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Several yeas ago I saw a device on TV that used the temperature differintal of the water/air to drive a coolant based system. He in-lined a small turbine to capture the refrigerant motion energy. It lit a 100W standard light bulb quite brightly. I never knew what became of it, and I have no clue how to track it. It used no compressor. It was shown on local news Eugene, OR in about '83-'85. One set of coils were floating in the Willamette river, a set of automotive flex hoses to shore (5') and a big ass set of coils in the air. A turbine in the lines between coils. I think it was double acting using both sides to get power.
I imagine the possibility of leaking freon & associatted chemicals into the water is what killed that idea. However I think it might work with a different medium like helium, propane, hydrogen as the main coolant fluid. Something begnin to the environment in the event of failure to contain. One wouldn't need much flow with a big absorbtion turbine and exchange system. Massive refrigeration as a byproduct might be possible. Any fluid dynatics geeks awake out there?
Just how big is the river flowing by? The Colombia? The Mossouri, The Snake.... How big is big?
If you have permission to anchor an undershot on a flexline to allow for rise and fall, one could get some power that way. Set it upstream, and close to the "whirlies" that show obvious water motion. Make it with adjustable depth of draft on the paddles for a govenor. Have wings that funnel more water into the wheel zone. Make the pontoons, or other floatation, super durrable. Run it deeper to get even more torque. There's a lot of if's in a row needed to make power this way. IF one could string together 20, 30, or more of these down a long straight line it would have to add up. Irrigation canals should do this or something else to get that power back instead of just letting it waste. Opps, sorry, got to thinking of conservation of power there. :-)
Good luck oh yee most blessed with water. I live on a rock pile/hardwood forst/south side of an Arkansas mountain. I could build some water diversion ditches and carve up the side of my considerable incline to gain head with very limited flow. That would only happen occasionally so I can't see it high on my list. I dream of some water flow!