pond airlift as hydroelectric generator?

[hrur_j]

Hello everybody!

A friend of mine recently pointed my attention towards so-called "airlifts" for pond water circulation and filtering.
The principle is very simple, you attatch a vertical tube in your body of water and pump air into the lower end of the tube, witch then bubble upwards inside the tube.
This motion of bubbles propells the water up thru the tube, allowing for a slightly elevated water ejection port, witch can then sift thru a filter down into the pond.

I am no engineer by any standards and i failed highschool physics, but it sparks ideas in my head..

Im thinking; air expanding under lesser pressure and therefore increase speed up thru the tube, hence; longer tube = greater available energy in form of higher water flow upwards.

Now, since not just a part of the water column in the tube is in motion, i dont need a constant stream of small bubbles, i can release large bubbles every x seconds to further increase momentum.

Now, if we were to place a vertical turbine blade in the center of this tube and use the rotational output to power a electrical generator, would it create enough to power a air compressor big enough to feed the system?
If so, would there be surplus energy available?

Ive also discovered that these airlifts are used for water wells, to pump water up from some depth.
This means that the top part of the above suggested system could end several metres above sealevel, and use that fall-heigh to power a oldschool water-wheel.

Now, i come with this idea for the simple reason; please shoot it down for me!
..i need to drop it or build it, but i cant drop it until i know.. :P
..and i dont have the cash for such experiments.. :P

The reason for this idea is mostly to circulate the water in the baltic sea (between sweden, finland, denmark, poland and so on) where the bottom fauna is on some parts dead and dying due to lack of oxygen, while the top living fauna is dying becuase (as i understand it) of lack of nuitrition.
So a generous swap of water would benefit the region for all life, for a very cheap price.

And if there were enough to generate electricity for land-based consumers, that should seriously motivate governments to build their own vertical hydroplants.. :P

So again, please shoot this down for me, so i can drop this idea thats been clinging to my head for too long now..

[jastewart]

You are describing a perpetual motion machine of the second kind, forbidden by the rules of entropy.
You will have to put in more energy than you can generate.

Perhaps you might be better served with wind turbines pumping water electrically.

Secondary to this, use wind to power your device, such as a venturi with high velocity wind section in it, and use the vacuum created to pump water, however, one can only lift water a distance of about 10 m.

A submersible pump will be able to move water (and nutrients) from the bottom more easily).

A scheme researched since the 1970's by the U.S.A. used changes in the density of water from cold to warm to produce circulation, and generate small amounts of energy in this way, OTEC is the acronym for this, but it will not work as well (and it worked inefficiently at best) at higher latitudes unless you combine this with some sort of solar heating of the surface waters, preferably in a surface impounded floating "tank". This surface impoundment could be made fairly large, depending on stability, and calmness of local seas (which may not be very likely at all). If the surface impoundment is made with a dark geofabric bottom, and still provided sufficient buoyancy, a saline gradient in the impoundment could be thus formed, allowing heat to be trapped (to at least a limited extent, depending on insular value of the impoundment bottom). Low quality heat (less than water normal boiling point at your indicated latitude) would be harnessed by Stirling cycle, or SCC cycle piston engine driving hydraulic accumulator. The stored hydraulic power can be efficiently used for pumping water, generating electricity, desalination, etc.

I have not heard of any system utilizing deep ocean water as the cold sink, with a shallow pond as a heat source, heat collector, but this has a better chance of working than attempting OTEC in the Baltic. Clearly geofabric lined pond with saline gradient can produce a useful upper thermodynamic temperature to drive Organic Rankine Cycle, Stirling, or SCC engines.