Sweden

Sweden

WindTech: Why wood works for modular, low-carbon towers

A Swedish company has come up with a novel design to make towers more cost-effective to manufacture, transport and install while reducing lifecycle LCoE. And it is made almost entirely out of wood.

Scaled… The conical sections for the 1:5 prototype are assembled in the factory, but the pre-curved modules will be easy to transport
Scaled… The conical sections for the 1:5 prototype are assembled in the factory, but the pre-curved modules will be easy to transport

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Engineering and industrial-design company Modvion is the driving force behind the reintroduction of wood as a structural material for turbine towers.

Set up in Gothenburg, Sweden, in 2016, the firm specialises in developing a novel, modular, segmented "full-wood" tower concept.

Modvion claims its patented technology for modular high towers delivers enhanced yields and a lower levelised cost of energy at less favourable wind sites, while potentially making wind power carbon-neutral.

The current main design focus is at a structure with 150-metre hub height for the 4-4.5MW volume class.

However, a first step is to install a 1:5-scale prototype tower with a 30-metre hub height on an island near Gothenburg this year, according to CEO Otto Lundman.

"The tower is produced in close collaboration with Moelven, a leading Nordic manufacturer that recently completed what is currently the world’s tallest timber building in Norway.

"We are about to finish all five tower sections for the prototype with them. We also work closely with Chalmers University of Technology in Gothenburg for developing the towers, site construction and a joint testing programme," Lundman explains.

Chalmers University and the Swedish Wind Technology Test Centre are the prototype customers. The university supplied a research turbine with a 16-metre rotor, which, according to Lundman, is well suited to study how the tower performs.

The downsized structure should become a stepping stone towards building the much larger 150-metre commercial tower. It is also expected to become a landmark for advanced tower designs using wood construction, once the concept has proved itself.

The firm recently secured a €500,000 financial injection from investors including Chalmers Ventures. The fresh capital will enable Modvion to grow the team and complete the pilot tower commissioning.

Modvion’s re-introduction of wood as a structural material for wind turbine towers comes seven years after now-defunct Germany-based TimberTower installed its octagonal-shape prototype in 2012 (see box).

Radically different

Chief technical officer Erik Dölerud explains that Modvion’s concept differs radically from the TimberTower design in several respects, specifically in the approach to the wood elements.

"A first key visual and structural difference is that our towers feature a circular cross-sectional shape," Dölerud says.

"Second, the structural materials used for Modvion towers are glue-laminated timber (GLT) and laminated veneer lumber (LVL), while TimberTower used cross-laminated timber (CLT) panels.

LVL is a loadbearing plywood structure created through laminating many very thin wood-veneer layers, making the Modvion towers 250% stronger than CLT-based equivalents."

This composition of many thin layers compensates for the natural imperfections in the material and gives the tower additional load-bearing capacity, Dölerud adds.

A third major difference is that Modvion’s modules are assembled into taller segments at ground level to minimise the use of the tallest — and most expensive — cranes.

There are similarities to the TimberTower design, though, says Dölerud. "All our tower modules are 300mm thick, comparable to TimberTower’s uniform panel thickness.

And similar to them, we deploy steel adapters at the base and top of the tower, and steel inserts for all other horizontal wood-steel-wood interface connections."

The relatively small diameter and hub height of Modvion’s scaled 30-metre tower meant all five roughly six-metre long conical cylindrical sections could be assembled in the factory.

This tower diameter cones from 2.5metres at the base to 0.8 metres at the top.

Pre-curved

The 150-metre tower has a 12.5-metre base diameter and cones towards 4.0 metres at the top. The plan is to deploy section lengths of 15-25 metres, Dölerud says.

Each individual section is, in turn, subdivided into four to eight identical pre-curved modules, with the number and width of the modules decreasing with growing height.

"We are currently evaluating the best possible section heights, with regards to opportunities to optimise mass and cost and considering specific customer requirements," he adds.

The vertical joining of all circle segments in each tower sections will be carried out on site, which means the modules can be transported to project sites on inexpensive flat-back trucks (see picture, left).

"For the actual joining process, we will deploy a commercial industrial glue, and a stable mobile jigging system for section pre-assembly, functionally comparable to solutions used for bolted-steel modular towers," Dölerud says.

"Steel is much stronger per volume unit than wood. But a positive side effect of the inherently thick wooden tower walls is that this creates a large surface area for each individual glue joint, which means these joints have very favourable material stress levels," he says.

The overall benefits of Modvion’s laminated timber technology increase with size, he claims, because the processed structural wood is stronger per mass unit than steel, and more cost-effective regarding specific load-bearing capacity.

Mass and cost reductions

"Our calculations indicate that the 150-metre tower will reduce mass by about 30% and cut manufacturing costs by roughly 40% compared with an equivalent tubular steel tower with a 6-7-metre base diameter, says Lundman.

"And wood is a natural product that can often be sourced locally, creating local jobs and other added benefits."

The wooden towers also offer additional environmental benefits compared with steel towers thanks to the lower-carbon manufacturing process.

Lundman estimates a saving of 2,000-tonnes of CO2-emission per tower up until deployment. Plus, carbon sequestration in the wood offers the potential to make a wind-power plant carbon neutral.

Lundman hopes to improve the product’s environmental credentials even more by further advancing the mass and cost-optimised design, together with additional overall reductions in road transport-logistics and installation time and costs.

"This is also a main contributing reason behind Modvion’s overall strategy to maximise the share of wood in its tower designs," he adds.

Following the 30-metre prototype, the company plans its first commercial-scale tower with a 110-metre hub height by 2021. This could become the world’s tallest timber structure and, at the same time, have the lowest carbon footprint of all wind-turbine tower installations.

Design life

The development team is currently discussing specific requirements for a certified 25-year product design life with a certification body, and is confident this goal could be met.

"One main contributing factor is that wood offers excellent fatigue and load properties compared with steel, where material fatigue is a known limiting factor."

Beyond the first commercial installations, Modvion intends to continue raising hub heights and dimensions, and explore even longer product design-life.

First all-wood design — TimberTower built prototype but went bust

German manufacturer TimberTower installed its all-wood tower with a 100-metre hub height near Hanover, Lower Saxony, with a 1.5MW Vensys 77 turbine atop.

The main construction elements were multiple flat tapering cross-laminated timber (CLT) panels of various sizes (width and height) assembled into complete ring sections into an octagonal shape integrated with a lattice tower frame.

Another key design characteristic was the CLT cross-wise lamination of multiple 30mm and 40mm wood layers into 300mm thick panels built in a sandwich structure.

Industrial glue was used to provide permanent structural bonding strength in all vertical element-joining surfaces.

TimberTower had planned two further project variants for 2.5-3.0MW turbines, each featuring a 140-metre hub height.

The first of these proposed variants was a scaled-up 12-sided wood-steel hybrid tower with an 80-metre bottom section in wood, supplemented by three tubular-steel sections.

The second was an all-wood 132-metre structure, supplemented by a five-metre steel adapter at the tower top providing a steel-steel yaw-bearing interface.

None of these projects was realised as the company went out of business.

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