Compressed air/bouyancy generator

[hrldjmsn]

Hello, I had good luck getting set straight on a seemingly good but inherently flawed idea a few months ago on this site. I have another seemingly (to me) good idea and am hoping for helpful input.

    The idea is essentially to sink compressed air in deep water, release it at depth and use the energy of its bouyancy, the trip back up, to make electricity. The catch is how to overcome the energy cost of compressing the air in the first place:

Imagine-

  -a floating platform over deep water

  -a very long length of bicycle style chain (how long it needs to be is an essential question)

  -a generator unit on top of the platform

  -the chain draped over the platform, threaded through alternator and free cogs on either side of platform.

  -equally-weighted underwater cages attached to either end of the chain. Cages have large bags on top, open at the bottom to catch bubbles released from below by compressed air tanks. Cages also could/should have lots of extra weight, (rubble or something).

  -alternator plus batteries runs compressor and tank exchange system at top.

Start:

  -cage 1 is down(chain fully out below platform on cage 1 side), cage 2 is up. Both have empty bags.

  -Tank in cage 1 releases air to fill bag. Cage 1 begins to rise while cage 2 descends, turning generator.

  -When cage 1 nears surface platform, valve on bag opens and releases air.

  -At the same time, valve on tank in cage 2 opens to release air, filling bag on cage 2.

  -When tanks near empty, they are replaced with full ones at the top by a robotic arm/some kind of shuttling device. Empty tanks are refilled by compressor.

The crux: (Assuming the basic concept doesn't have anything major wrong with it.)

  -How long of an ascent/descent does it take to overcome the energy cost of compressing the air. If it is too long, then the air at depth will not be buoyant enough to lift the lowered cage. I imagine that would have to be very deep indeed, so hope that it will work, at least in principle. Great depth could also make it unfeasible because of distance from points of use. It would be fantastic, although unlikely, if it worked at a small enough depth to be feasibly used in large man-made tanks/deep pits/lakes.

  Right, there's my idea, thanks for reading and thinking about it.

[Roamer195]

This is how dolphins and whales cross the oceans without using up much of their energy reserves.

They compress and internal bladder which causes them to sink. Their bodies then "glide" forward through the water as they're sinking.

When they reach a certain depth, they expand the air bladder and continue gliding forward as they float upward toward the surface again.

It only takes a momentary muscle flexing to manipulate the bouyancy bladder at the high and low peaks of the glide.

Of course, the overall horizontal movement is at right angles to gravity, not directly resisting it.

[finnsawyer]

Won't work.  Here's another take on it.  Suppose you design your tanks with a piston on one end and just let them be full of air at atmospheric pressure.  The weight is such that a tank just starts sinking.  As the tank sinks the water pressure will force the piston in, compressing the air and reducing the buoyancy, causing the tank to fall faster.  When it reaches the bottom the air will be at maximum pressure and could be used to provide power.  When the air is exhausted you pull the tank to the surface.  The trouble is, all the buoyancy is gone.  The energy you need to raise the tank is at least equal to the energy you got from the compressed air and likely more, allowing for the usual friction losses.  This would work only as a one shot deal.  On the other hand since the pressure surrounding the tank is the same as the air pressure you could get the system to work for any depth, even seven miles, if the air hose could handle it.  So maybe there's merit in it as a one shot deal, if the system and tanks are cheap enough.

Two other possibilities: Use a hydraulic fluid instead of air and/or hang weights from the tank that are then dropped so the tank can be economically recovered, in which case why not just use weights?

[ghurd]

Why bother with the tanks? Just run a hose down.

The energy to get the air down is going to be more than it makes on the way up.

G-

[hrldjmsn]

I wasn't thinking of getting any energy from compressing air, but from decompressing it at depth and then using its bouancy to lift a weight...then drop the weight...then lift it,etc. Got the idea from seeing archeological divers lifting big pieces of wreckage by attaching a bag to them and then filling the bag with bubbles from their tanks. The bag and the tank and the wreckage all sank naturally, giving up potential energy, then decompressed air used its potential energy to lift them back up. This is a common method for lifting things off the sea floor, so their must be numbers somewhere on the air volumes needed at which depths for what masses. As you increase the depth you loose potential energy from the air, but not till you reach equal or greater pressure on the outside as on the inside  of the tank. And as it rises it gets more and more powerful.

  My main doubts come from not being able to pin where the energy would be coming from if it did run a surplus. Would have to be something pretty esoteric; warp drive concept of changing the local shape of space-time or something. But really, that's what our muscles or any other engine does. I know this is crazy and probably won't work, but there seems to be something to it... appreciate that your taking the time to explain it. Thanks!  

[finnsawyer]

In order to fill the bags at depth the air will have to be compressed relative to atmospheric.  So think of running a hose down from the surface and using a compressor.  You have to expend energy to fill the bags enough to get positive lift.  The load rises to the surface.  The bags are used to help inflate the submerged bags for the next load.  But their maximum pressure is just equal what is needed to inflate the submerged bags.  So, the air from them must be run through the compressor, which does less work than before, but still puts in net energy.  So each lift requires the input of energy (at least equal to what it takes the raise the object against the Earth's gravitational field).  Sorry, no free lunch.

[hrldjmsn]

I understand that it would cost more than you would get to push air down a hose, but that is not what I am proposing, I am proposing compressing the air into tanks and then sinking the tanks. Once the tanks sink below the level that the compressor could have pushed the air to, then there ought to be a surplus of potential energy available from the air in the tank. Unless that is also the depth at which the released air is no longer bouyant... but following the reasoning of the piston tank example, there is maybe a zone where the greater pressure aids the tank in expelling the air, but the air is still bouyant....as you can tell, I am out of my league (no pun intended) but just want to be sure that you understand what I am thinking before we write it off. -H

[finnsawyer]

Since the tanks are assumed perfectly rigid at least down to working depth, there is no effect of the water pressure on the internal air pressure.  The tank absorbs the difference in pressure.  (Think submarine).  So, you can only get out the same or less energy than you put in initially.  If the tanks can collapse, then the pressure of the water can be transmitted to the air inside, but this is basically the same as the piston idea.  If the tank ruptures, all is lost.

[picmacmillan]

hi geo ...as i was reading this i was thinking of something along the lines of an aerator for a fish tank...puts air back in to the water...you could fill a bladder or whatever with that maybe....i would also think air may not be the best tool here...i remember while working on mathematical solutions in school...while i was bored one day i figured out how big a helium balloon had to be to lift me up off the ground...i am 165 lbs and the balloon had to 8'(feet) in diameter...i didn't do any science of using helium in water though...just a thought that maybe another medium other than air could be used although air is cheaper than most others..pickster

[finnsawyer]

Yeah, and as the balloon lifts you up it must expand to continue displacing the same weight of air, as the air density, unlike water, decreases with altitude.  In water a tank containing a vacuum can be buoyant.  In fact, since air has weight, you could fill the tank to some pressure (read that as weight of air), allow the tank to sink to some desired depth, release the air and hence lower the weight of the tank, and have the tank bob back to the surface.  But this won't get you any net increase in energy.  I think I've gone as far with this thread as I can go.

PS:  The air in the tank would only flow into the bladder if the pressure in the tank exceeds that of the surrounding water.  An aerator for a fish tank must also overcome the water pressure and hence do work.  No free lunch there either.

Question:  Why don't they increase the air pressure in a submarine to equal that of the water pressure?  Then the sub could go to any depth.  It seems this could be done safely if done slowly enough.