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Comment by Cthulhu_ | Hacker News Reader

And with this one, currents - mainly ebb and flow - are caused by the gravity of the moon pulling on the oceans.

But yeah, it will have an effect; the other question to ask is, how significant is this effect, and how does it compare to other forms of power production, e.g. fossil fuels?

The global warming caused by fossil fuels has a bigger effect on the oceans; with decreasing salinity due to the melting of ice caps / glaciers, the gulf stream is slowing down and will eventually stop, causing distribution of heat across the planet to stop, causing the northern seas to cool down and the southern ones to heat up.

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Tidal flows are a pretty massive energy reservoir - basically you’ve got all the potential and kinetic energy of the moon’s orbit to draw from. That’s (according to my crude Wolfram Alpha based research) about 3e28J of kinetic energy plus 7e28J of potential energy, for a total of 10^29 Joules. Technically a non renewable resource, but you would be hard pressed to make a dent in that energy budget with any kind of human scale extraction project.

Technically a non renewable resource...

How's that? Can we consume it? What would be the reaction? Moon altering its orbit?

Yes this technically degrades the moons orbit. Do it long enough and hard enough and the moon will hit the earth. Shouldn't be of any concern though, because the kinetic energy the moons movement is carrying is insanely high.

Huh. Would Ice 9 also make the moon crash?

This doesn't degrade the moon's orbit lol. The energy is there regardless of whether we are using it or not, it will go somewhere, this is just making "useful" work out of it (if anything humans do to the environment, other than going extinct, is really value added for the environment)

I think it’s the rotational inertia of the earth, not the kinetic energy of the moon. Since the earth is spinning faster than the moon orbits, the tides actually are pulled ahead of the bulge the moon creates, transferring energy from the earth to the moon.

Eventually the earth will be tidally locked to the moon (just like the moon is to the earth), and the moon will be in a higher orbit.

I guess the turbines will speed this up? As they’ll keep the bulge farther ahead of the moon.

EDIT: Wikipedia link https://en.m.wikipedia.org/wiki/Tidal_acceleration

That's true - for some reason I was thinking of tides as following the moon's orbit, when of course they're caused by the earth spinning underneath the moon (tides occcur daily, not monthly. Always check your working against reality, kids).

Weirdly, the resulting pool of energy is pretty much the same size - wikipedia quotes the rotational kinetic energy of the Earth as being 2.138E29J.

So we're still talking about an insane amount of energy.

If you use 17.7TW as the total power consumption of all of humanity, if we were to somehow derive ALL of our energy needs from tidal generation, it would take 358 million years to deplete the rotational energy of the earth.

Would the turbines make the rotation decay faster, or would they just put some of the energy of rotation decay that happens anyways on a slightly different path towards heat?

I think this is targeting ocean currents, which are solar-powered convection between the tropics and the poles, rather than tidal currents.

Currents, on the large scale, are not caused by tides. Surface currents (down to 300m or so) are driven by prevailing winds, and deep currents driven by differences in temperature and salinity (which affect water density), both also affected by Coriolis effect.

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