Choosing Your Wind Turbine Tower at Criticaltowers.com

Choosing the Right Tower

Choosing the right tower for your location is one of the most important decisions you will need to make before you can install your 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. Sandown Wireless supplies towers made by U.S. Tower, Inc. manufacturers of freestanding and guyed towers. We will look at each of these and discuss the advantages and disadvantages of each design.

Choosing the Correct Tower Height

The general rule of thumb for placing wind turbines is that the bottom of the blades should be at least 30 feet above any obstruction within 500 feet. Some people use 300 feet but 500 feet is safer. Consider a wind turbine with a 20 foot diameter rotor. The minimum tower height would be 40 feet above the height of obstructions - 30 feet + 10 feet (half the rotor diameter). It must be stressed that this is a MINIMUM HEIGHT! Higher is better. If you have trees that are 40 feet high, the minimum tower height would be 80 feet.

Obstructions (Courtesy Iowa Energy Center)

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 placing a turbine upwind of a building the minimum distance from the building should be twice the building height.
2. When placing 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 placing 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 wind speed. A change in wind speed from 10 to 12.5 MPH doubles the power in the wind. Because even small changes in wind speed 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!

Lattice Freestanding Tower (U.S. Tower, Inc. Model W)

Modern Freestanding towers have 3 legs with diagonal bracing. Three leg towers usually have legs made from pipe or round tubing and angle steel braces connecting the legs. The braces give the appearance of "lattice work", hence the name. One advantage of freestanding towers is that they take up less room than guyed towers. For a turbine the size of the Bergey Excel 10 kW or Aerostar 6 Meter, a typical foundation for a 100 foot freestanding tower is a concrete block about 4' deep and 16 feet square. The tower base itself is only about 12 feet across. For some soil conditions it is preferable and less costly to use a pier type foundation. Such a foundation consists of three pads, usually 6 to 8 feet below the surface with smaller diameter round piers protruding above ground.

US Tower, Inc. Model W (Aerostar 6 Meter Turbine)

US Tower, Inc., Lakeville, MA

Guyed Towers

Guyed towers typically consist of a slender structure supported and stiffened by cables called "guy wires". The tower can be fabricated as a lattice type structure, typically solid round rod or tubing or it can consist of a single tube such as the one shown on the image at the right. Permanent guyed towers have 3 sets of guy wires arranged 120° apart. "Tilt" guyed towers must have 4 sets of guy wires arranged 90° apart. Guyed towers are very efficient from a design point of view. This efficiency results in a simple tower which uses a minimum of steel. The cost of a guyed tower is generally less than for other types of towers.

Tilt guyed towers have a hinge at the base. A winch or tractor is used to raise and lower the tower. One advantage of tilt towers is that no crane is required for assembly, so installation costs can be lower. Another advantage is that the turbine can easily be serviced without having to climb the tower. The tower can also be lowered in situations where severe weather, such as a hurricane, is expected. An owner could also lower the tower if he or she were going on vacation at a time when hurricanes or other dangerous weather might be expected. US Tower, Inc. tilt down towers are available with an electric winch kit so that raising the tower is so easy anyone can do it. Pull out a single pin and press the button on the winch and the tower is on the ground in a few minutes.

Guyed towers take more room that other types of towers because the guy wires are spread out from the tower base. A typical 100 foot tilt down tower has about a 70 foot spread at the base.

Guyed Tilt Down Tower being raised

Tower Installation

In most jurisdictions, tower installation requires a permit from your local building department. In order to secure the necessary permit your tower installer must provide the building department with plans showing the tower and foundation. In most municipalities, the tower and foundation plans must be stamped by a Professional Engineer licensed to practice in your state. Having the plans and foundation reviewed and stamped by a local engineer could cost you thousands of dollars. Sandown Wireless working with U.S. Tower, Inc. has engineers available who are licensed to practice in every state in the U.S. so we can provide you with the necessary plans for a very reasonable cost.

Foundation Installation for 100' Tower

Vibration Testing

Towers, like all physical objects vibrate at certain frequencies. These frequencies are called "Natural Frequencies". A tuning fork is an often cited example. Another example is a playground swing (although technically a pendulum, the example serves to illustrate the point). It takes very little energy to excite an object at its natural frequency. Once a swing is moving it doesn't take much energy to keep it moving or to increase the arc. In order to keep the swing moving it is necessary to push it at the end of its arc. The "pushes" must occur at just the right time. Obviously, if you were to push the swing while it was still coming toward you, you would slow the swing not keep it moving. Another way of saying this is that the "push" must be in "resonance" with the frequency of the swing.

The blades of a wind turbine are a large heavy rotating mass. If the rotational speed of the blades happens to correspond to the natural frequency of the tower, the tower will begin to vibrate. If the blades were to continue to be in resonance with the tower, the amplitude of the vibrations could increase and, if there were not enough damping in the tower, eventualy damage the structure. It is therefore important to ensure that the natural frequency of the tower does not correspond to the rotational frequency of the turbine rotor.

Many small wind turbines operate at varying rotational speeds. If the turbine you are considering uses an inverter, it almost certainly uses a variable frequency alternator. Because the rotational speed of the blades varies with wind speed it is quite likely there will be times when the rotor speed corresponds to a tower frequency. When this happens, there may be some tower vibration. The good news is that, because the wind velocity is constantly changing, the rotor speed will also change so the rotor RPM will probably not operate at the excitation frequency for a very long time, usually just a few seconds. As soon as the rotor speeds up or slows down a little, the tower will no longer be in resonance. In other words, the rotor will usually "pass through" the tower natural frequencies quite quickly. The U.S. Tower, Inc. Model W tends to be stiffer than competing freestanding towers so it's natural frequencies are higher than these other towers and therefore less likely to be excited by the slow turning turbine rotors.

Another type of of small wind turbine uses an induction generator. These turbines always turn at the same rotational speed. Because this speed is known, it is easy to design the tower so that it never becomes excited by the turbine.

US Tower, Inc. Vibration Modes Test