I am wondering about the concept of water flowing from one place to another due to air pressure and have a question about the limitations in the real world. Don't understand me wrong, I am not trying to create a Perpetuum mobile, I am trying to figure out what are the limits of such a concept.

So imagine that 2 water tanks (see scatch below), one higher than the other, are connected with a tube and underpressure is created for brief time period(s), so that water flows from the bottom tank to the top (an analogy would be draining the fuel from a car by sucking on the tube). Imagine that also there is another tube between the top and the bottom tank that lets water flow down and create electricity through a generator. I am aware that such construction can not generate net energy over long periods, else it would have been already built. But what are the reasons it can't work?

I know one limit is that the maximum height difference between the 2 tanks is limited by the air pressure to $\approx 10-15$ m. But this doesn't prohibit the operation of such a mechanism in small dimensions. What else is there that I am missing? Tnx

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  • $\begingroup$ Because of frictions, resistance, and other losses you cannot generate as much or more power than the motor takes to turn the generator, no matter how they are connected. $\endgroup$ Jun 11, 2021 at 11:02
  • $\begingroup$ See; en.wikipedia.org/wiki/Laws_of_thermodynamics $\endgroup$ Jun 11, 2021 at 11:05
  • $\begingroup$ Thanks, my actual mistake, as Dale points out in the answer is, that the siphoning will only work as I describe (the motor running not continuously, but only to cause an initial underpressure for sustained flow), if my second tank is below the first. Making the rest of my idea obsolete $\endgroup$
    – NeStack
    Jun 11, 2021 at 14:46

1 Answer 1


One thing that you are missing is that your upper tube only moves the fluid upward when the motor, M, is actively running. When the upper motor is not pumping then fluid will be siphoned down. You mention siphoning fuel, but seem to not realize that after the initial sucking the siphon only works if the receptacle is lower than the source. You should test this at home yourself, but with water instead of fuel.

This system only moves fluid clockwise when M is actively running, and G will always produce less power than M consumes. Even if both M and G are 100% efficient. This is because fluid friction will waste some energy in heating the fluid and the container and pipe walls.

  • $\begingroup$ Thanks a lot for finding my actual mistake in the idea! You are right, that I didn't realize the siphoning (of water or fuel) works only if the second tank/container is lower than the first. I'm a little bit embarrassed :) $\endgroup$
    – NeStack
    Jun 11, 2021 at 14:44

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