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I read an article on energy forms and sources that made me think. Energy comes from somewhere and is limited in various senses. It's most obvious for fuels: we burn coal and oil and at some point we'll have used up all that we have.

We use solar power, and at some point in time the sun will run out of fuel and we won't have that source anymore (although we'll be dead before that, but that is not the point of this question).

Then the article mentioned tidal power, mainly as water power plants making use of the fact that the moon rotates around the earth and causes tidal waves / water movements.

How does our use of tidal power affect this "resource" ? What is there to "run out of"? Does our exploitation of tidal "gravitational" power have any effect on how long it will last?

For comparison, our use of oil has a direct effect on how long it will last, but not our use of solar power, since it will be there whether we use it or not.

Could the tidal power theoretically be there forever, if the moon just keeps moving around the earth?

As far as I know, the moon is slowly moving away from earth and at some point will leave its orbit. So that would be the end of tidal power in this form.

Sorry if there are several questions in one, but it all belongs together.

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    $\begingroup$ As far as I know, the moon will not leave earth's orbit, but it will eventually stop raising tides due to tidal lock. But the Sun raises tides too. $\endgroup$
    – user108787
    Jul 11, 2016 at 21:21
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    $\begingroup$ Uhm, is my concept flawed or is the presumption gravitational energy equals tidal energy wrong? $\endgroup$
    – Zaibis
    Jul 12, 2016 at 5:54
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    $\begingroup$ @Zaibis One flaw - as with everything, you need a potential gradient to extract useful work. In this case, as long as there's a difference between the Moon's orbital speed and the Earth's rotational speed, there exists a gradient to exploit; once the Earth's rotation is tidally locked with Moon's (which will probably not happen - the system will be disrupted long before that happens, once the Sun turns into a red giant), there's no more energy to extract from that. $\endgroup$
    – Luaan
    Jul 12, 2016 at 8:13
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    $\begingroup$ @Luaan: Uhm, my question was if tidal energy and gravitational energy are interchangably the same thing as OP sounds to me like presuming it. I don't get where you answer my comment with yours... and I have to admit I'm not getting it at all what you are trying to tell me. Sorry if this is due to my lack of knowledge. $\endgroup$
    – Zaibis
    Jul 12, 2016 at 8:20
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    $\begingroup$ @Zaibis Let me try in a different way - there is no potential tidal energy, and it's not related to gravitational energy. However, tidal forces are gravitational - basically, gravity mediates the energy exchange between the angular momentum of the Earth and the orbital momentum of the Moon (in the simplest scenario - reality is quite a bit more complicated of course). A tidal power plant indirectly steals momentum from the Moon and the Earth. But it doesn't "steal" Earth's gravitational energy. $\endgroup$
    – Luaan
    Jul 12, 2016 at 11:57

3 Answers 3

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The moon does work on the Earth because the Moon orbits around the Earth at a different speed than the Earth's rotation. So, as time passes, different parts of the Earth feel the maximum strength of the Moon's pull, leading to oscillations in the height of the Earth's surface, which can be exploited for energy extraction.

The Earth has always been extracting energy from the difference between the Moon's orbital period and the Earth's rotational period. Whatever energy humans extract will be tiny compared to natural tidal forces. There are two side effects of the changing tides: (1) the Earth's rotation is slowed down, and (2) the Moon is pushed into a higher orbit further from the Earth. Eventually, the Earth's rotation will slow down so much that one rotation will take the same time as a Moon orbit (this is called tidal locking). At this point, no more energy can be extracted from tidal forces because the Earth's surface is no longer changing with time. This will not happen for a very long time since the length of Earth's rotation is increasing by about 2 milliseconds per century.

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    $\begingroup$ Great answer, just a minor nitpick: the tides aren't extracting energy from the moon's orbit, since the total energy of the earth-moon gravitational system is going up over time. The energy comes from the rotational kinetic energy of the Earth. $\endgroup$
    – knzhou
    Jul 12, 2016 at 3:55
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    $\begingroup$ @Zaibis : it's not that bad, we will probably change our leap year calculation logic by the time it became relevant. Even today, leap years are not just the ones divisible by four: years divisible by 100 but not divisible by 400 are not leap years. So, we'll probably just add another such exception with "if the year is divisible by 5000 ... " or something like that. $\endgroup$
    – vsz
    Jul 12, 2016 at 6:22
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    $\begingroup$ In 50,000 years humans will be living on multiple planets, moons, and space stations throughout the solar system. I imagine by that time we will have come up with a Sol-universal way of telling time, much like the inventions of calendars and time zones. $\endgroup$
    – Mark H
    Jul 12, 2016 at 7:24
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    $\begingroup$ I get the weirdest conversations beneath my answers. $\endgroup$
    – Mark H
    Jul 12, 2016 at 10:03
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    $\begingroup$ "Whatever energy humans extract will be tiny compared to natural tidal forces." I'm sure that's what the first iron miners thought. And oil tycoons. $\endgroup$
    – corsiKa
    Jul 12, 2016 at 18:08
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Ultimately, the short answer is yes.

The moon is slowly moving away from the Earth, at a rate of 4 centimeters per year. This is due to it being in a slightly higher orbit than equilibrium.

At the same time, the Earth, being a satellite of the sun, is having its rotation rate slowed, until it will eventually be tidally locked with the sun.

Both of these effects, however, happen on a timescale that is longer than the age our sun will likely attain. This means that, although the Earth will one day no longer have tides, from either the sun or the moon, it will not happen before the sun expands into a red giant and consumes the Earth/Moon system.

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It is all question of scale. If we could harness tidal energy at large scale and more effectively it would slow down the earth faster but at the scales practical now it is not even comparable to topographic changes due to erosion of the continental shelves which brake the tides.
Shifting of tectonic plates and mountain ranges could affect the balance of earth as a perfect sphere and change the pull of moon more.
Inconsistency of density of top layers of earth act like ballast to change the rhythm of tides.

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