With my farm I bought, it has a decent stream that flows year round that runs along the 800' of frontage I have near the road. My driveway goes over this by means of a 40' long x 5' diameter plastic pipe. Over the last few days, I've done some measurements to see the suitability for hydro in the future.
The stream flows about 100-200 GPM on most days that it's not raining. In the summer in a dry spell, it might get down to 50 GPM. In the ~48 hrs after a rain, 300-600 GPM is expected. During a light rain or a medium snow melt day, I estimated about 1800 GPM. On the high snow melt days, it fills half the tube....so probably 1-2 m^3/second.
The downside is I don't have much drop. Over the 800', it's about 8-9 feet. If I just look at end of the pipe into the stream, I could get about 1'. The 100' leading up to the end of the pipe is about 21", so that would be about my reasonable limit for doing a "run of the river" type design with either a 4" or 6" pvc pipe. 4" at that length would only be good for about 100 GPM, 6" could do 300 GPM.
After much thought, I don't think I have enough to get this past the "toy" stage and reasonably grid tie it. With the GPM * head(feet) / 10 = watts formula, I'm probably 10-60 watts for an undershot Poncelet wheel (1 foot * 100-600 GPM). A run of the river style will be throwing a lot of water away give the minimal GPM I could get through the pipe consistently year round and how long the pipe would need to be. If I came across a bunch of free 6" pipe and I did the whole 800' at 150 GPM, then I could do 100 watts, but it would also be ugly on what is a fairly visible stream by the neighbors.
Grid tie would be fairly easy if I did a good job matching the MPPT voltages to the same Enphase IQ8+ microinverters that I plan on buying for my solar array (future post
).