Choosing the Correct Tower Height
Choosing the right tower for your location is one of the most important decisions you will need to make before you can install your Aerostar wind turbine. By far, the most important consideration is tower height. It is essential that the turbine be located high enough to be out of wind turbulence. If the tower is too low, power production will suffer, in some cases dramatically. Next on the list is the type of tower. Towers can be guyed, freestanding or monopole types. We will look at each of these and discuss the advantages and disadvantages of each design.
The image above shows how obstructions can have an effect well downstream of the obstruction. The effect of the building extends to a distance of 20 times the height of the building and to an elevation of 2 times the height of the building. A 30 foot high building could create turbulence up to 60 feet high and for a distance of 600 feet from the building. A few general rules of thumb are:
1. When siting a turbine upwind of a building the minmum distance from the building should be twice the building height.
2. When siting a turbine downwind of a building the minimum distance should be 10 times the building height and preferably 20 times the building height.
3. When siting a turbine downwind of a building the minimum turbine height should be twice the height of the building.
The power in the wind increases with the cube of the windspeed. A change in windspeed from 10 to 12.5 MPH doubles the power in the wind. Because even small changes in windspeed have dramatic effects on power production, turbulence has the potential to substantially reduce the effectiveness of a wind turbine.
Obstructions sometimes cause the wind to make rapid and dramatic shifts in direction. A wind turbine will try to follow these changes. It is usually easy to tell when a wind turbine is running in turbulence because it will be constantly yawing in an attempt to follow the wind.
Obstructions also cause wind shear. Wind shear is a large change in wind velocity with height. When a turbine operates in high wind shear, the upper blade will be subjected to a different wind velocity than the lower blade. Both turbulence and wind shear can place heavy loads on a turbine and tower, potentially shortening the life of the components.
It has been said that placing a wind turbine on too low a tower is like putting solar collectors in the shade. Don't do it!
All Aerostar wind turbines use induction generators. Many other small wind turbines use permanent magnet (PM) generators. The advantage of an induction generator is that no inverter is required. The generator produces power essentially identical to that provided by any utility company. Because the generator frequency must be constant (60 Hz) the rotor always turns at the same speed. This makes the turbines very quiet in high winds when the rotor of a PM generator is turning very fast. Because of the constant speed operation of an induction generator, only towers manufactured by Aerostar should be used. If the natural frequency of the tower happens to match the rotor speed, the rotor would constantly be putting energy into the tower resulting in damage and structural fatigue to the tower and turbine. With PM generators, this problem may be less severe because, as the wind speed varies, the rotor RPM varies so the excitation frequencies may not last very long. The disadvantage of PM generators is that, because of the widely variable RPM, the turbine will almost always cause the tower to shake at some wind speed. This can result in noisy rattling and, if severe enough, structural damage. With an induction turbine, if the tower is designed for the turbine, it will never cause damaging vibration, no matter what the windspeed is.