There's a lot of buzz these days about 'renewable energy', and with Germany's recent decision to close down their nuclear plants by 2012, activists are talking about moving to completely 'renewable energy'. But, why is it called that? What is actually being renewed?
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Renewable energy is energy made from a renewable resource. A renewable resource is one which will (or can easily be made to) reappear given ordinary processes that will run pretty much unchanged for millions of years.
Thus, copper is not a renewable resource because we mine it from the ground and don't put it back where we got it; wood is a renewable resource because trees will just grow again (as long as we don't let all the topsoil wash off).
Hydroelectric power is renewable because rain keeps falling in catchment areas. Nuclear power is not renewable because radioactive isotopes don't appear in the ground again.
It is entirely possible to exhaust a renewable resource: you can cut down all your trees, empty out your reservoir, etc.. What makes them renewable is not that they're inexhaustable, but that they'll be back. (At least given some sensible precautions.)
Basically, you can't "run out of wind", therefore the wind energy is renewable, because the wind will blow again, but you can "run out of trees", therefore wood is not renewable.
One could argue that philosophically you can run out of wind, but that would be very difficult and isn't really possible, at least will not be caused by turning the wind into energy.
The energy source is what is being renewed and this isn't really a part of the process of turning the energy source to the energy. That's what might have confused you.
The source is considered to renew itself. It also makes a nice contrast to "consumable" sources.
More accurately, the sources are renewed by (a) the sun (b) coriolis energy (c) geothermal processes.
Renewable energy is more along the lines of being a layman's term. Speaking in terms of physics with regards to the laws of thermodynamics, energy can not be created or destroyed. It should rather be coined as 'recyclable energy' but that is also borderline of being a layman term.
The term renewable energy references the growth or procurement and usance of sustainable resources to provide a form of energy to meet our needs and wants.