Multiphase-Simulation of Membrane Humidifiers for PEM Fuel Cells
STAR Global Conference
Sebastian Bilz, Vladimir Buday, Carolus Gruenig, Thomas von Unwerth
Vienna, March 17-19, 2014
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers 1
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CFD Simulation of Fuel Cell Humidifiers Overview
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Introduction
Simulation Methodology
Single-Duct Modelling
Modelling of a Complete Humidifier
Conclusion/Summary
Outlook/Further Work
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CFD Simulation of Fuel Cell Humidifiers Overview
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Introduction
Simulation Methodology
Single-Duct Modelling
Modelling of a Complete Humidifier
Conclusion/Summary
Outlook/Further Work
4
• Electrolyte of the fuel cell has to be hydrated to ensure proton conductivity
• An option: humidifier
• But: water management of a PEM fuel cell has to be well controlled to secure power
in every operating point → CFD simulation
Introduction Task/ Purpose
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
proton exchange membrane
hydrogen
electrical load gas with high
relative humidity
air gas diffusion layer
electrode with catalyst
anode cathode bipolar plates
cf: A. Vlath: Dreidimensionale dynamische Modellierung und Berechnung von PEM- Brennstoffzellen-systemen. 2009 cf: Perma Pure
Development of a methodology to simulate a fuel cell humidifier with Star-CCM+
Examination of this methodology under different operating conditions
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Introduction Functional Principle of the Humidifier
Compressor Humidifier
Anode
Cathode
Fuel cell
Dry Humid
Humid exhaust Dry exhaust
Air
H2
Position of the humidifier in the cathode stream:
Setup of the humidifier:
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Dry air Humid air
Liquid water or gas
with high humidity
Bundle of pipes
Exhaust stream
Transport of moisture
through the membrane
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CFD Simulation of Fuel Cell Humidifiers Overview
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Introduction
Simulation Methodology
Single-Duct Modelling
Modelling of a Complete Humidifier
Conclusion/Summary
Outlook/Further Work
7
Example water-to-gas-humidifier:
Δx
Δa
aA=f(λ)
aI=f(λ)
Liquid water
Membrane
Low humidity High humidity
Simulation Methodology Calculation of the Resulting Flux
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Procedure:
1. Writing the activity of both sides into tables → interpolate them → conditions are available on
both sides of the membrane
2. The resulting flux is calculated by assuming a linear gradient
3. This flux is added to the inner region and subtracted from the outside
Air
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CFD Simulation of Fuel Cell Humidifiers Overview
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Introduction
Simulation Methodology
Single-Duct Modelling
Modelling of a Complete Humidifier
Conclusion/Summary
Outlook/Further Work
9
Geometrical data is taken from the humidifier:
Single-Duct Modelling Design of the Simulation Model
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
• Inner fluid: cathode stream, fluid region with shell region on the wall for evaporation
• Membrane: properties of Nafion 115® for heat transfer, solid region
• Outer fluid: liquid water or cathode exhaust, fluid region (shell region – condensation)
254mm
4m
m
1,4
mm
0,1
27
mm
H2O
H2O
Air
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• Example: Temperature of water 40°C
Volume flow of the cathode stream 357 l/min
• Mass Fraction (flow direction Z):
Single-Duct Modelling Results of the Water-to-Gas-Humidifier
Gas inlet Gas outlet
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Mass f
raction o
f H
2O
[-]
Coordinate Z [m]
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Gas inlet Gas outlet
Single-Duct Modelling Results of the Water-to-Gas-Humidifier
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
• Example: Temperature of water 40°C
Volume flow of the cathode stream 357 l/min
• Temperature (flow direction Z):
Coordinate Z [m]
Ga
s tem
pera
ture
[K
]
R
ela
tive
Hu
mid
ity [
-]
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Variation of water temperature and volume flow of the cathode stream:
Volume flow of the cathode stream [l/min]
Ma
ss f
ractio
n o
f w
ate
r a
t th
e o
utle
t [-
] Single-Duct Modelling Validation of the Water-to-Gas-Humidifier
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
TH2O = 80°C
TH2O = 60°C
TH2O = 40°C
Low change in temperature of the inner fluid means negligible error
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• Example: Temperature of the exhaust 70°C,
Volume flow of the cathode stream 60l/min
• Mass fraction (flow direction Z):
Inlet of the
cathode stream
Single-Duct Modelling Results of the Gas-to-Gas-Humidifier
• Temperature (flow direction Z):
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Outlet of the
cathode stream
Inlet of the
cathode stream Outlet of the
cathode stream
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Single-Duct Modelling Validation of the Gas-to-Gas-Humidifier
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Variation of temperature of the outer gas with constant mass fraction
Mass fraction H2O at the outlet of
cathode stream Deviation to validation data
Mass fraction of the inner gas is influenced by the mass fraction of the outer gas
Agrees with validation data
Simulation
Data sheet
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CFD Simulation of Fuel Cell Humidifiers Overview
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Introduction
Simulation Methodology
Single-Duct Modelling
Modelling of a Complete Humidifier
Conclusion/Summary
Outlook/Further Work
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers 16
• Realistic description of the outer fluid (flow profile)
• Interaction of the pipes
Transfer of the moisture in dependence of the position of the pipe
Increase of accuracy
Model: regular array Reality: irregular array
Modelling of a Complete Humidifier Motivation
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Outlet:
air
Inlet: air
Inlet: water
Outlet:
water
• Fluid region water: 150 pipes modelled
• Solid/fluid region air: 4 representing pipes, transfer of the results to the surrounding
pipes
Modelling of a Complete Humidifier Design of the Simulation Model
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
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Mass fraction: Temperature:
Ma
ss fra
ctio
n H
2O
at th
e o
utle
t [-
]
Volume flow of the cathode stream in [l/min] Volume flow of the cathode stream [l/min] Te
mp
era
ture
at th
e o
utle
t [K
]
Modelling of a Complete Humidifier Results in Comparison with the Single Duct
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
data sheet
single duct
humidifier
single duct
humidifier
Temperature of gases at the outlet agree with data of a single duct,
But: constant offset in mass fraction of 0.02
No significant difference between the results of the positions of the 4 pipes
Prospect: study of a gas-to-gas-humidifier
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CFD Simulation of Fuel Cell Humidifiers Overview
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Introduction
Simulation Methodology
Single-Duct Modelling
Modelling of a Complete Humidifier
Conclusion/Summary
Outlook/Further Work
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• Implementation of a suitable simulation approach
• Computation of multiphase flow with condensation and evaporation
• Calculation of the mass transfer through a semipermeable membrane with the
help of field functions and tables
• Calculation of both operational strategies (gas-to-gas, water-to-gas) possible
• Achieved result accuracy
• Trend of the results agree with validation data, error sources are known
• But: no validation data for temperature or relative humidity
• Modelling/ simulation of complete humidifier
• Not feasible in terms of require computational efforts
• No advantage in simulation of the entire humidifier in operation with liquid
water in comparison to the single-duct strategy
• Calculation with the help of the water content of the membrane → matches water
management in a fuel cell → first step to simulate an entire fuel cell
CFD Simulation of Fuel Cell Humidifiers Conclusion / Summary
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
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CFD Simulation of Fuel Cell Humidifiers Overview
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
Introduction
Simulation Methodology
Single-Duct Modelling
Modelling of a Complete Humidifier
Conclusion/Summary
Outlook/Further Work
22
• Measurement to get more validation data (heat transfer, temperature, relative
humidity)
• Further development of the methodology to reduce errors
Extension to consider other physical effects (membrane swelling, crossover of
gases)
• Computation of an entire gas-to-gas-humidifier, comparison with single-duct-model
CFD Simulation of Fuel Cell Humidifiers Outlook / Future Work
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
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Thank you very much
Sebastian Bilz
IAV GmbH
Rockwellstraße 16, 38518 Gifhorn
www.iav.com
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers 24
Appendix Comparison of Parameter: Model and Data Sheet
Parameter Data Sheet Model
Length 10‘‘=254 mm 10‘‘=254 mm
Inner diameter of the pipe - Measured: 1,4 mm
Membrane material Nafion Nafion 115 (based on membrane
thickness)
Properties of the membrane (porosity,
tortuosity, Diffusion coefficient)
- taken from literature, empirical
equations
Volume flow of air 71-500 l/min 71-500 l/min
Inlet air temperature - 298,15 K
Volume flow of water - 4,5E-3 (negligible)
Inlet water temperature1 40, 60, 75, 80°C 40, 60, 80°C
Air temperature or relative humidity at
outlet
- computed
Mass fraction of water of air at outlet Dew point temperature computed
Water temperature1 at the outlet - computed
Mass fraction of water of the exhaust
stream at the outlet2
- computed
1: temperature of exhaust stream in case of a gas-to-gas humidifier
2: only in case of gas-to-gas humidifier
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers 25
Appendix Interpolating of Activities
Activity is written into a table at the inner membrane surface
Interpolated with field function
Values pertain for each volume element and change in z-direction (from inlet to outlet)
Values are equal in the radial direction
Resulting flux can be calculated in the inner and outer fluid region to subtract and add the
same value
Inner Activity
Inner fluid
Membrane
Outer fluid
© IAV · 03/2014 · Sebastian Bilz · cd-adapco Global User Conference · Vienna · CFD Simulation of Fuel Cell Humidifiers 26
Small volume flow:
Large volume flow:
Inlet
Outlet
Outlet
Appendix Comparison of the Fluid Film Thickness
Small volume flow → fluid film thickness grows → saturated gas near the outlet
Large volume flow → minor fluid film thickness, remains constant → unsaturated
gas near the outlet
Inlet