Durability is the key to wind turbine engineering. The huge blades are highly exposed to the elements and must withstand everything nature throws at them for decades, but when they reach the end of their operational life, that durability becomes a problem.
Throwing blades into landfills takes up huge amounts of space and wastes materials that could be useful when extracted and given new life. The thing is, wind turbines are so durable that breaking them down and recycling them presents significant challenges. In a study published today in the Cell Press journal Chem Circularity, researchers present a new approach that could overcome these challenges.
By rapidly heating turbine blade waste with short bursts of energy, the authors broke it down into silicon carbide and graphene, two valuable manufacturing materials that can be used to improve the mechanical and thermal properties of new blades. And compared to current recycling techniques such as chemical dissolution and heat recovery, this new approach reduces greenhouse gas emissions by 92% and 83% respectively.
“It can be used to turn low-value waste into high-value waste,” lead author Hao Bi, an environmental engineer at Tsinghua University in Beijing, told Gizmodo.
The leaves keep turning
Wind turbine blades are usually made from fiberglass, which contains silica, and a carbon-rich polymer resin. When these materials reach super-hot temperatures, carbon can react with silica to form silicon carbide, an extremely hard, heat-resistant compound. Carbon atoms can also rearrange into a flat, honeycomb-like lattice called graphene, coveted for its strength and ability to conduct heat and electricity.
To extract these materials from turbine blade waste, Bi and his colleagues used a technique called flash joule heating. He explains it as “high current electrical impulse applied to conductive raw materials to increase their temperature to 2,000 degrees Celsius [3,632 degrees Fahrenheit] or even higher in milliseconds.”
Flash Joule heating has already been used to convert plastic waste and biomass waste into graphene and other nanostructured carbon, and it has proven successful in extracting useful materials from turbine blade waste as well.
The researchers first created a mixture of powdered blade waste and nearly pure elemental carbon, then subjected the mixture to Joule heating over several experiments. A typical experiment turned about 56% of the mixture into a silicon carbide and graphene composite consisting of 34% silicon carbide and 66% graphene.
Adding a small amount of these products – just 0.3% by weight – to the resin made it stiffer and stronger, improving its resistance to deformation by 27% under tension and 9% when bent. At 0.1% by weight, the resin’s thermal diffusivity – a measure of how quickly heat spreads through a material – increased by 41%.
Thus, silicon carbide and graphene extracted from retired wind turbine blades could be used to make new blades even more durable, but scaling this process presents a whole new challenge.
Next step: Scaling
Bi and his colleagues were only able to test small batches of the blade waste and carbon mixture. The largest was just 10 grams – well below a practically relevant processing scale.
His team has already designed and built a larger reactor to process a 200 gram batch, but it still requires optimization. In particular, Bi emphasized a need to ensure that this new reactor can heat the material uniformly and produce consistent product quality.
“I would describe this work as a promising proof of concept with initial scale-up results, but it is difficult to predict when it could serve a real wind farm,” he said.
On a commercial scale, the researchers estimate that their blade recycling approach could make a profit of about $80 per ton of blade material, given the high value of the recovered silicon carbide and graphene. Conventional recycling methods require wind farm operators to pay for waste processing. Pyrolysis costs about $460 per ton, while chemical dissolution costs about $1,800 per ton, according to Bi.
This would help stimulate a closed-loop turbine blade manufacturing system, making this critical renewable energy source more sustainable. But again, the road to scaling remains long.