A Study of Two-Phase Flow in a Polymer Electrolyte Fuel Cell Tang.M.Z, K.E.Birgersson

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1 A Study of Two-Phase Flow in a Polymer Electrolyte Fuel Cell Tang.M.Z, K.E.Birgersson Engineering Science Programme, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Road, Singapore Abstract In this study, the single phase flow of gas in GDL is studied. The effect of porosity and regularity of geometry on the permeability of GDL is analyzed by using Incompressible Navier-Stokes model in COMSOL 3.4, in order to present study methods and modeling methods in two-phase flow. Finally, modeling method and prediction results of twophase flow in GDL is introduced. 2. Introduction to fuel cell Fuel Cell can directly transform fuel energy to electricity without combustion process, so fuel cell has better efficiency than normal energy generating method. There are many kinds of fuel cell, such as PEMFC (Polymer Electrolyte Membrane fuel cell), DMFC (Direct Methanol Fuel Cell), etc. Here PEMFC is studied PEM Fuel Cell PEMFC s Chemical reaction is2 4 4, Hydrogen is fed at the anode side and is oxidized. Oxygen is fed at the cathode side and is reduced. Proton generated at anode will be conducted to cathode through PEM (proton exchange membrane). The electron released by the hydrogen has to travel through outer circuit to cathode and electrical current is generated Advantage of Fuel cell PEMFC system is very clean. It is only run on pure hydrogen and only produces water. Fuel cell systems also have better efficiency than combustion engines, because it converts chemical energy in the fuel directly to the electricity 2.3. Disadvantage of Fuel Cell Besides its storage, support and control problems, water management is also a big problem for fuel cell. If the water is moved too fast from the fuel cell, electrolyte will dry out and performance diminishes. Not efficient remove of water will cause electrode flooding, which also affect the performance of fuel cell. The accumulation of liquid water in electrodes can severely hinder the performance of PEMFC; the accumulated water reduces the ability of reactant gas to reach the reaction zone. 1

2 3. Introduction to GDL GDL (Gas Diffusion Layer) is a porous media in PEMFC. Gas can pass through the pours of the GDL, so GDL assist distribution of gas. It is also a path that water can go through. Two materials are typically used as gas diffusion layers in PEMFC; carbon cloth and carbon paper. Both of them are fabricated from carbon fibers. This table and picture of Toray carbon paper GDL, cited from [1]. 4. Numerical Simulations The domain of interest is a pore network model L0=200μm long in x direction and L1=200μm long in y direction. The process to be simulated is movement of air through the GDL. Air is injected from the bottom boundary at y=0. The flow of gas inside the model is governed by the incompressible Navier-Stokes equations: t ρ μ ρ p 0 0 Where μ is the viscosity, u is the velocity, ρ is the density of air and p is the pressure. Darcy s law; k is permeability. Gravity force has been neglected. At y=0, inward velocity boundary condition is used: u =u0 The top boundary y=200μm P0=0 Pa The boundary at x=0 μm and x=200 μm are symmetric boundary. 2

3 Group1 This is regular uniform porosity geometry. Under this geometry, 3 cases are studied: GDLThickness, 2e -4 m Width, 2e -4 m Case1: Case2: Case3: Porosity, 66.75% Porosity, 59.6% Porosity, 52% Pore size, 5e -6 m Pore size, 5.5e -6 m Pore size, 6e -6 m T=60 o C; ρ=1.068 kg/m 3 ; μ=2.01e -5 Pa.s; Inlet velocity: 1e -6 to 1e -3 m/s Group2 Random arrangement GDLThickness, 2e -4 m Width, 2e -4 m Case 4 Case5 Porosity, 66.75% Porosity, 52% Pore size, 5e -6 m Pore size, 6e -6 m T=60 o C; ρ=1.068 kg/m 3 ; μ=2.01e -5 Pa.s; Inlet velocity: 1e -6 to 1e -3 m/s Mixture of different size of block Group3 Case 6 Porosity, 52% GDLThickness, 2e -4 m Width, 2e -4 m Pore size, 5e -6 m, 6e -6 m T=60 o C; ρ=1.068 kg/m 3 ; μ=2.01e -5 Pa.s; Inlet velocity: 1e -6 to 1e -3 m/s 3

4 5. Simulation Result 5.1 Regular and uniform pores Pressure Velocity Field 5.2 Irregular and single size Pores 5.3 Irregular and non-single size pores 4

5 5.4 Data analysis In this part, several graphs are plotted in order to find out how the porosity of GDL affects permeability. By Darcy s Law The graphs are plotted by using equation: The first plot is for the 3 cases in Group 1. We can see that the permeability increases with the increase of porosity. (Pa) 1.80E E E E E E E E E E E E E E 03 Series1 Case1 Series2 Case3 Series3 Case2 U (m/s) permeability 7.00E E E E E E E E+00 0% 20% 40% 60% 80% Permeability Vs. Porosity We can see that permeability is rising with the increase of porosity. 5

6 Blow is the plot for group 2, irregular and single size pore. We can see that they also follow Darcy s Law, they are straight lines. 2.00E E+01 (Pa) 1.00E E+00 Series1 Case5 Series2 Case6 0.00E E E E E E E E 03 u (m/s) Now we take out Case 3, case 5, and case 6. All the 3 cases have same porosity of 52%, graph is plotted in the same way. 30 (Pa) Series1 Case3 Series2 Case5 Series3 Case E E E E E E E 03 u (m/s) We can see from the graph above that irregularity and non-uniform will lower the permeability of GDL with same porosity. 5.5 Short Summarization 1) Permeability will decrease with the decrease of the porosity; 2) Irregular geometry also follows Darcy s Law; 3) Regular Shape has better Permeability than the irregular shape with same porosity, which means in regular geometry, effective diffusion coefficient is larger; 6

7 4) If the size of the blocks in the geometry is not uniform, the permeability will be lower than uniform geometry. 6. Introduction to two- phase flow in GDL Water management is an important factor in PEMFC. The conductivity of the proton conducting membrane is largely affected by water content. Normally the water is generated at the cathode, but it is not enough for fuel cell to keep a high conductivity. Therefore, additional water has to be supplied to cell. However, if too much water is accumulated in the porous medium, reactant transport will be hindered due to partly blocked reactant passages increases. An increased mass transfer resistance and a subsequent decrease in performance are due to partial water condensation in the porous media. The picture is cited from [1], showing water droplet formation. From the picture above, we can see images of condensation in PTFE-treated carbon paper. First, the liquid water has formed as droplets. It can be seen that over time, with greater levels of liquid water present, the droplets have connected and travelled toward areas of greater liquid accumulation. The permeability of GDL under two-phase regime is a function of time, as water will accumulate over time. Time-dependent Level Set model in COMSOL can be used to simulate the process. With level set method, we can represent boundaries and interfaces using fixed mesh. Thus we can observe the droplets formation in fuel cell. The geometry of two phase model will be similar to one phase model, but an initial interface has to be defined on the geometry. As what have been done for the one-phase flow, Darcy s Law will be studied. Permeability vs. time curve can be plotted. How the permeability change with time will be studied. Moreover, the effect of geometry and porosity on the formation of droplet will also be studied. 7

8 6.1Prediction The water will attach the wetted wall and overtime, the water droplet will be formed, according to experiments. The water droplet will change the effective geometry or the GDL, porosity will decrease, and effective diffusitivity will be changed. Further as the accumulation of the water, the regularity of the geometry will also be affected. All the effects will result in decrease in permeability and large resistivity for gas to travel. References: [1] B. Sundén, M. Faghri, Transport phenomena in Fuel cells, WIP Press [2] COMSOL 3.4 model Library. [3]N. Holmström, J. Ihonen, A. Lundblad, and G. Lindbergh, The influence of the Gas diffusion layer on water management in polymer electrolyte fuel cells, [4] Teng Zhang, E.Birgersson. Analysis of the Gas Diffusion Layer in a PEM Fuel Cell, [5] Satish Kandlikar, RITThomas Trabold, GMJeffrey Allen, Visualization of Fuel Cell Water Transport and Characterization under Freezing Conditions, MTU, Feb