Chemical reacting transport phenomena and multiscale models for SOFCs

Transcription

1 Chemical reacting transport phenomena and multiscale models for SOFCs Updated version for group seminar Martin Andersson Dept. of Energy sciences Lund University, Sweden Heat Transfer 2008, 9-11 July, Maribor

2 Agenda Introduction to Fuel Cells (FC) Market potential Different types Modeling at different scales Current and Future research Conclusion

3 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

4 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

5 from: IEA 5

6 from: IEA 6

7 Fuel Cells

8 Alkali Fuel Cell (AFC) Working temperature: C Lifetime: 8000 h Efficiency: 60 % Electrolyte: Space missions Vulnerable to CO 2 poisoning Potassium hydroxide

9 Proton Exchange Membrane (PEMFC) 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

10 Phosphoric acid (PAFC) Working temperature: ~200 C Overall Efficiency: 85 % 40% electricity Electrolyte: Phosphoric acid Price: 4000$/kW Due to their platinum catalyst

11 Molten carbonate (MCFC) Working temperature: >650 C Combined with gas turbine Overall Efficiency 90% 60% electricity Stationary use Internal reforming possible

12 SOFCs Working temperature: C Combined with gas turbine Overall Efficiency: >85 % 70% electricity Stationary use Internal reforming is possible Current Cost: $/kw Vulnerable to sulfur poisoning

13 SOFC - reactions exothermic exothermic exothermic endothermic exothermic

14 Modeling at different scales System scale Component scale Flow/diffusion morphologies Material structure Functional material levels ~10 2 m ~10 1 m ~10-3 m ~10-6 m ~10-9 m

15 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

16 Modeling at different scales Microscale Theoretical knowledge Macroscale Empirical data

17 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]

18 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.

19 Modeling at microscale Prediction of concentration over potential Dusty Gas Model (DGM) Ficks Model (FM) Stefan-Maxwell Model (SMM) Does not consider Knudsen diffusion Possible to use SMM in COMSOL Multiphysics

20 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)

21 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)

22 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.

23 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

24 SOFC multiscale FC operation depends on interaction between: Mass transport Heat transfer Electrochemical/chemical reactions Multi-phase fluid flow

25 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

26 COMSOL Multiphysics (FEM) Define a geometry (1D, 2D, 3D) Boundary conditions Subdomain conditions Adjust mesh Time dependent / Stationary conditions

27 COMSOL Multiphysics (FEM) 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

28 COMSOL Multiphysics (FEM)

29 COMSOL Multiphysics (FEM)

30 COMSOL Multiphysics (FEM)

31 COMSOL Multiphysics (FEM)

32 COMSOL Multiphysics (FEM)

33 COMSOL Multiphysics (FEM)

34 COMSOL Multiphysics (FEM) x=0.2

35 COMSOL Multiphysics (FEM) x=0.2

36 Future research Compare different materials Design optimisation Add CH 4, CO, CO 2 to the model (internal reforming) Current density as a function of conditions inside the FC

37 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

38 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

39 Questions??? Clarifications??? Comments???