Role of the "Anode Exhaust" subsystem in PEMFC Simscape model and hydrogen flow path decision

Hello,

I am working with the PEM Fuel Cell (PEMFC) model in MATLAB/Simscape and I have a question about the "Anode Exhaust" subsystem.

In the model, part of the hydrogen supplied to the anode is consumed in the electrochemical reaction, and the remaining unused hydrogen can either:

1. Be recirculated through the hydrogen recirculation subsystem, or
2. Be vented out via the Anode Exhaust subsystem.

My questions are:

• What is the exact role of the Anode Exhaust subsystem in the Simscape PEMFC model?
• How does the Anode Exhaust subsystem itself work internally ?
• How does the model determine whether the unused hydrogen flows to the recirculation path or to the exhaust?
• Is this decision based on a pressure control mechanism, a flow control setting, or is it a fixed split defined in the model parameters?

If possible, I would appreciate any documentation or references explaining the internal logic of this gas flow split in the MATLAB/Simscape implementation.

Thank you in advance for your help.

https://fr.mathworks.com/help/simscape/ug/pem-fuel-cell-system.html

Recirculation subsystem

Anode exhaust subsystem

Answer 1

[Yifeng Tang]

In most PEMFC system, the anode side doesn't vent to the outside like the cathode side. Unused hydrogen are too precious to be wasted, so a recirculation system brings it back to the anode gas channel. The anode fuel supply side is therefore a "closed" system, meaning there is no regular mass transfer to the outside. As a result, as the membrane become moist during the operation, water vapor can be transported to the anode side and may condense as liquid. Water vapor can reduce the H2 mole fraction for reaction and water liquid may accumulate and clog pipelines. So it's common to see a purge system on the anode side, which is responsible to getting rid of the excess water from time to time (and waste some H2 in the process). The anode exhaust in this example model is a placeholder for that. You should be able to observe in the simulations how H2O fraction increases on the anode side and decide whether it's necessary to open the purge valve and bring the H2 concentration back up.