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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.)

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Thanks; this was basically what I suspected. Renewable energy is not strictly renewable. –  Jez May 30 '11 at 19:41
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@Jez - The Second Law of Thermodynamics says that nothing is renewable, strictly speaking. Since we don't care overly much about the entropy of outer space, we can use the word "renewable" in the context of our planet in an informative way. –  Rex Kerr May 30 '11 at 20:40
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Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable (naturally replenished).

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.

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As you said, you can indeed 'run out of wind'. When you convert wind energy into electricity you are removing energy from the atmosphere. If anything, I'd actually say wood was more renewable than wind energy; you can plant new trees and have a pretty much endless cycle. –  Jez May 30 '11 at 18:23
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You are being too strict here. The OP has asked the definition of renewable energy, not whether it's wrong. I can't argue = you are right, but you are missing one point. You are talking about "taking the energy from the wind and not giving it back to the universe, therefore running out of wind", then you go on saying you can "plant new trees and they will grow." No, they won't grow, if you don't give the energy back, like the first scenario. You are not being consistent in your statements. –  RiMMER May 30 '11 at 18:34
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@Jez: No, you can't run out of wind, not even if you use it to produce energy. Wind is simply air movement determined by different air pressions and heat (the Sun), it's not something concrete. There will always be wind until the sun and the air will exist, so for a long time yet. Trees can be re-planted, sure, but how long does it take to a tree to become "adult" and therefore useful for wood? Not to mention that burning wood is more polluting than using wind to produce energy. –  Alenanno May 30 '11 at 18:48
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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.

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@Jez: And your point is? –  Marcin May 30 '11 at 18:29
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@Jez If the sun burns out, life on Earth is finished. If oil runs out, we can turn to something else. I'd say that's a large enough difference to squash your quibble. –  Matthew Read May 30 '11 at 19:33
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@Jez: That's a specious analogy, and you know it. The sources that we use are renewed are from other sources, and that is the renewal that occurs. That renewal will continue to occur for longer than the length of recorded history (and in fact longer than the time in which hominids are estimated to have existed). If that's too short for your plans, I suggest you take your tardis elsewhere. –  Marcin May 30 '11 at 21:13
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@Jez: It's not a misnomer, technically, or otherwise. The supply is actually being renewed. –  Marcin May 30 '11 at 21:44
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@Jez: If something is renewed, then it is capable of being renewed. The word for being capable of being renewed is "renewable". –  Marcin May 30 '11 at 21:47
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