Why isn't a hydroelectric dam considered perpetual motion?

lundyjb

I am familiar with the second law of thermo, so i realize that there is an answer. My reasoning is that the water is flowing because of gravity, which turns the dam turbines. So where is the energy being removed from the flowing water? I mean its not changing the earths gravity right?

mfb

You use the potential energy of water - and you need the sun to get water back to the top (via evaporation and rain).

You lower the total energy content of the gravitational field of earth+water.

lundyjb

So its kind of a manifestation of "solar" energy?

NascentOxygen

Harnessing hydro energy is solar energy. So is wind energy, and wave energy (and coal energy if you wish to stretch the time line for "renewable" a bit).

Buckleymanor

Yes so it's solar, but what is the manifestation of solar energy the big bang?
You can't just stop at it's solar that does it!
Stretch the time line and it's becomes rather hard to come to a reasonable answer.
We don't know.

mfb

The sun uses nuclear energy - and this is not renewable, it will end (for our sun) in about 5 billion years. And if you wait long enough, most hydrogen in our universe will be fused, and all stars died. In that way, solar energy is not really "renewable" - but a billions of years are way beyond our timescales.