After reading the following news about a Norwegian town generating energy from the tide, http://story.news.yahoo.com/news?tmpl=story&u=/nm/20030920/sc_nm/environment_energy_moon_dc ,
I remembered a question I always had and always forget to follow up on. Does harnessing the energy from tides offer “resistance” of any type and therefore slow down the moon and the earth’s rotation more than it would on its own, however slightly? For some reason I can’t wrap my mind around this one.
I keep thinking “energy is always conserved” and I imagine tidal energy is ultimately transformed, but I wonder what it’s transformed into when it’s not harnessed by us? Waste heat that escapes into space? If considered as a unit, why doesn’t the earth/moon system maintain its rotational momentum - space is practically a vacuum. I am asking because I know the earth’s rotation has been slowing down ever so slightly over the eons.
Sorry it appears I have several questions in there, I hope someone can make sense of them and answer them.
>> Does harnessing the energy from tides offer “resistance” of any type and therefore slow down the moon and the earth’s rotation more than it would on its own, however slightly?
Yes, the amount would be negligible but yes. The liquid and solid tides within the earth are always a bit behind the Moon which causes them and so slow it down.
sailor’s been into the grog again.
The energy imparted to the ocean by the moon’s gravity is used up anyway, whether by the weather or just sloshing around. Or moving paddles. More energy for paddles = less for weather and sloshing.
I’m gonna claim that harnessing power this way will in fact lessen the amount the tides slow the rotation of the earth.
See if my logic makes sense. Water sloshing around on the earth is a wasteful process, with energy ultimately ending up as heat(as you pointed out). Since I can see no way that the energy needed to cause the sloshing is ever put back into the rotation of the earth, once that rotational energy is converted to sloshing, we’ve lost that bit of rotation for good.
Now start putting things in the water that resisit the sloshing, and you get less of it…and so, you lose less rotational energy to it. So not only does this water wheel prevent the burning of fossil fuels, it will help prevent our days from getting longer. Sounds like a good idea.
Now to you doubters, if you wish to say my logic is flawed, which it occasionally is, please make sure you point out exactly where it is wrong.
DAMN, and now on preview, I see mangeorge already beat me to the idea…
No, I disagree. If there were no friction and water could move around without expense of energy then the system would not be slowing down. Add a little friction and the system starts to slow down. Add more friction and it slows down faster. The greater the angle between the water and the Moon, the greater the slowing force.
Imagine a railroad track around the equator and a train running in sync with the moon. If the train is always right under the moon it will not slow the moon down but it will not gain any energy from it. As the train moves back it begins to feel the pull of the moon more and more but, at the same time the moon feels the pull back in the same proportion. The greatest angle would be when the train sees the moon on the horizon. Then it would get the strongest pull and would exert the most action on the moon. Using the tides is no different. It increases the angle of the mass of water with respect to the moon.
I see where you are going with this but look at the extreme opposite of frictionless water. If the water was completely solid, with no movement at all, you also wouldn’t have any losses(lets avoid gravity wave losses, let’s stick with Newton). My thought is that by constraining the water better, it is closer to my extreme example.
I guess there is a bell curve here with frictionless water at one end and solid water at the other end, with our water wheel somewhere on either side of the middle.
The pull of the moon’s gravity would tend to lift the train when it was just below, the same amount of energy as the pull as the train fell behind. The train’s mass doesn’t change. All that changes is the distance between the moon and the train. Might be something there. Lemme think about it.
The Earth rotates faster than the moon orbits, so the tidal bulge beneath the moon lags a little because of friction–and there are offcenter bulges a bit in front of the moon, in the sense of rotational direction. The moon pulls on both the nearside and backside bulges, and the effects are opposing, but the nearside effect is a bit stronger. So, the Earth is slowed, and the moon is pulled forward–which carries it into a higher, slower orbit. Angular momentum is conserved.
There would be no such effect if there were no lag due to friction.
This isn’t so complicated as you guys are making out.
Suppose I set up two huge tidal reservoirs, one in California, and one in Maine.
In Maine, I let the tide go out freely, and then draw power by retarding its return into my reservoir.
In California, I catch the tide after it comes in, and draw power by retarding it as it goes out.
On one side I am making the bulge larger, and on the other I am making it smaller. Trivially, I know. But, if this leading and following bulges pulling on the moon differently nonsense is true, I just got a free lunch, by doing it in both directions.
There ain’t no setch thang as a free lunch.
I can harness the tides because the turbulence of the oceans is an energetic system. That system converts the energy of the rotation of the Earth into heat, and energy of orbital velocity of the Moon. Since electrical energy is likely to induce far higher temperatures than tides, the net efficiency of the exchange into, and dissipation of heat will increase. Therefore either the earth will slow down faster; or the moon will speed up faster, or both, but it ain’t about whether you retard the tidal bulge, or move it forward.
By “free lunch,” I guess you mean you think that your system restores the state of the Earth/moon system, so you would draw off energy indefinitely if the “nonsense” were true?
I think that the “angle between the water and the Moon” is nonsense.
Gravity causes the water to move. Moving the water increases the thermal energy of the Earth at the expense of the kinetic energy of the Earth. Taking power out of the tidal kinetic system and producing electricity would cause it to convert to heat more efficiently, therefore it would slow the Earth’s rotation faster. The “Angle between the water and the Moon” is not a factor. If it was a factor, you could overcome it by using the power at different angles. But you can’t.
Then how do you explain the transfer of angular momentum to the moon?
Laser ranging has verified the moon’s receding–which, with the slowing of the Earth, will eventually result in a tidal lock of the Earth, just as the moon has tidally locked to the Earth.
Only two things will affect the Earth’s “tug” on the Moon (and vice-versa), mass and distance. Even though the bodies of water do move to and fro relative to the Moon, over time the distance balances. The Moon see’s the Earth as a whole, and how we move things around here, how we alter the expediture of energy, shouldn’t have any effect on the Moon.
Norway is cooling the ocean, and maybe reducing global rainfall a bit, but I doubt that they’re slowing the Moon any.
I’m not sure what you mean by “see’s the Earth as a whole” but I suspect you mean that the effect of the Earth’s mass on the moon is as if the entire mass were concentrated at a single point, at the center of mass.
That’s only a first approximation, and isn’t true in general. Consider the Earth/Sun system–the center of mass of that system is inside the Sun, but we are more drawn to the center of the Earth rather than the center of mass.
In the case of the moon/Earth system, the shape of the Earth, beyond just mass and distance, does influence the movement of the moon. That’s the whole idea behind tidal slowing and tidal locking.
The freedom of movement of the tides does affect the rotation of the earth and the orbital distance of the Moon quite substantially over millions of years;
if there were no continents the tides would flow more smoothly and slow down the Earth less.
This would also cause the moon to recede more slowly.
so taking tidal power out of the seas does affect the ease of movement of the seas, and takes energy out of the Earth/ Moon system.
Where else would the energy come from-
oh yes- the Earth - Sun system must lose energy too.
