Recycling fibre composites used in wind turbine blades

The volume of wind turbine blades to be recycled over the next few decades is phenomenal : some sources estimates range up to 800,000 tonnes per year (globally) in 2050 [1]. Sending all this to landfill would be a challenge as well as a huge waste. Unfortunately the blades are usually made of carbon fibre composite which makes recycling difficult. However, it is possible and if it is possible for wind turbine blades it should be possible for other uses of fibre composites such as aircraft wings, body parts for cars and so on.

Fibre composites materials, typically glass fibre or carbon fibre, are valuable for their high strength to weight ratio and resilience to fatigue. These valuable properties come from the strength of the fibres and the bond between the fibres and the surrounding material which is typically a polymer. The difficulty with recycling fibre composites is in separating out the fibres without losing strength. 

Wind turbine blades are usually made of carbon fibre because it is more stiff than glass fibre although it is more expensive. Fatigue resilience is also particularly important because wind turbine blades are subjected to cycling loads as the vanes rotate. These cycles can cause cracking and carbon fibre is more resilient than glass in this respect. However even carbon fibre blades do not last forever – typically 20 to 25 years - and then they must be either be sent to landfill or recycled. (It is also possible to repurpose the blades. There are some examples of blades used for bike shelters and in playgrounds on the BBC website here but these do not generate electricity.) Since the blades are very large and carbon fibre costs 18 times as much as glass fibre (per tonne: $23,600 vs. $1300 (Table 1 and 2 from [2]), material recycling is much preferred. 

To recycle the fibres they first have to be separated from the surrounding material. The main techniques used to do this are:

Mechanical – this means chopping the material into small pieces and sorting them. This is the least costly of the three methods compared here [2]. However, the fibres become short which effectively loses strength. This method is analogous to recycling paper by chopping it into small pieces and separating the fibre from the rest of the pulp. Typically the paper (cellulose) fibres become unusably short after 4-5 cycles. (https://recycled-papers.co.uk/green-matters/lifecycle-of-recycled-paper). For carbon fibre, particularly where high strength is needed, it is not possible to separate usable fibres with this method. Hence the final strength (and value) of the resultant material is zero. [1]. 

Chemical – this means dissolving the polymer matrix surrounding the fibres and is called solvolysis. This method is the most expensive [2] but effective, especially with carbon fibre (100% yield cf. 56% yield for glass fibre [2]). Given that carbon fibre is also more expensive than glass fibre this method is the most cost effective approach giving the highest net value [2] Fig 2).

Pyrolysis or other combustion including microwave assisted pyrolysis (MAP). Burning off the polymer without damaging the fibres is easier with glass fibre because this is more heat tolerant. 

Overall, the best recycling option to retain strength (least loss) is chemical recycling of carbon fibre as shown in this chart. The losses are calculated as 1- (fibre yield x retained relative strength of fibres) and expressed as a percentage.

 

Data from [2] tables 1 and 2 (converting from strength retained (yield * relative strength retained) to strength lost). For carbon fibre, loss is 100% from mechanical recycling because no usable fibres can be recovered. Chemical recycling works well for carbon fibre because 100% of fibre can be recovered at 100% of the original strength. This is the most cost effective recycling method.

Summary

Carbon fibre composites are used for many applications, including wind turbine blades, where high strength, stiffness and fatigue resistance are needed. Increasing use of renewable energy from wind is expected to generate large volumes of carbon fibre waste from retired wind turbine blades. Ideally this would be recycled. 

Carbon fibre is very expensive compared to glass fibre which favours recycling even where this is relatively expensive. For recycling, it is critical to separate the fibre from the surrounding polymer matrix without losing strength. The most cost effective way to do this is to use solvolysis - dissolving away the polymer leaving the fibres behind. Other methods are cheaper but lose value by losing or damaging the fibres.

[1] Liu and Barlow (2017) Wind turbine blade waste in 2050 in Waste Management https://doi.org/10.1016/j.wasman.2017.02.007

[2] Liu, Meng and Barlow (2022) Wind turbine blade end-of-life options: An economic comparison in Resources, Conservation & Recycling https://doi.org/10.1016/j.resconrec.2022.106202