The tower of a wind turbine serves to elevate the main part of the machine up in the air. It carries the nacelle and the rotor. For a horizontal axis machine the tower must be at least high enough to keep the blade tips from touching the ground as they rotate. In practice, towers are usually much higher than that. Winds are nearly always much stronger as elevation above the ground increases and they are less turbulent. All other things being equal, the tower should be as high as practical. Choice of tower height is based on economic trade off of increased energy capture versus increased cost.
The principal options for large wind turbines may be either tubular steel towers, lattice towers, or concrete towers. Guyed tubular towers are only used for small wind turbines.
Tubular steel towers
Most large wind turbines are delivered with tubular steel towers, which are manufactured in sections of 20-30 m with flanges at either end and bolted together on the site. The towers are conical, i.e. with their diameter increasing towards the base, in order to increase their strength and to save materials at the same time.
Photograph © NEG-Micon A/S 1998 .
Lattice towers are manufactured using welded steel profiles. The basic advantage of lattice towers is cost, since a lattice tower requires only half as much material as a freely standing tubular tower with a similar stiffness. The basic disadvantage of lattice towers is their visual appearance, although that issue is clearly debatable. Be that as it may, for aesthetic reasons lattice towers have almost disappeared from use for large, modern wind turbines.
Photograph © Nordex A/S 1998 .
Guyed Pole towers
Many small wind turbines are built with narrow pole towers supported by guy wires. The advantage is weight savings, and thus cost. The disadvantages are difficult access around the towers which make them less suitable in farm areas. Finally, this type of tower is more prone to vandalism, thus compromising overall safety.
Photograph © Soren Krohn 1999 .
Some towers are made in different combinations of the techniques mentioned above. One example is the three-legged Bonus 95 kW tower of the photograph, which may be said to be a hybrid between a lattice tower and a guyed tower.
Photograph © Bonus Energy A/S 1998 .
Below some considerations for the tower are examined:
- Cost considerations: The price of a tower for a wind turbine is generally around 20% of the total price of the turbine. For a tower around 50 m height, the additional cost of another 10 m of tower is about 10,000 euros. It is therefore quite important for the final cost of energy to build towers as optimally as possible. Lattice towers are the cheapest to manufacture, since they typically require about half the amount of steel used for a tubular steel tower.
- Aerodynamic considerations: Generally, it is an advantage to have a tall tower in areas with high terrain roughness, since the wind speeds increases farther away from the ground, as we learned on the page about wind shear. Lattice towers and guyed pole towers have the advantage of giving less wind shade than a massive tower.
- Tower stiffness: One of the primary considerations is the overall tower stiffness, which also has a direct effect on its natural frequency. There is a distinction between so called stiff and soft towers. Stiff towers are those whose fundamental natural frequency is higher than that of the blades passing frequency. They have the advantage of being relatively insensitive to motions of the turbine itself, but being heavy, they are also costly. Soft towers are those whose fundamental
natural frequency is lower than the blade passing frequency. A further distinction is commonly made: a soft tower's natural frequency is above the rotor frequency as well as being below the blade passing frequency. A soft-soft tower is one whose natural frequency is below both the rotor frequency and blade passing frequency. The soft towers are generally less expensive than the stiffer ones.
- Structural Dynamic considerations: The rotor blades on turbines with relatively short towers will be subject to very different wind speeds, and thus different bending, when a rotor blade is in its top and in its bottom position, which will increase the fatigue loads on the turbine.
- Occupational safety considerations: The choice of tower type has consequences for occupational safety, as shown in page Occupational safety.
Choosing between low and tall towers
Obviously, you get more energy from a larger wind turbine than a small one, but if you take a look at the three wind turbines, which are 225 kW, 600 kW, and 1,500 kW respectively, and with rotor diameters of 27, 43 and 60 m respectively, you will notice that the tower heights are different as well.
Clearly, a 60 m rotor can not be fit into a tower of less than 30 m. But if we consider the cost of a large rotor and a large generator and gearbox, it would surely be a waste to put it on a small tower, because we get much higher wind speeds and thus more energy with a tall tower. Each metre of tower height costs money so the optimum height of the tower is a function of:
1. Tower costs per metre (10 m extra tower will presently cost about 10,000 euros)
2. How much the wind locally varies with the height above ground level, i.e. the average local terrain roughness (large roughness makes it more useful with a taller tower)
3. The price the turbine owner gets for an additional kWh of electricity.
Manufacturers often deliver machines where the tower height is equal to the rotor diameter. aesthetically, many people find that turbines are more pleasant to look at, if the tower height is roughly equal to the rotor diameter.
- Tower construction: steel tubular, steel lattice, concrete