MadSci Network: Engineering |
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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