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

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

Three-dimensional multi-physics modelling and structural optimization of SOFC large-scale stack and stack tower

Multi-physics modelling of the Solid Oxide Fuel Cell (SOFC) stack requires significant computational resources. Design optimization of large-scale stacks and stack towers has always been a challenge in recent years. This study establishes a three-dimensional multi-physics model based on a two-step coupling using the BP neural network. The comparison between the novel model and the traditional fully coupled model in both accuracy and computing resource requirements are explored. The novel method has high effectiveness for modelling the large-scale stacks. Based on this, planar SOFC 50-cell stacks and 150-cell stack towers are simulated. The results show that, the flow uniformity of fuel distribution of the stack towers can decrease more than 30% comparing with the 50-cell stack, which leads to significant deterioration of the voltage and temperature distribution. The parameters of manifold and buffer area and channel height of the stack tower is optimized to achieve better uniformity of flow and voltage distribution and lower temperature gradient simultaneously.     

Flow distribution in the manifold of PEM fuel cell stack

In this study, the pressure variation and the flow distribution in the manifold of a fuel-cell stack are simulated by a computational fluid dynamics (CFD) approach. Two dimensional stack model composed of 72 cells filled with porous media is constructed to evaluate pressure drop caused by channel flow resistance. In order to simplify this model, electrochemical reactions, heat and mass transport phenomena are ignored and air is treated as working fluid to investigate flow distribution in stacks. Design parameters such as the permeability of the porous media, the manifold width and the air feeding rate were changed to estimate uniformity of the flow distribution in the manifold. A momentum-balance theory and a pressure-drop model are presented to explain the physical mechanism of flow distribution. Modeling results indicate that both the channel resistance and the manifold width can enhance the uniformity of the flow distribution. In addition, a lower air feeding rate can also enhance the uniformity of flow distribution. However, excessive pressure drop is not beneficial for realistic applications of a fuel-cell stack and hence enhanced manifold width is a better solution for flow distribution.     

Effect of cathode inlet manifold configuration on performance of 10-cell proton-exchange membrane fuel cell

A 10-cell proton-exchange membrane fuel cell (PEMFC) stack with 10 cathode flow channels is employed to investigate the effect of airflow inlet manifold configuration on the overall performance. Four different types of airflow inlet manifold with a 90° turn are considered. First, the flow patterns according to the manifold configuration are numerically sought. The computational result for the improved inlet manifold predicts about 8.5% increase in the uniformity of the airflow distribution. The experiments are carried out to confirm the numerical predictions by measuring actual airflow distributions through the fuel cell stack. The polarization curve and the power curve for the 10-cell PEMFC are also obtained to determine the effect of inlet manifold configuration on the actual performance. The maximum power output increases by up to 10.3% on using the improved airflow inlet manifold.     

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.     

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

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

Design and analysis of SOFC stack with different types of external manifolds

In this study, a four-cell stack of anode-supported planar solid oxide fuel cells (SOFCs) was designed and simulated to investigate the flow/heat transport phenomena and the performance of the SOFC stack. This SOFC stack was designed based on the external manifold types with one side open toward the cathode inlet and components such as base station, manifold, end plate, press jig, and housing. To investigate the performance of the SOFC stack, a step-by-step heat and flow analysis was conducted. First, the separator, functioning as a current collector and a gas channel, was designed to have repeated convex shapes. As the boundary of the flow passage was periodic in both streamwise and transverse directions, only a small part of the flow channel was simulated. In the case of simple homogeneous porous media, the computational results for flow resistance could be expressed by following Darcy's Law. Subsequently, these calculation results from the separator flow analysis were used in the housing and stack analysis. Second, the flow of the cathode region in the housing of SOFC stack was analyzed to verify the flow uniformity in the cathode channel of the separators. Finally, a stack analysis was executed using the electrochemical reaction model to investigate the performance and transport phenomena of the stack. Owing to the uniformity in flow and temperature, each SOFC cell exhibited similar contours of reactant gases, temperature, and current density. In the case of two different fuel utilizations with different flow rates, the low fuel utilization performed slightly better than the high fuel utilization.     

Study of cell voltage uniformity of proton exchange membrane fuel cell stack with an optimized artificial neural network model

The cell voltage uniformity of the proton exchange membrane fuel cell stack, which may consist of tens or hundreds of cells in series, plays a significant role in the stack's lifetime and performance. But it is challenging to predict the multi-cell voltages and the uniformity with a physics-based model due to complex stack geometry and huge computation efforts. In this work, we develop an artificial neural network model to estimate the steady-state cell voltage distributions of a 60 kW 140-cell stack. The optimized and well-trained model can efficiently reproduce the 140-cell voltages at different operating conditions with the error of less than 2 mV. The increased cathode gas pressure improves the cell voltage consistency and stack performance, while the voltage uniformity worsens with ascending load current. The efficient model prediction of cell voltages is beneficial for accurate evaluation of fuel cell performance, health state, and reliability.     

Numerical simulation and analysis of thermal stress distributions for a planar solid oxide fuel cell stack with external manifold structure

A three-dimensional numerical model based on the finite element method (FEM) is constructed to calculate the thermal stress distribution in a planar solid oxide fuel cell (SOFC) stack with external manifold structure. The stack is composed of 5 units which include cell, metallic interconnect, seal and anode/cathode current collectors. The temperature profile is described according to measured temperature points in the stack. It can be clearly seen that the maximum stress concentration area appears at the corner of the components when the stack is heated from room temperature (RT) to 780 °C. The effects of stack components on maximum stress concentration have been investigated under the operation temperature, as well as the thermal stress simulation results. It is obvious that the coefficient of thermal expansion (CTE) mismatch between the interconnect and the seal plays an important role in determining the thermal stress distribution in the stack. However, different compressive loads have almost no effect on stress distribution, and the influence of glass-based seal depends on the elastic modulus. The simulation results can be applied for optimizing the structural design of the stack and minimizing the high stress concentration in components.     

A key geometric parameter for the flow uniformity in planar solid oxide fuel cell stacks

Intensive CFD calculations are performed for the flow distribution in planar solid oxide fuel cell (SOFC) stacks with different number of cells. The calculations are based on 3D models with realistic geometric and operational parameters. The effects of design parameters, such as the channel height and length, the height of the repeating cell unit and the manifold width, on the flow uniformity are examined. The CFD results demonstrate that the ratio of the outlet manifold width to the inlet manifold width (α) is a key design parameter that affects the flow uniformity. The physical origin for the effect of α on the flow distribution is discussed and a simplified 2D model with the critical details of the flow physics is developed. The 2D model provides quality result for the optimal value of α and is easy to use for the broad engineering society.     

Optimization of entrance geometry and analysis of fluid distribution in manifold for high-power proton exchange membrane fuel cell stacks

Uniformity of fluid distribution in the manifold is extremely essential to enhance the output performance and prolong the lifetime of high power proton exchange membrane (PEM) fuel cell stack. The entrance effects are usually ignored in the existing studies focusing on the fluid distribution at stack level, which cannot thoroughly guide the high-power fuel cell stack development. In this study, the effects of entrance geometry on the fluid distribution in manifold for a high-power fuel cell stack are investigated using computational fluid dynamics (CFD) method. Optimizations of the intermediate zone configuration in upper endplate and inlet tube diameter are conducted under different current densities. Results show that the fluid distribution in manifold is strongly influenced by entrance geometry which determines the generation of vortexes. The mass flow rates in unit cells near the entrance of the stack with diffuser-type intermediate zone are enhanced compared to the non-diffuser-type intermediate zone. The coefficient of variation (CV) of mass flow rate decreases dramatically as the ratio of inlet tube to manifold hydraulic diameter (RITMHD) increases and then raises. The experimental and simulation results are useful in guiding the design of high-power PEM fuel cell stacks to seek higher power density.     

Air and H2 feed systems optimization for open-cathode proton exchange membrane fuel cells

In this study, air and H₂ feed systems optimization for open-cathode proton exchange membrane fuel cells (PEMFCs) has been evaluated. For air feed system, a spoiler was introduced. The air velocity distribution, polarization curve, single-cell voltage distribution, and temperature distribution of the 11-cell open-cathode fuel cell stack with blowing, blowing-spoiler, and drawing air feed system were assessed. On this basis, the influences of the distance between the fan and stack with different air feed systems were investigated. The results show that the application of the spoiler could solve the problem of low air velocity in the middle of the stack and increase stack performance by 7.3%. And drawing air feed system could enhance the heat dissipation capacity of the stack and the uniformity of temperature distribution, resulting in the 7.9% stack performance increase. Optimization of the distance between the fan and stack enhances the full development of turbulence and the rate of heat transfer. In addition, the effects of four different H₂ feed systems and the flow direction between air and hydrogen on the fuel cell performance were also investigated. It is beneficial for open-cathode PEMFC to be operated with the location of the H₂ inlet and outlet staggered in two different endplates for better stack performance and single-cell voltage uniformity. Evidence also shows that the higher performance also could be obtained when the flow direction of air and hydrogen is vertical with lower ohmic resistance, charge and mass transfer resistance. The study contributes to the design of the open-cathode fuel cell stack to get better performance and reliability.     

Geometric strategies for attainment of identical outflows through all of the exit ports of a distribution manifold in a manifold system

The focus of this investigation is to identify strategies whose application is capable of perfecting manifold design to achieve the same rate of mass outflow through each of the exit ports of a distribution manifold. A quantitative systematic study based on numerical simulation is performed in which each of eight proposed strategies is evaluated with regard to its capability for producing the same per-exit-port mass outflow. Each of the strategies is geometric in nature and is characterized by geometrical parameters which can be varied systematically in order to attain outflow uniformity. A quantitative metric, the standard deviation from uniformity of the individual outflows through the exits of the distribution manifold, was used to identify the degree of mass outflow uniformity that was achieved by the use of the various parameter values which characterize each strategy. The end result of the study is the identification of the certain strategies that are most effective for the attainment of the goal of outflow uniformity. These are: (a) enlargement of the cross-sectional area of the distribution manifold (Section 4.1), (b) variation of the cross-sectional areas of the outflow channels (Section 4.5), (c) linear tapering of the cross-sectional area of the distribution manifold, and (d) non-linear tapering of the cross-sectional area of the manifold by means of quarter-elliptical contouring of the manifold wall (Section 4.3).     

Experimental study of clamping effects on the performances of a single proton exchange membrane fuel cell and a 10-cell stack

The contact pressure distribution is known to have significant influences on the contact ohmic resistance, porosity of gas diffusion layers (GDLs) and performance of the proton exchange membrane fuel cell (PEMFC) consequently. This work experimentally investigated the effects of various combinations of bolt configuration and clamping torque on the corresponding contact pressure distributions and performances of a single PEMFC and a 10-cell stack. The pressure-sensitive films (FUJI-FILM I&I) were used to visualize the contact pressure distributions under three different clamping torques and three different bolt configurations in the experiments. The importance of the proper stacking design was clearly demonstrated by these contact pressure images. The mean value and the fluctuation intensity of the contact pressure were extracted statistically from the data of pressure-sensitive films. A non-dimensional pressure fluctuation intensity, which indicates the relative dispersion to its mean value, was proposed to gauge the uniformity of the contact pressure distribution, similar to the definition of the turbulence intensity in fluid mechanics. The experimental results showed that, for the single cell under the current experiment conditions, the larger mean contact pressure tends to yield the higher maximum power, regardless of the bolt configuration and the applied torque. The uniformity of the contact pressure distribution, the ohmic resistance and the mass transport limit current had highly linear correlations with the mean contact pressure. In the case of the 10-cell stack, the effects of various combinations of bolt configuration and clamping torque on its performance and the mass transport limit current could not be reflected by the stack mean contact pressure only. Increasing the mean contact pressure improved the uniformity of the contact pressure distribution and reduced the contact ohmic resistance, in general. However, the maximum power did not increase monotonically with the mean contact pressure and no linear correlation was found. The detailed contact pressure distribution may have important influences on the local electrochemical reactions and heat and mass transfer processes involved in the stack.     

Multi-scale design simulation of a novel intermediate-temperature micro solid oxide fuel cell stack system

This paper presents a multi-scale simulation technique for designing a novel intermediate-temperature planar-type micro solid oxide fuel cell (mSOFC) stack system. This multi-scale technique integrates the fundamentals of molecular dynamics (MD) and computational fluid dynamics (CFD). MD simulations are carried out to determine the optimal composition of a potential electrolyte that is capable of operation in the intermediate-temperature region without sacrifice in performance. A commercial CFD package plus a self-written computational electrochemistry code are employed to design the fuel and air flow systems in a planar five-cell stack, including the preheating manifold. Different samarium-doped ceria (SDC) electrolyte compositions and operating temperatures from 673 K to 1023 K are investigated to identify the maximum ionic conductivity. The electrochemical performance simulation using an available 5-cell yittria-stablized-zirconia (YSZ) mSOFC stack shows good agreement with our experimental results. The same stack design is used to predict a novel SDC-mSOFC performance. Feasibiulity studies of this intermediate-temperature stack are presented using this multi-scale technique.     

Design and numerical analysis of a planar anode-supported SOFC stack

In the present study, numerical simulations are conducted to examine the flow characteristics and attributes of electrochemical reactions in the stack through three-dimensional analysis using finite volume approach prior to the fabrication of the SOFC stack. The stack flow uniformity index is employed to investigate the flow uniformity whereas in the case of electrochemical modeling, different mathematical models are adopted to predict the characteristics of activation and ohmic overpotentials that occur during electrochemical reactions in the cell. The normalized mass flow rate is found almost same in each cell with flow uniformity index of 0.999. The calculated voltage and power curves under different average current densities are compared with experimental results for the model validation. The changes in the voltage and power of the SOFC stack, current density, temperature, over potential and reactants distributions in relation to varying amounts of reactants flow are also examined. The current density distribution in each cell is observed to vary along the anode flow direction. The temperature difference in each cell is almost same along the flow direction of reactants, and the irreversible resistance showed an opposite trend with a temperature distribution in each cell.     

Shunt current calculation of fuel cell stack using Simulink

Presence of shunt current in fuel cell stacks can lead to corrosion and power loss problems. The objective of this paper is to develop a stack-level model for calculation of shunt currents. The simulation model was based on electrical circuit, and created using Simulink^® software. The Simulink^® results were validated by using PSpice^® software and comparing with experimental data of an electrolyzer stack. The Simulink^® model is also used to evaluate the effectiveness of a protective current method to reduce the shunt current. The protective current method was found to effectively reduce shunt current of a 100-cell stack. The Simulink^® model also shows that shunt current is highest at end cells of the 100-cell stack examined, suggesting extra care be applied to end cells for corrosion prevention. Monte Carlo simulation was performed to examine sensitivity of variance in voltage, manifold, channel and cell resistance on calculated shunt current. The sensitivity analysis shows that calculated shunt current is most sensitive to variance in manifold resistance and followed by variance in voltage, electrolyte and channel resistance. The calculated power loss due to shunt current is within 1% for the conditions examined.     

Constructal PEM fuel cell stack design

This paper describes a structured procedure to optimize the internal structure (relative sizes, spacings), single cells thickness, and external shape (aspect ratios) of a polymer electrolyte membrane fuel cell (PEMFC) stack so that net power is maximized. The constructal design starts from the smallest (elemental) level of a fuel cell stack (the single PEMFC), which is modeled as a unidirectional flow system, proceeding to the pressure drops experienced in the headers and gas channels of the single cells in the stack. The polarization curve, total and net power, and efficiencies are obtained as functions of temperature, pressure, geometry and operating parameters. The optimization is subjected to fixed stack total volume. There are two levels of optimization: (i) the internal structure, which accounts for the relative thicknesses of two reaction and diffusion layers and the membrane space, together with the single cells thickness, and (ii) the external shape, which accounts for the external aspect ratios of the PEMFC stack. The flow components are distributed optimally through the available volume so that the PEMFC stack net power is maximized. Numerical results show that the optimized single cells internal structure and stack external shape are “robust” with respect to changes in stoichiometric ratios, membrane water content, and total stack volume. The optimized internal structure and single cells thickness, and the stack external shape are results of an optimal balance between electrical power output and pumping power required to supply fuel and oxidant to the fuel cell through the stack headers and single-cell gas channels. It is shown that the twice maximized stack net power increases monotonically with total volume raised to the power 3/4, similarly to metabolic rate and body size in animal design.     

Effect of thermal cycling on durability of a solid oxide fuel cell stack with external manifold structure

A 5-cell stack with external manifold is thermal cycled between room temperature and 750 °C fifteen times. The electric performances after each cycle are measured and compared. The stack has an initial peak output of 328.44 W and shows excellent stability in thermal cycling. The average operating voltage degradation rate is only 0.8% corresponding each thermal cycle. A cell from the stack is randomly chosen for electrochemical evaluation. Its performance is found to be comparable to a cell which is not thermal cycled. Post-test examination shows deterioration of cathode contact materials at points of contact and cracks throughout the oxide layer between corrugated and bipolar plates to be the main causes of the degradation.     

Pressure drop and flow distribution in parallel-channel configurations of fuel cells: Z-type arrangement

A general theoretical model based on mass and momentum conservation has been developed to solve the flow distribution and the pressure drop in Z-type configurations of fuel cells. While existing models neglected either friction term or inertial term, the present model takes both of them into account. The governing equation of the Z-type arrangement was formulated to an inhomogeneous version of the U-type one. Thus, main existing models have been unified to one theoretical framework. The analytical solutions are fully explicit that they are easily used to predict pressure drop and flow distribution for Z-type layers or stacks and provide easy-to-use design guidance under a wide variety of combination of flow conditions and geometrical parameters to investigate the interactions among structures, operating conditions and manufacturing tolerance and to minimize the impact on stack operability. The results can also be used for the design guidance of flow distribution and pressure drop in other manifold systems, such as plate heat exchanges, plate solar collectors, distributors of fluidised bed and boiler headers.