Influence of Parallel Flow Field Design on the Performance of PEM Fuel Cells

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INFLUENCE OF PARALLEL FLOW FIELD DESIGN ON THE PERFORMANCE OF PEM

FUEL CELLS

E. Barakat1, K. Ahmed

2, M. Ahmed

1,*, Ali K. Abdel-Rahman

1 and Ahmed Hamza H. Ali

1

1Department of Mechanical Engineering, Assiut University, Assiut 71516, Egypt

2Department of Mechanical and Industrial Engineering, Qatar University, Qatar

*Corresponding Author: [email protected].

ABSTRACT

Flow field design in PEM fuel cells has an important influence on both the power density and

efficiency of fuel cell systems. In order to effectively utilize the active area, flow distribution and current

density should be as homogenous as possible. In addition, pressure losses should be minimized. The main

objective of this work is to provide a comprehensive study of the flow field characteristics for different

parallel flow channel configurations. The investigated channel configurations are parallel with various

cross-sectional configurations. For each channel configuration pressure drop, species mass fraction,

velocity, and current distributions are numerically predicted. In addition, performance curves are

presented and discussed. The predicted performance parameters are compared with the corresponding

available experimental data. Based on the computed results the optimum channel design is selected.

Detailed results are presented and discussed. The present work would help PEM fuel cell manufacturers,

companies and researchers to choose the optimum configuration for their applications and products to

achieve the best possible performance.

KEYWORDS: PEM fuel cell, PEM flow field design, PEM fuel cell performance

1 INTRODUCTION

Proton Exchange Membrane (PEM) fuel cells can be considered as the best candidate to replace the

fossil fuel job in different applications. Modeling and optimization of the fuel cell and its components are

critical tasks to the fuel cell researchers and companies. The flow field design plays an important role in

controlling the PEM fuel cells performance. It can be used to direct the gas flow into the gas diffusion

layer through a channel system, provide uniform reactant flow over the active area, water management,

and make the cell mechanically stable (Li & Sabir 2005). Typical flow field designs are parallel,

serpentine, interdigitated and many other modified and/or combined configurations of channel designs.

The geometric parameters of flow field design that have a significant effect on the performance of fuel

cells include; active area, pattern type, cross-sectional shape, flow channels area, flow channel to the rib

width ratio, and the flow channel aspect ratio.

Many researchers studied the influence of such parameters in order to improve the fuel cell

performance. Wang et al.(2008) studied the effects of two flow field design (on cathode side) parameters

such as the flow channel aspect ratio, and cross-sectional area, on the cell performance and local transport

phenomena. They found that the channel aspect ratio and cross-sectional area had little effects on the cell

performance. Bachman et al. (2012) experimentally studied the effect of channel length on the

performance of fuel cell. In their work, three lengths of 5, 15 and 25 cm were selected to be investigated.

They concluded that the longer the channel length the better the cell performance. Camci et al. (2011)

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studied numerically, using COMSOL software, the effect of making obstacles inside the flow channels of

the bipolar plates. In addition, the effects of the inlet velocity, the outlet pressure, and the channel length

on the fuel cell performance were investigated. Results indicated that increasing outlet pressure, inlet

velocity and depth of the obstacles through the channels enhanced the cell performance. Wang et al.

(2009) studied numerically using a two-phase 3-d model, the effect of active area on the cell performance

using interdigitated and serpentine flow channel design. They found that in general increasing the active

area significantly improved the cell performance. In addition, in the case of using parallel flow field, the

performance at low voltages did not increase with increasing size of active area. They reported that this

could be partially due to their 100% humidified anode and cathode reactant feed gases. Other study was

conducted by (Wang et al. 2009) in order to study the effect of widths of flow channel and rib for the

various flow channel area ratio on the cell performance. They found that at higher operating voltages, the

cell performance is independent of the above reported parameters and the operating conditions while at

lower operating voltages, these parameters affect cell performance.

Su et al. (2005) studied experimentally and numerically the effect of parallel and serpentine flow

field on the cell performance. They found that the cell performance using serpentine design is better than

that using parallel one. Shimpalee et al. (2006) studied the effect of serpentine channel path length on the

cell performance using five different flow-field designs at constant active area of 200 cm2. They found

that the shorter path length gave more uniform current density distribution and less condensed liquid

water than the longer path. Karvonen et al. (2006) numerically investigated the effect of new distributor

channels (header) on the performance of a parallel flow channel design. They studied the flow distribution

across the channels and the total pressure drop across the flow field. They mentioned that using such

design could significantly reduce the pressure drop. Yan et al. (2006) experimentally studied the cell

performance of two parallel, two interdigitated, and one serpentine flow field channel. They

found that in a serpentine flow field the electrochemical reaction was raised as a result of

increasing in the channel numbers, the channel lengths and the number of corners.

Typically, the parallel channel configurations have a small hydraulic resistance and thus does not

generate a large pressure drop across the flow channel as reported by Karvonen et al. (2006). On the other

hand, the parallel channel flow field designs often have a non-uniform flow distribution (with respect to

other designs such as serpentine or interdigitated) and these configurations are consequently more

susceptible to flooding. However, these problems can partially be avoided with a careful design of the

flow field system. The objective of the present work is to find the optimal configuration of cross section

in order to achieve a better performance of fuel cells. Therefore, a complete 3-d PEM fuel cell model is

used to study the effects of the flow field channel designs on the cell performance for different parallel

flow fields. The local transport characteristics and the performance of cells with triangle, trapezoid (with

different aspect ratios), and half circle channels are compared to cell with a regular rectangular channel

design. A uniform flow distribution with the parallel channel system is presented, and the main result is to

show that uniform flow distributions can be achieved with parallel channel flow fields with relatively

slight changes in the flow field design.

2 MODEL GEOMETRY AND BOUNDARY CONDITIONS

2.1 MODEL GEOMETRY

A 3-d numerical model of one-channel cell is proposed in the current work to compare the channel

design effects on the cell performance by considering polarization curves of the cells. Details of the

governing equations are given in appendix. Different cell models are considered, depending on the

channel designs. A based reference model is developed using one channel based on the dimensions used

by Cheng et al. (2007), as shown in Table 1. The model shown in Figure 1 considers all fuel cell

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components; anode current collector, anode flow channel, anode gas diffusion layer, anode catalyst layer,

membrane, cathode catalyst layer, cathode gas diffusion layer, cathode flow channel and cathode current

collector. Mesh independency tests are performed to ensure that the model results are independent of the

grid size. A 130,000 cells mesh shown in Figure 2 is found to provide a sufficient resolution in the first

based reference model

Table 1: Cell dimensions

Geometrical parameters Units Value

Gas channel length mm 50.0

Height of gas channel mm 1.0

Width of the gas channel mm 1.0

Width of the cell mm 2.0

Thickness of catalyst layer mm 0.01

Thickness of gas diffusion layer mm 0.30

Thickness of current collector mm 2.0

Thickness of membrane mm 0.178

Figure 1: A single PEM fuel cell. (a) Complete cell with parallel flow channel and (b) model of one

channel

Figure 2: Model meshing

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2.2 BOUNDARY CONDITIONS

Anode and cathode boundary conditions are set as reactant species velocities at the inlets to model

the momentum transport. Reactant species mass fraction boundary conditions are applied at the inlets and

outlets for modeling the mass transport and diffusion phenomena. The motivating force for the

development of the electric current is the applied electric potential difference between the anode and

cathode electrodes. The anode voltage is set to zero (grounded) and the cathode voltage is set as the cell

operating voltage that take various volts starts from open circuit voltage. Polarization curves are obtained

by setting the cell voltage at constant value, solving the model, and integrating the local current density

value along the active layer, which gives the cell current density. Model boundary conditions are

summarized in Table 2.

Table 2: Model boundary conditions

B.C. Type Location Value Units Ref.

Velocity-

inlet

Inlet anode flow

channel face

Anode inlet gas velocity 0.3 m/s

(Cheng et

al. 2007)

Anode inlet mass fraction of H2 0.445

Anode inlet mass fraction of H2O 0.555

Inlet Cathode flow

channel face

Cathode inlet gas velocity 0.5 m/s

Cathode inlet mass fraction of O2 0.212

Cathode inlet mass fraction of N2 0.709

Cathode inlet mass fraction of

H2O 0.079

Pressure

outlet

Outlet anode flow

channel face Anode outlet gas pressure 0

Pa

(gage)

Outlet cathode flow

channel face Cathode outlet gas pressure 0

Pa

(gage)

Wall

The terminal and

upper anode current

collector face

Specified electric potential 0

volts (ANSYS

Inc. 2011) The terminal and

lower cathode

current collector

face

Specified electric potential 0.95 –

0.3

All Outer cell faces Thermal condition constant

temperature 323 k

(Cheng et

al. 2007)

3 MODEL SOLUTION

3.1 SOLUTION TECHNIQUE

The physical model presented in this work is comprehensive and includes the transport equations of

gaseous species, ionic charge, energy, mass, momentum, and water in the dissolved phase within the

membrane as shown in appendix. These equations are solved over a 3-d computational domain. The

physical model is solved with a commercially available CFD package, ANSYS FLUENT 14.0 with a

PEM fuel cell add-on module, adapted to include the transport relations relevant to PEMFCs (ANSYS

Inc. 2011). The monitored residuals for converged solution are set at 1E-6 for; pressure, velocities and

current density. The history of calculated current density with iterations for cell voltage 0.6 volts is

shown in Figure 3. The model physical parameters, shown in Table 3, are kept constant throughout the

study. These parameters are needed to fully describe the transport processes taking place within the fuel

cell. These parameters are available in the literatures as reported in Table 3.

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Figure 3: Convergence history of the predicted current density

Note: the negative sign due to the current direction

Table 3: Model physical parameters

Physical parameters Units Value Ref.

Cell operating temperature 0C 323 (Cheng et al. 2007)

Cell operating pressure pa 101325 (Cheng et al. 2007)

Open-circuit voltage volts 0.95 (Obayopo et al.

2011)

Anode Reference exchange current density A/m2 10000 (ANSYS Inc. 2011)

Cathode Reference exchange current

density

A/m2 20 (ANSYS Inc. 2011)

Current collector Electric conductivity 1/ohm-m 4000 (Cheng et al. 2007)

GDL Electric conductivity 1/ohm-m 300 (Cheng et al. 2007)

Anode exchange coefficient 1 (Iranzo et al. 2011;

Iranzo et al. 2010;

Arvay et al. 2011)

Cathode exchange coefficient 1 (Iranzo et al. 2011;

Iranzo et al. 2010;

Arvay et al. 2011)

GDL Porosity 0.5 (Cheng et al. 2007)

CL Porosity 0.112 (Cheng et al. 2007)

3.2 MODEL VALIDATION

The I-V curve for the current numerical results and that experimentally obtained by Cheng et al. (

2007) are in good agreement as shown in Figure 4. There is only a minimal difference between the

simulation results and the experimental data. The standard deviation in the predicted current density

based on the definition made by Giri and Bannerjee (1975) is found to be 0.008 A/cm2.

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Figure 4: Comparison of the present base model and experimental results obtained by Cheng et al. (2007)

4 RESULTS AND DISCUSSION

4.1 EFFECT OF FLOW DIRECTION

The most common arrangements for flow paths within the anode and cathode channel are counter-

flow, parallel-flow and mixed. A counter-flow fuel cell is one in which the direction of the flow of one of

the working fluids is opposite to the direction to the flow of the other fluid. In a parallel flow, both fluids

in the channel flow are in the same direction. A mixed flow can be achieved by making the anode and

cathode in different channels.

The fuel cell performance is measured by polarization curves which relate the cell voltage to both;

the current and power densities. The counter flow channel shows better performance at all current

densities than parallel flow channel as shown in Figure 5 and Figure 6. The counter flow increases the

performance by 11% compared to that obtained using parallel flow. In addition, the parallel flow channel

produces maximum output power of 0.24 W/cm2 at 0.50 A/cm

2, while the counter flow channel produces

maximum output power of 0.28 W/cm2 at 0.55 A/cm

2, with a 19% increase in the maximum output

power.

The counter flow channel shows a better mass fraction of the reactants species and better current

distributions as well. Figure 7 shows mass fraction of H2 and O2 in a plane centered at the flow channel

in both cases at 0.50 A/cm2, the maximum power point. It is observed that the hydrogen mass flow rate is

less than the needed amount by the chemical reaction at this point, which explains the missing of

hydrogen mass fraction at channel end.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5

Cel

l volt

age

Vcell (

volt

s)

Current density i (A/cm2)

Present model

Experimental Cheng et al. (2007)

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Figure 5: Polarization curve for different flow arrangement

Figure 6: Power density curve for different flow arrangement

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

Cel

l volt

age

Vcell (

volt

s)


I-V counter flow

I-V parallel flow

0

0.05

0.1

0.15

0.2

0.25

0.3

0 0.2 0.4 0.6 0.8 1

Pow

er d

ensi

ty p

(W

/cm

2)


I-P counter flow

I-P parallel flow

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Figure 7: Mass fraction of H2 and O2 in both cases at 0.50 A/cm2 (a) H2 mass fraction parallel flow (b) H2

mass fraction counter flow (c) O2 mass fraction parallel flow (d) O2 mass fraction counter flow

4.2 EFFECT OF CHANNEL LENGTH

This study aims to investigate the effect of channel length on the cell performance. The base fuel

cell design that used before in model validation is used for this study with changing channel length to

various lengths; 40, 80, 100, 150, 200, 250 mm. The results of this study show that the shorter channel

lengths achieve a higher performance at all current densities than that of the longer channels. Table 4

shows the maximum power and the corresponding current density for each length.

Figure 8 and Figure 9 show the effect of channel length on the cell performances. Based on the

figures, increasing the channel length significantly reduces the performance of fuel cell for all studied

cases. Figure 10 shows the effect of channel length on the current density at various cell voltages. It is

obvious based on the figure that increasing the channel length results in a significant reduction in the

current density. This trend is observed for all values of voltages in the range of 0.4, 0.6, 0.8, and 0.9 volts.

Table 4: The maximum power and the corresponding current density for each length

Channel length (mm) 40 80 100 150 200 250

Maximum power (W/cm2 ) 0.33 0.28 0.26 0.24 0.241 0.22

Current density (A/cm2) 0.64 0.57 0.56 0.56 0.51 0.40

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Figure 8: Polarization curve for different channel lengths

Figure 9: Power density curve for different channel lengths

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Cel

l volt

age

Vcell (

volt

s)


length 40 mm

length 80 mm

length 100 mm

length 150 mm

length 200 mm

length 250 mm

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Pow

er d

ensi

ty p

(W

/cm

2)


length 40 mm

length 80 mm

length 100 mm

length 150 mm

length 200 mm

length 250 mm

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Figure 10: Effect of channel length on the current density at various cell voltages

4.3 EFFECT OF FLOW CHANNEL CROSS-SECTION

For each cross-section, the channel land length (1 mm) remains the same. In addition, the channel

height (1 mm) remains the same except for half circle case. The constant cross section constrain does not

adapt with the same channel land length so the same area is chosen. The same boundary conditions,

channel length and all other physical properties are used for all cases to eliminate any other factors to,

solely, find the effect of flow channel configuration. Table 5 shows the designed cross-sections.

Figure 11 and Figure 12 show the effect of different designed cross-sections on the performance

curves. The reversed trapezoidal cross-section is the highest performance for the same channel land

length and the same height, because it is the largest area of them.

Table 5: Cross-section designs

Configu

ration

Rectangular Triangle Trapezoidal Reversed

trapezoidal

Half circle

channel

height

1 mm 1 mm 1 mm 1 mm 0.5 mm

channel

land

length

1 mm 1 mm 1 mm 1 mm 1 mm

Dimensi

ons and

Shape

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

40 mm 80 mm 100 mm 150 mm 200 mm 250 mm

Cu

rren

t d

ensi

ty i

(A/c

m2)

0.9 volts 0.8 volts 0.6 volts 0.4 volts

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Figure 11: Polarization curve for different cross-sections

Figure 12: Power density curve for different cross-sections

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

Cel

l volt

age

Vcell (

volt

s)


Half circle

Trapezoidal

Triangle

Rectangular

Reversed trapezoidal

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.2 0.4 0.6 0.8 1

Pow

er d

ensi

ty p

(W

/cm

2)


Half circle Trapezoidal

Triangle rectangular

Reversed trapezoidal

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5 CONCLUSIONS

The effects of varying the flow field designs on the PEM fuel cells have been numerically

investigated. The flow direction in anode and cathode, channel length, and channel cross section

geometry are considered in the present work. The governing equations are solved with a commercially

available CFD package, ANSYS FLUENT 14.0 with a PEM fuel cell add-on module (ANSYS Inc. 2011).

Model validation is carried out by comparing the predicted results with the available experimental data.

Results indicate that counter flow direction in anode and cathode achieves a better performance of fuel

cells. In addition, a shorter length is recommended since it provides the maximum possible output power.

For the same channel land length and channel height, the reversed trapezoidal cross section shows the

best cell performance.

6 ACKNOWLEDGEMENT

Authors acknowledge Misr El Kheir foundation for its financial support provided to carry out

attend ICCE 2013 conference.

REFERENCES

ANSYS Inc., 2011. ANSYS FLUENT 14.0 fuel cells module manual.

http://cdlab2.fluid.tuwien.ac.at/LEHRE/TURB/Fluent.Inc/v140/flu_fcm.pdf.

Arvay, A. et al., 2011. Convergence criteria establishment for 3D simulation of proton exchange

membrane fuel cell. International Journal of Hydrogen Energy, 37(3), pp.2482–2489. Available at:

http://dx.doi.org/10.1016/j.ijhydene.2011.11.005.

Bachman, J. et al., 2012. Experimental investigation of the effect of channel length on performance and

water accumulation in a PEMFC parallel flow field. International Journal of Hydrogen Energy,

37(22), pp.I7172–I7179. Available at:

http://linkinghub.elsevier.com/retrieve/pii/S0360319912018253 [Accessed April 28, 2013].

Camcı, T. et al., 2011. MODELING OF GAS FLOW CHANNELS FOR PROTON EXCHANGE

MEMBRANE FUEL CELLS. In IGEC. pp. 381–388.

Cheng, C.-H., Lin, H.-H. & Lai, G.-J., 2007. Design for geometric parameters of PEM fuel cell by

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Karvonen, S. et al., 2006. Modeling of flow field in polymer electrolyte membrane fuel cell. Journal of

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APPENDIX

This section discusses the model parameters based on ANSYS FLUENT fuel cell module manual

(Arvay et al. 2011; Iranzo et al. 2011; Iranzo et al. 2010; ANSYS Inc. 2011). The transport of gas

mixtures in the gas channels and in the electrodes conforms to the mass, momentum, species conservation

principles, and electrochemical reactions. The 3D fluid flow and heat transport is solved using CFD

techniques, which include conservation of mass, momentum and energy that can be expressed in the

Naviere-Stokes equations are written as follows:

(1)

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The conservation equation in this form states that the rate of change quantity plus the transport

due to convection plus the transport due to diffusion is equal to the source. is the transported quantity

(energy, momentum), t is time, A is the surface area, V is volume, is the diffusivity coefficient , S is the

source term.

Equations for the surface over-potential, the driving force behind the electrochemical reactions

within the fuel cell simulation, are solved according to the following two equations

(2)

The first equation describes the difference between the phase potential of the solid materials which

governs the transport of electrons through the GDL porous material and current collectors. The second

equation describes ionic transport of ions through the membrane. represents the electrical

conductivity ( ); is the electric potential (volts); is the volumetric transfer current (

) in the solid or membrane.

Transfer currents are computed within the catalyst layers using the Butler-Volmer function.

(4)

(5)

is the specific active surface area ( ). is the reference exchange current density per active

surface area . The quantities in the brackets represent the local species concentration );

is the concentration dependence; is the transfer coefficient. F is Faradays constant and R is the

universal gas constant. The quantity is the local surface over-potential or activation loss which is

computed using the following equations where is the open circuit voltage.

(6)

(7)

Mass conservation is obeyed using volumetric species mass terms.

(8)

(9)

(10)

Where represents the species source term (Kg/s.m3); is the molecular mass of the species

(kg/kmole). The sign of the equations indicate that hydrogen and oxygen are consumed while is

generated. The electric current is conserved by obeying the following equation.

(3)

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(11)

Volumetric sources for thermal energy are required because not all of the chemical energy is

converted to electrical work. This is accounted for by using a thermal energy (rate of enthalpy change, )

equation

(12)

where is the net rate of enthalpy change (J/s) due to the electrochemical reactions.

is the product of the transfer current and the over-potential in the anode or cathode.

is the ohmic resistivity of the conducting media and I is the current (A). is the rate of enthalpy

change due to phase changes of the water.

Flow within the porous media of the GDLs and catalyst layers is modeled by adjusting the source

term by adding a negative source which represents fluid flow pressure drops in the species equations and

calculating species diffusivities.

( 13)

Si indicates the source in i direction. is the kinematic viscosity (m2/s), the fluid density (kg/m

3),

are the velocity and magnitude of the velocity (m/s), C2 is the inertial resistance factor (m-1

).

Influence of Parallel Flow Field Design on the Performance of PEM Fuel Cells

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