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humility

A question about gravity and tidal energy

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So I was thinking about how long the earth core will remain molten and the how much power is produced by the earth's core compared the amount of energy earth receives from sunlight and what would happen if geothermal energy was overused, pretty straight forward, the earth's core solidifies and that led me to thinking about tidal energy, which is powered by the moon rotating around the earth. And what happens if tidal energy is overused. The tide ceases. But then I thought "Hmm there has to be something else being used up, otherwise that would just be providing far too much energy considering the moon's orbit won't finish decaying for billions more years.  Can it really be a constant energy source for that long?"

 

So what am I missing here?

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2 hours ago, humility said:

So what am I missing here?

An appreciation of the magnitudes involved, which would come from doing some calculations of the energies of these processes.

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humilty's post mentions "tidal power".  This is sometimes cited, nowadays, as a potential source of "Green Energy".  That's to say, harnessing the ebb and flow of the tides in the oceans, to drive turbines, thereby generating electricity.

As long as this electricity is employed for purely terrestrial purposes, I can't see that it would do any harm.  After all, the tides are created by the gravitational influence of the Moon, as it orbits the Earth.

And the Moon doesn't care how its gravitational influence is employed on Earth.  Whether by making the Earth's oceans  just rise up and fall down again, creating pure kinetic energy.  Or by having part of this kinetic energy, converted , via turbines,  into terrestrial electrical energy.

Either way, the Moon's energy "expenditure" would seem to be the same. So the Moon  would keep orbiting the Earth at the same distance and speed.  

The turbines on Earth, couldn't affect the Moon, could they?

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1 hour ago, Charles 3781 said:

humilty's post mentions "tidal power".  This is sometimes cited, nowadays, as a potential source of "Green Energy".  That's to say, harnessing the ebb and flow of the tides in the oceans, to drive turbines, thereby generating electricity.

As long as this electricity is employed for purely terrestrial purposes, I can't see that it would do any harm.  After all, the tides are created by the gravitational influence of the Moon, as it orbits the Earth.

And the Moon doesn't care how its gravitational influence is employed on Earth.  Whether by making the Earth's oceans  just rise up and fall down again, creating pure kinetic energy.  Or by having part of this kinetic energy, converted , via turbines,  into terrestrial electrical energy.

Either way, the Moon's energy "expenditure" would seem to be the same. So the Moon  would keep orbiting the Earth at the same distance and speed.  

The turbines on Earth, couldn't affect the Moon, could they?

Tidal interaction between the Earth and Moon transfers angular momentum from the Earth to the Moon, slowing the Earth's rotation  and raising the Moon's orbit. Turbines intended to extract energy from  the tides would increase the ocean tide- Earth drag. This would magnify this transfer. In effect, the energy comes at the cost of the Earth's rotation.

But as Swanson alluded to, a comparison of the comparative magnitudes would be informative as to the overall effect on the rotation.

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2 hours ago, Charles 3781 said:

Either way, the Moon's energy "expenditure" would seem to be the same. So the Moon  would keep orbiting the Earth at the same distance and speed.  

The turbines on Earth, couldn't affect the Moon, could they?

 

Well, very slowly now mind you, the Moon is gradually moving away and the length of Earth's day increasing. If the Sun wasn't going to expand first, the Earth would end up being tidally locked to the Moon, with the Moon permanently in one spot of the world's sky.

Relative to the large amount of energy available and already being spent creating a traveling bulge, the impact of conventional tidal power will be pretty minuscule.

 

Should note that there are solar tides as well.

 

cross-posted

Edited by Endy0816

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On 10/14/2020 at 11:49 PM, Janus said:

Tidal interaction between the Earth and Moon transfers angular momentum from the Earth to the Moon, slowing the Earth's rotation  and raising the Moon's orbit. Turbines intended to extract energy from  the tides would increase the ocean tide- Earth drag. This would magnify this transfer. In effect, the energy comes at the cost of the Earth's rotation.

But as Swanson alluded to, a comparison of the comparative magnitudes would be informative as to the overall effect on the rotation.

Are you sure this is correct? Harvesting the energy would as you say increase the ocean tide Earth drag, but the added resistance should reduce the angular momentum transfer, should it not? 

Generally speaking the turbines should reduce the net energy transfer, not increase it.

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9 hours ago, J.C.MacSwell said:

Are you sure this is correct? Harvesting the energy would as you say increase the ocean tide Earth drag, but the added resistance should reduce the angular momentum transfer, should it not? 

Generally speaking the turbines should reduce the net energy transfer, not increase it.

The reason the tidal interaction transfers angular momentum from Earth to Moon is that the drag between the Earth and tides causes the tidal bulges to be pulled "ahead" of the Moon. If you removed this drag completely, the bulges would remain lined  up with the Moon, and there would be no momentum transfer.  Increasing the drag would allow the spinning Earth to pull the bulges even more out of line, increasing the tendency transfer energy to the Moon.

As far as energy goes: Most of the energy lost by the Earth through its slowing rotation is not transferred to the Moon, but is lost as heat.( the Moon gains less orbital energy than the Earth loses rotational energy).  So it doesn't really work to think of energy "used" by the turbines as being taken away from energy given to the Moon.  Look at it this way.  If we just increased the drag without generating electricity, we would increase the amount of energy the Earth loses in rotational energy both by transferring some to the Moon, and by waste heat.  By extracting some of that energy by converting it to electricity,  you are making use of a part of that energy that would have been lost as heat.

 

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1 hour ago, Janus said:

The reason the tidal interaction transfers angular momentum from Earth to Moon is that the drag between the Earth and tides causes the tidal bulges to be pulled "ahead" of the Moon. 

Ahead? Do the bulges not lag behind?

1 hour ago, Janus said:

 If you removed this drag completely, the bulges would remain lined  up with the Moon, and there would be no momentum transfer.  

Agree. Idealized case. And the bulges would be greater.

1 hour ago, Janus said:

  Increasing the drag would allow the spinning Earth to pull the bulges even more out of line, increasing the tendency transfer energy to the Moon.

Increasing the drag "completely" (idealized case) and you get no bulge, or at least none that moves. Increasing it would tend to put it more out of line but decrease it. Does decreasing it not have more effect than putting it further out of line?

 

1 hour ago, Janus said:

 

As far as energy goes: Most of the energy lost by the Earth through its slowing rotation is not transferred to the Moon, but is lost as heat.( the Moon gains less orbital energy than the Earth loses rotational energy).  So it doesn't really work to think of energy "used" by the turbines as being taken away from energy given to the Moon.  Look at it this way.  If we just increased the drag without generating electricity, we would increase the amount of energy the Earth loses in rotational energy both by transferring some to the Moon, and by waste heat.  By extracting some of that energy by converting it to electricity,  you are making use of a part of that energy that would have been lost as heat.

 

Agree. That's how I saw it also except for seeing the effect on momentum and energy transfers both as reduced

Isn't there a dynamic tidal effect on the Earth's Magma, but because the resistance is so much greater than that of the Ocean's the effect on those transfers is comparatively negligibly, despite the much greater resistance and mass?

I always know I'm going to learn something when I disagree with you.

Edited by J.C.MacSwell

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19 hours ago, J.C.MacSwell said:

Ahead? Do the bulges not lag behind?

Agree. Idealized case. And the bulges would be greater.

Increasing the drag "completely" (idealized case) and you get no bulge, or at least none that moves. Increasing it would tend to put it more out of line but decrease it. Does decreasing it not have more effect than putting it further out of line?

 

Agree. That's how I saw it also except for seeing the effect on momentum and energy transfers both as reduced

Isn't there a dynamic tidal effect on the Earth's Magma, but because the resistance is so much greater than that of the Ocean's the effect on those transfers is comparatively negligibly, despite the much greater resistance and mass?

I always know I'm going to learn something when I disagree with you.

No. You are right. I was thinking in the Earth frame of reference on that point. 

 

20 hours ago, Janus said:

The reason the tidal interaction transfers angular momentum from Earth to Moon is that the drag between the Earth and tides causes the tidal bulges to be pulled "ahead" of the Moon.

The rest of what I stated I still think holds. (But I wouldn't bet against you) Where am I wrong? 

Edited by J.C.MacSwell

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On 10/14/2020 at 8:34 PM, swansont said:

An appreciation of the magnitudes involved, which would come from doing some calculations of the energies of these processes.

To be honest I could never quite grasp how to comprehend electricity. Amps, Watts,  Volts, Watt hours, Quads, I can never quite figure out what does what or means what. So when I read that there are 3 trillion wattseconds of energy in the tides, that doesn't sound like a lot to me. I just thought "Thats just 30 million lightbulbs at one time, thats a very small number. But a single Tidal Turbine Produces 25 gigawatts a year,  thats 79 watts a second,  so you would half the amount of tidal energy in the oceans after just 19 million turbines built. Which is a lot, but there is more than enough space in the oceans for them. There are already over 300k wind turbines in the world and those can only go in very specific locations."

 

But I think the responses largely solved my curiosity regardless.

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11 minutes ago, humility said:

To be honest I could never quite grasp how to comprehend electricity. Amps, Watts,  Volts, Watt hours, Quads, I can never quite figure out what does what or means what. So when I read that there are 3 trillion wattseconds of energy in the tides, that doesn't sound like a lot to me. I just thought "Thats just 30 million lightbulbs at one time, thats a very small number. But a single Tidal Turbine Produces 25 gigawatts a year,  thats 79 watts a second,  so you would half the amount of tidal energy in the oceans after just 19 million turbines built. Which is a lot, but there is more than enough space in the oceans for them. There are already over 300k wind turbines in the world and those can only go in very specific locations."

 

But I think the responses largely solved my curiosity regardless.

gigawatts a year is not a proper unit. gigawatt-years would be a unit of energy.

It’s not so much having a feel for any particular source, but a comparison to see if one is bigger.

For example, the rotational kinetic energy of the earth is about 2 ×10^29 J while our energy use is about  5.67×10^20 J per year. Somehow tapping into rotation, for example, will not cause a rapid depletion of that energy

You can do the same for the sources you are worried about, to see if they would run out.

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12 hours ago, swansont said:

gigawatts a year is not a proper unit.

Sure it is.  Bob (owner of Bob's solar farms) was known to sink all his profits into expansion of the business, and thus Bob's solar farms increased its energy production capacity at an average rate of 40 megawatts per year over the last decade.

Edited by Halc

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3 minutes ago, Halc said:

Sure it is.  Bob (owner of Bob's solar farms) was known to sink all his profits into expansion of the business, and thus Bob's solar farms increased its energy production capacity at an average rate of 40 megawatts per year over the last decade.

Not a proper unit in the context of its use in the discussion. I apologize for the imprecision.

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11 hours ago, swansont said:

Not a proper unit in the context of its use in the discussion. I apologize for the imprecision.

No worries...just don’t let it happen again...

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