Q: Could the tidal forces of the Sun and Moon be used to generate power directly?

The original question was: If a machine enclosed in a building could, as I think it could, detect the tidal forces of the sun and moon upon the Earth could the effect be used to extract useful energy from those forces and what would the effect be on the motion of the Earth?


Physicist: “Useable” energy; yes.  But probably not too useful.  You can calculate the maximum total energy caused by lunar and solar tides on something the size and mass of a building, and you find that it’s really, really tiny.

To generate power from tidal forces directly, in a self contained building, you’d want to lift something very heavy when the gravity is “turned down”, and then lower it and generate power when gravity is “turned up”.  That difference is not big.  It’s about 1 part in 3 million at best (even less for the Sun).  Unfortunately, motors and generators that are more than 99.99996% efficient just aren’t made, so that tiny “1 part in 3 million” may as well be zero since it won’t cover energy lost to inefficiency.

The approximate relative sizes and distances of the Earth and Moon.

The Earth, Moon, and the space between them (to scale).  This is why you don’t notice the difference in gravity.

A building the size of the Burj Khalifa could generate an absolute maximum of around 100MJ of energy, twice daily (two tides), using perfectly efficient machines.  It would just be a huge block of iron that is lifted and dropped at the right times.  So under unrealistically ideal conditions, such a building could daily provide electricity to the United States for not quite half a millisecond.

However, it is both possible and feasible to harvest the tide for power.  Basically you find a bay with a single small inlet and you put free flow hydroelectric turbines in the inlet to catch the ocean tide flowing in and out.  You don’t have to build a huge weight and a bunch of machines to raise and lower it, you just anchor some turbines on the bottom of a bay.  Much easier.  There are a bunch of these already in use, and they tend to be far more compact and powerful than wind turbines.  Water, being denser, carries a lot more energy and packs a much bigger punch than air.

Ultimately, all of the energy gained from tidal forces has to come from somewhere, and that somewhere is the rotational kinetic energy of the Earth.  So, when you generate power from tides you literally slow the Earth down.  But that slowing isn’t something to worry about.  The oceans, and the crust of the Earth itself, are already doing more than their part.

Right now the “tidal bulge” of the Earth leads the Moon (just because it takes a little time for things to move around), and the effect of this is to slow the rotation of the Earth while speeding up the Moon’s orbit.

Fun fact: the day was once about a third as long, and the Moon was more than ten times closer.

The Earth is stretched slightly by the lunar tidal force.  But the Earth turns, and it and the oceans don’t snap back into shape instantly, so there’s always a little extra mass leading the Moon.  The pull of this extra mass speeds up the Moon in its orbit and slows the turning of the Earth.

Every attempt to harvest energy from the tide comes down to keeping extra mass higher than it “should” be for a little extra time, and this amounts to making the tidal bulge just a tiny bit bigger.  But even if we “captured the tide” along every coast in the world, our efforts would be statistical noise compared to the sloshing of the entire ocean.

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5 Responses to Q: Could the tidal forces of the Sun and Moon be used to generate power directly?

  1. Jasper A. Visser says:

    Almost sounds like you’re describing the plot of one of the earliest Transformers episodes (“The Ultimate Doom, parts 1-3”). Megatron pulls the planet Cybertron into close orbit of Earth, and then harvest the ocean waves for energy. Yes, by some miracle, it costs so little energy to pull planets around that this actually produces a net benefit. 🙂

    So, by this logic, we only need to pull the moon closer and tidal energy might be a usable energy source.

  2. Jessica says:

    There’s been some interesting research and trials around wave power. Although the amount of power generated is miniscule (at best) it’s still an interesting way to harvest alternative energy. Time will tell where it ends up in terms of efficiency.

  3. Kopernik2 says:

    In about 50 billion years the Moon in its orbit will be beyond the grasp of Earth’s gravity. Not too soon to make plans. If there is an Earth.

  4. William says:

    Considering that the entire history of life on this planet is two and a half billion years, if there’s anything still alive on Earth 50 billion years from now it won’t be anything like anything we’ve ever seen before.

  5. Scott says:

    50 billion!?? The current theories I’ve been reading say it will only be about 5 – 7 billion until the sun uses up its fuel. Before even that happens the sun will swell into a red giant engulfing Mercury, and Venus, and very close to earth if not engulfing it too. The habitable zone will be pushed out toward the outer planets possibly making them potentially habitable for a while. Unless we have learned to colonize other planets or move our own in another 3 or so billion years Earth will be cooked along with everything else still living there. I say we are much more likely to wipe ourselves out before the sun does. Our bones and artifacts will be dug up and studied by the next form of intelligent life evolved from surviving bacteria or spores. Those future scientists would shake their heads in pity at our easily preventable demise only to find themselves sharing the same fate — only this time much sooner because they are able to reverse engineer technology left behind by us.

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