[hatbert]

> FCs don't operate via the carnot cycle so in theory can be 90+% efficient in electricity generation.

Sorry, this is wrong.

FCs are not heat engines, so the thermodynamic limitations of heat engines are not relevant to fuel cells. That said, thermodynamic limitations do set an upper bound on the efficiency of fuel cells. For hydrogen fuel cells, the theoretical upper bound is 83% [1]. That's for just the process of turning hydrogen into electricity, and that does not include the process for producing hydrogen in the first place. "90+%" efficiency is strictly impossible in theory or practice.

> i think the carnot efficiency maxes out somwhere 40-50% so no matter what you do (and we've been doing this for 100 years now)

It must be noted that 50% thermal efficiency is a practical upper bound for a real electrical power plant, not a theoretical one. Some combined-cycle gas turbines do exceed 50% efficiency under some operating conditions.

[1] http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/electrol.h...