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.     

CFD investigating the effects of different operating conditions on the performance and the characteristics of a high-temperature PEMFC

The effects of different operating conditions on the performance and the characteristics of a high-temperature proton exchange membrane fuel cell (PEMFC) are investigated using a three-dimensional (3-D) computational fluid dynamics (CFD) fuel-cell model. This model consists of the thermal-hydraulic equations and the electrochemical equations. Different operating conditions studied in this paper include the inlet gas temperature, system pressure, and inlet gas flow rate, respectively. Corresponding experiments are also carried out to assess the accuracy of this CFD model. Under the different operating conditions, the PEMFC performance curves predicted by the model correspond well with the experimentally measured ones. The performance of PEMFC is improved as the increase in the inlet temperature, system pressure or flow rate, which is precisely captured by the CFD fuel cell model. In addition, the concentration polarization caused by the insufficient supply of fuel gas can be also simulated as the high-temperature PEMFC is operated at the higher current density. Based on the calculation results, the localized thermal-hydraulic characteristics within a PEMFC can be reasonably captured. These characteristics include the fuel gas distribution, temperature variation, liquid water saturation distribution, and membrane conductivity, etc.     

Modeling of a planar solid oxide fuel cell based on proton‐conducting electrolyte

A 2D computational fluid dynamics (CFD) model is developed to study the performance of an advanced planar solid oxide fuel cell based on proton conducting electrolyte (SOFC‐H). The governing equations are solved with the finite volume method (FVM). Simulations are conducted to understand the transport phenomena and electrochemical reaction involved in SOFC‐H operation as well as the effects of operating/structural parameters on SOFC‐H performance. In an SOFC based on oxygen ion conducting electrolyte (SOFC‐O), mass is transferred from the cathode side to the anode side. While in an SOFC‐H, mass is transferred from the anode to the cathode, which causes different velocity fields of the fuel and oxidant gas channels and influences the distributions of temperature and gas composition in the cell. It is also found that increasing the inlet gas velocity leads to an increase in the local current density and a slight decrease in the SOFC‐H temperature due to stronger cooling effect of the gas species at a higher velocity. Another finding is that the electrode structure does not significantly affect the heat and mass transfer in an SOFC‐H at typical operating voltages. Copyright © 2009 John Wiley & Sons, Ltd.     

Influences of gas relative humidity on the temperature of membrane in PEMFC with interdigitated flow field

A fundamental understanding of the water balance of a fuel cell during operation is crucial for improving the cell performance and durability. The humidification in the anode or cathode has an important effect on the flow characteristics and cell efficiency. Three-dimensional steady mathematical model based on the electrochemical, current distribution, fluid motion continuity equation, momentum and energy equation, boundary layer theory has been developed to simulate PEMFC with interdigitated flow field using the computational fluid dynamics (CFD). Effects on the current density and temperature differences have been simulated and analyzed respectively, when the humidification in the anode or cathode is from 0% to 100% respectively. The numerical results show that the humidification strongly influences the current density and temperature difference so as to affect the cell efficiency. Under the same operation conditions and low humidification conditions, anode humidification can better enhance the performance of the battery and improve the extent of PEM humidification.     

Three‐dimensional modelling of catalyst layers in PEM fuel cells: Effects of non‐uniform catalyst loading

Improving the performance of polymer‐electrolyte membrane (PEM) fuel cells depends on the optimization of catalyst layer composition and structure for large active surfaces. Modelling studies provide a valuable tool for investigating the effects of catalyst layer composition and structure on the electrochemical and physical phenomena occurring in PEM fuel cells. Previous modelling studies have shown that the distribution of electrochemical reactions in catalyst layers is highly dependent on the complex interaction of activation and ohmic effects as well as contributions from transport limitations and variations in local and overall current densities. In this paper, three‐dimensional, multicomponent and multiphase transport computations are performed using a computational fluid dynamics (CFD) code (FLUENT^TM) with a new PEM fuel cell module, which has been further improved by taking into account the detailed composition and structure of the catalyst layers using the multiple thin‐film agglomerate model. The detailed modelling of reactions in the catalyst layers is used to determine methods of improving the effectiveness of catalyst layers for a given platinum loading. First, available data on catalyst layer composition and structure are used in CFD computations to predict reaction rate distributions. Based on these results, spatial variations in catalyst loading are then implemented in CFD computations for the same overall catalyst loading to investigate possible performance gains. It is found that grading catalyst loading towards the membrane in the anode and the gas channel inlet in the cathode provides the most beneficial effects on the fuel cell performance. Thus the results suggest that significant savings in cost can be attained by reducing the platinum loading in underutilized regions of the catalyst layers, while at the same time improving the performance. Copyright © 2009 John Wiley & Sons, Ltd.     

Three-dimensional,two-phase,CFD model for the design of a direct methanol fuel cell

This study presents a computational fluid dynamics (CFD) model for modelling gas evolution and current distribution in a direct methanol fuel cell (DMFC). The improved two-phase model includes a new sub-model for estimating the interface mass transfer without empirical correlations. Simulation results in a horizontal channel of the DMFC agree with typical trends reported in the literature for bubbly flows. The increase in inlet flow rate is found to lead to a decrease in the gas content in the outlet of the anode channels. A case study illustrates applications of the CFD model for modelling gas evolution and current distribution in a DMFC with a parallel flow-field design. Simulation results with a improved two-phase model provide an explanation of experimental observations of a transparent DMFC with parallel channels. An improved three-dimensional CFD model includes all relevant phenomena and is valuable for gas management in a DMFC design.     

Effects of operating and design parameters on the performance of a solid oxide fuel cell–gas turbine system

We present a steady‐state thermodynamic model of a hybrid solid oxide fuel cell (SOFC)–gas turbine (GT) cycle developed using a commercial process simulation software, AspenPlus?. The hybrid cycle model incorporates a zero‐dimensional macro‐level SOFC model. A parametric study was carried out using the developed model to study the effects of system pressure, SOFC operating temperature, turbine inlet temperature, steam‐to‐carbon ratio, SOFC fuel utilization factor, and GT isentropic efficiency on the specific work output and efficiency of a generic hybrid cycle with and without anode recirculation. The results show that system pressure and SOFC operating temperature increase the cycle efficiency regardless of the presence of anode recirculation. On the other hand, the specific work decreases with operating temperature. Overall, the model can successfully capture the complex performance trends observed in hybrid cycles. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical analysis on the effects of manifold design on flow uniformity in a large proton exchange membrane fuel cell stack

The size and configuration of manifold can affect the flow characteristics and uniformity in proton exchange membrane fuel cell (PEMFC) stack; then its efficiency and service life. Based on the simulation results of a single fuel cell considering electrochemical reaction, a stack model with 300 porous media is established to numerically investigate the performances of a large commercial PEMFC stack. The effects of manifold width and configuration type on the pressure drop and species concentration are studied by computational fluid dynamics (CFD). The results show that the uniformity for most cases of U-type configuration is better than those of Z-type configuration. For U-type configuration, a very good uniformity can be obtained by selecting anode inlet manifold width of 20 mm and anode outlet manifold in range from 25 to 30 mm; the uniformity is bad for all cathode inlet manifold width, relatively better uniformity can be achieved by adjusting cathode outlet manifold width. For Z-type configuration, bad uniformity is found for cathode inlet and outlet manifold with all width; a relatively good uniformity can be obtained with suitable anode manifold width of 35 mm. The research can provide some references to improve gas distribution uniformity in large PEMFC stacks.     

Mini CHP based on the electrochemical generator and impeded fluidized bed reactor for methane steam reforming

The paper presents a configuration of mini CHP with the methane reformer and planar solid oxide fuel cell (SOFC) stacks. This mini CHP may produce electricity and superheated steam as well as preheat air and methane for the reformer along with cathode air used in the SOFC stack as an oxidant. Moreover, the mathematical model for this power plant has been created. The thermochemical reactor with impeded fluidized bed for autothermal steam reforming of methane (reformer) considered as the basis for the synthesis gas (syngas) production to fuel SOFC stacks has been studied experimentally as well. A fraction of conversion products has been oxidized by the air fed to the upper region of the impeded fluidized bed in order to carry out the endothermic methane steam reforming in a 1:3 ratio as well as to preheat products of these reactions. Studies have shown that syngas containing 55% of hydrogen could be produced by this reactor. Basic dimensions of the reactor as well as flow rates of air, water and methane for the conversion of methane have been adjusted through mathematical modelling.The paper provides heat balances for the reformer, SOFC stack and waste heat boiler (WHB) intended for generating superheated water steam along with preheating air and methane for the reformer as well as the preheated cathode air. The balances have formed the basis for calculating the following values: the useful product fraction in the reformer; fraction of hydrogen oxidized at SOFC anode; gross electric efficiency; anode temperature; exothermic effect of syngas hydrogen oxidation by air oxygen; excess entropy along with the Gibbs free energy change at standard conditions; electromotive force (EMF) of the fuel cell; specific flow rate of the equivalent fuel for producing electric and heat energy. Calculations have shown that the temperature of hydrogen oxidation products at SOFC anode is 850 °C; gross electric efficiency is 61.0%; EMF of one fuel cell is 0.985 V; fraction of hydrogen oxidized at SOFC anode is 64.6%; specific flow rate of the equivalent fuel for producing electric energy is 0.16 kg of eq.f./(kW·h) while that for heat generation amounts to 44.7 kg of eq.f./(GJ). All specific parameters are in agreement with the results of other studies.     

Cathode flow field design for nitric oxide/hydrogen fuel cell in cogeneration of hydroxylamine and electricity

A three‐dimensional model is developed for an NO/H₂ fuel cell cogenerating hydroxylamine and electricity. The model describes the distribution of velocity and concentration on the cathode side of the fuel cell, making the assumption of fuel excess on the anode side. The developed CFD‐based model is used for evaluation of various cathode flow field designs. Simulation results indicate the possibility of enhancing the fuel cell performance by decreasing the maldistribution of fields. A new cathode flow field design is developed for NO/H₂ fuel cell with an improved performance for both energy and hydroxylamine production. Copyright © 2016 John Wiley & Sons, Ltd.     

Three‐dimensional nonisothermal modeling of solid oxide fuel cell coupling electrochemical kinetics and species transport

A three‐dimensional (3D) nonisothermal model is developed and applied for anode‐supported planar solid oxide fuel cell (SOFC). The mass and momentum, species, ion, electric, and heat transport equations are solved simultaneously by implementing the electrochemical kinetics and electrochemical reaction as volumetric source terms. The interconnect land limits the O₂ transport under the land and lowers the local current density under the land. The effects of interconnect land width and cathode substrate thickness on SOFC cell performance are quantified in this study. Cathode stoichiometry is found to have a large effect on the SOFC cell temperature distribution. Under low‐cathode stoichiometry, significant temperature gradients are seen in the SOFC cell. Higher‐cathode stoichiometry is beneficial for lower temperature and more uniform current density distribution in SOFC cell. Co‐flow and counter‐flow arrangements are investigated and discussed with the model. Counter‐flow arrangement is found to induce a high temperature and high current density region near the H₂ inlet. On the other hand, co‐flow arrangement leads high temperature and high current density to occur relatively downstream, a slightly lower maximum temperature on cell and considerably more uniform current density distribution. A 67.2‐cm² SOFC cell is simulated considering the side cooling effect. The side cooling effectively lowers the cell temperature, at the same time, causes temperature, current density, and fuel utilization nonuniformity in the across multichannel direction. Because of the strong coupling of the in‐plane current density distribution and temperature distribution, limiting the locally high temperature and temperature gradient is critical for achieving a more uniform current density distribution in anode‐supported planar SOFC.     

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.     

Investigation on no-vent filling process of liquid hydrogen tank under microgravity condition

In order to investigate the no-vent filling performance under microgravity, the computational fluid dynamic (CFD) method is introduced to the study, where a model aiming at filling a liquid hydrogen (LH₂) receiver tank is especially established. In this model, the solid and fluid regions are considered together to predict the coupled heat transfer process. The phase change effect during the filling process is also taken into account by embedding a pair of mass and heat transfer models into the CFD software FLUENT, one of which involves liquid flash driven by pressure difference between the fluid saturated pressure and the tank pressure, and the other one indicates and calculates the evaporation–condensation process driven by temperature difference between fluid and its saturated state. This CFD model, verified by experimental data, could accurately simulate the no-vent filling process with good flexibility. Moreover, no-vent filling processes under different gravities are comparatively analyzed and the effects of four factors including inlet configuration, inlet liquid temperature, initial wall temperature and inlet flow rate, are discussed, respectively. Main conclusions could be made as follows: 1) Compared to the situations in normal gravity, the no-vent filling in microgravity experiences a more adequate liquid–vapor mix, which results in a more steady pressure response and better filling performance. 2) Inlet configuration seems to have negligible effect on the no-vent filling performance under microgravity since liquid could easily reach the tank wall and then cause a sufficient fluid-wall contact under any inlet condition. 3) Higher initial tank wall temperature may directly cause a higher pressure rise in the beginning, while this effect on the final pressure is not significant. Sufficient precooling and reasonable inlet liquid subcooled degree are suggested to guarantee the reliability and efficiency of the no-vent fill under microgravity.     

Effects of operations and structural parameters on the one-cell stack performance of planar solid oxide fuel cell

A three-dimensional mathematical model coupling the electrochemical kinetics with fluid dynamics is developed to simulate the heat and mass transfer in the one-cell stack of planar solid oxide fuel cells (SOFCs). Based on flow uniformity analysis, the distributions of temperature, current density, overpotential loss and other performance parameters in various operating parameters are obtained using a commercial CFD code (Fluent) coupled with the external subroutines programmed by VC++. Numerical flow data are observed in good agreement with experimental results reported in the literature. Results show that the one-cell stack in counter flow case has the advantages in better uniform current density and temperature distributions of PEN (Positive/Electrolyte/Negative) structure in the width direction, higher power output, fuel utilization factor and fuel efficiency than that in co-flow case. For counter flow case, better thermoelectric characteristics are observed in the temperature gradient, power output, fuel utilization factor and fuel efficiency with the decrease in the fuel inlet flow rate or the anode porosity. Increasing the air inlet flow rate and decreasing the fuel inlet temperature will reduce the temperature gradient; power output, fuel utilization factor and fuel efficiency are enhanced with the increase of the air inlet temperature and the decrease of the anode pore size and thickness.     

Performance of tubular Solid Oxide Fuel Cell at reduced temperature and cathode porosity

The effect of decreasing the inlet temperature and the cathode porosity of tubular Solid Oxide Fuel Cell (SOFC) with one air channel and one fuel channel is investigated using Computational Fluid Dynamics (CFD) approach. The CFD model was developed using Fluent SOFC to simulate the electrochemical effects. The cathode and the anode of the cell were resolved in the model and the convection and conduction heat transfer modes were included. The results of the CFD model are presented at inlet temperatures of 700 °C, 600 °C and 500 °C and with cathode porosity of 30%, 20% and 10%. It was found that the Fluent-based SOFC model is an effective tool for analyzing the complex and highly interactive three-dimensional electrical, thermal, and fluid flow fields associated with the SOFCs. It is found that the SOFC can operate in the intermediate temperature range and with low porosity cathodes more efficient than at high temperatures given that the transport properties of the cathode, anode and the electrolyte can be kept the same.     

Effect of inlet flow maldistribution on the thermal and electrical performance of a molten carbonate fuel cell unit

This study investigates the temperature and current density distributions in a molten carbonate fuel cell unit when the inlet flows of the anode gas and the cathode gas are mal-distributed in eight patterns. The two-dimensional simultaneous partial differential equations of mass, energy and electrochemistry are solved numerically. The results indicate that the maldistribution of anode and cathode gases dominates the current density field and the cell temperature field, respectively. Moreover, the non-uniform inlet flow slightly affects the mean temperature and mean current density, but worsens the distribution of temperature and current density for most maldistribution patterns. According to the results, the variations of the cell temperature in Pattern G and the current density in Pattern D are 12% and 37% greater than those in the uniform pattern when the deviation of the non-uniform profile is 0.25. Consequently, the effect of non-uniform inlet flow on the temperature and current density distribution on the cell plane is evident, and cannot be neglected.     

Buoyancy driven flow in a hot water tank due to standby heat loss

Results of experimental and numerical investigations of thermal behavior in a vertical cylindrical hot water tank due to standby heat loss of the tank are presented. The effect of standby heat loss on temperature distribution in the tank is investigated experimentally on a slim 150 l tank with a height to diameter ratio of 5. A tank with uniform temperatures and with thermal stratification is studied. A detailed computational fluid dynamics (CFD) model of the tank is developed to calculate the natural convection flow in the tank. The distribution of the heat loss coefficient for the different parts of the tank is measured by experiments and used as input to the CFD model. Water temperatures at different levels of the tank are measured and compared to CFD calculated temperatures. The investigations focus on validation of the CFD model and on understanding of the CFD calculations.The results show that the CFD model predicts satisfactorily water temperatures at different levels of the tank during cooling by standby heat loss. It is elucidated how the downward buoyancy driven flow along the tank wall is established by the heat loss from the tank sides and how the natural convection flow is influenced by water temperatures in the tank. When the temperature gradient in the tank is smaller than 2 K/m, there is a downward fluid velocity of 0.003–0.015 m/s. With the presence of thermal stratification the buoyancy driven flow is significantly reduced. The dependence of the velocity magnitude of the downward flow on temperature gradient is not influenced by the tank volume and is only slightly influenced by the tank height to tank diameter ratio. Based on results of the CFD calculations, an equation is determined to calculate the magnitude of the buoyancy driven flow along the tank wall for a given temperature gradient in the tank.     

Performance analysis of a solid oxide fuel cell with reformed natural gas fuel

In the present study a two‐dimensional model of a tubular solid oxide fuel cell operating in a stack is presented. The model analyzes electrochemistry, momentum, heat and mass transfers inside the cell. Internal steam reforming of the reformed natural gas is considered for hydrogen production and Gibbs energy minimization method is used to calculate the fuel equilibrium species concentrations. The conservation equations for energy, mass, momentum and voltage are solved simultaneously using appropriate numerical techniques. The heat radiation between the preheater and cathode surface is incorporated into the model and local heat transfer coefficients are determined throughout the anode and cathode channels. The developed model has been compared with the experimental and numerical data available in literature. The model is used to study the effect of various operating parameters such as excess air, operating pressure and air inlet temperature and the results are discussed in detail. The results show that a more uniform temperature distribution can be achieved along the cell at higher air‐flow rates and operating pressures and the cell output voltage is enhanced. It is expected that the proposed model can be used as a design tool for SOFC stack in practical applications. Copyright © 2009 John Wiley & Sons, Ltd.     

Electrical/flow heterogeneity of gas diffusion layer and inlet humidity induced performance variation in polymer electrolyte fuel cells

A three-dimensional single-flow channel computational model is used to investigate the performance characteristics of polymer electrolyte fuel cells (PEFC). The combined influence of non-uniform interfacial contact resistance (ICR) and inlet relative humidity (RH), along with the heterogeneous flow properties of the gas diffusion layer (GDL) on the PEFC performance is evaluated. The study considers combinations of full and partial humidification of anode and cathode reactants. Results reveal heterogeneous GDL with non-uniform ICR distribution results in a slight ∼4.4% reduction in current density at 0.3V compared to the homogeneous case. However, under same electrical/flow heterogeneities, the current density is observed to increase by ∼19% to ∼1.3A/cm² under fully humidified anode and partially humidified cathode (i.e., RH_a|RH_c = 100%|60%) as compared to ∼1.1A/cm² under symmetric RH_a|RH_c = 100%|100%. Interesting observations are made on the temperature distribution and cathodic water fractions. The variation in anodic inlet humidity is observed to have no impact on temperature distribution in the membrane, whereas variation in cathodic inlet humidity is effective in reducing the temperature in the channel regime with a 4K (kelvin) difference among all the cases. It is noted here that the overpotential map is not an indicator for performance loss, at least at full inlet humidity. This parameter is observed to depend on water concentration in the cathode. The study provides a detailed analysis of the distribution of reactant mass fraction, water concentration, current density, temperature, cathodic overpotential, and cell performance for all the simulated cases.     

Systematic assessment of the anode flow field hydrodynamics in a new circular PEM water electrolyser

In this work, we investigated the key underlying flow characteristics of a circular unit cell proton exchange membrane (PEM) water electrolyser. In particular, we focused on investigating anode flow field design using computational fluid dynamics (CFD) tool. Transient, 3D single phase fluid flow simulation results were presented, and in-house experiments were conducted for validation against CFD simulation data identifying key performance parameters of the PEM water electrolyser: uniform water distribution, pressure drop and hydraulic retention time. The effects of the water flow rate, inlet and outlet sizing and different number of inlet and outlet configurations were considered. The main observation from the study was discussed to provide insight into the factors affecting the flow pattern. Among the studied flow field design cases, it was found that the average pressure drop decreased with increase in number of inlets, also flow profile can be grouped into different set, depending on number of inlets. The correlation between pressure drop and mean velocity profile for different inlet and outlet configurations provides a useful basis to properly design the high performance PEM water electrolyser.