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Chemical reacting transport phenomena and multiscale
models for SOFCs
Martin AnderssonDept. of Energy sciencesLund University, Sweden
Heat Transfer 2008, 9-11 July, Maribor
Updated versionfor 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
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)
M. Andersson HEAT 2008
• Working temperature: ~80°C • Current stack cost: 2000 $/kW• Electrolyte: Solid polymer• Transportation sector
– Fast start-up time– High power to weight ratio
• Vulnerable to CO poisoning
Phosphoric acid (PAFC)
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• Working temperature: ~200°C • Overall Efficiency: 85 %
– 40% electricity• Electrolyte: Phosphoric acid
Price: 4000$/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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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 (<nm)
– Atom/Molecular level• Mesoscale • Macroscale (mm>)
– 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.
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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)
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.
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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
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)
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
M. Andersson HEAT 2008