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In the setting of large-scale electricity generation, I was wondering why one would want to burn hydrogen in a gas turbine instead of using it to power a fuel cell. Aren't fuel cells more efficient because they're not bound by the Carnot limit? Two reasons I can think of are:

  1. Fuel cells are limited in their power output
  2. Fuel cells can't be started as quickly as gas turbines

Are these points correct and relevant? I guess the second point is only relevant for peaking power plants.

In a similar vein, I've read in one of Smil's books that fuel cells are somehow limited (in what sense I'm not sure anymore but I believe it was power output) by their "active area". I'm having difficulties learning more about this "active area". Can someone maybe expand on that?

Thanks a lot.

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I've read in one of Smil's books that fuel cells are somehow limited (in what sense I'm not sure anymore but I believe it was power output) by their "active area". I'm having difficulties learning more about this "active area". Can someone maybe expand on that?

I've not worked directly with large-scale fuel cell installations (never heard of one, actually) but I did some undergraduate lab work involving methanol fuel cells which seem to share the same principle. I imagine the limiting factor that probably causes problems with large scale-up is increasing mass transport to and from surface area. Unlike a turbine which acts on volumes of fluid, a fuel cell acts on area fluxes of fluid.

Consider a bizarre branch of history where fuel cells were invented and perfected long before combustion turbines, I imagine engineers would be amazed at how a "combustible" fuel in a turbine can act as its own catalyst once it reaches sufficiently high temperature and pressure. In contrast, fuel cells require careful slow flow of fluid through a fragile membrane which acts as a sort of two-dimensional catalyst that facilitates the proton-stripping reaction. Fuel cell membranes can't be cut up and dissolved into the fluid bulk to facilitate the reaction; the membrane must form a perfect electrically-conductive seal to prevent mixing of the electron-donating fuel and the electron-accepting oxidizer fluids.

Since each unit of energy produced requires one unit volume of fuel fluid pass through two stagnant boundary layers on both sides of the membrane, the amount of energy lost to friction in the supporting piping is significant no matter how large the array of membranes. In contrast, a unit of fluid passing through a gas turbine has a lower probability of passing near a surface film of a pipe wall or turbine blade the more the turbine machinery and its supporting piping is enlarged.

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Hydrogen combustion has some advantages over fuel cells, namely rate of consumption allows significant power output.

  1. Uses existing technology, gas turbines can be easily modded to burn any gaseous fuel (Methane, propane, etc)
  2. power to weight ratio is superior
  3. Direct power: Fuel cells convert to produce electricity, the power must be delivered as electric power and used as such. Turbines can run a direct transmission to provide mechanical power (For ship, etc)

However hydrogens energy density makes it less efficient compared to gas/oil.

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