While steel and concrete are the popular materials for wind turbine tower construction, a new design is demonstrating the potential of wood.
German tower manufacturer and project developer TimberTower installed its first octagonal-shape wooden prototype tower of 100-metre hub height combined with a 1.5MW Vensys 77 turbine in 2012 in Hanover, Lower Saxony.
The company is trialling another design at another wind farm, in Heidelheim, Bavaria. This will feature five hybrid 12-sided wood and tubular-steel towers, offering a 140-metre hub height and a wider 80-metre base, made from wood supplemented by three tubular steel tower segments. These TimberHybrid towers are designed for 2.5MW Vensys nacelles with 112-metre diameter rotors. The project is expected to complete this year.
A third design is planned for a forthcoming project, for which it owns the rights to develop. This will comprise three turbines of 2.5-3.0MW in North-Rhine Westphalia, with 140-metre hub heights. For this, the firm will use a largely wooden tower section of 132 metres and a five-metre long steel adapter to accommodate the yaw bearing. The remaining three metres is from the yaw bearing bottom interface with the tower up to the centre of the rotor.
The main aim in developing a tower entirely made from wood was to display the full benefits of the material for structural and cost reasons.
"We are in contact with the forest land administrations of different German states, and we are pleased that TimberTower has already signed some land-lease contracts," says Carlo Schroder, the company's head of engineering. "Acquiring construction permits always takes time, but for our first project in particular we had to do a lot of convincing and answering questions from authorities unfamiliar with wooden tower designs," Schroder says. The company is also in discussions with French developer InnoVent about delivering 18 towers for two projects in France.
TimberTower gained a type certificate for a 20-year design life, through German certification body TUV Nord. This matches the certified design life of tubular steel and concrete-steel hybrid towers.
This was not an easy task, given the novelty of the design. "One of many difficulties we faced is that while there are German industry standards for bridges and other wooden structures in place, these tend to be conservative and, as we found, sometimes completely out of date," Schroder says.
"However, based upon our own simulations, calculations and measurements, we are convinced that up to 45 years' design life is a realistic assumption," he adds, because of wood's good damping characteristics and the near absence of materials fatigue.
TimberTower engaged a wood construction expert to develop and validate the seemingly straightforward but structurally and dynamically complex connection between the wood slots and perforated sheet-metal solution. It also engaged another expert for the highly stressed structural glue bonding joints. "A complicated and unexpectedly time-consuming factor was that, as this joining method is a new invention, it was neither described nor validated in any existing German DIN or international ISO industrial standard," says Schroder. "The overall testing and validating process therefore involved multiple tests in a specialised university laboratory to prove the strength, characteristics and long-term performance of this type of connection. Including type certification approval, this process took 13 months in total."
TimberTower engineers and external experts developed a functionally comparable joining solution for the steel adapter (with octagonal-shape) bottom flange, which serves as an interface between the tower top and yaw bearing, which requires a circular mounting base. The adapter used for the 100-metre tower of the 1.5MW turbine contains a total of 176 perforated steel sheets welded to the adapter bottom flange, which weighs 7.6 tonnes and has a height of 3.65 metres.
The company operates an advanced monitoring system in the prototype tower currently installed, which continuously measures and monitors damping, moisture content inside the tower and wooden elements, as well as fatigue loading. The collected data is then compared with tower simulations and serves as input for further tower optimising and new product developments.
Using wood has a number of environmental benefits. Wood is a natural product, requires limited energy input to turn it from raw material into a ready product, and it is almost fully recyclable at the end of operating life, Schroder says. The use of wood also promotes local employment, especially in forested wind market regions where raw material can be grown, harvested and processed in the area.
He also points to the easy, cost-effective transportation using standard flatbed trucks or 40-feet containers for tower component delivery. The mass of individual compact lightweight tower wall elements is only in the 650-5,500kg range. This eliminates typical road transportation bottlenecks and the need for heavy-duty equipment often required for tubular-steel or hybrid concrete-steel towers.
Successfully entering the market with a new, innovative and unusual product represents a formidable challenge, but Schroder is confident of the benefits over concrete and steel, and low costs. "Wood as raw material has a favourable price level, while price fluctuations are minor by comparison. The same is true when wood is compared with concrete, again on raw materials basis. Our towers can be up to 20% cheaper compared with concrete-steel hybrid towers. Our next steps are constructing a wind farm with five towers of 140-metre hub height each and develop plans for 160-metre towers."
Innovative design is often prone to scepticism, but if TimberTower succeeds in convincing key parties of the overall benefits of using wood in tower construction, this will pave the road for potential future commercial success. Hybrid towers, with wooden bases and using standard steel tower top sections for large hub heights, could become a particularly interesting alternative to the currently dominant concrete-steel hybrid solutions.
CONSTRUCTION DETAIL - TIMBERTOWER PROTOTYPE PIECES
The prototype TimberTower tower has an octagonal cross-sectional shape, measuring 7x7 metres at the base, and gradually narrows to 2.9 x 2.9 metres at the top.
There are 54 wall panels, all tapering cross-wise laminated structures, 300mm thick, and varying in length from 3.75 to 15 metres, up to 2.72 metres wide.
"Individual elements consist of eight layers plus a grey PVC outer top layer to ensure long-term protection against weather, including moisture, rain, snow, and icing. The laminated wooden part is a sandwich structure consisting of six 4mm layers and two 3mm layers. In ready form they are placed in a pattern with 2x4mm + 1x3mm + 2x4mm + 1x3mm and again finally 2x4mm layers," Schroder explains.
The 4mm layers are incorporated with fibre orientation in a vertical direction when mounted in the tower, while the fibres for the 3mm layers sit horizontally in tower-mounted position. The "cross-wise lamination", switching between vertical and horizontal fibre orientation, is essential for optimised loads.
Additional wooden sections are in 15-metre segments, assembled from multiple wooden elements mounted on a temporary timber structure, which acts as a precision assembly tool.
Then industrial glue is applied between the element's vertical joining surfaces, providing the main permanent structural bonding strength.
Six temporary segments are used for a 100-metre tower, each providing pre-fabricated lightning and ladder systems, attachment points, power-cable holders and electrical pre-installation units.
For the highly stressed horizontal interfaces between the segments, TimberTower developed an unusual joining solution comprising many closely interspaced perforated steel sheets and matching slots machined in the matching opposing element side.
A pre-assembled row of steel sheets are welded on the flange of the foundation, which will be glued with the wooden elements. Before new segments are assembled, glue is applied to all protruding steel sheets, which are then slid into the corresponding slots of the lower segment. This provides a strong permanent bonding connection between all vertical segments. Additional tension strength is provided by what Schroder calls "glue-bridging" between each perforated metal sheet sandwiched between the two opposing sides of a matching wood slot.