Tidal Power Costs

[wacki]

When you apply resistance to an already spinning electric motor, it effects the current draw of the motor.

What I'm wondering, when Tidal power starts to become a more popular form of clean power, will their be a similar effect. And if so, what will the effects be of Tidal power? I understand that the effects of harvesting the sea are probably insignificant, but this is more of a theoretical question. Lets say (remember its hypothetical) tidal power become so popular, that the intensity of the tides are changed. Would this effect the orbit of the moon? Basically what is the cost of tidal power?

 

[ZeroNine8]

I make no representations of the values used to be correct, though I believe them to be reasonably accurate (google):

If you consider the earth-moon system by itself, the total kinetic energy from rotation of the moon about the earth is approximately 3.79E+28 Joules (this does not count the rotation of the earth on it's axis)

This translates to 1.05E+22 kilowatt-hours

A DOE forecast predicts that worldwide energy consumption to be 22 trillion kilowatt-hours (per year) by the year 2020

Since tidal energy is generated mostly by the lunar influence (~70% due to the moon, ~30% due to the sun), we can assume this energy comes from the rotation of the earth-moon system about its center of mass. Even if we say that tidal energy comes 100% from the earth/moon, 22 trillion kwh is only 0.000000208968% of the available energy in the earth-moon system. This amount of energy consumption effectively slows down the rotation of the moon by 2.77358E-08 rev/yr (it slows down more than this already).

This is a grossly simplified version of how it works, but it's enough to show that we're not talking about a significant influence even if we were to generate 100% of our predicted future energy consumption from tidal power.

 

[wacki]

Ya, I figured the energy would be insignificant.

But, just to clarify, I assume from your previous post the slowing down of the moons orbit is one of the costs of tidal power.

What I mean is, my question wasn't exactly how much of a cost would tidal power have on the moons orbit if it did effect the moons orbit, but rather does tidal power effect the moons orbit no matter how insignificant the effect really is. And my question really wasn't directed directly at the moon either, but merely at tidal powers costs. Basically what are they.

 

[uart]

I think you'll find that the energy lost from tidal action slows the Earth and not the Moon ZeroNine8.

Interestingly enough the action of the tides actually serves to increase the Moon's orbital KE rather than reduce it, strange as that might first seem. Here's the reason : As the Earth is spinning faster than the Moon it causes the tidal bulge to constantly be pushed slightly ahead of the moon. This gives the Earth a slight "dumb-bell" shape that stays synchronized to the Moon but slightly phase advanced. So the gravitational pull between the Earth and the Moon is not precisely radially directed but instead has an ever so slight tangential component, directed so as to retard the Earths rotation and increase the Moons rotational energy. Over time this causes small but constant transfer of rotational energy from the Earth to the Moon. Many people (sometimes even Physicists with PhD's I've found) wrongly attribute the slowing of the Earth primarily to this effect. Actually it does slow the Earth but is just one tiny component of the various ways in which the Earth losses rotational KE and is small compared to some other factors.

I any case what ZeroNine8 says about the amount of Energy that can be extracted compared to the total rotational energy stored in the Earth is quite correct. I dont know the exact figure (because I dont know exactly how the Earths density varies with radius to calculate it exactly) but I'm estimating it to be about 1E+29 to 2E+29 Joules. Also remember that tidal friction and Wind friction slow the Earth in any case, whether or not we try to extract energy from them or not they still lose heaps of energy just the same. Extensive Tidal and Wind energy farming might very slightly increase the Energy loss over what already occurs anyway, but it would be by a fairly insignificant amount (relatively speaking).

 

[hairygit1]

There is an effect locally -
or at least there can be.

Where tides are affected to a small degree in relation to the coastline. Different species have evolved to survive in certain tidal conditions. where those conditions change suddenly due to a tidal barrier, things could go catastrophic on a very local scale for the existing wildlife.