Learning Forum

[Raúl Sotillo]

Hello everyone!

I am trying to simulate a planar SOEC (Solid Oxide Electrolyser Cell) where the inlet is H2O, to produce H2 (cathode) and O2 (anode) at 800ºC (1073K). I am using the Fuel Cell and Electrolysis module, and defining it for Electrolysis. I am using ANSYS 2022.

The layers I am using are, from top to bottom (and represented on the figure).
- Cathode Interconnector
- Cathode Channel (defined by the gap in the cathode interconnector)
- Cathode Gas Diffusion Layer
- Cathode Active Layer
- Electrolyte
- Anode Active Layer
- Anode Gas Diffusion Layer
- Anode Channel (defined by the gap in the anode interconnector)
- Anode Interconnector

It is a simple geometry, 20x20cm, with straight channels. The channels are flowing in cross flow, so the direction of the anode channel is perpendicular to the direction of the cathode channel.

I have not defined my own source terms, but instead, and according to the ANSYS Guide, I have selected where each one of them takes place (in which area) and I expect those terms to change accordingly for an Electrolyser.

Also, the voltage I am defining it with values of +1.3V for the tab in the Anode, and 0V for the tab in the Cathode, the upper and the bottom parts of the interconnectors, defining like that the voltage difference. I've set the OCV Voltage to +0.84V.

Nevertheless, when I run the simulation, and even though it successfuly generates H2 and O2, I get very high values for the Current density (A/m2) in areas where it should be low. I am trying to replicate and compare with the following article: http://dx.doi.org/10.1016/j.apenergy.2014.04.104

The current should have a different profile. Attached you can find the image, comparing at left the article results and on the right my results. Also, my values are clearly different.

- Am I considering right the signs on the source terms? Should I edit something for the electrolysis mode in this Module?
- I am sure there has to be a problem with the sign of the values (+/-) somewhere. Anyone has any clue?
- Are there any boundary condition I haven't set up?

Also, the quantity of H2 and O2 produced ans the Temperature are very much dependent on the Initial conditions for simulating... Making the results not very trustable.

I am 3 weeks away of presenting my thesis and a small help in this matter would mean so much to me. I could also menction that help on my thesis :)

Thank you so much! And cheers from Spain.
Raúl

[Aleksey Gerasimov]

Hello Raul,

The main confusion for SOEC comes from a different definition of anode and cathode between fuel cells and electrolyzers. We have a chapter in the User Guide that explains this point. Unlike in the PEMWE and AEC models where notation is conventional, The SOEC notation inherited the very same notation as the SOFC. Naturally, it might create some confusion for the placement of potential boundary conditions.

The best way, to look into this issue consistently is by looking at the location of the OER (Oxygen Evaluation Reaction) and HER ( Hydrogen Evaluation Reaction). Please also consider prescribing voltage directly in the FC&E panel. The software will apply the voltage consistently, and then when everything becomes clearer, you can change to prescribing the potential boundary conditions directly at terminals.

Hope this helps and please let me know if you have any further questions.

Please note that the current densities in the order of 0.5 A/cm2 (5000A/m2) are perfectly achievable in solid-oxide electrolysis and fuel cells.

Kind regards,

Aleksey.

 

[Raúl Sotillo]

Hello Aleksey. Thanks so much for the answer. This solves a few months of doubts about this; in the guide, it was somehow explained but I couldn’t properly catch it. I also dropped a question on that topic here.

How can I set up the voltage from the PC&E Panel? I can only see how to set up the OCV, which in my case I’ve set up to +0.84V. And then I always indicate that on the Boundary Conditions.

Kind regards,
Raúl

[Raúl Sotillo]

 

 

 

[Aleksey Gerasimov]

Hello Raul,

Yes, you are right. For the Unresolved SOFC/SOEC approach you can specify voltageor current b.c.'s directly in the FC panel, but the FC&E module does not offer such possibility yet. I have logged an enhancement request for this issue.

For the time being, please anchor the voltage to 0V at the hydrogen evolution side (Cathode - in conventional notation, Anode - in the FC&E panel), then please set the required voltage value, say 1.4V at the oxygen evolutoin side ( Anode - in conventional notaion, Cathode - in the FC&E panel)

Hope this helps,

Aleksey.

[Raúl Sotillo]

Hi Aleksey, Thank you for the answer, it helps a lot.

The program calculates V_total = V_cathode (conventional, H2 side) - V_anode (conventional, O2 side). Therefore, when I include a positive value for the potential, then my V_total is negative. This can generate problems since my OCV is designed as positive (+0.84V).

Shall I consider a negative voltage of, let's say, -1.4 V in the O2 side? Maybe I should state that the OCV is -0.84V?

I hope you can hep me with this matter.

Thank you,

Raúl