Expert: Paul Soderman - 11/16/2008

Question
Hello,
1. Would it help an Offshore Wind Turbine to have more than three blades?
2. If I had a coaxial propeller design, both spinning the same direction, would that help the Wind Turbine operate a larger Generator ?

Answer
Alexander
1.  Hard to say.  Wind turbine performance depends on many factors such as mission (power required), wind speed, rotational speed, height above ground, number of blades, fixed or variable pitch, twist, blade shape, and rotor diameter.  If blade number was the deciding factor we could say that 4 is better than 3, 5 is better than 4, 6 is better than 5, and so on.  But blade number is not the deciding factor.  I suggest that a good parameter to start with is efficiency.

Wind turbine efficiency is discussed here:

http://www.ftexploring.com/energy/wind-enrgy.html

The following website discusses diameter, efficiency and rotational speed of an aircraft propeller:

http://www.jefflewis.net/aviation_theory-theo_prop_eff.html

At this website:

http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm

the author relates the recommended prop diameter for a given horsepower available to blade number.  I'm not sure how accurate it is, but it is a good calculation to consider.  I can't be more specific in my answer without knowing something about your turbine.  But it seems there is information out there that can help.  Try google: wind turbine efficiency.

2. Coaxial rotors spinning in the same direction would have flow interactions that would be very important and deleterious.  So the answer to your question is - probably yes.  But it may not be the best way to generate energy.
Paul