Living off the grid, Part IV: Catching the Wind

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A turbine must have a clear shot at the wind, so a well-designed tower is crucial for an effective system. Building too short a tower is the most common mistake home owners make. The higher one goes above ground, the stronger and "smoother" the wind is. Smoother air means air that is free from the turbulence that occurs from obstructions such as trees and buildings. It is always a better investment to get more tower instead of a larger wind turbine. Mark feels that if he had gone only 20 or 30 feet higher in his 70-foot tower, he could almost double the output of the generator. As a rule of thumb, the tower should be at least 30 feet above any obstacles within 500 feet.

Before you run out and install a tower on your roof, cleverly thinking that you can cut 30 feet off your tower height, remember that towers vibrate alot . The vibrations could seriously damage both your house and your nerves. Wind towers should always be installed on the ground.

Since the wind does not always blow when you want to listen to your stereo, an "off-grid" system typically includes turbine-charged batteries so that electricity is available at any time throughout the day, plus a regulator to protect the batteries from overcharging. A battery system will usually also have an inverter to change the DC electricity from the turbine and batteries into AC electricity that most of our appliances consume.

Windy Enough for You?

How much power is available from the wind? The main factor is speed. In 1927 Alfred Betz of Germany computed a formula for how much power the wind can generate. He concluded that the power in the wind is proportional to the cube of the wind speed. For example, if the wind speed doubles, the available power increases eightfold. That is, a 20-mph wind will yield eight times the power of a 10-mph one.

Another important factor in the amount of power possible is the area of the rotor. A larger rotor will intercept a larger area of wind, thus providing more power. Although we do have control over what size rotor we use, we don't have control over the wind speed. So it is essential to know how much wind is necessary for a wind system to be practical.

Since wind speeds change constantly (changes of up to 30 mph are possible in 5 seconds or less), it is important to talk of average wind speeds. Areas with average monthly wind speeds of 12 to 16 mph or above are excellent wind sites. For a utility-tied system, speeds of 9 mph or above are economical. Finally, for places where there is no utility and wind is your only option, areas with very low wind speeds might still be more economical than relying on a diesel generator.

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