## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### overpotentials calculation fuel cell

• Diogo Martinho
Subscriber

Hi all,

I need to define a udf where I implement butler Volmer equation. But to calculate the current density I need to calculate the overpotentials of my cell. Do you have any idea of how to get this parameter out of the electrical potential equation? Since this current density will then be used to calculate the gas phase generated at the surface.

• Aleksey Gerasimov
Subscriber

Hello Diogo,

Is there any special reason for writing a UDF instead of using our readily-available Potential or Electrolysis model? By the sound of it, you are trying to model an electrolysis or an electrowinning cell. Is that correct? The activation potentials do not necessarily need to be calculated explicitly, they can be computed inversely. It all depends on the approach.

Kind regards,

Aleksey.

• Diogo Martinho
Subscriber

Dear Aleksey,

Two main things: I would know exaclty what is implement (which I could also know by reading the book, but at the same time a lot of things would be implemented that I wouldn't want) and also to learn how to implement such UDS and UDF by myself using Fluent.

Yes, but how can I do that, or how can I learn how to do that inversely calculation? Because I understand that it would be much better to do it inversely than explicitly.

Hope this makes sense,

Diogo

• Aleksey Gerasimov
Subscriber

Hello Diogo,

It is still not clear what kind of problem you are trying to solve. I appreciate that the use of the dedicated fuel-cell modules might not be justified if you are not modelling fuel cells.

Please consider taking an advantage of the simple Potential model. It might be sufficient if you are modelling a single potential. However, typically, fuel cells and electrolyzers with membrane require a solution of two potential equations: one for protonic transfer and another one for the electronic potential.

We cannot elaborate deeper on the exact implementation details due for IP reasons, but when it comes to overpotentials, I can only say that you can use the value of potential from a previous iteration.

My recommendation would be to have a clear set of equations that you are planning to solve and then you might need to use certain submodels and assumptions in order to close the set and solve them.

Please also note that a dual-potential PEM water electrolysis model is available in 2022R2 as a beta feature, you can activate beta features to see it under the Potentail model in the Tree.

Hope this helps, Aleksey.

• Diogo Martinho
Subscriber

I am modelling an alkaline electrolyzer cell :) .

So, yes I would need two potential equations. And how will I relate Butler-Volmer as a source term of such equation.
I am trying to couple an Eulerian model with these two potential equations where Butler-Volmer would be a source term on these potential equations.

Best regards,

Diogo

• Aleksey Gerasimov
Subscriber

Hello Diogo,

Is this electrolysis cell with or without membrane, do you have porous regions in your electrolyser or is it just a couple of solid electrodes in an aqueous soluiton?

Best regards,

Aleksey.

• Diogo Martinho
Subscriber

I have porous regions. I will have one as a membrane (porous region), then the gas compartments, then catalyst layers (porous) and solid electrodes.