Why Not Use heavy Bouys w/ Pumped Water Energy Storage

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https://youtu.be/iGGOjD_OtAM

I’m assuming for a similar reason the thing in the video is impractical?

Otherwise would having either Bouys or maybe a boat with a bunch of weights on it add to the efficiency? The rising water will lift it. Move the boat/weights/bouy out of the water reservoir

Need energy? Drain the water. Water gone? Lower the weights.

More effort than it’s worth? Potential issue being the water reservoir not filling all the way, but perhaps there are ways to engineer around it.

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It reduces your storage.

It occupies space that the water would otherwise go into, so you can pump less total water. It acts a lot like keeping a little bit of water in there that you never pump out.

The issue is that it is pointless.

As stated by Archimedes’ Principle (circa 246 BC) a floating object displaces its equivalent weight of the fluid in which it is floating. If you have a 40 ton floating buoy then the space it takes up below the water line would contain exactly 40 tons of water. So if you pump water into a reservoir with a heavy buoy in it then you might as well just be pumping water without the buoy, the water level will reach the same point with exactly the same effort. To retrieve the energy you just need to let the water back out through a turbine that drives a generator. No need to bother with buoys, cables, etc.

Pumped water storage is a thing that gets used successfully to store energy.

The issue is that it takes a lot of water pumped up over quite a distance to store significant amounts of energy.

Actually building special infrastructure with tanks and stuff simply would not be efficient.

The only way to make it really work economically is to find a per-existing mountain or big hill that either already has a likely lake on top or can be converted to hold a lake at the top and bottom that you can pump water between.

Those hills are not exactly all over the place and building a pumpe storage site would destroy much of the environment already there.

However quite a few places have them build like this to store energy created during the day by solar etc and release it during the evening and early morning when people are awake and using energy but solar is not working at 100%.

To get you an idea of the simple scale issue involved.

* One Watt is one joule per second.
* A joule is newton time a meter.
* A liter of water weighs about 1 kilogram
* A cubic meter of water is about a metric ton.
* An hour has 3600 seconds.
* Gravity on earth is about 9.8 m/s², but can be rounded to 10 m/s² for rough calculations.

All that together means lifting a cubic meter of water by one meter represents an energy of 10 kilojoule which is 2.77 watthours.

Filling an Olympic swimming pool (50 m x 25 m) to a depth of 3.6 meters and pumping that up a 100 meter represent an energy of 4.5 gigajoule which is 1250 kilowatthours.

The average American consumes about 30 kwh a day.

You can see how it would be difficult to scale this up.

There are only so many lakes you can put on top of mountains to create pumper storage facilities.

Trying something silly with cranes and concrete blocks or ships and ballast would simply not work.

You need something that can get big and is really low maintenance.

It’s already be said, but I think it’s worth repeating. Adding a buoy/boat to the cannot improve efficiency, not even a little.

The energy to lift the boat comes from the pumps as they fill the reservoir.

If the boat **drops with the water** as you let water out, then the energy of the boat dropping is recovered by the turbine as the water flows through it.

If you **hold the boat up high until the water is gone** and then drop the boat, then the energy of the boat dropping is **NOT** recovered by the turbine as the water flows through it. Yes you can now use a generator connected to a cable (or something) to recover the energy as you drop the boat, but it’s **the same amount of energy** that the hydroelectric turbine missed out on when the boat didn’t drop with the water.

In both cases, there is no gain in energy. The pump that raised the water level supplied the energy to lift the boat. In the second case, a whole lot of extra equipment was added to get exactly the same energy back out of the system that the hydroelectric generator could have gotten anyway.