by windgat on Sat Jul 19, 2008 5:15 pm
Like all compromises, its hard! Unless you have variable pitch blades, they will be optimised for a particular wind speed. 2km/h wind has such low power density that its really not worth worrying about it. Working with 20km/h = 5.5m/s:
Wind power available is about 0.6 Av^3 (or 0.6 x A x v x v x v).
So for a 3m diameter: area A = 7sq.m, and power is 0.6 x 7 x 166 = approx 700watts.
Now that is the kinetic energy in the wind, of which 0.6 is available for conversion = approx. 420watts.
A good generator will get 80% of that into electricity = 335W.
So now you can follow the above calculation with various wind speeds and diameters. You will see why manufacturers often quote power at 10m/s! (Other deceptive advertising will quote the rating of the generator - nothing to do with the power actually produced). You will also see why small systems (<1m diameter) are very limited.
A turbine larger than 3m diameter is big, and therefore heavy. Safety considerations, such as consequences of large blades becoming detached in a storm, and also engineering issues, such as how to get something that heavy onto the top of a pole, are behind my thinking about suitability for suburbia. If you have a small holding or large garden, and space to lower the pole horizontally, that is different.