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

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

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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

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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

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5

from: IEA

6

from: IEA

Fuel Cells

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Alkali Fuel Cell (AFC)

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

Proton Exchange Membrane (PEMFC)

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• 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)

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

– 40% electricity • Electrolyte: Phosphoric acid

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

Molten carbonate (MCFC)

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• 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

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SOFC - reactions

exothermic

exothermic

exothermic

endothermic

exothermic

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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

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Modeling at different scales • Microscale ()

– Global flow field – Empirical factors from mirco/mesoscale

can be used for macroscale modeling

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Modeling at different scales • Microscale

– Theoretical knowledge • Macroscale

– Empirical data

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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]

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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.

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Modeling at microscale

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

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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)

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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)

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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.

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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

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SOFC multiscale • FC operation depends on interaction

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

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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

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COMSOL Multiphysics (FEM)

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

conditions

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• 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”

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COMSOL Multiphysics (FEM)

COMSOL Multiphysics (FEM)

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COMSOL Multiphysics (FEM)

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COMSOL Multiphysics (FEM)

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COMSOL Multiphysics (FEM)

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COMSOL Multiphysics (FEM)

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COMSOL Multiphysics (FEM)

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COMSOL Multiphysics (FEM)

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x=0.2

COMSOL Multiphysics (FEM)

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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

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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

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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

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Questions ???

Clarifications ???

Comments ???

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