A new rotation for turbine blades

Extreme conditions need extreme solutions. That’s why the enormous blades driving our wind turbines are marvels of modern technology. But they aren’t immune to entropy. So what happens when they wear out?

According to Dr Ali Hadigheh of the University of Sydney’s School of Civil Engineering, modern advanced composite materials answer a pressing need.

“Wind turbines have blades of anywhere between 30m and 100m in length. These must be strong. They must be lightweight. And steel and aluminium can’t do both,” Hadigheh told Cosmos.

“Carbon fibre composites are considered a ‘wonder’ material – they are durable, resistant to weathering and highly versatile – so much so that their use is projected to increase by at least 60% in the next decade alone.”

Smaller blades are generally made of fibreglass, while the largest use carbon fibre. Both rely on the inherent strength of resins holding long fibres woven in alignment with the applied forces, but they’re not invulnerable.

Ultraviolet light is a challenge, though resistant coatings have reduced that problem. Then, there are lightning strikes and the rapidly shifting stresses associated with storms.

“Mostly, they go through a lot of cycles,” Hadigheh explains. “They are constantly turning. So, just as you see cracks and dings in a car after a few years, turbine blades also experience wear and tear.”

Fibre composites are as strong as they are because of their finely tuned construction. But the constant stress the blades must endure can turn a minor crack into a structural failure.

“If you put one of these composites indoors with minimal stress loads, they’ll last centuries. But the constant cycling of stress loadings causes separation between fibre and epoxy.”

A fibre can snap. That tiny crack can let in rain. In turn, that causes the crack to enlarge and the larger crack can trigger more fibres around it to snap.

“That’s why these blades have specific service lives, a point at which you know you will need to replace them. And that’s a huge task,” Hadigheh says.

The Clean Energy Council released its “Winding Up” wind turbine recycling report in May. It found a turbine has a design life of about 25 years. And about 600 turbines across Australia are now more than 15 years old.

Globally, it’s been estimated that some 500,000 tonnes of carbon and glass fibre waste will be produced by the renewable energy sector by 2030.

Turbine blades aren’t the only problem. Aircraft, mobile phones, road vehicles, boats – even utility poles – now use these durable materials regularly. That durability is a problem. It means most waste composites are currently burnt or buried.

So, the race is on to find ways to recover as much as possible, as cheaply as possible.

Carbon and glass fibres must be separated from the epoxy resin encasing them, then they must be sifted, sorted and realigned. And it must be done in an environmentally friendly way.

“From the earliest stage, we looked at different recycling pathways, which method would give us better performance in terms of the cost return as well as reduction in CO2 emissions,” says Hadigheh.

A study by Hadigheh and his former PhD student Dr Yaning Wei found much of the infrastructure needed to do the job is already in place. And the most significant challenge – realigning the retrieved fibres – now has a patented solution.

“We use very, very weak acids and pyrolysis (high-temperature decomposition without oxygen),” he explains. “These methods lead to substantially lower CO2 emissions than landfill and incineration.”

Thermo-chemical processing is an industrial technique used around the country.

“All the equipment is there. The technology is there. And we have the knowledge here at UTC. Now we just need some interest from the industry to invest in the configuration and optimisation needed to incorporate this process into their workflow.”

He says that using weak acids to break down the resins containing glass and carbon fibre helps maximise the length of the retrieved fibres. And pyrolysis is achieved at the relatively low temperature of 420⁰C. “So that reduces the energy input to the system, so it saves both on cost and CO2 emissions,” he adds.

The decomposed resin produces gases that can fuel the pyrolysis process (a feedback loop the industry already exploits). Other byproducts include oils that can be used in other products, such as asphalt, and biochar, which can be used in concrete.

“Nothing will be lost,” says Hadigheh. “You have to make it cost-effective to get as much return from your investment as possible.”

The clean fibres will be too short to produce the extreme strength needed for replacement turbine blades. But Hadigheh says they can be used to make components at least as strong as steel – at a fraction of its weight.

“Once recycled, the fibres are like hairs piled on the ground at a hairdresser. They are tangled, random, out of form. And it’s hard to separate them – they’re one-tenth the thickness of a human hair.”

They’re also brittle and too much handling will break the fibres into even shorter lengths. So keeping them as long as possible to be woven into new structural matting is imperative.

“We developed a patent for that,” says Hadigheh. “We can align these recycled fibres in any direction we want.”

The fibres are picked up by water and carried through different channels. Hydrodynamic forces align them before being deposited on a rolling mesh. The repurposed fibres are left behind in the desired precise pattern as the water drains away.

“We did tests on these samples,” he says. “The tensile strength for virgin carbon-fibre composite is about 1,300 megapascals. Randomly aligned recycled short fibres can achieve less than 100 Mpa. But aligned short fibres get up to about 450 Mpa – about that of steel or aluminium, but in a much lighter form.

“That means that if you’re already using steel or aluminium, you can replace it with this much lighter product. And it’s recycled.”

The team is examining how the filaments could be applied in different ways, such as through 3D printing or as a strengthening agent for concrete.

“It could also replace the steel reinforcement inside concrete,” he says. “They are corrosion resistant. So you could reduce the risk of corrosion in something like a bridge in a coastal area, for example.”

Dr Hadigheh says the next step is to commercialise the process.

“While awareness of everyday consumer recycling is increasing and plastic waste is in the spotlight, Australia must urgently consider wide-scale recycling of new generation construction materials before they mount up as another waste problem and are put into the ‘too hard basket’,” he concludes.