what does it means when we consider two mode for load current change in PEM FC .
Transient and steady state . What do these attached pictures means as the load current?
You want to look at how the fuel cell behaves under different conditions. Steady state mode is the preferred mode of operation of a FC. Under this mode you get the actual performance of the system. In dynamic mode, or in other words transient mode, process that are short lived can be observed, such as current spike etc, that can very much determine the life span of a FC.
Steady state mode exists an optimal operational condition, but transient state tests its short time perforance not an averge one.
Indeed this is a language abuse. You cannot say "steady state load current variation". Steady state means that the dynamical system at hand is at steady state, i.e. all dynamics are dead (dx/dt = f(x, u, t) = 0).
If the operational conditions change, then there is a transient stage to a new steady state. Also, if the load changes and the Fuel Cell is controlled (if there is an open loop controller), then the controller will stabilize the FC to a new steady state such that the load set point is satisfied.
It is always interesting to study both modes. Varying the operational conditions, waiting for steady state allows for instance to plot the map current or power vs. potential. Transient modes are interesting in that you cannot observe dynamical effects at steady state.
Finally following the previous comment, there are sets of optimal operating conditions, whereby, e.g., for a given load, you have maximal electrical efficiency. But there are also optimal transient conditions, whereby you have optimal control trajectories which will bring, for instance, optimally and dynamically the FC to optimal steady state conditions.
If you want to fully understand these concepts I suggest you to study textbooks on dynamical systems, process control and process optimization in the general framework (these are multi disciplinary, mathematically sound, topics), that is, not only applied to FC or any other process
Hello,
If I understand correctly your question. The steady state is when the answer given to a load change will be stabilized. The transient is when the time between the moment you change the desired value and the steady state. Concerning FC system, the transient response is very important, since a bad transient response of the moto-compressor group, for example, can lead to temporary oxygen starvation which will degrade the FC. This is due to the fact that the FC system has a low transient response (few second), mainly because of the air supply system or the pipes.
I hope it helped you a little
Regards
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