that hydrogen dissolution on the pore-ionomer and hydrogen surface reaction on the platinum significantly affect cell performance. The influences of water on ionomer hydrogen concentration, platinum surface hydrogen concentration, and surface reaction rates are investigated under the voltage range of 0.9–0.4 V. The increase of water content can improve the net adsorption rate and benefit electrochemical reaction. Besides, the anode water is better for the uniformity of surface reaction rate distributions while not affecting the hydrogen distributions inside the ionomer and on the platinum surface. Additionally, the positive influence of water on CO poisoning is investigated in terms of surface reaction. The water's beneficial effect on decreasing platinum loading is also discussed. The results show that 20 % water can reduce the platinum loading by 20 %.

Numerical simulation of a PEM fuel cell: Effect of tortuosity parameters on the construction of polarization curves

Fuel cells (PEMFCs) are considered a clean alternative for the production of electricity. To improve their efficiency, it is necessary to understand the transport phenomena that determine their performance. This work analyzes the effective diffusion coefficients in the gas diffusion and catalyst layers (GDL and CL, respectively) based on theories such as effective medium approximation and percolation theory of a 3D multiphase nonisothermal model of a single channel PEMFC. We calculate polarization curves with different effective diffusion models and tortuosity factors m, n using OpenFOAM. The best model that approaches to experimental data is the Tomadakis-Sotirchos model with n = 4.0. The exponent m that represents the tortuosity due to the geometry has a high impact on the polarization curve construction compared to the exponent n. High values of diffusibility do not mean high values of current densities showing that there are other phenomena involved, such as water flooding.

Optimization of porous media flow field for proton exchange membrane fuel cell using a data-driven surrogate model

Flow field design plays a key role in proton exchange membrane (PEM) fuel cells due to its decisive influence on reactant gas transfer, liquid water discharge, electron conductance, and heat transfer. Porous media flow field (e.g. metal foam) shows promise to improve cell performance in the concentration polarization regime and to benefit uniform distributions of oxygen, liquid, current density, and temperature, etc. In this study, a data-driven surrogate model based on support vector machine (SVM) is applied to optimize porous media flow field geometry. The training data is obtained from a validated three-dimensional (3D), multi-phase non-isothermal model. For better characterization, the complex structure is simplified as a 3D structured mesh consisting of a set of fibers with each fiber perpendicular to each other at the joint. The results show that the data-driven surrogate model based on SVM predicts similar results as the 3D physical model. The genetic algorithm (GA) is further used for optimization, in which the data-driven surrogate model is selected as a fitness evaluation function. The optimal values obtained by the surrogate model are verified by the 3D physical model, indicating that the proposed data-driven surrogate model is effective in the design and optimization of porous media flow field.

Three-dimensional multi-phase simulation of PEM fuel cell considering the full morphology of metal foam flow field

It is well-known that flow field design is of primary importance to optimization of proton exchange membrane (PEM) fuel cell. Traditional channel-rib flow fields, e.g. parallel or serpentine channels, always lead to non-uniform distributions of reactant gas, liquid, current density and so on between the channel and rib regions. Metal foam materials with high porosity (>90%) have been proposed as alternative flow fields for PEM fuel cells. In this study, influences of metal foam flow field on the transport phenomena coupled with the electrochemical reactions in PEM fuel cell are investigated using a three-dimensional (3D) multi-phase non-isothermal model. Specifically, the full morphology of metal foam flow field is taken into account in the 3D simulation after validated against experimental permeability data. The full morphology inclusion enables capture of the detailed gas flow from the flow field into the gas diffusion layer (GDL) and the current collection at the metal foam/GDL interface. In addition, compared with the conventional channel-rib flow fields, the metal foam design greatly increases fuel cell performance in the high current density regime. In addition, the oxygen and current density distributions in PEM fuel cell with the metal foam flow field are more uniform than those in the conventional one. Though the current collection area at the GDL surface is much smaller in the metal foam flow field, the relevant Ohmic loss won't increase significantly due to the improved physical contact by the fine pore structure of metal foam over the GDL.

Three-dimensional multi-phase simulation of cooling patterns for proton exchange membrane fuel cell based on a modified Bruggeman equation

A three-dimensional multi-phase non-isothermal model of proton exchange membrane fuel cell (PEMFC) is developed based on a modified Bruggeman equation. The developed model is validated with experimental results and it shows the modified model can more accurately simulate actual working conditions of PEMFC at medium and high current density. The influences of different cooling patterns on the performances of PEMFC are explored. Results reveal that different gas flow patterns have roughly the same effect on the temperature distribution of the membrane in the case of the cooling water counter-current flowing. Under the counter-current gas flow pattern, pattern 3 is the best cooling pattern, that is to keep the cooling water flow direction consistent with the cathode gas flow direction. Under the co-current gas flow pattern, pattern 6 is the best cooling pattern, that is to keep the cooling water flow direction consistent with the gas flow direction. Meanwhile, the temperature distribution of the cathode and the anode is basically symmetrical at the interface of the cathode gas diffusion layer and the cathode catalyst layer. Additionally, the optimal cooling patterns need to ensure the cooling water co-current flowing, and it has obvious advantages of system integration and cost reduction.

Investigation of current density spatial distribution in PEM fuel cells using a comprehensively validated multi-phase non-isothermal model

In this study, a three-dimensional (3D) multi-phase non-isothermal model of proton exchange membrane (PEM) fuel cell is present to investigate the current density spatial distributions under different output voltages, temperatures, current densities and relative humidities (RH). Two sets of experimental data are selected for validation, including those from the Los Alamos National Laboratory in the United States and the University of Waterloo in Canada. Reasonable agreements are achieved between the model prediction and experimental measurements, indicative of the validity of this 3D model. In addition, it is found that under a higher RH of 50%, most electric current is produced near the cathode inlet, which is primarily due to the local availability of abundant oxygen and hence small transport polarization. Low current occurs near the outlet of the cathode air flow, as a result of oxygen consumption by the oxygen reduction reaction (ORR). Under a lower RH of 25%, the high current density region shifts to the middle of the fuel cell, which is primarily attributed to hydration of the dry membrane by water production. In our case study, the operating temperature has little impact on the current density distribution.

Three-dimensional simulation of a new cooling strategy for proton exchange membrane fuel cell stack using a non-isothermal multiphase model

In this study, a new cooling strategy for a proton exchange membrane (PEM) fuel cell stack is investigated using a three-dimensional (3D) multiphase non-isothermal model. The new cooling strategy follows that of the Honda’s Clarity design and further extends to a cooling unit every five cells in stacks. The stack consists of 5 fuel cells sharing the inlet and outlet manifolds for reactant gas flows. Each cell has 7-path serpentine flow fields with a counter-flow configuration arranged for hydrogen and air streams. The coolant flow fields are set at the two sides of the stack and are simplified as the convective heat transfer thermal boundary conditions. This study also compares two thermal boundary conditions, namely limited and infinite coolant flow rates, and their impacts on the distributions of oxygen, liquid water, current density and membrane hydration. The difference of local temperature between these two cooling conditions is as much as 6.9 K in the 5-cell stack, while it is only 1.7 K in a single cell. In addition, the increased vapor concentration at high temperature (and hence water saturation pressure) dilutes the oxygen content in the air flow, reducing local oxygen concentration. The higher temperature in the stack also causes low membrane hydration, and consequently poor cell performance and non-uniform current density distribution, as disclosed by the simulation. The work indicates the new cooling strategy can be optimized by increasing the heat transfer coefficient between the stack and coolant to mitigate local overheating and cell performance reduction.

Modeling of the effect of ionomer volume fraction on water management for proton exchange membrane fuel cell

Proton exchange membrane fuel cell (PEMFC) has been recognized as the most promising energy conversion device due to its outstanding merits, such as high energy efficiency, low pollution, low operating temperature and low noise. Due to the complicated process of water production and transportation in porous media, the water concentration can affect the distribution of species and ionic conductivity, thus, the water management has been regarded as one of the most critical issues for PEMFC. In this paper, a three dimensional multiphase non-isothermal model for the PEMFC performance prediction has been developed to study the effect of ionomer volume fraction on the performance of PEMFC. The numerical results show that the ionomer volume fraction of catalyst layer has an appreciable effect on the performance of PEMFC and water distribution in porous electrodes, the higher ionomer volume fraction in catalyst layer can be helpful to avoid the flooding problem in porous electrodes.

Modeling of hydrogen alkaline membrane fuel cell with interfacial effect and water management optimization

In this study, a whole-cell 3D multiphase non-isothermal model is developed for hydrogen alkaline anion exchange membrane (AAEM) fuel cell, and the interfacial effect on the two-phase transport in porous electrode is also considered in the model. The results show that the insertion of anode MPL, slight anode pressurization and reduction of membrane thickness generally improve the cell performance because the water transport from anode to cathode is enhanced, which favors both the mass transport and membrane hydration. The effect of cathode MPL is generally insignificant because liquid water rarely presents in cathode. It is demonstrated that slight pressurization of anode, which might not lead to apparent damage to the membrane, can effectively solve the anode flooding and cathode dryout issues.

Matching of water and temperature fields in proton exchange membrane fuel cells with non-uniform distributions

In this work, a three-dimensional multiphase non-isothermal model incorporated with a capillary-extended sub-model in gas channels is used to investigate the coupled phenomena of water and thermal transport in proton exchange membrane fuel cells. Distributions of water and temperature along the flow path in the channel are highlighted and the pros and cons of various operating temperatures are elaborated. In addition, this work also sheds light on the impacts of temperature variations of bipolar plates induced by non-uniform cooling conditions, which have been overlooked by most previous works. An important phenomenon of water distribution, dry-out at inlets and flooding at outlets (DIFO), is observed and this non-uniform distribution is revealed to be greatly influenced by the operating temperature, inlet relative humidity and gas flow stoichiometry. Moreover, temperature variations of bipolar plates are shown to exert remarkable impacts on water distribution. Consequently, optimum matching between water and temperature fields is proposed to be of vital importance in fuel cell design, e.g., strong cooling at the inlet and weak cooling at the outlet are demonstrated to be a feasible way of mitigating the problem of DIFO.