Turbines installed on 120 meter high hybrid towers

Hamburg, December 19, 2007. At the Iven site in the German state of Mecklenburg-West Pomerania, Nordex has erected the first two N90/2500 turbines on hybrid towers. The hybrid tower comprises a concrete tower with a height of around 60 meters, which is mounted directly on the base at the location and then pre-stressed. It bears the three steel tower sections of the modular tower with a total height of a further 60 meters

With the technical progress being made in producing more efficient turbines, it is possible to generate electricity from wind power in non-coastal locations just as economically as in coastal locations provided that the turbines are positioned at an elevated hub height. To achieve the necessary rigidity at hub heights of over 100 meters and to suppress resonance frequencies caused by turbine rotation, the lower part of the tower must have a diameter of over four meters. As this makes the tower too large to be carried on a conventional low-bed trailer, road transport is not a viable option.

Nordex has solved this logistic and resonance frequency problem arising with towers with a hub height of over 100 meters by developing a special concrete/steel hybrid tower. The advantage of this is that the concrete tube is produced on site, thus dispensing with the need for overland transportation, while the standard tower sections can be carried on conventional vehicles. At the same time, the overall system has an adequate resonance frequency as the diameter of the concrete element fitted in the lower part of the hybrid tower is freely adjustable.

All told, 800 cubic meters of concrete and 130 tons of armoring steel were used for each of the two towers in Iven. The concrete for the tower is prepared at the site free of any joints and pre-stressed over the entire height of the tower using external tendons. This process ensures systematic pre-stressing of the concrete tower over a period of 20 years. At the same time, the pre-stressing tendons can be easily examined for any damage on a permanent basis and, if necessary, replaced. The lowest section of the steel tower section is placed on top of the concrete tower head and fastened by means of anchor bolts.

As the diameter of the tower base is around 8 meters, it is possible to align the transformer, the main converter and the medium voltage switchgear on a single level. This greatly simplifies the assembly of these components after completion of the tower and enhances accessibility for monitoring and maintenance.