Close up - Installation heights hit 200 metres and beyond

GERMANY: Over the past 18 months the trend to combine high towers with large rotors has grown steadily. Windpower Monthly technology correspondent Eize de Vries visited a project being developed by Juwi near the city of Mainz, where the turbines reach a total installation height of 200 metres.

An ATS tower under construction
An ATS tower under construction

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In Germany, trend setting turbines with large rotors and high towers for low and medium wind speed inland sites are considered a key enabler in the country's ambitious renewable energy transition process. The "informal" 200-metre total installation height limit appears to have been dropped and is opening up fresh opportunities in areas previously considered unfit for wind power generation, such as forested regions.

However, John Kop, sales director of Advanced Tower Systems (ATS) stressed that while the prospect of even better future yields is attractive, economic viability remains key.

Initially founded as a subsidiary of Dutch engineering consultancy Mecal, ATS today operates as a joint venture with German renewable energy company Juwi. The two build towers and projects for the German market under the trade name ATS Construction, while product licenses have been awarded to international partners in Spain and Mexico.

One of the Juwi's latest projects under construction is a 24-turbine inland wind farm at Gau-Bickelheim near Mainz, in the central German state of in Rhineland-Palatinate. The project aims to maximise energy yield and comprises 2.4MW Kenersys turbines with a 110-metre rotor diameter and 145-metre hub height, adding up to a 200-metre total installation height.

The prefabricated ATS towers consist of multiple slender concrete elements, each 0.5–4 metres wide and 16 metres high that are easy transportable by road on standard flatbed trucks. Each square cross-section comprises four identical cylindrical-shaped 90-degree corner elements and four flat coning elements that fit in between. The latter decrease in width for each added layer, while corner elements have standardised dimensions.

The base diameter of most tubular steel towers is often restricted to around 4.2 metres, essential for enabling road transportation. Such restrictions do not exist for ATS towers. Kop said: "When fully assembled, the tower base measures 8 metres or more between the parallel sides, but actual size depends upon turbine power rating, rotor diameter and hub height. A wide concrete tower base allows very high, stable towers."

Tower construction commences with the full bottom ring of segments mounted atop a concrete foundation. Each additional ring is fully assembled and fixed by grouted connections at ground level before being hoisted into position. The assembly process is completed by putting a concrete transition piece on which to accommodate the steel tower part and post tensioning all segmented concrete rings with steel cables inside the tower. Kop said: "Our strategy is to extend the concrete section to an elevation where it can accommodate a steel tower with 4.2-metre bottom diameter."

Inside the tower base is huge and rather empty, with ample room left despite a power electronic converter in the centre and an elevator. In future the transformer will also be moved inside the tower base.

For future inland wind projects Juwi aims to utilise similar or lower rated Kenersys turbines but with an enlarged 120-metre diameter offering 205-metre total height and even better yields per megawatt installed. Juwi believes building inland at these, and, if necessary, even bigger heights for the industrialised southern part of Germany provides a more economical alternative compared to offshore wind power generation in the North Sea.

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