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I'm not a physicist, and this question is bugging me .. :)

Is it possible to create a closed power-generation system, using upper and lower water-reservoirs where a continuous flow from the upper to the lower turns a waterwheel (or series of..) with an elongated axle(s) to which several generators are attached with bevel-gears, by using the power generated by some of the generators to pump water back to the upper reservoir at the same rate as the outflow, and the remaining generators feed the grid?

[edit] Based on the comments and answers:
(and I'm not fooling about, I really want to understand)
Say you have one waterwheel with one generator attached which gives say 50% efficiency (can return 50% of the used water to the upper reservoir, all friction, heat-loss, etc, taken into account); Now you add 2 more generators to the waterwheel's elongated axle: the output of two of these generators will return 100% of the water, and the output of the third is available for use. There can be a cascading set of waterwheels each with generators attached..
Why would this not work, what am I not seeing?

[PS:]Wasn't it Archimedes who said something about a long-enough lever and a place to stand and he will move the earth? Meaning that a small input (muscle-power in this case) can have much larger output. A waterwheel is nothing less than a lever, and so are the gears, ...

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  • $\begingroup$ Do you mean something which generates energy from nothing, basically? $\endgroup$
    – CL22
    Apr 15 '15 at 6:38
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    $\begingroup$ Pumped hydro is possible. Perpetual motion not possible. $\endgroup$
    – Nick Alexeev
    Apr 15 '15 at 6:38
  • $\begingroup$ @Jodes: no, - gravity ... $\endgroup$
    – slashmais
    Apr 15 '15 at 6:39
  • $\begingroup$ To use the energy from flow of water to put the water back, with energy to spare, is creating energy from nothing. That is perpetual motion, which is impossible $\endgroup$
    – CL22
    Apr 15 '15 at 6:40
  • $\begingroup$ @Jodes: even with several wheels where you can adjust the radiuses (==lever-lengths), as well as that of the bevel-gears? $\endgroup$
    – slashmais
    Apr 15 '15 at 6:43
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Say you have one waterwheel with one generator attached which gives say 50% efficiency (can return 50% of the used water to the upper reservoir, all friction, heat-loss, etc, taken into account).

Got it. Remember, generators work by turning the power from the input shaft into electrical energy. The power is the speed of the wheel multiplied by the force it's pushing against (the torque of the generator, plus drag).

Now you add 2 more generators to the waterwheel's elongated axle: the output of two of these generators will return 100% of the water

No, it won't. The additional generators will (assuming they're collecting power) place additional drag on the wheel. It will slow down and all the generators will draw less power. The sum of the collected power by all the generators will be less than the input power from the falling water.

and the output of the third is available for use. There can be a cascading set of waterwheels each with generators attached.. Why would this not work, what am I not seeing?

Every collecting generator is more drag. They can't be attached for free.

Wasn't it Archimedes who said something about a long-enough lever and a place to stand and he will move the earth? Meaning that a small input (muscle-power in this case) can have much larger output. A waterwheel is nothing less than a lever, and so are the gears,

That is the cite, but he didn't say how far he could move it. Levers let you trade force against distance, or in the case of a generator, rotational speed against torque. We can lower the torque so the water spins the wheel faster. But power is the product of speed and torque. The levers don't let us make them both bigger at the same time.

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  • $\begingroup$ now this is a nice ELI5(see reddit) thank you. $\endgroup$
    – slashmais
    Oct 31 '20 at 6:28
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The problem is electrical resistance and conservation of energy and friction. You lose force due to mill friction. You lose electrical energy due to electrical resistance. Lets say you have 25% of the energy left. You would need 100% to lift the water back to the top.

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Yes, this is possible. You can totally build the described contraption and it will probably even run.

It will however, not run indefinitely.

"feeding the grid" means to take energy out of the system, which means it will eventually stop its operation and that assumes a machinery without any other "losses" (see @Jimmy360's answer on that)

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This is possible but not 100% efficient, so you will always get back less power than you put into it - at best, maybe 80% with a large-scale plant designed for best efficiency.

So why would you want to?

If you can buy electricity at 3p/kWh overnight, and sell it at 30p/kWh during the early evening peak demand, then even factoring in the losses, that's good business.

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