...for a power collector, without distinction of the flow's nature (air,water...gravity fall),you can see at : www.geocities.com/iacob_alex/Some_Drafts/text012.JPG
The main idea is to play torque difference,on the same side of the fulcrum.
We can use " the long arm-short arm" scheme in any flow.
The two masses (M1and M2) have the same weight ,size and shape (spherical).
All the Bests ! / Alex