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The sky’s the limit

In close cooperation with Henkel, the German start-up company Timber Tower GmbH implemented a pilot project that could herald a new era for the wind power industry.

Reaching up to the heavens to a height of some 100 meters, a wind turbine tower of spruce – the first of its kind – has now been built near Hanover in Germany. Yet no dowels, no nails, and no screws or bolts are used to hold this timber construction together. Instead, the support tower has been adhesive-bonded using a special process – with various wood adhesives of the Purbond brand manufactured and marketed by Henkel Industrial Adhesives. The associated turbine-generator set is due to be hooked up to the grid in December of this year.

The project offers great promise for the future because switching to wood raises the prospect of increased tower heights. From the point of view of the operator, this aspect is particularly interesting because, at an onshore site, every additional meter gained means roughly one percent more in revenue. As tubular steel towers are made by assembling massive rings that have to be transported under highway bridges, however, with this technology the ring diameter is always going to be a limiting factor. Hence, the maximum height of construction currently lies at 110 meters. Wood is a potential problem-solver here as the components can be moved with relative ease to the site where they are then assembled to create the larger-sized parts required.

Close cooperation with Henkel
As a renewable resource, moreover, timber is in many respects superior to conventional materials such as steel and concrete. The service lifetime alone of the new system is likely to be double that of conventional solutions, according to the new German start-up company TimberTower GmbH, which implemented this project in close cooperation with Henkel.

In a direct comparison, steel and concrete are prone to much faster fatigue development. Particularly under heavy strain, both materials lose much of their already meager elasticity, causing them to embrittle. And that is why wind turbines tend to be disconnected from the grid after around 20 years. Wood does not have this handicap. The structure-strengthening effect of the glue also helps, with the result that the TimberTower generator with its capacity of around 1.5 megawatts should be able to supply electricity to around 150 households in Germany’s Lower Saxony for the next 40 years.

20 to 30 percent cheaper to make
“The way we see it, a timber tower offers nothing but advantages,” says TimberTower’s General Manager, Holger Giebel. “It has greater stress resistance, can be built significantly higher and is 20 to 30 percent cheaper to make. The Purbond adhesive joints had to prove their suitability right from the start of the construction project, with each one duly receiving a certificate of worthiness from the DIBt, Germany’s institute of building technology.

Now a white waterproofing membrane has been placed around the octagonal, wooden outer shell to protect the tower from the elements. The massive cross laminated timber located underneath are bonded with Purbond. Bolts were only used in the initial assembly phase in order to secure the structure. Once the tower was complete, all the mechanical fixtures were removed and the stabilizing inner construction was then adhesive-bonded from the inside using an especially developed application technique involving the 2-component casting resin, Purbond CR 421. The adhesive penetrates deep into the pores of the wood and interconnects the cross laminated timber panels with the perforated steel plates. The timber tower weighs more than 90 metric tons, with the top-mounted gondola and rotor blades adding another 100 t.

“One of the most innovative projects in the history of wood”
“This wind turbine is one of the most innovative projects in the history of wood,” says Walter Stampfli, General Manager of Purbond. It is already looking like it won’t be the last, because the expertise that Henkel particularly has gained from this project is gradually gaining renown in the relevant engineering communities. Back in 2008, the first eight-storey timber building was constructed in London with Purbond adhesives, while the first ten-storey high-rise was built in Melbourne. And in Canada a construction project involving a 30-storey building is also in the offing.

“We have the most advanced adhesives for the manufacture of load-bearing timber elements and constructions able to meet the standard specifications applicable in virtually all countries,” says Stampfli, alluding to the development results achieved over the last two years. “Moreover, we can offer technical support wherever this is required and have established ourselves as an internationally recognized source of expertise for public authorities, standards institutions and approval agencies almost everywhere in the world. As a consequence, we are ready to roll on all kinds of projects.”

Purbond
Henkel acquired the Swiss company Purbond back in early 2011. Purbond is one of the world’s leading suppliers of polyurethane adhesives for load-bearing engineered wood applications and timber constructions. Since the takeover, the brand has continued to be managed as an independent business.

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