The Power of the Wind

There a lot of different wind turbines out there. How do I know which one is the one for me. How can I estimate its power production or reliability?

Size Matters!

There is only so much power in the wind! Period. No matter what a manufacturer claims, you cannot defy physics. The swept area of a rotor will determine its maximum potential energy yield. The larger the diameter of rotor swept area, the larger the potential energy available. The energy available in the wind is as follows:

1/2 (air pressure) x 3.14 (pi) x (blade length squared) x (wind velocity cubed)

As you can see, every time the wind velocity doubles, the energy available increases 8 times! Much of your power will come from high wind speeds. Turbine weight is another factor in determining a good wind turbine. The heavier they are, the more robust they generally are.

What's my wind speed

Wind map of North America - created by Archer and Jacobson
Map created by Archer and Jacobson Check your area on the North American wind map to the right (Click the image for a larger version). Please note, speeds indicated on this map are for 80 meter height. Wind speed will be less at lower heights.

How tall should my tower be?

There is more wind the higher you go. Install as tall a tower as you can afford. For each 10m increase in height, you will gain 25% more energy output. Taller towers will often get your turbine out of turbulence. Turbulence will decrease the energy yield and can shorten the life span of your unit. A good rule of thumb is to site your tower 10m above any surrounding object with in 100m.

Betz limit

As formulated by German Physicist Albert Betz in 1919, 59% of the available energy in the wind can extracted. No wind turbine has come close to the Betz limit, most are in the 25- 30% efficiency range.

Check out Paul Gipes "Wind Energy Basics" for more information, it's an excellent book!

Wind Speed Calculation

Griggs-Putnam Index

The Griggs-Putnam Index is one of several methods of estimating the prevailing wind speed at a site by observing the growth patterns of trees. Strong winds will deform trees and shrubs so that they indicate an integrated record of the local wind speeds during their lives. The effect shows up best on coniferous evergreens because their appearance to the wind remains relatively constant during the year. Deciduous trees shed their leaves in the winter and thus change the exposed area tremendously. If average wind speed is high but still below some critical value, above which deciduous trees cannot survive, they will not indicate relative differences in wind speeds very well, although they do show distinctive wind damage.