Chemical reacting transport phenomena and multiscale ... Chemical reacting transport phenomena and multiscale

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  • Chemical reacting transport phenomena and multiscale

    models for SOFCs

    Martin Andersson Dept. of Energy sciences Lund University, Sweden

    Heat Transfer 2008, 9-11 July, Maribor

    Updated version for group seminar

  • Agenda

    • Introduction to Fuel Cells (FC) – Market potential – Different types

    • Modeling at different scales • Current and Future research • Conclusion

    M. Andersson HEAT 2008

  • Fuel Cells

    • Principle discovered in 1838 • High efficiency • No pollution • Fuel Cells are classified according to

    their ionic conductor (electrolyte) – AFC, PEMFC, PAFC, MCFC, SOFC

    M. Andersson HEAT 2008

  • Fuel Cells

    • Prices needs to be lowered for commercialisation: – 50 $/kW for “normal” cars – 135 $/kW for delivery vans – 200 $/kW for buses

    • First big market ???

    • Toyota claims they can build FC stacks for 500 $/kW

    M. Andersson HEAT 2008

  • 5

    from: IEA

  • 6

    from: IEA

  • Fuel Cells

    M. Andersson HEAT 2008

  • Alkali Fuel Cell (AFC)

    M. Andersson HEAT 2008

    • Working temperature: 20-250°C • Lifetime: 8000 h • Efficiency: 60 % • Electrolyte: Potassium hydroxide • Space missions • Vulnerable to CO2 poisoning

  • Proton Exchange Membrane (PEMFC)

    M. Andersson HEAT 2008

    • Working temperature: ~80°C • Current stack cost: 2000 $/kW • Electrolyte: Solid polymer • Transportation sector

    – Fast start-up time – High power to weight ratio

    • Vulnerable to CO poisoning

  • Phosphoric acid (PAFC)

    M. Andersson HEAT 2008

    • Working temperature: ~200°C • Overall Efficiency: 85 %

    – 40% electricity • Electrolyte: Phosphoric acid

    Price: 4000$/kW – Due to their platinum catalyst

  • Molten carbonate (MCFC)

    M. Andersson HEAT 2008

    • Working temperature: >650°C • Combined with gas turbine • Overall Efficiency 90%

    – 60% electricity • Stationary use • Internal reforming possible

  • SOFCs • Working temperature: 600 – 1000°C • Combined with gas turbine • Overall Efficiency: >85 %

    – 70% electricity • Stationary use • Internal reforming is possible • Current Cost: 12000 $/kW • Vulnerable to sulfur poisoning

    M. Andersson HEAT 2008

  • SOFC - reactions

    exothermic

    exothermic

    exothermic

    endothermic

    exothermic

    M. Andersson HEAT 2008

  • Modeling at different scales

    • System scale ~102 m • Component scale ~101 m • Flow/diffusion morphologies ~10-3 m • Material structure ~10-6 m • Functional material levels ~10-9 m

    M. Andersson HEAT 2008

  • Modeling at different scales • Microscale ()

    – Global flow field – Empirical factors from mirco/mesoscale

    can be used for macroscale modeling

    M. Andersson HEAT 2008

  • Modeling at different scales • Microscale

    – Theoretical knowledge • Macroscale

    – Empirical data

    M. Andersson HEAT 2008

  • Modeling at microscale

    •Diffusion at atomistic scale [Å-nm] –Density Functional Theory (DFT) •Oxygen ion-hoping phenomena inside YSC electrolyte –Molecular Dynamics(MD) •Mass transport of gases inside porous structures –Lattice Bolzmann Method (LBM) [example]

    M. Andersson HEAT 2008

  • Modeling at microscale

    Porous anode SOFC structure:

    Lattice Bolzmann Method (LBM) is used to calculate a steady state mole fraction variation in a typical porous geometry.

    M. Andersson HEAT 2008

  • Modeling at microscale

    •Prediction of concentration over potential –Dusty Gas Model (DGM) –Ficks Model (FM) –Stefan-Maxwell Model (SMM) Does not consider Knudsen diffusion

    M. Andersson HEAT 2008

    Possible to use SMM in COMSOL Multiphysics

  • Modeling at mesoscale

    • Simulation of open circuit voltage – Kinetic Monte Carlo (KMC)

    • Multiphysics processes in cathode/electrolyte interface considering geometry and detailed distribution of the pores – Finite Elements Method (FEM)

    M. Andersson HEAT 2008

  • Modeling at macroscale

    • Commercial codes are used to solve momentum, mass, energy and electrochemical kinetics – COMSOL Multiphysics

    • Finite Element Method (FEM) – FLUENT, CFX, STAR-CD

    • Finite Volume Method (FVM)

    M. Andersson HEAT 2008

  • Modeling at macroscale

    SOFC anode supported button cell:

    Dusty gas model is used in FLUENT(FVM) to calculate the velocity profile (m/s) within the anode compartment.

    M. Andersson HEAT 2008

  • Integration issues • Multiphysics modeling considers

    interaction between two or more physical disciplines

    • Hierarchical methods – Starts at smaller scale

    • Hybrid and Cocurrent method – Solve for several scales at same time

    M. Andersson HEAT 2008

  • SOFC multiscale • FC operation depends on interaction

    between: – Mass transport – Heat transfer – Electrochemical/chemical reactions – Multi-phase fluid flow

    M. Andersson HEAT 2008

  • COMSOL Multiphysics (FEM)

    • User friendly • Powerful • Ability to model several physical

    phenomena simultaneously – The free variable in one mode can be

    used as input in another, for example temperature, velocity, pressure.

    • Many post processing options

    M. Andersson HEAT 2008

  • COMSOL Multiphysics (FEM)

    • Define a geometry (1D, 2D, 3D) • Boundary conditions • Subdomain conditions • Adjust mesh • Time dependent / Stationary

    conditions

    M. Andersson HEAT 2008

  • • Our model: – Intermediate temperature anode supported

    SOFC (T = 700°C) – Current density, inlet temperature and

    velocity for fuel need to be assumed/specified (“input”)

    – Mass fraction, temperature distribution, heat transfer etc. are the “output”

    M. Andersson HEAT 2008

    COMSOL Multiphysics (FEM)

  • COMSOL Multiphysics (FEM)

    M. Andersson HEAT 2008

  • COMSOL Multiphysics (FEM)

    M. Andersson HEAT 2008

  • COMSOL Multiphysics (FEM)

    M. Andersson HEAT 2008

  • COMSOL Multiphysics (FEM)

    M. Andersson HEAT 2008

  • COMSOL Multiphysics (FEM)

    M. Andersson HEAT 2008

  • COMSOL Multiphysics (FEM)

    M. Andersson HEAT 2008

  • COMSOL Multiphysics (FEM)

    M. Andersson HEAT 2008

    x=0.2

  • COMSOL Multiphysics (FEM)

    M. Andersson HEAT 2008

    x=0.2

  • Future research

    • Compare different materials • Design optimisation • Add CH4, CO, CO2 to the model

    (internal reforming) • Current density as a function of

    conditions inside the FC

    M. Andersson HEAT 2008

  • Future research

    • Microscale modeling can be used to calculate the “input” parameters for macroscale model in COMSOL Multiphysics

    • Better understanding of phenomena at anode Triple Phase Boundary (TPB) – ionic, electronic, porous

    M. Andersson HEAT 2008

  • Conclusions

    • SOFCs can be described at different scales

    • Multiscale models are promising – Understanding of heat- and mass

    transport and chemical- and electrochemical reactions

    – Lower cost for development, i.e., the commercialisation of fuel cells will be promoted

    M. Andersson HEAT 2008

  • Questions ???

    Clarifications ???

    Comments ???

    M. Andersson HEAT 2008