Numerical investigation of a novel compound flow-field for PEMFC performance improvement

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 0 3 2 e1 5 0 3 9

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Numerical investigation of a novel compound flowfield for PEMFC performance improvement Yousef Vazifeshenas*, Kurosh Sedighi, Mohsen Shakeri Faculty of Mechanical Engineering, Babol Noushirvani University of Technology, Babol, Islamic Republic of Iran

article info

abstract

Article history:

To investigate the effectiveness of a novel compound flow field design concerned in PEM

Received 17 June 2015

fuel cell, computational fluid dynamics (CFD) approach is employed. A novel compound

Received in revised form

flow field design along with typical serpentine and parallel designs is verified through three

26 July 2015

dimensional simulations. The numerical results for different reactant flow fields yield to

Accepted 19 August 2015

observation of parameters like current density, membrane mass fraction of water and etc.

Available online 26 September 2015

Comparison of the results revealed that the way reactants are distributed on the surface is substantial. Driving the polarization curve for the three flow fields, it is seen that the

Keywords:

parallel flow field demonstrated weaker performance in comparison to the other two

PEM fuel cell

models. The main reason is associated to insufficient distribution of the reactants.

Compound flow field

Analyzing the polarization curve of the compound design along with other contours it can

Polarization curve

be said that this design can be a good candidate for preventing the flooding phenomena

Current density

while its performance is approximately the same as the serpentine type. So employment of

Flooding

the so-called design can be strongly recommended for high current density operating condition. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Pollution and cost problems which affect the world life today are making the researchers and the scientific community work to achieve sustainable development and keep on progressing in a more respectful way towards our planet. Obtaining electric energy from clean fuels with high efficiency and no CO2 emissions is a fundamental concern. Fuel cells are the device through which this aspect finds the most likely solution [1,2]. Amongst the different types of fuel cells, Polyelectrolyte Membrane Fuel Cells (PEMFC) seem to be the ones which show features that adapt better to mobile applications [3e6]. Experimental investigation in the so-called field

exposes different problems. In this situation modeling gains importance, as it may help to diagnose possible malfunction sources or problematic points when actual experimental techniques cannot reach and even to discard unnecessary experiments and to point to those which are actually essential [7,8]. Among the different modeling strategies and tools available, Computational Fluid Dynamics (CFD) is arising special interest due to its powerful capabilities for fuel cell performance evaluation and parametric design optimization [9]. Although the working temperature of fuel cells is a key parameter for categorizing them, some researchers are interested in low temperature fuel cell [10e15] and some others work on high temperature ones [16,17]. Despite this, there is still hard work to do to optimize high temperature

* Corresponding author. E-mail address: [email protected] (Y. Vazifeshenas). http://dx.doi.org/10.1016/j.ijhydene.2015.08.077 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 0 3 2 e1 5 0 3 9

PEMFC performance. For instance, although some works devoted to study flow channel geometry influence exist [18,19], none which specifically deals with high temperature PEM fuel cells has been found. Flow fields play a very important role inside a fuel cell. They supply reactants and remove products from the system. Fluid flux, heat and mass transfer processes have a very important effect on a fuel cell performance and flow channels are responsible to enable efficient energy and mass transport [20]. Moreover, homogeneous current density which is interested can be achieved through a good reagent spread and diffusion over the electrode surface. Therefore, a good and effective flow field design based mainly on their geometry and dimensions optimization is crucial to obtain uniform fuel distribution for proper fuel cell operation [21e23]. Differences up to 50% of output power density have been observed just due to an adequate flow channel distribution [24]. Some researchers [25] also investigated the flow field channels effect through having multiple structural bifurcations of the channels. They used 3D numerical simulation approach with commercial tool Fluent software to see this effect on fuel distribution uniformity. Numerical tools were also admitted by others for flow field investigation [26]. They concentrated on both straight and serpentine channels and studied the 3D models. In their models, the flow direction was also a variable. Additionally, the cost can be decreased by the optimization of the fuel cell process. Several models have been developed for optimization of the electrochemical kinetics of the process. However, little attention has been on the optimization of the flow-field design in the bipolar/end plates of the fuel cell [27,28]. Here, in this article attention is paid to the improvement of flow field designs through employing three dimensional numerical simulation of fuel cell. CFD approach is preferred for modeling the electrochemical reactions of PEM fuel cell for different flow field designs. In order to develop the typical serpentine and parallel designs, a compound design of these two considering the benefits of each one is suggested. The polarization curve, current density contours, membrane water content contour and some other parameters are employed for deep understanding of the design development in comparison with the typical designs (see Fig. 1).

Flow field designs The very important role of supplying reagents and removing products from fuel cell system is mainly concerned to the flow fields. Performing different functions, bipolar plates are one of the key components of a fuel cell, and essential for an effective performance of the system. Although some of these functions are better associated with physical-chemical and fluid-dynamic phenomenon, they are closely related with the BPs themselves, and with the flow channel geometry in particular. Thus, they provide the necessary mechanical support for the stack, keep the different reactants isolated from each other distributing them on the catalyst surface of the MEA through the gas diffusion layer, and help to manage the water and heat generated inside the cell. In order to develop the serpentine design the compound model is suggested. This flow field type offers low pressure

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paths at the exit region. Hence water removal can be available because of easy gas transferring through the channels. This idea is numerically studied here to be approved.

Model development The current model is a three-dimensional single-phase numerical mass-transfer unified model for PEMFC. The steadystate performance is just a special case where all the time dependent terms in the governing transport equations would be nullified. The fuel cell behavior would be studied for different flow-field designs (serpentine, parallel, and compound) in bipolar/end plates of the fuel cell and the results would be compared. The overall performance of the flow-field design would be judged based on the steady state behavior of the fuel cell. This would help in finding the best possible flowfield design for the bipolar/end plates of the fuel cell. The simulation domain consists of cathode and anode flow-fields corresponding to the cathode and anode sides of the fuel cell, respectively, separated by the membrane electrode assembly (MEA) in between. The MEA consists of cathode and anode porous gas diffusion layer and catalyst layer, on each side of the membrane. The active area of the domain was a square of side 5 cm. The cathode and anode flow-field were essentially the machined channels in the bipolar/end plates. These help to distribute the reaction gases uniformly over the reaction surface. In this work three different flowfield designs were studied. Before simulation, 3D modeling of the geometry was done using Gambit software. 1st of all the flow field designs geometry were modeled in 2 dimensions and by extending them into the 3rd dimension, the geometry was prepared. After floe field geometries, the GDL, Catalyst and Membrane layers were added between the flow fields. The dimension of the channel width, land width, and channel depth was 1 mm. The cathode and anode gas diffusion layers were the electrodes made with porous carbon cloth that helps to diffuse the reactant gases from the bipolar/end plate flowfield channels towards the reaction catalyst layer, and also diffusion of byproduct water from the reaction site.

Governing equations The governing equations for this unified model are presented here. The transport equations include equations of conservation of mass, momentum and species transport in the domain. These equations are implemented zone-wise in the model domain. The equation for conservation of mass in general form can be described as: vr þ V$ðr! y Þ ¼ Sm vt

(1)

where r is the density of the gaseous mixture; t the time; ! v the velocity vector; and Sm is the source term for the continuous phase. This source term corresponds to the production or consumption of reaction species in a particular zone. Since all the electrochemical reactions take place only in the cathode and anode catalyst layers, the value of Sm is only non-zero for these layers and is given by:

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Fig. 1 e The geometries of the article a: parallel design, b: serpentine design and c: compound design.

Sm;c ¼ So2 þ Sw;c

(2)

Sm;a ¼ Sh2 þ Sw;a

(3)

Fig. 2 e Grid independency verification.

where the subscripts ‘c’ and ‘a’ refer to the cathode and anode sides, respectively, and Sx (x ¼ H2, O2, w (water)) represent the source terms for species x in the catalyst layer. The density, r mixture is calculated using the compressible gas technique (ideal-gas behavior) and is given for multi-component system as:

Fig. 3 e Comparison of the polarization curves for the three designs.

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¼ ! vðr! yÞ þ V$ðr! y! y Þ ¼ VP þ V$ t þ S p þ Sm;k ! y vt

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(5)

! where t is the stress tensor and S p is the momentum source term. For laminar flow through porous media, the momentum ! source term S p is proportional to velocity and is given by Darcy's law:   m ! ! y Sp ¼  b

(6)

where m is the viscosity of the gaseous mixture given by massweighted mixing law: m¼

X

mi mi

(7)

i

Fig. 4 e Comparison of the power curves for the three designs.

r¼

Pop þ P P mi RT i M i

(4)

where Pop is the operating pressure; P the local relative (or gauge) pressure, mi the mass-fraction of species i, and Mi is the molecular weight of species i. The conservation of momentum equation in general form can be described as:

and b is the permeability of the medium. The value of b was taken to be 1012 m2 for the electrodes and catalyst layer. The species transport equations were written in general form for each of the species H2, O2, N2, and H2O vapor. On the anode side, the species transport equations are:    v ! rmH2 þ V r! y mH2 ¼ V$ J H2 þ SH2 vt

(8)

v ! ðrmw;a Þ þ Vðr! y mw;a Þ ¼ V$ J w;a þ Sw;a vt

(9)

Fig. 5 e Velocity contours for a: parallel design, b: serpentine design and c: compound design.

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where Ji is the diffusion flux of species i. The diffusion flux of species i is given by Maxwell relationship as: N1 X ! Ji¼ rDi;j Vmj

(13)

j¼1

where Di,j is the binary diffusion coefficient of species i in species j, and N is the total number of species in the mixture. The binary diffusion coefficients (Di,j) were calculated using the kinetic theory of gases. The source terms for each of the species transport equations exist only in their respective catalyst layer and are given by:

Fig. 6 e Flow streamlines on the end corners of the compound design.

 SH2 ¼

and for the cathode side the species transport equations of O2, N2, and H2O are:    v ! rmO2 þ V r! y mO2 ¼ V$ J O2 þ SO2 vt

(10)

   v ! rmN2 þ V r! y mN2 ¼ V$ J N2 vt

(11)

v ! ðrmw;c Þ þ Vðr! y mw;c Þ ¼ V$ J w;c þ Sw;c vt

(12)



Iðx; yÞ MH2 Acv 2F

(14)

aðx; yÞ Iðx; yÞMH2 O Acv F

(15)

Iðx; yÞ MO2 Acv 4F

(16)

 Sw;a ¼

 

SO2 ¼

Sw;c ¼



 1 þ 2aðx; yÞ Iðx; yÞMH2 O Acv 2F

(17)

where I(x, y) is the local current density; F the Faradays constant; a(x, y) the local net water transfer coefficient per proton;

Fig. 7 e Mass fraction of water in cathode channel current density contours for a: parallel design, b: serpentine design and c: compound design.

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and Acv is the specific surface area of control volume element in the respective zone of the domain.

Results and discussion Different flow field designs were numerically investigated. To compare the effects of using various flow field types, all assumptions were the same. The simulations were performed for atmospheric pressure and constant temperature of 343 K for all cases. The reactant gases were fed to anode and cathode side in fully humidified condition. The polarization curve and contours of the flow fields were applied here for more understanding of the effects. In order to verify the grid independency of the problem, different hexahedral grid systems were tested and the result is show in the below Fig. 2. It is concluded that utilizing grid size of 690,000 leads to grid independent results. The polarization curve of the three designs shows sensible difference of the parallel flow field performance (see Figs. 3 and 4). As it is depicted the parallel flow field which shows the weaker performance would normally suffer from bad maldistribution. The media is not forced to move through definite paths, so the most low pressure drop direction is preferred. On

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the other hand, it is seen that the compound design performs as well as the serpentine design. In power curve also it is noted that the power driven from the compound design is the same as the serpentine one. This is achieved while the end section experiences parallel type channels. To see the superiority of the compound design, it should be defined that whether it can handle the water management better than the serpentine design or not. To achieve that, velocity, current density, water mass fraction in cathode channels and membrane water content contours should be analyzed. Observing the flow velocity contours it should be said that the serpentine and compound designs offer more uniform reactant distribution in comparison to the parallel design. Actually in parallel design the velocity magnitude cannot assure that the reaction product would not cause problem. It means that along with bad distribution of reactant on the active area, the velocity of the reactants is very low. On the other hand, in compound design it is seen that the flow on the end section corners accelerated since the flow section area decreased in the so-called regions. This is admitted in Fig. 6 clearly. This parameter can be the key factor for this design superiority in working in high current density condition where water production is a critical concern. Humidity of the reactant gases along with the water produced from fuel cell reactions are known as two sources of water which should be considered in flow field configuration.

Fig. 8 e Current density contours for a: parallel design, b: serpentine design and c: compound design.

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The balance between flooding and dehydration of the channels is more critical in cathode channels since it experiences oxygen reactions. So Fig. 7 represented the mass fraction of water in cathode channel as said. As it is seen, for b and c designs the water mass fraction is enhanced gradually from entrance toward exit section of the flow field. But the parallel design didn't have such trend. Actually enhancement of the water production is responsible for the aforesaid situation and an ideal design should be able of dealing with the water exist in the channel paths. Associating this situation to the results of Figs. 5 and 6 it can be concluded that the compound design can more efficiently be able of removing the product water and prevent the performance decrement in high current density conditions. After analyzing the previous parameters, Fig. 8 displays the current density contours. It shows that the serpentine and compound designs offer more uniform current density from inlet toward outlet of the paths on the active area while the parallel design didn't obey this rule. Although the parallel design showed high current density amounts, its nonuniformity was remarkable. So it can be concluded that the serpentine and compound designs are superior in this aspect of verification (see Fig. 9). For more clarification of the compound design characteristics, membrane water content was also compared for

the three designs. Focusing on these results it is concluded that the parallel design did not experience uniform and sufficient water content in mid section of its membrane. So this justifies the poor performance of this design. On other view, it is seen that both b and c designs display a promoting trend in water content. It is clear that existing more water than what is needed for making the membrane hydrated, blocks the porous zones and lowers the performance. So analyzing the water content contours it is comprehended that the compound design experiences more uniform water content which does not impose great water concentration gradient.

Conclusion In the present investigation numerical approach was employed for better understanding of the flow field effects on improvement of the PEM fuel cells performance. 3D simulation was an effective tool for verification of the design development. Assessing phenomenon happening in fuel cell, better vision of designing is possible. Flooding was a feasible issue which is prevented by utilizing this novel compound flow field whilst the PEMFC performance is not reduced. Considering all the above results it is concluded that:

Fig. 9 e Membrane water content for a: parallel design, b: serpentine design and c: compound design.

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 Numerical methods and simulation tool can be promising techniques for PEM fuel cell investigations.  Changing the flow field designs still can be determined as an applicable way for performance improvement.  The parallel design has the lowest current density and power in comparison with the other designs.  Enhancement of the reactant velocity at the end section of the flow field can be achieved by reduction of the cross section area.  Better discharge of the water at the end section of the flow field can effectively be a good treatment for flooding of the channels.  Compound design can perform as well as the typical serpentine design, and also in some aspects can be a better choice than the serpentine one.

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