Is it possible to generate electricity using only bouyancy?

Question

In theory, is the type of machine below possible, or is it a form of perpetual motion machine?

You create a hollow tube, full of air, that stretches from the Ocean surface to the bottom of the ocean, and line it with turbines. You then drop a heavy but bouyant weight down the tube, expel it somehow at the bottom, and allow buoyancy (a.k.a. gravity) to carry it back to the surface. You then drop it over and over in order to generate electricity.

Answer 1

Interesting idea. As another answerer points out, wave (and some versions of tidal) schemes are essentially using buoyancy. A few thoughts;

1. You make a rig inside of your tube similar to an elevator where the lowering mass generates electricity on its way down. You would have to lift the elevator car back up after you took the mass off, but it would be much lighter without the mass in it. This way you are getting power out of the mass' (gravity) trip down as well as its (buoyant) trip up.

1a. It just occurred to me - if you could have the rising mass outside pull the elevator car up (as well as generating electricity) then you overcome the extra complexity required to get the car back to the top.

2. You get the mass out of the tube and into the water by having a water lock at the bottom (like an air lock in space ships, but with water), so you are not having to continually flood and pump down the tube over and over.

3. You maximize the buoyancy part of the trip (and get around having to handle big objects) by making the mass a collapsed air bag on the way down that you fill up with air once you have it out in the water. Perhaps you could use air compressors powered by the electricity generated from the falling elevator contraption. Or, if you can find some cheap chemical reaction that generates a lot of gas you use that to fill the bag. Chemical reactions don't care about the amount of pressure they are producing against.

4. The engineering required to build this hollow tube is pretty extreme. My gut feel is that the device would have to function for a long time to payoff the engineering/construction costs.

5. It shouldn't be too hard to do a potential energy problem for the two different legs (up and down) to get an idea of the kind of power levels we are talking about. While idle speculation doesn't bother me in the slightest, doing the math on this would be too close to actual work for me. Any takers?

All in all seems like it could work. The only problem is that fishy smell of perpetual motion.  The law of conservation of energy always finds a way to send perpetual motion schemes down in flames. What are we missing?

Answer 2

So buoyancy carries the weight back to the surface. But you left out a step, removing the water from the tube! Pumping water against the pressure at the bottom takes a lot of energy. That would take a lot more energy than you could recover.

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Answer 3

I guess so, though you would have to make it reach the bottom somehow, it sounds like you would have to use other energy to do that, therefore making it not perpetual motion

...also, you may have to think about rotation problem, and sea ceatures in the way.

Answer 4

The concept of utilizing the vertical desplacement of floats riding on waves has been around for a couple generations. It is technically feasible, however not yet found to be practical to be of use apart from limited, particular applications.

Answer 5

Your problem is with the "expell it somehow" portion of your explanation. While you are expelling it, you are in effect lifting the average height of sea level upwards in order to make room for your object. This expulsion will require more energy than you will derive by letting it float back to the surface, and while it will be floating back to the surface, you will be dropping average sea level back down to your starting point. Sorry, but this does not qualify as a perpetual motion machine. But don't get insulted, since they are impossible anyway.

Answer 6

http://www.darvill.clara.net/altenerg/wa... easier to use the Waves on the top