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