Fuel Cell Modeling with...

© 2011 ANSYS, Inc. November 28, 2014

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PEM Fuel Cell Modeling with ANSYS-Fluent

Sandeep Sovani, Ph.D.

Director, Global Automotive Industry April 8, 2014


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• Geometry Model

• Physics Model

• ANSYS-Fluent PEMFC Module

• Model Validation with Experiments

• Stack Simulation

Contents


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Geometric Model ANSYS-Fluent uses a Resolved Electrolyte Model

All of the following zones are resolved, i.e. have individual meshes

anode

cathode gas diffusion layer

flow channel

current collector

catalyst layer

membrane

catalyst layer

gas diffusion layer

flow channel

current collector

Not

to sc

ale!

electrolyte

Coolant Channel


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Geometric Model

membrane

catalyst layer

gas diffusion layer

flow channel

current collector Also called “bipolar plate“ or “interconnect”. Solid material, e.g. graphite. Used to conduct electrons to/from the external circuit. Gives structural stability.

Channel to provide fuel (anode) and oxidizer (cathode) and to transport away the reaction products.

Porous medium to allow diffusive flow of fuel (anode) and oxidizer (cathode) and to permit transport of electrons.

Also called “electrode“. Porous medium to allow diffusive flow of fuel (anode) and oxidizer (cathode) and to permit transport of electrons. Partially filled with catalyst material, e.g. Platinum, and membrane material.

Proton conducting polymer material, e.g. Nafion.

Coolant Channel Coolant flow


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Physics Model

• Fuel Cell Modeling requires to calculate

– fluid flow with reacting species

– convective/conductive heat transfer (w/o radiation)

– mass transfer

Standard ANSYS Fluent

ANSYS Fluent Fuel Cell Module

– heterogeneous electrochemical reactions

– transport of electric current driven by electric potential

– multiphase flow (water condensation within the PEMFC)


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A general purpose CFD solver is ideal for modeling

PEM fuel cells, however, some additional sub-models are needed

PEMFC multi-physics

– Laminar/Turbulent/Transitional Fluid Flow

– Heat Transfer

– Species Transport

– Chemical Reaction

– Multiphase Flow

– Electrochemistry

– Electric Potentials (current transport)

Physics Model

General Purpose CFD Code

Additional Sub-Models Needed


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Additional submodels needed for complex PEMFC multi-physics

• Electrochemical submodel - model current density, voltage, species sources/sinks at the MEA surfaces

• Electrical submodel - model current and voltage distribution in porous and solid conducting regions

• MEA submodel - predict electrical losses and water flow in MEA

• Porous Media Multiphase Flow submodel -model liquid water and oxidizer flow in porous cathode diffusion layer

• Thin Film Multiphase submodel - model flow of liquid water in cathode gas flow passages

Physics Model


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The PEMFC module is a separately licensed-managed add-on module

− Included with FLUENT distribution − Fully supported and documented − Available for SERIAL and PARALLEL Fluent

ANSYS-Fluent PEMFC Module


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Fluent’s PEMFC Module can be used for

– Single cell simulation

– Stack simulation

– Steady state simulation

– Transient simulation

– Computing current for fixed voltage

– Computing voltage for fixed current

One simulation per data point on the I-V curve



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Detailed modeling of MEA with dual potential model from Kulikovsky, Divisek and Kornyshev*

Compute current/voltage from specified voltage/current

Capabilities of modeling contact resistance and Joule heating, cooling channels, etc.

Membrane water transport

Phase change and liquid water transport in porous media, clogging to gas diffusion and reaction sites

Robust solution procedure and fast convergence

Fuel Cell Specific Graphical User Interface (GUI) set up * Kulikovsky et al., J. Electrochem. Soc. 147 (3) (2000) 953-959

ANSYS-Fluent PEMFC Module: Key Features


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Parallelized computing

User-Modifiable properties, e.g. gas diffusitivity, electrolyte conductivity etc. by using User Defined Functions (UDF) written in C

Automated stack set-up

Multicomponent diffusion

Temperature dependent leakage current

Non-isotropic electrical and thermal conductivities in the gas diffusion layer

Validated by experiments

ANSYS-Fluent PEMFC Module: Key Features


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PEMFC: Domains modeled

anode

cathode gas diffusion layer

flow channel

current collector

catalyst layer

membrane

catalyst layer

gas diffusion layer

flow channel

current collector

coolant channel

coolant channel

coolant channel

coolant channel

Membrane Electrode Assembly

MEA

H+ H+

−+ +→ eHH 442 2

OHeHO 22 244 →++ −+

e-

e-

e-

e-



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Equations Solved



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Electric Sub-Model

Two electric potentials are computed

• Solid phase potential (e- transport in conducting solid)

• Membrane phase potential (H+ transport in MEA)

Advantages

• Account for current transport in all regions

• Facilitate modeling of contact resistance at material interfaces



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Electrochemical Sub-Model

Ran and Rcat are calculated using the Butler-Volmer function



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Liquid Water Transport

Inside the membrane • water content • diffusion model with consideration of the osmotic drag • MEM-CAT interface: Springer et al. (1991) and Eaton (2001, account for

phase change )

Outside the membrane • water saturation in GDL • fine mist in flow channel (vw = vgas) • condensation/vaporization • capillary diffusion and surface tension in porous zones



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PEMFC: Solution Procedure Specify solid phase potential BCs at cathode current collectors:

Cell voltage Vcell or average current density Iave

Specify inlet temperature boundary conditions

Solve the system of equations for u, v, w, p, yi, T, φs, φm, s, λ

For prescribed Vcell:

For prescribed Iave:

Get polarization curve (Iave, Vcell)

mem

aaave A

dVRI ∫=

contactexternalcathode|scellV φ=



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Post-Processing

ANSYSs-Fluent’s standard post-processing features are all available with the PEMFC module, e.g. Contour plots, vectors, iso-surfaces, graphs, etc.

Variables available for post-processing

• Standard quantities – Pressure, X,Y,Z Velocities, Temperature, – Species mass (or mole) fractions

• PEMFC specific scalars – UDS-0 Solid Phase Potential (Volts) – UDS-1 Membrane Potential (Volts) – UDS-2 Liquid Saturation (Liquid Water Volume Fraction) – UDS-3 Water Content



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Post-Processing

More PEMFC specific scalars available for post-processing



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Schematic diagram of the test cell of Mench et al [1]: all numbers in mm

2.54

71.12

Channel depth: 3.18; width: 2.16 Inlet

Outlet

70.99

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

I (A/cm2)

V (V

)

Data: 1.50 equiv.Fluent: 1.50 equiv.Data: 2.25 equiv. Fluent: 2.25 equiv.

Computed (lines) and measured (symbols) global polarization curves for cathode stoichiometry of 1.5 and 2.25 equiv.

50 cm2 MEA

Model Validation vs. Experiments


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0.0

0.2

0.4

0.6

0.8

1.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

x/L

I (A

/cm

2 )

Exp. 0.85 V Exp. 0.80 V Exp. 0.70 V Exp. 0.65 V Cal. 0.85 V Cal. 0.80 V Cal. 0.70 V Cal. 0.65 V Exp. 0.55 V Exp. 0.45 V Exp. 0.40 V Exp. 0.35 V Cal. 0.55 V Cal. 0.45 V Cal. 0.40 V Cal. 0.35 V

Computed (lines) and measured (symbols) local current density distributions for cathode stoichiometry of 1.5 equiv.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

x/L I (

A/c

m2 )

Exp. 0.85 V Exp. 0.80 V Exp. 0.70 V Exp. 0.65 V Cal. 0.85 V Cal. 0.80 V Cal. 0.70 V Cal. 0.65 V Exp. 0.55 V Exp. 0.45 V Exp. 0.40 V Exp. 0.35 V Cal. 0.55 V Cal. 0.45 V Cal. 0.40 V Cal. 0.35 V

Computed (lines) and measured (symbols) local current density distributions for cathode stoichiometry of 2.25 equiv.



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Inlet

Outlet

48.36

50.0

0.8 0.84

Unit: mm

25 cm2 MEA of Liu et al (2005)

3-channel serpentine gas flow channel for both anode and cathode sides

Anode and Cathode outlet pressure = 1 atm

Air and fuel stoichiometric ratio = 2

Cell temperature = 353 K



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Global IV polarization for fully humidified fuel (H2)

Liu et al (2005)

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 0.8I (A/cm2)

Pote

ntial

(V

)

Experiments

Simulation

Air humidity = 100 %

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 0.8I (A/cm2)

Pote

ntial

(V

)

Experiments

Simulation

Air humidity = 50 %

ANSYS FC Module



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Global IV polarization for partially humidified fuel (H2)

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 0.8I (A/cm2)

Pote

ntial

(V

)

Experiments

Simulation

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 0.6 0.7I (A/cm2)

Pote

ntial

(V

)

Experiments

Simulation

Air humidity = 100 %

Air humidity = 30 %

ANSYS FC Module

Liu et al (2005)



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Stack simulation

415,000 FV per FC

MEA area 50 cm2

Anode Inlet Conditions • mass flow rate 1e-5 kg/s • YH2 0.2 • YH2O 0.8

Cathode Inlet Conditions • mass flow rate 1e-5 kg/s • YO2 0.22 • YH2O 0.22

Wall Temperature fixed at 80°C

Potentiostatic BCs: • 0.7V per cell : i ≈ 0.5 A/cm2


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Stack Simulation Channels

Fuel

Air


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Oxygen Consumption Cathode Catalyst

Stack Simulation


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Temperature Membrane

Stack Simulation


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Robustness and rapid solution convergence (Residuals) Stack Simulation


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Performance: Computing Time

Computing Time (h)

#CPUs 2nd

Solution

Single PEMFC 1 2:45

4-PEMFC Stack 4 4:10





Cluster of PCs with

2 Dual Core CPUs

2.8 GHz

8 GB Memory

Fast Interconnect

Note:

increase in • problem size :

8 times • computing time:

< 2 times

Stack Simulation


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Stack Simulation – Example 2

• 5 PEMFC stack with end plates (omitted in this picture).

• Close to 9.3 Mio mesh cells.


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Stack Simulation – Example 2

Temperature °C on outer walls Temperature °C in an anode

YH2 YO2


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Strengths of ANSYS-Fluent PEMFC Solution

• Detailed, accurate model • All zones resolved • Detailed physics sub-models

• Highly customizable • Most aspects of the module are user customizable • Detailed customization documentation

• Services

• ANSYS has extensive experience in consulting and funded development services for PEM Fuel Cells

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