Plastic in wind turbines?

Wind turbines are one surprising application where plastics are helping to fight the climate crisis. Darshil Shah from the University of Cambridge took Verner Viisainen to the wind turbine at the Wood Green Animal Charity in Godmanchester in Cambridgeshire to explain to him why plastics are essential for structures like the wind turbines.

Verner - Right Darshil, we're standing right next to the wind turbine, which parts are actually made of plastic?

Darshil - The tower is completely made from steel, but it's the blades where all the composite and the plastic materials are.

Verner - Why are plastics used for the blades?

Darshil - There are two key reasons why we have been making these out of reinforced plastics. Firstly, wind turbines become more efficient if the blade shape is particularly more aerodynamic. To form these efficient blade shapes, you need to be able to curve them and taper them and twist them in very specific profiles, which is manufacturable through reinforced plastics. The other key advantage of composite materials is also their great strength and stiffness to weight ratios. They're incredibly strong and stiff for their weight and this is important because the lighter the blade is, the whole turbine is more efficient in generating power, which is the whole purpose of a wind turbine.

Verner - You mentioned that these are not any old plastics, they're so-called 'composite materials'. So what are composite materials?

Darshil - In this case, we refer to them as reinforced plastics and usually they are fibre-reinforced plastics. The polymer itself can be synthetically derived from petrochemical sources. In this case, it is based on an epoxy resin. The fibres themselves are what are actually providing the stickiness and the strength. In this case, it is fiberglass that is being used. This blade is only about 25 meters in length weighing about 1 tonne to 1.5 tonnes, but blades can get much, much larger. Today's largest wind turbine is getting to about 14 megawatts of rated power with the blades themselves being 4x the length of this blade, but around 28 times the power capacity of this turbine.

Verner - These blades are made of plastic and we've heard the plastics have quite a lot of emissions associated with them, but also the wind turbines are providing us with renewable energy. So would you say this is a good use of plastics?

Darshil - I think this is definitely one of the better uses of reinforced plastics. The two main areas where plastics are using a lot of energy and consumption in their processing is firstly to do with their petrochemical sourced nature. These need to be converted into polymers. Firstly, and then for the fibres, if it's carbon fibre, you need to process these at really high temperatures. Therefore a lot of energy is going into the production of these high-performance fibres and they are reducing the amount of tower material you're requiring. So it is a good use of the resource, but we can do better.

Verner - It's a bit of a rainy and chilly day today. Should we move somewhere a bit warmer to continue the chat? Yes, definitely. I would like that.

Verner - We've come away from the wind turbine because it was a very dreary and rainy day today. And to cheer us up, I bought both of us some doughnuts, which we're about to enjoy. Then I wanted to run you through this analogy and see what you thought. 'Plastics to the earth are a bit like doughnuts to human health. We know that doughnuts are not very good for us if we eat too many of them, but they serve an important function because they taste oh-so-good.' This is a bit like plastic. If we use too much of them, they end up in the oceans and cause a lot of pollution and emissions. We need to be careful in the way that we use our plastics and use them in moderation a bit like how we should approach eating doughnuts. What do you think of this analogy?

Darshil - I think I can work with it. I like it. Just like we have doughnuts as a treat, I suppose we need to be more selective where we are using plastics. And if we think about it, plastics have really fantastic properties and that's why we do use a lot of it. We need to get better at both considering end-of-life possibilities of these materials, as well as on the other scale, how can we make more of these from bio-based resources?

Verner - We typically see plastic as a problem because it accumulates as waste, but this plastic in the wind turbines could keep spinning for many years. So why might this plastic represent a problem then?

Darshil - Partly because wind turbine blades have a defined lifespan. Usually that tends to be 20 to 25 years after which a wind turbine is decommissioned. It is completely brought down and you just replace it with a new one. We consume or at least produce two and a half million tons of reinforced plastic materials, specifically for wind turbine blades, every year.

Verner - How do these techniques for recycling reinforced plastics compare to the techniques used to recycle typical plastics?

Darshil - That's an interesting question because the classes of the polymers that are used in reinforced plastics, particularly for wind turbines and the plastics you get in packaging are different. The packaging plastics fall into a category referred to as 'thermoplastics', which effectively when you heat them they start to melt and then you can mould them into a specific shape. When you take away the heat, it retains the shape but amazingly you can then reheat these materials and form them into a new shape again. In contrast, the polymers that are used in wind turbine blades tend to be 'thermal sets'. These initially start off as liquids and then a polymerization process leads to a hardened and cured material, which is the shape that you end up with following moulding. Thermal sets are slightly more difficult to recycle because once they form this cured and hardened material, it's not a reversible process. Meaning the only way to deal with them tends to be either incineration or traditionally landfilling. There are some methods in which you can reuse some of these polymers to downcycle. For example, integrating them for making injection moulded materials. The complexity of recycling becomes even more increased when you introduce fibres. It's related to the problem that we have in conventional products of mixed plastics. If you have too many different types of materials together, you need to first spend energy in separating these materials beforehand.

Verner - So do we need to start making wind turbines out of thermoplastics to make recycling them feasible?

Darshil - Again, that's a fascinating question. There have been some efforts to make wind turbine blades with thermoplastics. The challenge with thermoplastics is that they usually start off as solids, which you then need to heat up. That's all-and-well if the component size is quite small, but if it's a wind turbine blade that is 25 meters long or 100 meters long, then that becomes really almost impossible because then you need compression moulding machines or presses and heated presses that will facilitate the fabrication of these large components. Therefore the possibility of making them with thermoplastics is quite small. Thermosets for a while will remain.

Verner - Thank you so much Darshil, this has been illuminating. Shall we head back to Cambridge now?

Darshil - Yes and have our doughnuts.