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Quite recently the company Rosch has developed a new kind of power plant that supposedly utilizes the buoyancy effect to generate electricity. The apparatus consists of a vertical conveyor belt with buckets attached to it. The whole construct sits inside a cylinder of water. Through a compressor, air is blown into the upturned buckets which decreases their density and allows them to rise. At the top the air is released and the buckets go back to the bottom of the cylinder. This turns the conveyor belt and a generator that produces electricity. A schematic is shown below.

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Now this technology is very controversial and many claim that it is a fraud but the company has had a test facility running for quite a while and generated enough electricity to power both the compressor and a number of household appliances.

I have spent a lot of time trying to figure out problems with the system because it seems that it does not conserve energy. Sure it uses gravity but still there is no loss of energy elsewhere, like the loss of potential energy in a hydroelectric facility for example. I simply do not understand where the energy is coming from. Would this not count as a perpetual motion machine?

(If this is more related to engineering feel free to migrate the question)

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  • $\begingroup$ @Qmechanic: ah, I followed the link, no it's not duplicate! This one is (to the common folk) different and more tricky! The question is very interesting, much more interesting than all the crazy quantum/string/gravity/etc questions. This is a company that pretends to produce something, it's some kind of civil responsibility to react on that. Especially since they call themselves green, "Save The Planet AG" - and maybe even want to or actually get subsidies from the EU etc. $\endgroup$
    – Ilja
    Apr 16, 2016 at 8:20
  • $\begingroup$ This post (v3) seems to be off-topic because it promotes non-mainstream physics. I close it as a duplicate, not because it is necessarily an exact duplicate, but to point in the right direction. $\endgroup$
    – Qmechanic ♦
    Apr 16, 2016 at 15:00

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This is just a trick, or, if it is meant seriously, a lie -- and upon visiting the site I tend to think, that it is meant seriously. I would not even exclude, that they want to get or actually get subsidies from the EU! This is the most important part of the question. If anyone knows how to find such things out, I would really welcome this!

This is the same as a reversed machine, pumping water upwards and pouring it into something that gets rotated.

In fact, here the water does get pumped upwards. By inserting some air you have to replace some water by it, the surface is lifted. You can see it best in the bucket - to fill the bucket with air means, to lift the water contained in the bucket to the surface.

You could, indeed, argue like this:

The gas expands on the way upwards. So in the bottom you need to replace less water when filling the air, and the rest of the replacement the air does itself by expanding.

This is a tempting thought. But where from comes the energy for the expansion? ... Aha, the air cools down! And here you have another problem: The pump will heat the air when compressing it to the desired pressure! And this energy gets reduced again through the expansion on the way up. Part of it, at least. Some is simply lost to the water and metal of the bucket.

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  • $\begingroup$ regardless, there is a net upward force on the one side of the conveyer belt applying torque to the generator $\endgroup$
    – Jaywalker
    Mar 22, 2016 at 10:52
  • $\begingroup$ yes, there is. And it does some work: $mg\Delta h$, where $m$ is the mass of the water displaced by the air. But this is the same work as the pump has to do, to replace the water by air (this lifts the surface, i.e. it's equivalent to liftig the original water out of the bucket to the surface) $\endgroup$
    – Ilja
    Mar 22, 2016 at 10:56

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