Can tidal power plants have an effect on the Earth's rotation?

I just read an interesting article on the project to build power plants that tap into tidal energy. It's really weird to see that article now because I was discussing that exact subject with Fabien on the Stevens Pass chairs last Saturday.

A couple comments:
- There is a power plant in France that works on this principle. It's been operating since 1966 and it's producing 550 million kWh a year. China operates eight similar plants, and Canada also has one.
- This energy is *NOT* renewable. It's basically gravitational potential energy. Thoses of you familiar with physics know about the principle of action and reaction, which in this case implies that such energy tapping would in return have an impact on the relative motions of the Earth and Moon. Of course this effect is very very small and probably safe to ignore but we do have a precedent: the Moon itself now always shows the same face to us because the dissipation of the tidal energy into deformations of the crust quickly forced it into the minimal energy position which is the one where the tidal bulge on the rock always faces the direction of the tidal force. This is exactly similar: the deformation was tapping from the tidal energy, which slowed down the rotation of the Moon. How long would it take for the Earth? I didn't make any calculations but I'm pretty sure that would be a huge number.


  • the link to the power plant doesnt work

  • Yes it does. It's just that this article hasn't been written yet.

  • Of course they would. Tides are caused by the interaction between the Earth's rotation and the gravitational pull of the Moon. The tides themselves introduce friction into this system and dissipate energy because of their drag on the coastlines and ocean floor. Even without any tidal power plants the tides are slowing the earth's rotation and causing the Moon's orbit to recede. However the effects are so small it takes thousands of years to be noticed. Tidal power plants would increase this effect in a microscopically small way.

  • Sure, I think that's what I was trying to say. You make a good point on the fact that the Earth has the same kind of energy dissipation on water and rock that synchronized the Moon's proper rotation with its orbit. That effect is probably huge when compared to what tidal plants could do.

  • Rotation is overrated anyway.

  • While this is an interesting topic for a discussion, you have the tail wagging the dog. The moon does not orbit the Earth because of the tides. Even if the power plants were to absorb all of the tidal energy in the sea it would have no impact on the moon's orbit. Furthermore given that the mass of water on the earth compared to it's mass as a whole is so small the effect if any would be negligable.

  • DetroitJ: I never said the "moon orbits the Earth because of the tides" but tidal energy is gravitational energy and if you pump it it's going to have an impact on the relative movements of the Earth and Moon, it's plain simple energy conservation. And I think I made it quite clear in the conclusion of the post that these effects would be negligible...

  • First, I'd like to see a proof that the so-called tidal effects in the Moon's crust made it rotate faster or slower. What was the original rotation rate? Why? I have seen theories that the Earth and Moon split apart early in their evolution, which would require synchronized rotation as an initial state (although conservation of angular momentum would make the moon speed up a bit on separation as it coelesced into a sphere). Is the current theory about the origin of the Moon any different?

    All seriousness aside for a moment, you should be a lot more concerned about the damming of the natural rivers of the Earth. Those lakes and powerplants have got to be ruining things. I'm pretty sure of it. And those Segways. Zipping around on electric thingies is just not natural. If all the Segways started driving in the same direction, it would totally screw up Daylight Savings Time. Then they would run off the edge of the contiinent, fall into the sea, and electrocute the whales. That would be bad.

    I'm moving to a cabin in Montana.

  • Bertrand: I think Roger's post "causing the Moon's orbit to recede." threw me off because you responded "Sure, I think that's what I was trying to say". I did not mean any disrespect.

    The tides (or lack thereof) would not cause the moon's orbit to recede. The mass of the world's oceans are included in it's overall mass. The moon's orbit would decay due to it's orbital velocity being decreased by space particcles, comets, whatever else is up there crashing into the moon. Classical mecanics descibes gravity in the following; the standard formula for gravity is:

    Gravitational force = (G * m1 * m2) / (d^2)

    where G is the gravitational constant, m1 and m2 are the masses of the two objects for which you are calculating the force, and d is the distance between the centers of gravity of the two masses.

    Given that G is a constant and m1, m2, and d would not be effected by any tides the gravitational force would be unefected.

  • Can somebody explain to me, the thousands upon thouands of high-rise buildings we have established around the earth, plus rows and rows of windmills, have no drag effect on global air currents?

  • Well, Aaron, everything has an effect, and no energy comes free, but the question is how large the effect. If you consider the area of cities and windmill farms and compare that to the total area of the Earth, it's negligible.

  • Ulu: you know nothing of our lives, which may very well be a lot more interesting than yours for all I know.

  • So when using tidal energy, are we slowing the moon down, or slowing the rotation of the earth?

  • Bigolnickthephysicsteacher: from what's been said above, I'm pretty sure it's slowing down the rotation of the Earth (by a very tiny amount of course). Also, it's established that tides transfer energy to the Moon-Earth couple and bring them farther apart. So I'd say that if you act against them by virtue of action and reaction you should be bringing them back together (relatively, I mean, the effect will be minuscule). Then again, no calculations done here, just hands reasoning so I may be wrong.

  • I think all the trees we cut dowl will allow the wind to blow more freely, so turbines are not a problem, tidal friction has been going on since water flowed on the earth, so tidal schemes are of no real effect. the main worry for me is all those solar panels, if we keep putting them out at the rate we are, we could end up using up all the sunshine and we could be plunged in to permant night! ;-))

  • Doubtless the tidal energy comes from somewhere and it's not free?

    The tides he have been dissipating energy on the Earth's coast for 3Byears approx.

    If we extract energy from the tides it will not make the slightest difference to the moon's orbit. The moon will slow down over time and thus recede from the Earth as it has been for the past 3E9+ years.

  • Mike, sure it will have *very little* effect, just like anything that diffuses the tidal energy, which is what I'm saying at the end of the post and what's been extensively discussed in previous comments. But you can't say that it won't "make the slightest difference" because it will.

  • Great attitude, Kelly. Good luck with that.

  • can anyone tell me whr i can find abt noise generated from tidal plants...any link or smthing...

  • you're all freaks . i can't believe you think that tidal power plants are effecting the earth's rotation!
    let me make this clear YOU ARE ALL ......LOSERS & STUPID UNEDICATEDFREAKS

  • Alyssa: sure, with a PhD in physics, I'm clearly not educated enough to understand those problems :) The part about being a freak is correct though.

  • Well I think that is to what everyone else thinks but if it were to change the earths rotation would it have a big impact? I mean how much do we really know about Tidal Energy? because that would really help the ansmer to the question. If scientists have the proof that it will change the Earths rotation than I completely disagree with the Tidal Energy, but if they think that it will help the enviroment and keep supplying us with energy than I personaly think that we should agree, but nothing I can say or do will help prove that it can or can not. It is really not up to us... Thank you for reading this.

  • Bertrand, did you find a conclusive study on the topic? I want to settle and argument I had at lunch today. Clearly we slow earths rotation, but I need to know if the centers of mass get closer or farther.


  • @Kevin: to move farther away from its current orbit, a satellite has to gain momentum so it would seem like as energy is dissipated, the system would move to a state of lower energy, and the Moon would get closer to the Earth. But this would be neglecting the complex interactions between the rotations of both bodies and how angular momentum transfers. While there are natural satellites that go this way (see, it is not the case for our Moon, which is getting away from us at a rate of 3.8cm/year (see

  • i am just doing this for a school project but after reading all of these comments and articles about tidal power i feel that it could have an impact on the earth. all through college we have been told that energy can not be created but transformed and transferred. so it makes sense

  • I did a quick calculation: A 500 MW tidal power station would have to operate for 300 million years to slow the rotation of the earth by 1 second. So at this level, I wouldn't say it was a problem.

  • WOW!!! is this really that I don't think so...I love school projects its wht got me to this site because I have to do a project on waves(tidal)....someone help(specifically: Bertrand Le Roy)

  • @Maddie: I'm sorry but I think I would be helping you more by telling you to do your own research. It's the whole point of school projects. :)

  • @ Phil: From your quick calculation, could you clarify if the rotation would be slowed by 1 second total, or if the rotation would be slowed by 1 second per day over 300 million years?

    The simple reality is that the earth's rotation is not renewable with absolutely no way to reverse any side effects that we inadvertently cause.

    PS: Thanks for posting this thread Bertrand!

  • Wow, you people are amazing. Wondering about the answer to bizarre questions like this that have absolutely no practical implications for our day-to-day lives is what keeps us human. And helps me procrastinate.

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