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Fuel Cell (PEMFC) CFD Simulation

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The fuel cell used in this simulation is a type of polymer electrolyte member fuel cell (PEMFC).

 

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Description

PEMFC Fuel Cell Problem Description

The present problem is going to simulate a fuel cell. The fuel cell used in this simulation is a type of polymer electrolyte member fuel cell (PEMFC). The model consists of two main cathode and anode segments, each of which has four layers, including a flow collector, a flow channel, a gas distribution area and a catalytic section, and the space between the anode and cathode layers are filled by the polymer membrane. In addition, since in fuel cell modeling, various types of gases react, the species transport module is also automatically activated in the simulation. The hydrogen gas enters the anodic channel inlet and the oxygen species enters the cathode channel inlet to allow the corresponding reactions to occur in the fuel cell and the polymer membrane is the pathway for the gas species to pass through.

PEMFC Fuel Cell Problem Description

Current collectors are made of solid material with thermal energy sources and electrical potential. The flow channels carry a mixture of gaseous species including oxygen and hydrogen, and water. The catalytic part consists of a porous medium with a porosity coefficient of 0.5 and contains mass sources, thermal energy, electrical potential, proton potential, saturated water, hydrogen, oxygen, and water. The gaseous diffusion zone comprises a porous medium with a porosity coefficient of 0.5 and contains mass sources, thermal energy, electrical potential, saturated water, and hydrogen, oxygen, and water gaseous species. The polymer membrane region also consists of a porous medium with a porosity coefficient of 0.5 and has thermal energy sources and proton potentials.

Also, since the water content within the model must be zero, a fixed value is used to define the value of water equal to zero. The purpose of this study is to investigate the fluid behavior and thermal conductivity of a polymer fuel cell and its effect on the mass fraction of gaseous species and the amount of electricity produced in the cell.

Assumption

Several assumptions have been used for the present simulation:

The simulation is steady-state, the solver is pressure-based, and the gravity effect is ignored.

Geometry & Mesh

The 3-D geometry of the present model is designed by the Design Modeler software. The present model has a symmetrical structure and consists of nine regions of seven specific fluid zones (cathode and anode flow channel, cathode and anode gaseous region, cathode and anode catalytic domain and polymeric membrane) as well as two solid zones. The solid zone consists of a cathodic and anodic current collector. The mesh of the present model is carried out by ANSYS Meshing software. The mesh type is structured and the element number is 142000.

CFD Simulation

Summaries of the problem definition and problem-solving steps are presented in the table:

Models
Laminar Viscous model
on Energy
species transport Species model
H2, O2, N2, H2O (v), air species
PEM

FC

Fuel cell and electrolysis models
Boundary conditions
Velocity inlet Inlet type
2 m.s-1 velocity magnitude anode
353 K temperature
O2 : 0 H2 : 0.7 species mass fractions
H2O : 0.3 N2 : 0
2 m.s-1 velocity magnitude cathode
353 K temperature
O2 : 0.2 H2 : 0 species mass fractions
H2O : 0.14 N2 : 0.66
Pressure outlet Outlet type
0 Pa gauge pressure anode
0 Pa gauge pressure cathode
wall Walls type
0 W.m-2 heat flux cc-an, wall-an-cc, wall-ca-cc, wall-membrane. wall-gdi-an, wall-gdl-ca, wall-cl-an, wall-cl-ca
0 electric potential
0 protonic potential
0 W.m-2 heat flux cc-ca
0.5 electric potential
0 protonic potential
Simple (PEMFC) Pressure-velocity coupling
second order pressure Spatial discretization
second order upwind density
second order upwind momentum
second order upwind H2
second order upwind O2
second order upwind H2O
second order upwind N2
second order upwind energy
first order upwind electric potential
first order upwind protonic potential
first order upwind water saturation
first order upwind water content
Initialization (PEMFC)
Standard Initialization method
0 m.s-1 velocity (x,y,z)
0.35 H2
0.1 O2
0.22 H2O
0.33 N2
353 K temperature
0 electric potential
0 protonic potential
0 water saturation
0 water content

PEMFC Fuel Cell

In the present simulation, a PEMFC type fuel cell is used. A schematic diagram of this type of fuel cell, including its constituents and the type of reactions present therein.

fuel cell

The boundaries and areas or zones created in the model with their chosen symbol are as follows:

anode gas channel

(PEMFC)

zone (fluid) an-flow
cothode gass channel zone (fluid) ca-flow
anode catalyst layer zone (fluid) cl-an
cathode catalyst layer zone (fluid) cl-ca
anode gas diffusion layer zone (fluid) gdl-an
cathode gas diffusion layer zone (fluid) gdl-ca
electrolyt membrance zone (fluid) membrane
anode current collector zone (solid) an-cc
cothode current collector zone (solid) ca-cc
up of anode current collector wall cc-an
buttom of cothode current collector wall cc-ca
sides of anode current collector wall wall-an-cc
sides of cothode current collector

(PEMFC)

wall wall-ca-cc
between anode gas channel & anode current collector wall wall-an-cc-an-flow
between cathode gas channel & cathode current collector wall wall-ca-cc-ca-flow
between anode catalyst layer & anode current collector wall wall-an-cc-cl-an
between cathode catalyst layer & cathode current collector wall wall-ca-cc-cl-ca
sides of anode catalyst layer wall wall-cl-an
sides of cathode catalyst layer wall wall-cl-ca
sides of anode gas diffusion layer wall wall-gdl-an
sides of cathode gas diffusion layer wall wall-gdl-ca
sides of electrolyt membrance wall wall-membrane

To activate the fuel cell equations, you need to enter the following commands in the Fluent Software console window to add Fuel Cell and Electrolysis (PEMFC) to the model’s section:

Boundary Condition

Since no proton current flows out of the fuel cell at the outer boundaries, the zero flux boundary condition for the membrane potential (φmem) applies to all external boundaries. For the solid potential (φsol), there are external boundaries on the cathode and anode which are in contact with the external electrical circuit and only the electrical current generated through these boundaries passes through the fuel cell. On other external boundaries, there is a zero flux boundary condition for the solid potential. For the outer contact boundaries, a constant value for φsol should also be used, such that if a value of zero is assumed for the anode segment, the positive value defined in the cathode segment is considered as the cell voltage.

(PEMFC)

Thus, to define the electrical potential and the proton potential in all the exterior walls of the present model, a specific flux equal to zero is used, while only for the two exterior walls above the anode collector and the bottom of the cathode collector, which determine the potential difference of the circuit in the cell, only the specific value is used to define the electrical potential. As stated above, the potential value in the anode is assumed to be zero and as a result, only 0.5 is defined for the cathode electrical potential. Also at the anode inlet, the mass fraction of hydrogen is 0.7 and water is 0.3 and oxygen and nitrogen are zero; while at the cathode inlet, the mass fraction of hydrogen is zero and water is 0.14 and nitrogen is 0.66 and oxygen is 0.2.

The following figure shows the fuel cell model in the present simulation with boundary conditions applied to it:

PEMFC module settings

The source terms in the potential equations are obtained by the following equations:

(PEMFC)

In the above relation, jref denotes the reference current density or ref. current density (A.m-2), [ ] and [ ]ref respectively represent the local gas species concentration and the reference concentration or ref. concentration (kmol.m-3), ɣ denotes concentration exponen (dimensionless), 𝛂 denotes the exchange coefficient (dimensionless), and F denotes Faraday constant of 9.65 * 107 C/kg mol. The values of these parameters are entered in the software for both the cathode and the anode section. These values are entered in the software as follows:

Cathode Anode (PEMFC)
0.004 30 ref. current density (A.m-2)
0.00086 0.04 ref. concentration (kmol.m-3)
1 0.5 concentration exponent
2 2 exchange coefficient

Also, η as the local surface over-potential is equal to the potential difference between the solid potential (φsol) and the membrane potential (φmem) as defined by the following equations:

In the above relation, Voc is called the open-circuit voltage equivalent to the electrical potential obtained by passing through the anode to the cathode portion, which is usually defined between 0.9 and 1.2.

(PEMFC)

The reactions behave as surface reactions in the two catalytic layers, and it is assumed that the diffusion flux of each reaction species is balanced by the product rate.

In the above relation, Di is mass diffusion of gaseous species or ref. diffusivity (m2.s-1) and yi represent the mass fraction of any gas at the center of the cell or surface.

PEMFC Fuel Cell

Joule Hating option indicates resistant heating, Reaction Heating option indicates energy generated in the chemical reaction, Bulter-Volmer Rate option determining the amount of flow transfer in catalytic layers, Membrane Water Transport option is the amount of water transfer through the membranes, and the Multiphase option is used for cases where fluid transport within the gas permeable layers is estimated.

In the header settings for the Anode and Cathode in the fuel cell module, areas related to building a fuel cell must be defined using the Zone or areas in the model. A fuel cell consists of four areas, including the Current Collector, the Flow Channel, the Porous Electrode, and the catalytic area (TPB layer), each of which is for both the cathode and anodic portions. Thus, according to the symbols already mentioned for naming districts, the area should be defined for the zones in the following headings:

Cathode anode  PEMFC
ca-cc an-cc current collector
ca-flow an-flow flow channel
gdl-ca gdl-an porous electrode
cl-ca cl-an TPB layer (catalyst)

It should be noted that since the two types of gaseous diffusion and the catalytic zone are porous media, the parameters specific to the porous media including porosity coefficient and viscous resistance can be defined when defining these regions.

In the Electrolyte header, the membrane area of the fuel cell must be defined, which is the center of the model building. In the Zone section, the Membrane area should be defined.

The electrical conductivity of the fuel cell membrane portion is calculated as follows:

In the above relation, β and ω represent the coefficient of the protonic conduction coefficient and the protonic conductance exponent, respectively. λ also indicates the amount of water in the cell (water produced) or the water content during the simulation.

PEMFC Fuel Cell

The Reports tab should define the area of the cross-sectional area of the central membrane or electrolyte in square meters, which is 2 * e-5 m2 in the present model. The electrolyte area is only used to calculate the Current Density in a fuel cell. Also, in the section of External Contact interfaces of the model, the outer surfaces or walls of the cathode and anode are defined so that according to the designation in this model, the walls or surfaces of the cc-an and cc-ca are defined as external contact surfaces. These surfaces are used to report the amount of voltage in the two ends of the fuel cell.

The Advanced tab is used to define specific fuel cells; for example, Contact Resistivity is used to deal with some of the cell contact surfaces, a jump in the electrical potential caused by incomplete conduction. The Coolant Channel mode is used for cases that the cooling channel exists in the fuel cell, and the Stack Management mode is used when multiple fuel cell units are combined to increase efficiency.

 

All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.

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