Want to build one of those old-style water wheels powered by a waterfall. Is there a more efficient way than the ones that were around since the start of the industrial revolution?
I guess it depends on what you mean by "better".
The drawbacks are that the water must be as clean as possible and you should use all water for maximum efficiency. You also have to source a decent pelton turbine.
For the ecosystem of your stream/river, this setup usually prevents fish traveling past your power plant. It seems to be general permaculture knowledge that the nutrients, which get washed out into the sea, travel back up in the form of small snails, crayfish and the like, which, it is believed, play an important part in keeping a land healthy.
A fairly new (about 10 years) system is known under the cumbersome name "gravitation water vortex power plant", where water is fed into a pool tangential to the edge of the pool. The water forms a vortex, as in your bathtub, and blades are inserted into the vortex to slow down the rotation. The blades are connected to a generator and give you the electricity.
The picture explains everything, but you can also watch it in action.
Since the water is fastest at the edges and slowest near the centre, fish and other freshwater creatures can travel past your power plant. (Also, not really applicable when you already have a waterfall, you don't need a large "head", i.e. a large difference in height.)
As a very rough estimate, a small stream (maybe 2-3 meters wide) can provide electricity for several households.
The Banki Turnine is often mentioned in connection with small scale hydro and DIY projects, because it’s far easier to build than a Pelton turbine.
Ossberger, one major supplier, claim 80% efficiency for a small scale turbine over a wide range of flows – this will, of course, be lower for a home built one.
This document seems to contain detailed instructions for building on, including the welding of the turbine itself.
Disclaimer: I'm not in hydro and have never built such a system, so I can’t really vouch for it.
An additional possibility would be to use a hydrodynamic screw – working principle:
The German Wikipedia article states efficiencies of typically 70%, 90% is possible. Apparently, screws handle load changes and part load operation very well, but are surprisingly noisy. Typically they turn quite slowly, 20-60 rpm. Also they are described as being fish-friendly, though I can't say how much so in regards to which fish.
As for the generator, scavenging a permanent magnet DC motor off an old treadmill is a good start. There a LOT of treadmills out there. When I got my camper I thought it would be nice to have some additional power from the wind to supplement my solar panel.
Put a listing on CraigsList asking for free treadmills for parts. The next day I had one with a good DC motor. Cost me nothing. I still have to complete that project, I would like to be able to put a water wheel on it, mount it on stakes in a creek or river in addition to putting it on a mast with wind blades instead if wind is more plentiful at a given site.