I came up with this idea a while ago. It's simple, easy, and inexpensive. It seems like a perfectly good, clean, renewable energy-thing. It can be placed anywhere (with gravity, at least). It can be made any size (within reason).
I never did the math, but I did test it to make sure it was possible to create a water-cycle using siphoning. Here's a world-class diagram, to give an idea of how it works:
What you're seeing are two cylinders. One filled with water, the other has a rotor, and an opening near the bottom. The siphoning tube takes the water out of the left cylinder, dropping it onto the rotor in the right cylinder (thereby turning the rotor). The water then drops to the bottom and goes back into the left cylinder to be recycled (for this part, you might need to also use a hinged-door, which opens the way you want water to flow and closes if water tries to go the other way).
You can make these things very big, and they grow
up mostly, like skyscrapers, so they don't take up much room.