A Princeton-led team has built a tabletop device that generates voltage directly from Earth’s rotation through its magnetic field. While the power output is orders of magnitude too small for practical electronics, the breakthrough suggests Earth’s spin could someday provide constant, fuel-free energy if the effect scales up. The team is now calling for independent labs to reproduce the results.
There is already tidal power generation slowing down the Earth’s rotation, though not by much.
What about the propeller type generators that spin both ways? How would they slow down earth’s rotation?
As the earth rotates, the oceans follow the moon’s gravitational pull (and the sun’s, to a lesser extent). From an outsider’s perspective it is a lump of water always bulging towards the moon, and the earth rotates underneath this lump.
By placing a resistance to the free movement of the tides you are siphoning a very small amount of energy from the rotation of the earth as you are restricting the passage of the earth through that lump of water.
So it doesn’t matter if your generators spin both ways on the rising and falling tides, you are still restricting movement.