Manmade Harbor Turns the Tide on Clean Energy

But it lengthens the day by slowing the earth's rotation...

Just think of the impact on the moon (or will it be on the earth?) if the rotation comes to a complete halt. Oh, I think we have a concept for the next action movie!

I've thought about this before... how could you get energy from the spinning earth?

Because of the conservation of momentum, it has to involve something off of the earth, in this case, the moon and sun. (You can't change the angular momentum of the earth with anything mounted on the earth any more than you can move an empty rail car by pushing from the inside.)

As the tidal friction slows the earth's rotation it transfers the angular momentum to the earth-moon system, raising the moon's orbit, and to a lesser extent to the earth-sun system, raising the earth's orbit. In the end, there will be less mechanical energy in the earth-moon-sun system because some of it was used to move water around or, in this case, make electricity.

If the earth were perfectly solid and inflexible, there would be no tides or energy loss. If it were made of a frictionless fluid, there would be tides but no friction, so again no energy loss. Since it is between these two extremes, there is a tidal bulge that due to friction is dragged in the direction of the earth's rotation.

The offset mass of this tidal bulge changes slightly the direction of the earth's gravity on the moon, affecting it's orbit and causing the momentum transfer. Capturing energy from the tides minutely increases this offset and increases the slowing of the earth's rotation.

Is this important? No, but it's interesting to analyze where the energy is coming from.

If the earth were perfectly solid and inflexible

or dry!

Excellent points. I find it to be amazing that the moon and earth once spun so fast that a day was only 6 hours long. Of course, that was several billions of years ago. As the story goes, it was the massive oceans and tidal effects that caused the spin rate to change and the distance between the two bodies to increase.

I would expect the human race to become extinct long before measurable significance of using tides to provide clean energy could be noticed. We already know the distance between the earth and the moon seems to be increasing a few millimeters per year, unless that is just some sort of wobble. But deriving energy from water trapped in a harbor certainly would not cause us to choke to death from air pollution.

I once owned a book titled, "What if the earth didn't exist", until I loaned it out. There were 6 or 7 ideas talked about including one where the axis of the earth was horizontal like the planet just beyond Jupiter. It made me appreciate this wonderful rock we live on.

I understand the astronauts left a corner reflector on the moon, so the distance to the moon can be accurately measured over time using lasers.

You are right, removing energy from tides is highly unlikely to have any measureable effect.

It's interesting that even the solid of the earth is affected by the tidal force.

https://en.wikipedia.org/wiki/Earth_tide

Strength of a solid increases as square of size whereas its mass increases as the cube, so a rock the size of a planet or even a small moon collapses under its own weight to a spherical shape as if it were a liquid.