Tower and Foundation: An Overview

1. Tower

A wind turbine generator tower is a component subjected to high loads. The nacelle often weighs several hundred tonnes and there is also stress from the rotor blades and the force from the wind.

A wind turbine generator tower, constituting 15 - 20 % of the costs, plays a substantial role in the economic feasibility of a project. However, higher towers also increase the returns – the height of the tower or hub height is a vital factor in the energy yield.

As a rule, which height is suitable for a wind turbine generator (WTG), depends on several factors (e.g. costs) and must be decided individually for each location.

Types of towers

The following types of towers are available. However concrete and steel towers are much more common than steel lattice towers:

Steel towers usually consist of two to four segments.
Concrete towers with climbing formwork are constructed on site and make transport and fitting easier (called in-situ concrete.) But great care must be taken at significant heights and in winter.
Pre-cast concrete towers. Here the segments are placed on top of one another on site and braced with steel cables in the wall.
Steel lattice towers are very common in India, but can also be found in other countries, like in the USA (western mills) and in Germany.
Hybrid towers consist of components of the above-mentioned types of tower.
Guyed poles are very common in small wind generators, as on the one hand they are light and on the other can be set up without a crane.

Higher towers over 80 m usually have a car or a lift on the inside of the tower to facilitate the ascent.

Wind turbines with steel lattice towers in Costa Rica. ©WWEA e.V.

2. Foundation

The foundation fixes the wind generator into the ground. In order to guarantee the stability of the wind generator, pile foundations or shallow foundations are constructed depending on how stable the subsoil is.

Plate foundations/shallow foundations: Here a large reinforced concrete plate under the earth forms the footing of the generator. It is one of the most commonly used types of foundation.
In pile foundations, the foundation plates (plate foundations) are fixed with piles into the earth. This is particularly necessary in soft subsoil.

The following foundations are used offshore. At the moment a lot of research is still being done and tried out here.

Gravity foundations are often used and placed on the sea floor. These, huge concrete weights for example, are so heavy and stable that they do not need any more fixation to the sea bed.
Tripod: Here the wind generator is put on a tripod. The piles below the tower are connected with a steel frame that distributes the tower’s forces over three steel piles, each of which are fixed approx. 10 - 20 metres deep in the sea bed.
Bucket foundation: This consists of a steel cylinder open towards the bottom which gave it its name. First of all the cylinder is placed on the sea bed and then pumped out. The negative pressure generated inside the foundation presses the foundation into the ground. The material at the bottom of the inside of the cylinder supports the foundation and fixes it to the sea bed.
Monopile: Here a single mast, a steel pile with a diameter of approx. 4 metres, is sunk into the sea bed. Depending on the bed, the monopile is driven approx. 10 – 20 metres deep into the sea floor.

3. References and Links

Gasch, Robert / Twele, Jochen: Windkraftanlagen: Grundlagen, Entwurf, Planung und Betrieb, 4. Auflage, Wiesbaden: Teubner 2005
http://www.teubner.de/index.php;do=show/sid=7293608945094a285b66a503142839/site=t/book_id=1738(German)
Gasch / Twele: Wind Power Plants. Fundamentals, Design, Construction and Operation Berlin: Solarpraxis 2002
http://www.fachbuch-erneuerbare-energien.de/gasch_engl.htm (English)
BWE Bundesverband WindEnergie / German Wind Energy Association
http://www.wind-energie.de/de/die-technik/

Text:
WWEA World Wind Energy Association
http://www.wwindea.org