Multiphase-Simulation of Membrane Humidifiers for PEM Fuel Cells

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 1

CFD Simulation of Fuel Cell Humidifiers Overview Introduction Simulation Methodology Single-Duct Modelling Modelling of a Complete Humidifier Conclusion/Summary Outlook/Further Work 2

CFD Simulation of Fuel Cell Humidifiers Overview Introduction Simulation Methodology Single-Duct Modelling Modelling of a Complete Humidifier Conclusion/Summary Outlook/Further Work 3

Introduction Task/ Purpose 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 hydrogen electrical load gas with high relative humidity proton exchange membrane air gas diffusion layer bipolar plates anode electrode with catalyst cathode cf: Perma Pure cf: A. Vlath: Dreidimensionale dynamische Modellierung und Berechnung von PEM- Brennstoffzellen-systemen. 2009 Development of a methodology to simulate a fuel cell humidifier with Star-CCM+ Examination of this methodology under different operating conditions 4

Introduction Functional Principle of the Humidifier Position of the humidifier in the cathode stream: Air Compressor Dry Humidifier Humid H 2 Fuel cell Anode Cathode Setup of the humidifier: Dry exhaust Bundle of pipes Humid exhaust Liquid water or gas with high humidity Dry air Humid air Exhaust stream Transport of moisture through the membrane 5

CFD Simulation of Fuel Cell Humidifiers Overview Introduction Simulation Methodology Single-Duct Modelling Modelling of a Complete Humidifier Conclusion/Summary Outlook/Further Work 6

Simulation Methodology Calculation of the Resulting Flux Example water-to-gas-humidifier: a A =f(λ) Liquid water Δx Membrane a I =f(λ) Δa Low humidity Air High humidity 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 7

CFD Simulation of Fuel Cell Humidifiers Overview Introduction Simulation Methodology Single-Duct Modelling Modelling of a Complete Humidifier Conclusion/Summary Outlook/Further Work 8

1,4mm 4mm 0,127mm Single-Duct Modelling Design of the Simulation Model Geometrical data is taken from the humidifier: H 2 O Air H 2 O 254mm 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) 9

Mass fraction of H 2 O [-] Single-Duct Modelling Results of the Water-to-Gas-Humidifier Example: Temperature of water 40 C Volume flow of the cathode stream 357 l/min Mass Fraction (flow direction Z): Gas inlet Gas outlet Coordinate Z [m] 10

Single-Duct Modelling Results of the Water-to-Gas-Humidifier Example: Temperature of water 40 C Volume flow of the cathode stream 357 l/min Temperature (flow direction Z): Gas inlet Gas outlet Gas temperature [K] Relative Humidity [-] Coordinate Z [m] 11

Mass fraction of water at the outlet [-] Single-Duct Modelling Validation of the Water-to-Gas-Humidifier Variation of water temperature and volume flow of the cathode stream: T H2O = 80 C T H2O = 60 C T H2O = 40 C Volume flow of the cathode stream [l/min] Low change in temperature of the inner fluid means negligible error 12

Single-Duct Modelling Results of the Gas-to-Gas-Humidifier 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 Outlet of the cathode stream Temperature (flow direction Z): Inlet of the cathode stream Outlet of the cathode stream 13

Single-Duct Modelling Validation of the Gas-to-Gas-Humidifier Variation of temperature of the outer gas with constant mass fraction Mass fraction H 2 O at the outlet of cathode stream Deviation to validation data Simulation Data sheet Mass fraction of the inner gas is influenced by the mass fraction of the outer gas Agrees with validation data 14

CFD Simulation of Fuel Cell Humidifiers Overview Introduction Simulation Methodology Single-Duct Modelling Modelling of a Complete Humidifier Conclusion/Summary Outlook/Further Work 15

Modelling of a Complete Humidifier Motivation 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 Reality: irregular array Model: regular array 16

Modelling of a Complete Humidifier Design of the Simulation Model Fluid region water: 150 pipes modelled Solid/fluid region air: 4 representing pipes, transfer of the results to the surrounding pipes Inlet: air Inlet: water Outlet: air Outlet: water 17

Mass fraction H 2 O at the outlet [-] Temperature at the outlet [K] Modelling of a Complete Humidifier Results in Comparison with the Single Duct Mass fraction: Temperature: data sheet single duct humidifier single duct humidifier Volume flow of the cathode stream in [l/min] Volume flow of the cathode stream [l/min] 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 18

CFD Simulation of Fuel Cell Humidifiers Overview Introduction Simulation Methodology Single-Duct Modelling Modelling of a Complete Humidifier Conclusion/Summary Outlook/Further Work 19

CFD Simulation of Fuel Cell Humidifiers Conclusion / Summary 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 20

CFD Simulation of Fuel Cell Humidifiers Overview Introduction Simulation Methodology Single-Duct Modelling Modelling of a Complete Humidifier Conclusion/Summary Outlook/Further Work 21

CFD Simulation of Fuel Cell Humidifiers Outlook / Future Work 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 22

Thank you very much Sebastian Bilz IAV GmbH Rockwellstraße 16, 38518 Gifhorn sebastian.bilz@iav.de www.iav.com 23

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 temperature 1 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 temperature 1 at the outlet - computed Mass fraction of water of the exhaust stream at the outlet 2 - computed 1: temperature of exhaust stream in case of a gas-to-gas humidifier 2: only in case of gas-to-gas humidifier 24

Appendix Interpolating of Activities Inner Activity Inner fluid Membrane Outer fluid 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 25

Appendix Comparison of the Fluid Film Thickness Small volume flow: Inlet Outlet Large volume flow: Inlet Outlet 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 26