Rule #1: there is no such thing as free energy. You can only convert energy from one type to another, and you always loose efficiency.
Maybe if you want to power a single LED or even a couple, you could put a padlle wheel under the return line anything more will require you to restrict the return flow or have more return flow, by pumping up more water(=more power needed) in the first place.
Other then a little science project with the kids, there is not much value, IMO
Waffleman