Usually, for a PEMFC, hydrogen is stored under high pressure and released as per the requirement of the cell/stack and the excess hydrogen gas which could not be diffused my the MEA is drained out through the hydrogen outlet port.
Whether the excess hydrogen is collected in a separate tank? or it is compressed and sent again to the high pressure tank/cell?
Especially in a FCV. (Why FCV's has two hydrogen tanks?)
Thanks in advance..
Two main anode exhaust gas management system exist. The Dead-End Anode mode (DEA) and the recirculation mode. Concerning FCV, DEA mode is mainly used due to its compactness and simplicity. In this mode, the anode is closed and the hydrogen is purged to remove water and nitrogen mainly...then the low quantity of hydrogen purged go directly into the air. Another advantage of this mode is that the flow of inlet hydrogen when the anode is closed (no purge) is known and directly related to the current.
Thanks for your valuable response Soichi & Selvarani. Clarified well!
In larger systems a hydrogen circulation pump is an asset, to circulate the anode stream at twice or more of the rate of use, ( especially at low stack outputs) The anode stream can have more than 50% N2 and as long as things are well mixed, the stack will be fine. If there are local zones that are H2 depleted, the rest of the stack will drive current through starved cells, and damage them. Circulation of the anode gas insures adequate H2 over all the MEA surface area. Purging is only necessary when the overall output suffers, usually under higher loads. As per the previous comments, the purged gas is vented, it is so full of N2 and water vapor that saving it is not viable.
Firstly I'd like to thanks soichi to his complete answer. I have to add that its also depend on what you want to do with the energy and where you want to put it. In theory, the DEA cannot be helpful with FCV's because we need more autonomy to the car, so we must ensure maximum hydrogen efficiency by using recirculation mode.
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