Many parts of windmills – the metal and the concrete – were already perfectly reusable. But the blades are made of a so-called composite, a composition of different plastics. Especially glass and carbon fibers glued with a synthetic resin. Thanks to these materials, the blades are relatively light and can easily be shaped into the desired shape.
Only, after about 25 years, when hairline cracks sometimes appear in the blades, they have to be replaced. In the Netherlands they are then usually incinerated by waste companies in kilns. Abroad, they sometimes also disappear underground, according to many shared images of a ‘mass grave’ of rotor leaves in the US three years ago.
‘Such images are close to my heart’, says materials science researcher Harald van der Mijle Meijer of TNO. ‘Under the ground, that plastic degrades and so all microplastics end up in the environment. You don’t want that, especially not with a product that should be sustainable.’
So he started a project within TNO to better reuse the blades – of which hundreds of thousands will be made in the coming years. Van der Mijle Meijer now has the result next to a highly advanced oven at the TNO research site in Petten, North Holland. Pure pieces of fiberglass and, even better, beautiful pieces of braided carbon fibres. “When this came out of that oven, it was a euphoric moment.”
Oversized boiler
It was already known that resin and fibers separate at about 500 degrees Celsius. But in this oven, which looks more like an oversized boiler, that process has been perfected. The resin runs off the fibers at a relatively low temperature and without oxygen. Oxygen is then briefly admitted, whereby the residues of the resin burn and a lot of heat is released. ‘You can of course reuse that heat in a factory’, says the researcher enthusiastically. ‘For example to heat the next oven.’
At first glance, the results do indeed look good, says material researcher Julie Teuwen of TU Delft in a response. She is not involved in the investigation. ‘It was already known in itself that you could get those fibers out again. But it’s nice to see that they can now do that in the Netherlands in such a way that the fibers can also be used industrially.’
That is indeed our eureka moment, says Van der Mijle Meijer. Satisfied, he shows a part with strange shapes and threads made from the recycled fibres. ‘A test that proves that it is indeed suitable for use in, for example, the car industry, for making dashboards and housing for headlamps.’
Now Van der Mijle Meijer hopes very much that it will really come to that. ‘That a company will be built that will have a large oven built that can accommodate the entire rotor blades, and that everyone will soon be driving around in electric cars that are partly made from recycled windmill blades.’