Journal Inseneeria
Mart Arro, E-Betoonelement
Since wind energy has become one of the most important sources of renewable energy, while also creating a completely new industry, it is important to understand how to collect wind energy most effectively. Until today, wind turbine towers have been mostly manufactured from steel; however, it is reasonable to use concrete for building more powerful and higher towers.
Many years ago, wind turbines were mainly used in agriculture; for example, for grinding grain and pumping water. Several of these human-made masterpieces have survived until today. However, the requirements for wind turbines and their towers have increased in time and, therefore, turbines have become much more powerful. Original turbines with the capacity of 5-30 kW have given way to modern turbines with the capacity of 5-6 MW. The height of wind turbine towers has also increased constantly: 10 metres in 1970, 20 metres in 1983, 50 metres in 1990, up to 85 metres in 2000, and up to 140 metres in recent days.
Higher wind is “better”
During the history of wind turbines, both horizontally and vertically rotating generators as well as turbines with one, two and three blades have been tested. Today, horizontally rotating three-blade turbines are most common since they have proved to be the most effective. So how is energy collected from wind? This can be calculated using a simplified formula P = KD2V3, where P is energy, K is constant, D is rotor diameter and V is wind velocity.
In other words, the energy collected from wind equals the square of the rotor diameter and the cube of wind velocity. Wind becomes stronger and its quality improved higher from the ground level since relief, buildings and forest cause friction and turbulence close to the ground level which, in turn, significantly reduce the good qualities of wind. Therefore, placing a rotor with a bigger diameter higher from the ground makes it possible to produce energy more efficiently.
90 m is the dividing point for height
Turbines with the capacity of 2.5-3 MW are becoming a standard for land-based wind turbines. This means a great challenge for towers. For example, a turbine with the capacity of 2.5-3 MW together with a rotor with the diameter of 100 metres weighs approximately 150 tons. When lifted to a tower that is 100-150 m high, the blades of such turbine make 10-20 rotations per minute. In such conditions, wind turbine towers have to bear great static and dynamic load. Dangers arising from the latter and material fatigue are what complicate the design of such towers. It is reasonable to use steel for building towers that are up to 90 m high and meant for turbines with the capacity of less than 2.5 MW. In the case of higher towers and more powerful turbines, it is rational for both economic and technical reasons to use precast concrete. Concrete structure provides sufficient rigidity in the case of large turbines and long blades, helping to control vibration. It is also easier to create a tower solution that is resistant to resonance. In the case of metal towers, sections are joined together by means of bolts and building one tower requires hundreds of bolts. In the conditions of constant vibration, the joints of bolts need to be checked on a regular basis. Concrete does not need this and, therefore, the maintenance of concrete towers is much cheaper. In addition to that, the elements of precast concrete towers are beautiful and they can be used to quickly erect a suitable tower for the turbine. Wind parks often have limited access; the roads are narrow and curved. In the design phase, the size of the elements is chosen so as to ensure easy access to the least accessible construction sites.
30 cm thick walls
Concrete is an environmentally friendly material with good availability and a relatively stable price. In the case of towers more than 100 m high, concrete gives a considerable price advantage as compared to steel since the diameter of the tower and the thickness of steel sheets used to build the tower increase considerably. The increasing mass of steel sections and problems with transportation increase the price of steel solutions. There are two types of precast concrete towers: 100-110 m high full concrete towers, where the concrete section reaches the turbine, and 100-150 m high hybrid towers which combine concrete and steel. The lower part of a hybrid tower is made of concrete and the upper part, which is approximately 50-70 m high, is made of steel. A hybrid tower combines the benefits of both materials. In its essence, precast concrete tower for a wind turbine is a conical tower, the walls of which are approximately 30 cm thick. This concrete tower is divided into cylinders approximately 10-15 m high which, in turn, are divided into segments, i.e. precast elements. The wider part of an element is 3.5 m wide and its height is 10-15 m, depending on the height of the cylinder. The maximum weight of an element is 40 tons. Precast elements are manufactured from self-compacting concrete at the plant. Concrete panels, i.e. segments that are delivered to the construction site are pre-assembled into the aforementioned 10-15 m high cylinders. These cylinders are lifted on top of each other, creating the concrete structure of the conical tower. A concrete adapter slab is placed on top of the tower and a wind turbine or a steel tower (in the case of a hybrid tower) is attached to this. The whole concrete tower is post-stressed from the adapter slab to its foundation. This is necessary to ensure that the tower is resistant to the dynamic load caused by the working turbine. For example, a 100-metre-high tower can weigh up to 1,500 tons and consist of approximately 70 precast concrete elements.
