[Bring_Back_Shantz]

Originally Posted by Regorium

Actually, you don't have to spend a ton of resources to produce hydrogen. Modern Electrolyzers along with a good gas compressor (to get pressure high enough to useful levels) is 75% efficient. That is, 75% of your input electricity will be stored in compressed hydrogen. Then you get 50% of that when you put the hydrogen in your fuel cell. Not as efficient as an electric car, but far more efficient than conventional ICE's.

4.8kWh/Nm3 of hydrogen roughly translates to 60% electrolysis efficiency including all the pumps and compression. It seems I was a bit optimistic when I said 75%.

The problem with what you're proposing is that you're forgetting that efficiencies are cumulative.

So you've got an electrolysis process that is at best 75% efficient and a fuel cell that is only 50% efficient. So at best you're 37.5% efficient.
Now if we assume that all of this power is produced by fossil fuel powered electricity sources (Because let's face it, if we assume solar, wind, hydro, etc, then the argument is completely moot as efficiency isn't really an isssue), then you're using power that comes from a source that is (optimsitically) 50% efficient.
So now you're down to <19% effeciency, which is lower than modern engines.