I'm imagining a canister for storing compressed gas connected to a fully inflated balloon. This is sea-level. The valve is open so air can move freely between the bag and the tank. Some heavy objects, let's say, bags of sand, are attached to the canister to overcome the buoyancy of the balloon and the contraption is tossed overboard and is allowed to sink into the sea.

I presume as the depth increases the gas in the canister/balloon would become increasingly compressed. If the balloon was too small there would be a depth beyond which the balloon would be completely deflated.

By some mechanism, at some depth, the valve is closed and the ballast is disconnected. I would like to retrieve the canister of compressed air by allowing it to float back to the surface but I would settle for having the canister tethered by some cable.

I suspect I am violating conservation of energy somewhere, but I can't shake the intuition that such a system could convert the potential energy of the sea-level ballast rather cleanly to compressed gas.

Why is this impractical?, I guess, is what I am wondering. Too much energy to move the heavy ballast material? Compressed gas not worth having? Gas compresses to liquid and messes up all the equations? Containment cannot work for the canister at surface retrieval due to material constraints? Or maybe the volume of the gas required to make this practical can't be submerged this way.

Edit: In thinking about how to edit this question to make it a physics question instead of an engineering question I fear I would have to change so much that the exchange in the comments below would lose its context. I knew the question felt a bit border-line as I was asking it, so I tried to put in a concrete example. That contributes to the 'engineering' feel of the question but it could have turned out to have been an acceptable question if read and answered in good faith. (see my comment on the accepted answer)

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    $\begingroup$ by what you are describing , you cannot harvest energy, you can just harvest compressed gas. The energy needed to set up the system is lost in recovering the canister for example, and frictions. $\endgroup$ – anna v Mar 20 '17 at 5:09
  • $\begingroup$ Yes, this would work. But it is the same as hydro-power from water flowing down hill. In your case the ballast is the "water" and you need to supply a steady flow of rock, or whatever you use. Eventually it will fail as you fill up the deeper canyons. You can imagine that in the limit, you have a shallow sea that covers all the Earth but a little sand island which is the remains of the last mountain top. Your energy comes from uplift from plate tectonics (so maybe from the energy of the Earth-Moon orbit and the Earth -Sun orbit), so it is in a long term sense, renewable. $\endgroup$ – C. Towne Springer Mar 20 '17 at 5:29
  • $\begingroup$ Related: physics.stackexchange.com/questions/556 $\endgroup$ – Kyle Kanos Mar 20 '17 at 10:01
  • $\begingroup$ @anna, I don't claim that it is a perpetual motion machine. The energy captured by such a system would come from the difference in potential energy of the ballast at sea level and at the point the canister's decent is terminated. The setup of the system and the energy expended at the surface to move the ballast, collect the canisters (assuming they could be buoyant) would not depend on the depth of decent. So why assume the energy would all be 'lost'? $\endgroup$ – John Breedlove Mar 21 '17 at 5:26
  • $\begingroup$ Not sure why this has been voted off topic... It's a physically consistent (but almost certainly uneconomical) concept for generating energy (though the "renewable" tag is questionable at best). If that doesn't fit on topic, it would be polite to explain why... $\endgroup$ – Flyto Mar 21 '17 at 8:09

Apologies for a brief answer, but:

What you have invented is a way of compressing gas by throwing rocks into the sea: the energy that is lost (partly converted into the potential of the pressurised gas) is the gravitational potential energy of the ballast.

If the can of compressed gas were still sufficiently buoyant, you might be able to harvest some energy from its ascent as well.

If you didn't have a need for compressed gas you could achieve the same, probably more efficiently, by attaching a cable to a big rock and dropping it in the sea, then detaching the cable from the rock and winding it back in.

My instinct is that you'd run out of rocks and/or fill up your bit of the sea fairly quickly, ie you'd go through a lot of ballast for not a lot of return, making it economically as well as environmentally prohibitive, but I haven't done the maths.

You may be interested to know that there are serious proposals for something slightly similar, not for generating electricity but for storing it in the form of compressed gas in containers on the seabed. See, for example, https://www.theengineer.co.uk/issues/25-april-2011/compressed-air-energy-storage-has-bags-of-potential/

  • $\begingroup$ Thanks for the link. That article got me thinking about a couple of limitations I had not initially thought of. First the heat energy in the gas will be lost to the water on decent and any large scale applications at the surface may have to reheat the air to use it in turbines. Secondly, I have to pay in energy to move the ballast above water at its full weight but it is only going to provide energy for compression for its weight minus its own buoyancy. $\endgroup$ – John Breedlove Mar 24 '17 at 1:21

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