The tower of a horizontal axis wind turbine provides the support upon which the turbine electromechanical components are held in the correct position to capture the most energy from the wind. For both onshore and offshore wind turbines, the tower must raise the rotor so that its blades are both clear of the ground and clear of any obstacles. It must also place the rotor hub at such a height that the tips of the blades are clear of any turbulent layer of air close to the ground as this can cause additional fatigue loading as well as reducing energy capture. The higher the turbine rotor is mounted at any site, the stronger the wind, so it is better to raise the nacelle on the highest tower possible. In some cases turbine developers will choose to place a rotor on a higher tower than usual if the gain in power is sufficient at the higher elevation.
While there is no rule, the tower height is usually somewhere between two and three times the blade length. This height provides the optimum balance between energy capture and cost. However, it will always be site sensitive. The height of a wind turbine is usually defined by the hub height which is the height at which the center of the rotor sits above ground or sea level.
The typical height for a 3 MW wind turbine tower is around 80 m, although hub height can stretch to 100 m for some turbines of this size. A commercial 7.6 MW turbine has a hub height of 135 m, while a project to test the feasibility of a 20 MW wind turbine proposed a hub height or around 150 m. Such large machines as this are only ever likely to be used offshore.
Wind turbine towers have been constructed in a variety of ways. Many early turbines used lattice steel structures. Lattice towers are relatively cheap to build and they produce little wind shadow. A wind turbine with a lattice steel tower is shown in Figure 6.2. However, they are visually unappealing and they have high maintenance costs. As turbine size grew lattice towers were considered to be less practical and when towers extended above 60 m tubular towers began to grow in popularity. Attempts have been made to construct these from concrete but that proved costly and in most countries the industry gravitated towards tubular steel towers. These towers, which are conical in form, have become the standard means of construction for tower heights of up to 80 m. A wind turbine with a tubular steel tower is shown in Figure
