An analytical analysis on the cross flow in a PEM fuel cell with serpentine flow channel

A serpentine flow channel can be considered as neighboring channels connected in series, and is one of the most common and practical channel layouts for polymer electrolyte membrane (PEM) fuel cells, as it ensures the removal of liquid water produced in a cell with good performance and acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross directly to the neighboring channel via the porous gas diffusion layer (GDL) due to the high‐pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area resulting in a substantially lower amount of pressure drop in an actual PEM fuel cell compared with the case without cross flow. In this study, an analytical solution is obtained for the cross flow in a PEM fuel cell with a serpentine flow channel based on the assumption that the velocity of cross flow is linearly distributed in the GDL between two successive U‐turns. The analytical solution predicts the amount of pressure drop and the average volume flow rate in the flow channel and the GDL. The solution is validated over a wide range of the thickness and permeability of the GDL by comparing the results with experimental measurements and 3‐D numerical simulations in literature. Excellent agreement is obtained for the permeability less than 10^?9 m², which covers the typical permeability values of the GDLs in actual PEM fuel cells. The solution presents an accurate and efficient estimation for cross flow providing a useful tool for the design and optimization of PEM fuel cells with serpentine flow channels. Copyright © 2010 John Wiley & Sons, Ltd.     

Application of Open Pore Cellular Foam for air breathing PEM fuel cell

Open Pore Cellular Foam (OPCF) has received increased attention for use in Proton Exchange Membrane (PEM) fuel cells as a flow plate due to some advantages offered by the material, including better gas flow, lower pressure drop and low electrical resistance.In the present study, a novel design for an air-breathing PEM (ABPEM) fuel cell, which allows air convection from the surrounding atmosphere, using OPCF as a flow distributor has been developed. The developed fuel cell has been compared with one that uses a normal serpentine flow plate, demonstrating better performance.A comparative analysis of the performance of an ABPEM and pressurised air PEM (PAPEM) fuel cell is conducted and poor water management behaviour was observed for the ABPEM design.Thereafter, a PTFE coating has been applied to the OPCF with contact angle and electrochemical polarisation tests conducted to assess the capability of the coating to enhance the hydrophobicity and corrosion protection of metallic OPCF in the PEM fuel cell environment. The results showed that the ABPEM fuel cell with PTFE coated OPCF had a better performance than that with uncoated OPCF.Finally, OPCF was employed to build an ABPEM fuel cell stack where the performance, advantages and limitations of this stack are discussed in this paper.     

An investigation of convective transport in micro proton-exchange membrane fuel cells

The flow phenomena in a serpentine microchannel segment attached to a porous transport layer in a micro proton-exchange membrane fuel cell is investigated. Due to the presence of a porous transport layer, the fluid flow for this configuration exhibits different characteristics compared with that through a simple serpentine channel. The pressure drop and friction factor variation in the channel is examined for various values of Reynolds number and radii of curvature. Also, the effect of variation of permeability is investigated. There are two modes of fluid transport in this geometry—one through the serpentine channel and other via the porous media. With increasing permeability, more fluid is convected through the porous transport layer.     

An experimental and numerical investigation on the cross flow through gas diffusion layer in a PEM fuel cell with a serpentine flow channel

A serpentine flow channel is one of the most common and practical channel layouts for a polymer electrolyte membrane (PEM) fuel cell since it ensures the removal of water produced in a cell with acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross to neighboring channel via the porous gas diffusion layer due to the high pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area altering reactant flow in the flow channel so that the resultant pressure and flow distributions are substantially different from that without considering cross flow, even though this cross flow has largely been ignored in previous studies. In this work, a numerical and experimental study has been carried out to investigate the cross flow in a PEM fuel cell. Experimental measurements revealed that the pressure drop in a PEM fuel cell is significantly lower than that without cross flow. Three-dimensional numerical simulation has been performed for wide ranges of flow rate, permeability and thickness of gas diffusion layer to analyze the effects of those parameters on the resultant cross flow and the pressure drop of the reactant streams. Considerable amount of cross flow through gas diffusion layer has been found in flow simulation and its effect on pressure drop becomes more significant as the permeability and the thickness of gas diffusion layer are increased. The effects of this phenomenon are also crucial for effective water removal from the porous electrode structure and for estimating pumping energy requirement in a PEM fuel cell, it cannot be neglected for the analysis, simulation, design, operation and performance optimization of practical PEM fuel cells.     

Optimization of anode performance in the ethanol electrochemical reforming for clean hydrogen production

Carbon-based fuel electrochemical reforming is considered as a promising hydrogen production method. Ethanol is one of the most appropriate carbon-based fuels. In this work, anode performance, especially the flow, ethanol electro-oxidization and energy consumption in the ethanol electrochemical reforming is numerically studied and experimental verified. Take the straight serpentine channel with square cross-section as a base structure in the electrochemical cell (EC), the effects of channel geometry and operating parameters are analyzed. Another five different configurations of flow channels, as well as another three different cross-sections are designed and explored. Results indicate that at the same cross-section area, the wider channel provides the higher effective area for proton transfer, and thereby improves the electrode reactions. The appropriate decrease of inlet velocity or increase of input voltage promotes the anode reaction and reduces the pressure drop in channel, while the operating temperature has the opposite effects on ethanol conversion and pressure drop. The arc channel is found optimal considering its highest ethanol conversion, although its pressure drop is a bit higher. The sector cross-section with uniform flow field distribution is found most favorable for the straight serpentine channel considering the ethanol electro-oxidization. These findings will favor the improvement of EC.     

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

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

Investigation of bio-inspired flow channel designs for bipolar plates in proton exchange membrane fuel cells

Proton exchange membrane (PEM) fuel cell performance is directly related to the flow channel design on bipolar plates. Power gains can be found by varying the type, size, or arrangement of channels. The objective of this paper is to present two new flow channel patterns: a leaf design and a lung design. These bio-inspired designs combine the advantages of the existing serpentine and interdigitated patterns with inspiration from patterns found in nature. Both numerical simulation and experimental testing have been conducted to investigate the effects of two new flow channel patterns on fuel cell performance. From the numerical simulation, it was found that there is a lower pressure drop from the inlet to outlet in the leaf or lung design than the existing serpentine or interdigitated flow patterns. The flow diffusion to the gas diffusion layer was found be to more uniform for the new flow channel patterns. A 25 cm² fuel cell was assembled and tested for four different flow channels: leaf, lung, serpentine and interdigitated. The polarization curve has been obtained under different operating conditions. It was found that the fuel cell with either leaf or lung design performs better than the convectional flow channel design under the same operating conditions. Both the leaf and lung design show improvements over previous designs by up to 30% in peak power density.     

Study of novel flow channels influence on the performance of direct methanol fuel cell

The existing flow channels like parallel and gird channels have been modified for better fuel distribution in order to boost the performance of direct methanol fuel cell. The main objective of the work is to achieve minimized pressure drop in the flow channel, uniform distribution of methanol, reduced water accumulation, and better oxygen supply. A 3D mathematical model with serpentine channel is simulated for the cell temperature of 80 °C, 0.5 M methanol concentration. The study resulted in 40 mW/cm² of power density and 190 mA/cm² of current density at the operating voltage of 0.25 V. Further, the numerical study is carried out for modified flow channels to discuss their merits and demerits on anode and cathode side. The anode serpentine channel is unmatched by the modified zigzag and pin channels by ensuring the better methanol distribution under the ribs and increased the fuel consumption. But the cathode serpentine channel is lacking in water management. The modified channels at anode offered reduced pressure drop, still uniform reactant distribution is found impossible. The modified channels at cathode outperform the serpentine channel by reducing the effect of water accumulation, and uniform oxygen supply. So the serpentine channel is retained for methanol supply, and modified channel is chosen for cathode reactant supply. In comparison to cell with only serpentine channel, the serpentine anode channel combined with cathode zigzag and pin channel enhanced power density by 17.8% and 10.2% respectively. The results revealed that the zigzag and pin channel are very effective in mitigating water accumulation and ensuring better oxygen supply at the cathode.     

Numerical study on applicability of metal foam as flow distributor in polymer electrolyte fuel cells (PEFCs)

In this paper, the effects of using porous metal foam based bipolar plates (BPs) are investigated under practical automotive fuel cell operations with low humidification reaction gases. Particular emphasis is placed on evaluating water management capabilities of metal foam based BP designs, compared to the traditional serpentine flow field BP designs. A three-dimensional, two-phase fuel cell model developed in a previous study is applied to 25cm² real-scale fuel cell geometries with metal foam and serpentine flow modes, and then successfully validated against the experimental data measured under different operating pressures and current densities. The detailed simulation results clearly elucidate advantages of using metal foam as flow distributor through extensive multidimensional contours of flow velocity, species, and current density.     

Modelling of a porous flowing electrolyte layer in a flowing electrolyte direct-methanol fuel cell

The flowing electrolyte-direct methanol fuel cell is a developing technology that may have practical uses in the future. Its main advantage over a direct methanol fuel cell is that it limits methanol crossover using a flowing electrolyte layer. The flowing electrolyte layer (or flowing electrolyte channel) involves an ion-conducting fluid that allows protons to be transported from the anode to the cathode, and flows through a porous material to wash away crossed-over methanol. In this study, the flowing electrolyte layer is modelled as a porous domain in ANSYS CFX. General flow behaviour and the effects of volume flux, channel thickness, and porous material properties are investigated. It is found that the flow has a flattened velocity profile with thin boundary layers that are virtually independent of volume flux and channel thickness. The pressure drop is mainly dependent on the volume flux and the permeability. It is recommended that cell performance could be improved by using a flowing electrolyte channel that is thinner, and selecting a sufficiently high volume flux and a sufficiently permeable porous material to achieve an optimal combination of pressure drop and methanol removal characteristics.     

Numerical investigation of transport phenomena in high temperature proton exchange membrane fuel cells with different flow field designs

In this work, a three-dimensional, non-isothermal, steady-state model for high temperature proton exchange membrane fuel cells with phosphoric acid polybenzimidazole membrane has been developed using computational fluid dynamics. The importance of the gas flow field design on the transport characteristics and cell performance is revealed by solving the mass, momentum, species, energy, and charge conservation equations. The numerical results show that the best cell performance is provided by the fuel cell with serpentine flow channel flow field. However, the pressure drop is also very high due to the large length of the serpentine channel. In addition, the velocity, oxygen mass fraction, and temperature distributions are unevenly distributed over the entire active area of the fuel cell having straight channels with small manifolds, especially at low cell voltages when a large amount of oxygen is required. The cell performance and durability can be significantly affected by the uniformity of the reactants within the fuel cell. It is suggested that the flow field configurations must be optimized to obtain uniform distributions of the reactants, maximize the cell performance, and minimize the pressure drop penalty. The present results provide detailed information about transport characteristics within fuel cells and give guidelines for design and manufacturing of current collectors.     

Experimental analysis of the effects of porous wall on flame stability and temperature distribution in a premixed natural gas/air combustion

In the present analysis, the flame stabilization and temperature distribution within a premixed burner contain porous wall are studied experimentally. The effects of inner diameter, length, and pore density of the porous wall, thermal load, equivalence ratio, and the inlet velocity of the fuel‐air mixture on these are studied. The fuel used in this study is natural gas and the porous wall is SiC (silicon carbide) ceramic foam. The experimental results clearly indicate that the axial temperature along the porous wall increases when the inner diameter of the porous wall decreases and its length increases. The porous wall temperature with an inner diameter of 40 mm, length of 66 mm, and pore density of 30 PPI (pores per inch) has the highest temperature among the examined states. The results of studying the effect of the porous wall on flame stability show that the flame stability limit has a direct relationship with the length and pore density of porous wall and an inverse relationship with the inner diameter of the porous wall. Also, it is found that the porous wall has the highest temperature causes the maximum flame stability limit.     

Effects of geometrical dimensions of flow channels of a large-active-area PEM fuel cell: A CFD study

Various flow field designs have been numerically investigated to evaluate the effect of pattern and the cross-sectional dimensions of the channel on the performance of a large active area PEM fuel cell. Three types of multiple-serpentine channels (7-channels, 11-channels and 14-channels) have been chosen for the 200 cm² fuel cell investigated and numerically analysed by varying the width and the land of the channel. The CFD simulations showed that as the channel width decreases, as in the 14-channels serpentine case, the performance improves, especially at high current densities where the concentration losses are dominant. The optimum configuration, i.e. the 14-channels serpentine, has been manufactured and tested experimentally and a very good agreement between the experimental and modelling data was achieved. 4 channel depths have been considered (0.25, 0.4, 0.6 and 0.8 mm) in the CFD study to determine the effects on the pressure drop and water content. Up to 7% increase in the maximum reported current density has been achieved for the smallest depth and this due to the better removal of excess liquid water and better humidification of the membrane. Also, the influence of the air flow rate has been evaluated; the current density at 0.6 V increased by around 25% when air flow rate was increased 4 times; this is attributed to better removal of excess liquid water.     

Influence of sloping baffle plates on the mass transport and performance of PEMFC

Sloping baffle plates are installed numerically in the flow channel of proton exchange membrane fuel cell (PEMFC) to promote the mass transport in the porous electrode and the fuel cell performance. The sloping angle of baffle plate on the mass transport and performance of PEMFC are investigated and optimized. The numerical results show that the sloping angle of baffle plate influences the velocity distribution, flow resistance in the flow channel, and the intensity of mass transport between the channel and porous electrode. Larger sloping angle increases the velocity in the vertical direction which brings stronger squeeze effect between the channel and porous electrode, but it also reduces the squeeze area and increases the flow resistance. An optimization for the sloping angle of baffle plate is carried out. The baffle plate with the sloping angle of 45° shows the best performance in PEMFC net power considering the pumping power caused by the pressure loss. The effect of the baffle plate number is also investigated and optimized. The fuel cell current density increases with the baffle plate number, but the increment rate is decreased. The pumping power increases almost linearly with the baffle plate number. The PEMFC with six sloping baffle plates installed in the channel is found to be optimal in terms of the net power.     

Cross-leakage flow between adjacent flow channels in PEM fuel cells

In polymer electrolyte membrane (PEM) fuel cells, serpentine flow channels are used conventionally for effective water removal. The reactant flows along the flow channel with pressure decrease due to the frictional and minor losses as well as the reactant depletion because of electrochemical reactions in the cells. Because of the short distance between the adjacent flow channels, often in the order of 1 mm or smaller, the pressure gradient between the adjacent flow channels is very large, driving part of reactant to flow through the porous electrode backing layer (or the so-called gas diffusion layer)—this cross-leakage flow between adjacent flow channels in PEM fuel cells has been largely ignored in previous studies. In this study, the effect of cross-flow in an electrode backing layer has been investigated numerically by considering bipolar plates with single-channel serpentine flow field for both the anode and cathode side. It is found that a significant amount of reactant gas flows through the porous electrode structure, due to the pressure difference, and enters the next flow channel, in addition to a portion entering the catalyst layer for reaction. Therefore, mixing occurs between the relatively high concentration reactant stream following the flow channel and the relatively low reactant concentration stream going through the electrode. It is observed that the cross-leakage flow influences the reactant concentration at the interface between the electrode and the catalyst layer, hence the distribution of reaction rate or current density generated. In practice, this cross-leakage flow in the cathode helps drive the liquid water out of the electrode structure for effective water management, partially responsible for the good PEM fuel cell performance using the serpentine flow channels.     

Two-dimensional numerical pore-scale investigation of oxygen evolution in proton exchange membrane electrolysis cells

Oxygen blocking the porous transport layer (PTL) increases the mass transport loss, and then limits the high current density condition of proton exchange membrane electrolysis cells (PEMEC). In this paper, a two-dimensional transient mathematical model of anode two-phase flow in PEMEC is established by the fluid volume method (VOF) method. The transport mechanism of oxygen in porous layer is analyzed in details. The effects of liquid water flow velocity, porosity, fiber diameter and contact angle on oxygen pressure and saturation are studied. The results show that the oxygen bubble transport in the porous layer is mainly affected by capillary pressure and follows the transport mechanism of ‘pressurization breakthrough depressurization’. The oxygen bubble goes through three stages of growth, migration and separation in the channel, and then be carried out of the electrolysis cell by liquid water. When oxygen breaks through the porous layer and enters the flow channel, there is a phenomenon that the branch flow is merged into the main stream, and the last limiting throat affects the maximum pressure and oxygen saturation during stable condition. In addition, increasing the liquid water velocity is helpful to bubble separation; changing the porosity and fiber diameter directly affects the width of pore throat and the correlative capillary pressure; increasing porosity, reducing fiber diameter and contact angle can promote oxygen breakthrough and reduce the stable saturation of oxygen.     

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.     

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.     

Modelling and optimization of reactant gas transport in a PEM fuel cell with a transverse pin fin insert in channel flow

A proton exchange membrane (PEM) fuel cell has many distinctive features which make it an attractive alternative clean energy source. Some of those features are low start-up, high power density, high efficiency and remote applications. In the present study, a numerical investigation was conducted to analyse the flow field and reactant gas distribution in a PEM fuel cell channel with transversely inserted pin fins in the channel flow aimed at improving reactant gas distribution. A fin configuration of small hydraulic diameter was employed to minimise the additional pressure drop. The influence of the pin fin parameters, the flow Reynolds number, the gas diffusion layer (GDL) porosity on the reactant gas transport and the pressure drop across the channel length were explored. The parameters examined were optimized using a mathematical optimization code integrated with a commercial computational fluid dynamics code. The results obtained indicate that a pin fin insert in the channel flow considerably improves fuel cell performance and that optimal pin fin geometries exist for minimized pressure drop along the fuel channel for the fuel cell model considered. The results obtained provide a novel approach for improving the design of fuel cells for optimal performance.     

Numerical and experimental study of forced convection in graphite foams of different configurations

Forced convection heat transfer in a channel with different configurations of graphite foams is experimentally and numerically studied in this paper. The physical properties of graphite foams such as the porosity, pore diameter, density, permeability and Forchheimer coefficient are determined experimentally. The local temperatures at the surface of the heat source and the pressure drops across different configurations of graphite foams are measured. In the numerical simulations, the Navier–Stokes and Brinkman–Forchheimer equations are used to model the fluid flow in the open and porous regions, respectively. The local thermal non-equilibrium model is adopted in the energy equations to evaluate the solid and fluid temperatures. Comparisons are made between the experimental and simulation results. The results showed that the solid block foam has the best heat transfer performance at the expense of high pressure drop. However, the proposed configurations can achieve relatively good enhancement of heat transfer at moderate pressure drop.