Low head turbines are available, all depends on the water volume ( torque available)
The NIADE from
www.waterturbine.com is in a sense an equivalent of a PowerPal geared for low HEADS and proportional water levels, it does need a draft tube to increase the HEAD, like the PowerPal units
You fail to inform about the water velocity -- important factor, indeed, to determine the water volume ( liter/second, or gallon/sec, or gallon/minute).
The probable diameter is not wild guessed, it needs to be defined by the water volume and head available.
A much better description of your site is needed to see what is best for it, a propeller or what.
Propeller could be one, also a multi blade propeller (like 12 to 24 blades) to attain high RPM (300 to 500 RPM).
Multi blade can be built using small blades and 8 to 12 inch plastic pipe housing ( like a Y coupling) that can work almost submerged -- limited power --.
You commented about a PowerPal unit, it needs a constant head for the draft tube to properly do its job, so "floating" maybe needed -- same for alike turbine --.
Depending on your design, one can use a Banki on a "FLOAT" to compensate for the water volume height variations if the intake can be "floated" as well.
I have seen some like that in South America.
The idea of the "Pie cut" angles and profile is one that many look for it to attain the maximum efficiency.
You could approach such profile by looking and examining the electric air fan propeller -- the angle is about OK for a fast RPM -- the volume defines the propeller area - with the head the power generated.
Guide vanes are recommended for increased torque output.
Nando
