Using pultrusion to manufacture wind turbine components – such as spar caps – can make operations more efficient. Lou Reade reports.
Wind energy continues to grow in importance, with the UK becoming one of its leading proponents. Just this year, Britain started running the world’s largest offshore wind farm – the £1 billion Walney facility off the coast of Cumbria, which generates enough energy to power 600,000 homes.
At the same time, turbine blades continue to get longer. GE Renewable Energy, for instance, recently revealed plans to manufacture a turbine blade more than 100m long.
Composites technology – especially using carbon fibre – will be a crucial factor in the production of the blades. And some suppliers, such as Exel Composites – which has a UK facility in Runcorn in the North West of England – say that blade manufacturers are increasingly turning to carbon fibre pultrusion as a way of manufacturing critical blade components more effectively. Although pultrusion in wind energy is not new, it could be more widespread, says the company.
“Pultrusion helps to raise the efficiency of making blade components in a number of ways,” says Dag Kirschke, product business owner for wind at the company.
He cites several reasons why it makes sense to ditch traditional methods like resin infusion – and adopt pultrusion – when making critical blade components like spar caps and spacers.
“The most important reason is cost,” he adds. “Using pultrusion means there is no need for a mould – which cuts out a large investment.”
As well as removing the mould, switching to pultrusion means subcontracting the whole process of making components like spar caps: pultrusion is not the kind of process that is brought in-house, so it is contracted out to a specialist. While this takes an adjustment in attitude, Kirschke says the idea of subcontracting is well-established in composite blade production – revealing that turbine producers already subcontract the manufacture of towers or generators, for instance.
“They’re used to subcontracting, but not always in the process of blade building.”
The ability to banish spar cap moulds from the factory floor also frees up extra space for a blade manufacturer to boost production: Kirschke says that removing 10 spar cap moulds from a factory might allow a blade producer to add two more blade moulds.
[caption id="attachment_11671" align="alignright" width="405"] Kirschke: “Pultrusion raises the efficiency of making blade components in a number of ways.”[/caption]
There are other knock-on savings to using pultrusion., Because pultrusion allows the blades to be made lighter, it means that major components like the gearbox can be downsized.
“If you save weight, you take out raw materials and labour – and will therefore be more competitive,” notes Kirschke.
Spar caps are a critical component that add stiffness and strength to the blade. While the outer shell of the blade is still made using traditional resin infusion, the underlying spar cap structure – if it can be made lighter using pultrusion – will help to cut the total weight of the final blade, and that of the final tower and turbine.
One more factor in pultrusion’s favour is that it leads to greater accuracy in part production.
“If you resin-infuse a long turbine blade, you don’t know exactly where you have all the weight of the resin,” says Kirschke. “You may get 5% less here, and 1% more there. But with pultrusion, the weight of each section will be the same.”
The wind of change
Despite these advantages, he says that many manufacturers have yet to embrace pultrusion – though several are investigating its potential.
“Many are still using resin infusion because they are familiar with it. Switching to pultrusion also means a complete redesign of the blade.”
Resin infusion is an established process, but one that is well understood. Pultrusion, on the other hand, requires parts to be completely redesigned – though this effort will result in a lighter and stronger end product.
“You can’t just replace an infused spar cap with an identical pultruded one,” says Kirschke. “At least, not if you want to take full benefit of the process. With pultrusion, you want to design the blade as light as possible.”
So, the blade shell need not be designed to be as strong, because the underlying spar cap – if made using pultrusion – can be made both lighter and stronger. “This in turn allows the shell to be made lighter,” he says.
A lighter, stronger blade makes perfect sense, as it allows longer blades to be mounted on a tower. Due to the longer length, it will then be able to sweep more area and generate more power. This will be a key factor in boosting the energy efficiency – and generation potential – of new wind farms.
As well as replacing other composite production processes, in order to make spar caps, pultrusion can also be used to manufacture more ‘mundane’ tower components such as ladders and gantries. Here, the pultruded composite components are more likely to replace existing steel parts.
For its part, Exel recently added a new string to its bow – taking over US-based pultrusion specialist, Diversified Structural Composites (which has since been renamed Exel Composites US). This means that the company now has pultrusion capability in all the major wind turbine production regions. It also has the ability to transfer spar cap production technology from the US company to its European facilities, including Runcorn – which has already produced carbon fibre-based spar caps for a number of years.
“Runcorn is already one of our strongholds in carbon pultrusion,” says Kirschke. “I assume that it will grow significantly over the next few years, to become be one of the major centres for our carbon spar cap production.” Others would include the USA, China, Finland and India.
Unfortunately, the shadow of Brexit hangs over everything: if it ends up imposing large tariffs on UK products, Exel says its plans may change. However, despite this uncertainty over future production plans, Kirschke is unequivocal when it comes to the benefits of the technique for blade components.
“If it can be made using pultrusion, you should do it,” he says.