[peppercorn (OP)]

So, imagine a gigantic diving cylinder...
Fill it with water and sink it to the bottom of the ocean.  Once there bolt it to the ocean floor (somehow) and have a mechanism where an electric motor actuates a whacking great piston to force the water out of the cylinder into the ocean (leaving a vacuum more or less).

How much potential energy does this device hold (per cubic something)?

Could it work in a similar way as pumped storage (ie. water up a mountain) for balancing grid loads?

Or would the seals on the piston just fail in no time at all?

Thanks   []

[Geezer]

One atmosphere supports a column of about 30 feet of water. So, at one mile down, the pressure would be about 176 atmospheres or around 2,640 PSI. That does not seem like too much for a decent seal to support.

Of course, if you could find an oil that was a lot less dense than water, you could use it to force the water out of the cylinder then use it to drive a motor at the surface. I suppose you would not even need a piston, so the cylinder could simply be an inverted chamber of any shape and size. However, the denser the oil is, the less pressure you'll get, but I suppose if the volume was great enough, that might not be a problem.

Alternatively, you could use one of these instead

http://en.wikipedia.org/wiki/Pumped_storage_hydroelectricity

[peppercorn (OP)]

I've realised I had somehow assumed that pushing an empty 1m³ box 100 metres under the water would need more work than lifting 1m³ of water 100m into the air!

Am I now right in thinking, in fact, they represent more or less the same work done?


This type of storage may *just* be useful in (non-grid) places by deep water with no high land.  Even then it is probably going to require a suitable undersea natural cave system, where air can be pumped against the pressure of water in a large space.

[Geezer]

I've realised I had somehow assumed that pushing an empty 1m³ box 100 metres under the water would need more work than lifting 1m³ of water 100m into the air!

Am I now right in thinking, in fact, they represent more or less the same work done?


This type of storage may *just* be useful in (non-grid) places by deep water with no high land.  Even then it is probably going to require a suitable undersea natural cave system, where air can be pumped against the pressure of water in a large space.

It's probably about the same amount of work.

I seem to remember that there are energy storage systems that store compressed air underground. I suppose an advantage of the underwater version would be that it delivers constant pressure because the chamber does not have a constant volume.

Compressed air is not a very efficient storage medium though. A lot of energy is wasted (as heat) in making it.

[peppercorn (OP)]

You mean this?
http://en.wikipedia.org/wiki/Compressed-air_energy_storage

here's my suggestion. NB. pumping water not air...

ATTACHMENT DELETED - as incorrect

[Geezer]

I'm not sure you need the pump. The air will be pressurized to the same pressure as the water.

Can't you just push air down the air line (which will force the water out of the cave) then let the air back out to do work as the cave fills with water?

[peppercorn (OP)]

I'm trying to avoid pumping air due to the adiabatic losses.

To be honest I think I got my concepts muddled up (I changed what I was drawing half way through).

The best method will probably be to get rid of the air line and suck all the water out of the cavern (need to move the pump to bottom of tube).  This is really the idea I started with - only using geological spaces rather than fabricating them.

The biggest problem is that the cavern might collapse as the it depressurizes.

[Geezer]

OK - I think I understand. So the pump would run as a turbine to produce power when you let the cavern refill?

If it was a few thousand feet down it would not need too much water flow to produce a fair amount of power. The risk of collapse might be a real problem.

[peppercorn (OP)]

Updated illustration:

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