Conference "Towers and foundations for wind turbines"
 
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In the meantime, wind energy has advanced to one of Germany’s largest renewable energy sources. 19,460 wind energy converters (WEC), with a total power rate of more than 22,247 MW, have been installed in 2007. These have helped reduce the annual CO2 emission by approx. 35 million tons. However, the structural engineers are faced with new challenges concerning the substructure designs of ever larger WECs.

Contents

Knowledge in reference to substructure design and assessment methods of wind energy converters (WEC). Special topics deal with shell buckling, fatigue detection and constructive details at connections.

Overview of different substructure concepts

Load assumptions

Design and construction of prestressed concrete towers
Prestressed concrete towers, cast-in-place or prefabricated, multi-megawatt category, hybrid structures (prestressed concrete tower, steel adapter, rotor substructure), hub heights of more than 130 m, offshore wind energy plants (WEP), floating gravity structures, framework structures out of prestressed high performance concrete, natural frequency, second order theory, bending-moment-curvature relations, physically non-linear material behaviour, fatigue verification, large number of load cycles, design working life, climbing formwork, external prestressing tendons, spread foundations or pile foundations

Tubular steel tower

Lattice towers for wind turbines
Overview about the construction of lattice towers for wind turbines. Beside the description of the structural system and the approach to load calculations the specific benefits and disadvantages of the tower construction will be presented.

Soil investigations and foundations of wind energy converters
Introduction, field and laboratory investigation of the subsoil, foundation systems and design concepts, design of raft foundations, design of pile foundations.

Defect of the supporting structure, foundation

Problems with the soil, the expert report isn’t complete, problems with the piles, problems with the foundation section, many cracks on top of the concrete.

Spannbeton-Fertigteil-Türme / Towers of precast concrete Designs of construction, loads and calculation, details, problems and experience

Overview Offshore wind turbines

Practical experience from erected offshore wind projects

Wind turbine towers

The tower of the wind turbine carries the nacelle and the rotor.
Towers 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 (battery chargers etc.)

Tubular Steel Towers
NEG-Micon Tubular towers Most large wind turbines are delivered with tubular steel towers, which are manufactured in sections of 20-30 metres 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.

Lattice Towers
Nordex Lattice Tower 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.

Guyed Pole Towers
Genvind 22 guyed pole tower 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.

Hybrid Tower Solutions
Bonus 95 kW Hybrid Tower Some towers are made in different combinations of the techniques mentioned above. One example is the three-legged Bonus 95 kW tower which you see in the photograph, which may be said to be a hybrid between a lattice tower and a guyed tower.

Cost Considerations
The price of a tower for a wind turbine is generally around 20 per cent of the total price of the turbine. For a tower around 50 metres' height, the additional cost of another 10 metres of tower is about 15,000 USD. 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.

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.

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 below, which are 225 kW, 600 kW, and 1,500 kW respectively, and with rotor diameters of 27, 43, and 60 metres, you will notice that the tower heights are different as well. 225 kW Turbine 600 kW turbine 1,500 kW turbine Height labels Clearly, we cannot sensibly fit a 60 metre rotor to a tower of less than 30 metres. 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. (See the section on wind resources ). Each metre of tower height costs money, of course, so the optimum height of the tower is a function of

1. tower costs per metre (10 metre extra tower will presently cost you about 15,000 USD)
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 kilowatt hour 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.

Quelle: Danish Wind Industry Association