Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Petrochemicals on 07/02/2023 19:49:57
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If we harness the power of the tides as is suggested by some as a source of renewable energy what effect will this have on the orbit of the moon?
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"Will tidal power cause the moon to crash into us?"
No.
If we harness the power of the tides as is suggested by some as a source of renewable energy what effect will this have on the orbit of the moon?
No.
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Tidal interactions are the result of the Earth spinning faster than the Moon orbits around it. The ultimate result is for rotational kinetic energy to be extracted from the Earth and transferred to the Moon which slowly increases its orbital distance from us. So tidal power ultimately comes from the Earth's rotation, not the Moon's orbit.
The rotational kinetic energy of the Earth is truly immense and it's already being drained by the Moon. Human use of that energy would be minuscule by comparison. Eventually, you'd expect Earth's rotation to slow down until a day was as long as a lunar orbit. At that point, the two are tidally locked and you can no longer use tidal power.
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Eventually, you'd expect Earth's rotation to slow down until a day was as long as a lunar orbit.
I think it is more likely that the earth would become tidally locked with the sun.
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The rotational kinetic energy of the Earth is truly immense.....Human use of that energy would be minuscule by comparison.
Kinetic energy: 259000YJ
Global annual energy consumption (https://ourworldindata.org/energy-production-consumption): 634PJpa
259000YJ/634PJpa = 409 billion years
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Eventually, you'd expect Earth's rotation to slow down until a day was as long as a lunar orbit.
I think it is more likely that the earth would become tidally locked with the sun.
The tidal effect of the Moon on the Earth is roughly twice that of the Sun's, Tidal forces fall off by the cube of the distance, so even with its great mass disadvantage, the much closer distance to the Moon more than compensates.
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I think it is more likely that the earth would become tidally locked with the sun.
You do now have me wondering if it is possible (barring the Sun's future death, as we are talking about very large time scales here) for the Earth-Moon system to evolve in such a way that the Moon ends up at the L1 Lagrange point. That way, the Sun, Moon and Earth could all be tidally locked to each other.
However, given that L1 is about 1.5 million kilometers from Earth and the Moon is currently only about 384,000 kilometers from Earth, that would require some substantial retreat from Earth on the Moon's part.
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The tidal effect of the Moon on the Earth is roughly twice that of the Sun's, Tidal forces fall off by the cube of the distance, so even with its great mass disadvantage, the much closer distance to the Moon more than compensates.
[slaps forehead] Of course. Since the tides are due to the moon (primarily) that should have been a hint to me that I was incorrect...
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The rotational kinetic energy of the Earth is truly immense.....Human use of that energy would be minuscule by comparison.
Kinetic energy: 259000YJ
Global annual energy consumption (https://ourworldindata.org/energy-production-consumption): 634PJpa
259000YJ/634PJpa = 409 billion years
According to this it is world energy usage is the best part (0.58zj) of a zeta joule per year and increasing. So call it 1zj, 1 x 10^21, or 259 million years.
https://www.theworldcounts.com/challenges/climate-change/energy/global-energy-consumption
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The moon is actually moving away from the earth in 1969 when they landed on the moon they put a reflective panel there and its been measured at 3.8 cms a year.It will be an extremely long time but it is moving away from the planet.
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"Will tidal power cause the moon to crash into us?"
No.
Quote from: Petrochemicals on 07/02/2023 19:49:57
If we harness the power of the tides as is suggested by some as a source of renewable energy what effect will this have on the orbit of the moon?
No.
But tidal forces will cause Phobos to crash into Mars!
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If we harness the power of the tides as is suggested by some as a source of renewable energy what effect will this have on the orbit of the moon?
Harnessing tidal power has the same effect on the moon as harnessing any hydroelectric power: None since it is using energy/forces already there, and which would otherwise be wasted as heat.
But tidal forces will cause Phobos to crash into Mars!
Yes and no.
1: This has nothing to do with the energy being harnessed or not.
2: (the yes part) Our moon is moving away because Earth's day is shorter than the month. Phobos is moving inward because the Mars day is longer than the month as defined by Phobos.
3) (the no part) Phobos will break up when it crosses the Roche limit and become rings, at which point the tidal effects will become negligible. There will never be some cataclysmic event where the moon crashes down onto Mars. The gravel from the ring will gradually fall in as small meteors, as is happening with Saturn's rings.
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Harnessing tidal power has the same effect on the moon as harnessing any hydroelectric power: None since it is using energy/forces already there, and which would otherwise be wasted as heat.
I'll have to disagree here. At least most tidal energy systems( that I am aware of) use turbines or some other means to extract energy from the tides going in and out. This has the result of slowing the water, and increases the drag between Ocean and the Earth*. This, in turn, has the effect of shifting the tidal bulge( albeit very slightly), Since it is the misalignment between the bulge and the Earth-Moon line that causes the transfer of angular momentum from Earth to Moon, an increase in this will result in an increase in the transfer.
Of course, the effect we'd have is exceedingly small, and would be swamped out by other factors ( such as the variability in the Earth's rotation due to other causes.)
*continental drift has caused this to change also. The arrangement of the continents has an effect on how much the Earth drags the bulge with it. Back in days of yore, when we had a super-continent, there was a smaller tendency to shift the bulge.
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Harnessing tidal power has the same effect on the moon as harnessing any hydroelectric power: None since it is using energy/forces already there, and which would otherwise be wasted as heat.
I'll have to disagree here. At least most tidal energy systems( that I am aware of) use turbines or some other means to extract energy from the tides going in and out. This has the result of slowing the water, and increases the drag between Ocean and the Earth*. This, in turn, has the effect of shifting the tidal bulge( albeit very slightly), Since it is the misalignment between the bulge and the Earth-Moon line that causes the transfer of angular momentum from Earth to Moon, an increase in this will result in an increase in the transfer.
Of course, the effect we'd have is exceedingly small, and would be swamped out by other factors ( such as the variability in the Earth's rotation due to other causes.)
*continental drift has caused this to change also. The arrangement of the continents has an effect on how much the Earth drags the bulge with it. Back in days of yore, when we had a super-continent, there was a smaller tendency to shift the bulge.
I'm not sure about that.
Imagine we froze the oceans.
The tidal movement would almost completely stop.
And so, though there may be essentially the same forces acting, the distances over which they act fall to near zero.
So nearly no work is done.
And tidal drag falls enormously (and so any changes it brings about are postponed.)
Now, we are not proposing to stop the motion of the seas, but, by penning it up being tidal barriers, we are restricting its movement.
Doesn't that reduce the rate at which the tides extract energy from the moon's motion?
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None since it is using energy/forces already there, and which would otherwise be wasted as heat.
I'll have to disagree here.
Well, it's a ternary effect, but I agree it can't be zero. Even drinking my coffee has a trivial effect on how fast the moon recedes. The question is, does it necessarily hasten the recession of the moon?
This has the result of slowing the water, and increases the drag between Ocean and the Earth.
Usually slowing something decreases the drag. A dammed river drags less on the river bed than does the free river. Less heat energy is expended, enough energy to power cities in fact.
This, in turn, has the effect of shifting the tidal bulge( albeit very slightly),
That it does, but does it increase the misalignment or the size of the bulge? We put a dam at the mouth of a bay, and the bay fills slower as the tide is rising, and empties slower as the tide goes down. That's less average bulge than the normal pattern. The water outside is slightly more confined, similar to a tiny adjustment to the continent, which can have a positive or negative effect as evidenced by historic tidal drag. So I'm suggesting the opposite effect of decrease of transfer of angular momentum from Earth to Moon.
I welcome counterarguments since I'm hardly an expert here. I suppose the best way to go about it is to suppose we put tidal power stations everywhere, which practically eliminates any tides (kind of like damming a river ever 2 meters), and that reductio-ad-absurdum drives the momentum transfer way down, not up.
Imagine we froze the oceans.
The tidal movement would almost completely stop.
Almost, yes. But the moon is frozen, and yet Earth still exerts a torque on it despite its slow angular movement. The cumulative effect of those forces is to slowly circularize its orbit over time. The moon is tide locked so we can't alter its spin rate much. Just a tiny bit since as the moon wanders further away, the length of our month gets longer and the moon's lunar day with it.
Doesn't that reduce the rate at which the tides extract energy from the moon's motion?
The tides don't extract energy from the moon's motion. The energy is from Earth's spin. The moon is just a sort of lever that gives us a place to send the conserved momentum. Energy is not conserved. 3% of it goes to the moon (maybe 5% PE, -2% KE), and the other 97% is radiated away as heat, or captured by our tidal stations if we have them.
Side thought:
Imagine if humans had evolved after full tidal lock. The moon hangs in one spot in the sky forever. How much longer would it take to come up with the theory of gravity with that contradictory thing hanging up there hovering much in the same way that a rock doesn't?
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The tides don't extract energy from the moon's motion. The energy is from Earth's spin.
Oops.
But my point stands.
If nothing moves, you don't do work.