Apologies for the delay in getting back - that storm caused quite a bit of tree damage. Another this weekend. Ho Hum.
We have a similar situation to some of you. A stream with a long run to reach 15-20m head. Probably 2-300m, and I'm looking at 90-110mm mdpe (alkathene). That will be the best part of 2k. We do have plenty of flow, though - stream is about 8ft wide and a foot to 18 inches in depth. I've calculated around 600watts at 60-70% efficiency.
Based on my research (early stages), I could use a pelton or Turgo wheel, utilising the full head but at lowish flow rates. I could also use a lower head but pull a lot more water and use a screw or prop type turbine.
It seems that for anything over 20m3 a day I will need an extraction license and maybe even a return licence for the water.
Despite the costs, which could approach 5k, 600W is around 15KW per day, which eliminates our electricity bills, and allows extra for some water heating, or possibly, looking forward, an electric vehicle. With that in mind, the payback time on the project is less than 5 years, and then we're earning money.
Clearly this is all theoretical, and will depend on me doing all the work, and a lot more research first.
I hadn't considered doing it and then approaching the authorities retrospectively, as I'd be concerned they'd tell me to rip it all out.
I started thinking about this by watching a pile of youtube videos, by the way. The best of which was Kris Harbour Natural Building. Worth a watch.
https://www.youtube.com/channel/UC5rT7F0PGNuD54rJ9kzgWzw