MadSci Network: Engineering
Query:

Re: Fuel Cell and Electrolysis efficiency?

Date: Fri Dec 10 15:49:19 2010
Posted By: Barry Kamrass, Consulting Engineer
Area of science: Engineering
ID: 1286229517.Eg
Message:

This is the perpetual motion question:  'if I use a motor to power a 
generator which powers the motor, why doesn't it run?'.

In this case, we're looking for the efficiency of the fuel cell.  This 
will vary, but let's just say that it's 90%.  Then, if you use 1 liter of 
Hydrogen gas to run the fuel cell, and somehow recover with perfect 
efficiency that Hydrogen gas, you'll get 0.9 liter back.  

Similarly, if you have an electricity-->Hydrogen gas converter with a, 
say, 90% efficiency and feed in 100 joules of electricity (=100 watt-
seconds), you'll get 0.9 liter of Hydrogen gas.

The first law of thermodynamics states: "you can't get back out more than 
you put in" and the second law states: "you can't get back out as much as 
you put it."  So it is with our fuel cells.


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