Eli5: How do hydroelectric power plants in the ocean not violate the law of conservation of energy?

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As far as I understand, turbines that are powered by ocean tides don’t take energy out of the ocean. Since the tides carry on endlessly regardless of if there is a turbine, how is hydroelectric power not an infinite energy source?

In: Physics
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The tides are caused by the gravity of the moon. As it orbits us, the tides are pulled around.

The energy of the tides is put in by the rotation of the Earth relative to the motion of the moon, and while this is technically a limited resource it will last for the foreseeable future.

So some people are skirting around the answer you’re looking for.

Tidal energy is created by the movement of the earth and moon etc (and waves from wind which is caused by the sun). And yes, the tidal energy machine DOES take energy from the system. And look at a wave powered harnesser if you get a chance, they’re nuts, they leave the sea still as heck behind them. It sounds daft but yes, the tides moving is taking kinetic energy from the moon via gravity etc.

We call it renewable because we are on about time frames so vast that it doesn’t make sense to talk about it as being depleted. The sun will boil the seas before it’s an issue. In the same vein, wind and solar aren’t eternal, but they’re renewable because they’re just not going to stop being sources until the planet dies

Taking energy from the ocean tides will increase the rate that the moon’s revolution slows down. (ie makes the moon move slower in orbit around the earth). The moon is gradually doing so already but this very very long process will be sped up slightly. The energy is not infinite but is so large that in human terms, it might as well be.

Hydroelectric energy (from dams) is energy from the sun that evaporates the water, leads to rain and the rain forms rivers etc. It isn’t infinite either – the sun will eventually burn out.

they don’t violate conservation of energy.
Hydroelectric planta simply slow down the rotation of earth to get energy.

But the energy extracted is miniscule compared to the energy that is stored in the earths rotation.

If enough plants, and enough time, earth rotation will slow, such that one half of the planet sees the moon, and the other not. At that point there will be no tides anymore, at least not from the moon.

They don’t violate conservation of energy because ultimately they do remove energy from the ocean – they convert the kinetic energy of a wave into electricity, leaving calmer water behind. So the kinetic energy that would have been turned into the sound and movement of a wave striking the coastline is instead converted into electricity.

So while it does remove some energy from the natural system, it is worth remembering how much – the every output by hydroelectric systems is such a tiny, tiny, tiny percentage of the overall energy that it is incalculably small. So any difference to the overall energy distribution of our ecosystem is going to be so small as to make no difference whatsoever.