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Error on COMSOL model gallery (SOFC)??

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Old   June 9, 2010, 17:49
Question Error on COMSOL model gallery (SOFC)??
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Diego Ubeda
Join Date: Mar 2009
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Hello everybody!!!

I am a COMSOL rookie that started working not so long ago on PEM fuel cell modelling. The thing is that, in order to create my own problem, I have taken as references the different examples about fuel cells and mass transport that I could find in the modelling guide. Although I got to simulate a full 3D PEM fuel cell, my problems started when I tried to verify my results. I realized then that if I calculated the total oxygen that is consumed in the cathode side (in mol/s) and did the same with the hydrogen in the anode, I could see that the hydrogen that is consumed is far from being twice the oxygen that has reacted (as it should be, according to the reaction stoichometry). I tried to check if I was doing something wrong but was surprised when I did the same test with the file "sofc_unit_cell" from the model gallery, where a SOFC is simulated. In this case, we have oxygen, water and nitrogen in the cathode and hydrogen and water in the anode and by calculating the inlet and outlet molar flows I could see that it happens more or less the same, with the difference that in this case hydrogen is being generated from the inlet to the outlet instead of being consumed!! You can also check this by representing the molar flux of each species through the flow channel. Oxygen molar flux decreases from the inlet to the outlet, which is ok. However, both water and nitrogen are consumed as they go through the flow channel. I don't find any sense to this because water doesn't react in the cathode (SOFC) and nitrogen is inert. About the anode side, hydrogen is generated!!

Summarizing, I see two problems:

- Results don't agree mass balances
- Some species are generated when they should be inert and hydrogen is generated when it should be consumed

I calculate the inlet and outlet flow of the species i like this: Gas_velocity*pressure/(R*T)*yi (integrating at the inlet or at the outlet boundary). We can represent the molar flux of any species using the same expression.

Am I right? Or I don't have idea of what I am talking about?
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