CFD Analysis of PEM Fuel Cell

Transcription

1 CFD Analysis of PEM Fuel Cell Group Seminar Munir Khan Division of Heat Transfer Department of Energy Sciences Lund University

2 Outline 1 Geometry 2 Mathematical Model 3 Results 4 Conclusions I 5 Pore Scale Modelling 6 Conclusions II Group Seminar, March 31, 2011/ 1

3 Geometry Dimension a Value z 50 x 2.4 y 1.62 t cat 0.01 t mem t gdl a all dimensions are in mm. Group Seminar, March 31, 2011/ 2

4 Water Saturation The common approach for modelling liquid water transport is the direct implementation of an empirical and semi-empirical correlations. Leverett suggested a semi-empirical approach using dimensional analysis to represent the capillary pressure - saturation behaviour of the porous media in the following non-dimensional form; P c γ ( ) k 1/2 = f (s) ε Udell later introduced a contact angle term and proposed the following form for f (s); J (s) = 1.417(1 s) 2.120(1 s) (1 s) 3 Group Seminar, March 31, 2011/ 3

5 Extended Leverett J-Function In terms of modelling the capillary transport in PEM fuel cells porous media, Pasaogullari and Wang proposed an extended version of Laverett J-function to account for the hydrophobicity of the porous media. ( ε ) 1/2 P c = γ cosθ J (s) k { 1.417(1 s) 2.120(1 s) (1 s) 3 if θ < 90 J(s) = 1.417s 2.120s s 3 if θ > 90 Group Seminar, March 31, 2011/ 4

6 Shortcomings of Extended Laverett Approach Even though the Extended Laverett Approach provides a good starting point, but; Origin of this approach doesn t represent the complex heterogeneous structure of PEM fuel cells. It only uses the average pore radius (k/ε) 0.5 and neglects the tortuous nature of the porous media. It is also limited in its ability to describe the capillary characteristics of the porous media with high grades of heterogeneity or mixed wettability. Group Seminar, March 31, 2011/ 5

7 Validated Capillary Pressure Based on the results of the experimental evaluation, a validated capillary pressure function is presented as; ( ) ( P c = γ 2 0.4C ε ) 1/2 K (snw ) T k wt%[ (wt%) snw snw 3 ] lns nw 0 < s nw < 0.50 K(s nw ) = wt%[ (wt%) 12.68snw snw 3 ] lns nw 0.50 < s nw < 0.65 wt%[ (wt%) 14.1snw snw 3 ] lns nw 0.65 < s nw < 1.00 Group Seminar, March 31, 2011/ 6

8 Comparison SGL 24BC Capillary Pressure, kp a P c = γ ( ɛ k ) 1/2 K (snw) P c = γ ( ɛ k ) 1/2 J (snw) Non-Wetting Phase Saturation, s nw Group Seminar, March 31, 2011/ 7

9 Comparison SGL 24CC Capillary Pressure, kp a P c = γ ( ɛ k ) 0.5 K (snw) P c = γ ( ɛ k ) 0.5 J (snw) Non-Wetting Phase Saturation, s nw Group Seminar, March 31, 2011/ 8

10 Comparison SGL 24DC Cappilary Pressure, kp a P c = γ ( ɛ k ) 0.5 K (snw ) P c = γ ( ɛ k ) 0.5 J (snw ) Non-Wetting Phase Saturation, s nw Group Seminar, March 31, 2011/ 9

11 Species Diffusion in Porous Media Usually the Bruggeman relation is applied to correct the species diffusion. f (ε) = ε τ Tomadakis and Sotirchos estimated the effective diffusivity using Monte Carlo Simulations as; [ ] ε α α = εp f (ε) = ε 1 ε p α = in-plane through-plane Group Seminar, March 31, 2011/ 10

12 Compressional Effects on Porosity ε c = [ ] ε o 1 + s TR Catalyst Layer Diffusion Layer Compression (MPa) Group Seminar, March 31, 2011/ 11

13 Thermal Conductivity Generally volume averaged thermal conductivity is used in CFD simulations, calculated as; k eff = εk v + (1 ε)k s But, this relation highly over predicts the temperature gradients. The proposed alternative is the volume weighted harmonic average; k s k v k eff = εk s + (1 ε)k v Group Seminar, March 31, 2011/ 12

14 Numerical Technique Pressure-Velocity Coupling Scheme SimpleC Spatial Discretization of variables Pressure Body Force Weighted Other QUICK Mesh Adoption Dynamic Mesh Adoption based on Pressure and Water Saturation Gradients (total cells = 0.47M) Convergence Criteria All variables < anode R andv = cathode R catdv Anisotropic electrical conductivity Anisotropic permeability of the porous media Group Seminar, March 31, 2011/ 13

15 Water Saturation 0.5 V (a) 10 mm (b) 18 mm (c) 28 mm (d) 40 mm Group Seminar, March 31, 2011/ 14

16 Temperature Distribution 0.5 V (a) 10 mm (b) 18 mm Group Seminar, March 31, 2011/ 15

17 Temperature Distribution 0.5 V (c) 28 mm (d) 40 mm Group Seminar, March 31, 2011/ 16

18 Oxygen (O 2 ) Distribution 0.5 V (a) 10 mm (b) 18 mm (c) 28 mm (d) 40 mm Group Seminar, March 31, 2011/ 17

19 Hydrogen (H 2 ) Distribution 0.5 V (a) 10 mm (b) 18 mm (c) 28 mm (d) 40 mm Group Seminar, March 31, 2011/ 18

20 Water Saturation 0.3 V (a) 10 mm (b) 18 mm (c) 28 mm (d) 40 mm Group Seminar, March 31, 2011/ 19

21 Temperature Distribution 03 V (a) 10 mm (b) 18 mm Group Seminar, March 31, 2011/ 20

22 Temperature Distribution 0.3 V (c) 28 mm (d) 40 mm Group Seminar, March 31, 2011/ 21

23 Oxygen (O 2 ) Distribution 0.3 V (a) 10 mm (b) 18 mm (c) 28 mm (d) 40 mm Group Seminar, March 31, 2011/ 22

24 Hydrogen (H 2 ) Distribution 0.3 V (a) 10 mm (b) 18 mm (c) 28 mm (d) 40 mm Group Seminar, March 31, 2011/ 23

25 Polarization Curve 1 Previous Case Current Simulations 0.8 Voltage (V) Current Density ( A/cm 2) Group Seminar, March 31, 2011/ 24

26 Conclusions Validated water saturation approach provides much better picture as compared to the conventional Leverett approach. The capillary pressure is decreased by increasing the temperature. Compression of the diffusion media results in increased capillary pressure. The porosity decreases by increasing the compression. No effects have been noticed for the two approaches for the thermal conductivities of the porous media. Anisotropic species diffusion and electrical conductivity result in decreased current density. Group Seminar, March 31, 2011/ 25

27 Pore Scale Modeling Group Seminar

28 Geometry (a) Random Pt Distribution (b) Calculation Domain Group Seminar, March 31, 2011/ Munir Khan, Div. of Heat Transfer/

29 Electro-Chemical Sources Figure: Both the source terms are in kg m 3 s Group Seminar, March 31, 2011/ Munir Khan, Div. of Heat Transfer/

30 Species Distribution in Nafion (a) Water Vapor Group Seminar, March 31, 2011/ Munir Khan, Div. of Heat Transfer/ (b) Oxygen

31 Water Distribution in Pore Space Group Seminar, March 31, 2011/ Munir Khan, Div. of Heat Transfer/

32 Oxygen Distribution in Pore Space Group Seminar, March 31, 2011/ Munir Khan, Div. of Heat Transfer/

33 Conclusions II There is no protonic potential drop because the assumed thickness is too large. The conductivity of the Nafion is a function of thickness ( κ = κ N117 (l N117 l) 10 5). Gradient Source terms are more stable than volumetric sources. More realistic approach of species diffusion. Group Seminar, March 31, 2011/ Munir Khan, Div. of Heat Transfer/

34 Thank You