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Serious error on SOFC from the model gallery?
Posted Jun 9, 2010, 12:57 p.m. EDT 3 Replies
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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 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?
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 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?
3 Replies Last Post Jun 11, 2010, 4:49 a.m. EDT
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Posted:
1 decade ago
Hi
No, you are wrong in your opinions. Oxygen is consumed and water is produced at cathode (SOFC principle), the nitrogen mass fraction is just the mass fraction balance (1-wO2- wH2Oc).. On anode hydrogen is consumed (SOFC principle, counter flow case) and the water mass fraction is just the mass fraction balance (1-wH2Oa).
Regards
Naveed
No, you are wrong in your opinions. Oxygen is consumed and water is produced at cathode (SOFC principle), the nitrogen mass fraction is just the mass fraction balance (1-wO2- wH2Oc).. On anode hydrogen is consumed (SOFC principle, counter flow case) and the water mass fraction is just the mass fraction balance (1-wH2Oa).
Regards
Naveed
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Posted:
1 decade ago
Hi,
First of all, thank you very much for your reply.
Ok, maybe I was wrong when I said that water should not be produced at the cathode, but I knew that hydrogen is consumed at the anode, so I think I didn't explain myself very well.
The thing is that I don't like to watch mass fractions because in my opinion they can lead to a misinterpretation. I mean, hydrogen mass fraction can drop from the inlet to the outlet because of two different reasons:
- Hydrogen is being consumed
- Water is being generated and then its mass fraction grows, but it doesn't mean necessarily that hydrogen is being consumed (I think that this is what is happening in the SOFC example).
That's why I like to analyze molar fluxes.
Tomorrow I will try to attach some figures to explain better what I'm trying to say and see if you (or someone else) can help me.
Thank you very much again!!!
First of all, thank you very much for your reply.
Ok, maybe I was wrong when I said that water should not be produced at the cathode, but I knew that hydrogen is consumed at the anode, so I think I didn't explain myself very well.
The thing is that I don't like to watch mass fractions because in my opinion they can lead to a misinterpretation. I mean, hydrogen mass fraction can drop from the inlet to the outlet because of two different reasons:
- Hydrogen is being consumed
- Water is being generated and then its mass fraction grows, but it doesn't mean necessarily that hydrogen is being consumed (I think that this is what is happening in the SOFC example).
That's why I like to analyze molar fluxes.
Tomorrow I will try to attach some figures to explain better what I'm trying to say and see if you (or someone else) can help me.
Thank you very much again!!!
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Posted:
1 decade ago
Well, I have to apologize for writing such an alarming post. After analyzing carefully all the fluxes, I've come to the conclusion that hydrogen is not generated but consumed at the anode, which means good news for me.
However, I still say that by integrating total fluxes at the inlets and outlets we can prove that molar flows we obtain don't agree with mass balances. You can have a look to the file I've attached.
¿Anybody can make any comment that helps me to clarify this?
However, I still say that by integrating total fluxes at the inlets and outlets we can prove that molar flows we obtain don't agree with mass balances. You can have a look to the file I've attached.
¿Anybody can make any comment that helps me to clarify this?
Attachments: