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

Pause due to head injury

I have not for posted a while, mainly because I have been seriously ill in hospital and have undergone 5.5 hours of brain surgery.  As a result my cognitive faculties are definitely below par especially my memory. This is inconvenient to say the least. I also have a poor sense of time.  I hope I will get back to normal soon. I have been recommended to maintain a normal routine as far as possible. This is easier said than done. A zoom meeting can be a struggle  (where has the chat gone?) I guess usability testing on these apps does not include use by people with head injuries. They are still a good test for cognitive function.

How cold can you go? Is 16°C reasonable?

As we enter a cold snap, and energy prices have risen again, we are more worried about how to manage our home heating. The easiest way to reduce bills is to turn down the thermostat. Do we really need 20°C or higher at home? Perhaps this is merely a social convention. David MacKay published ‘Sustainable Energy Without the Hot Air’ in 2009, in which he proposed turning down the thermostat at home to 16°C to reduce carbon emissions. At his publicity talks he explained how his household (with wife and two children) had successfully adapted to these conditions. He also incorporated the 16°C target in the 2050 calculator for scenario planning for net zero. This was the extreme level for one of the home energy 'levers'.

I find this idea scary; thick jumpers do not seem to keep my hands warm – even at 19°C I find my fingers stiffen so that I struggle to type unless I wear half-fingered gloves. I know many people are forced to endure such conditions through fuel poverty but I cannot imagine doing it from choice. Is this reasonable?

So I was intrigued to read about a group of 23 people in Belgium who decided to experiment with heating at home – can they turn down the setpoint on the thermostat and heat the body instead, generating less carbon emissions while still enjoying their living conditions and avoiding any ill effects? [1].

By the third season some were comfortable with the mean living room temperature as low as 14°C.

How much to disconnect your gas supply?

 

When you decarbonise your home by going electric, you can save more money by going off gas completely so as to avoid the standing charge (the fixed annual charge). In my region, this can save up to £115/year but in some places you can save twice as much [1].

To avoid the standing charge you have to get your meter removed. The meter is owned by the supplier and you have to get them to do this – DIY or Fred Bloggs the engineer are not allowed. The supplier charges are highly variable, from £0 to £100s or even more. For example, this story Why does gas supplier charge £486 to remove meter when others do it free? (Guardian) details a charge of £486 to disconnect a Quaker Meeting House.

These charges are a disincentive to changing to a heat pump. Paying the annual fixed charge can tip the balance between paying more for low carbon heating or less. However, paying the meter removal charge adds to the upfront cost which is already steep.

Some recommendations based on actual experience.

I asked my friends (in Transition Cambridge Energy Group) about their experiences of disconnecting. These are all anecdotes and I have omitted names. 

Solving under-occupancy with ‘adjustable housing’

One of the best ways to save energy is to move to a smaller house. Heating a larger house takes a larger amount of energy – floor area accounts for 70% of variation in space heating demand, as modelled by the CHM [1]. According to the Bedroom Standard (see below), 4.4% of homes are over-crowded but 69% are under-occupied [2]. If you allow one spare room then that number reduces to about 36% but that is still a lot! If everyone lived in a home the right size, we would overall need less heating energy. Obviously this is a hopeless ideal – but could we get some of the way there?

What’s wrong with water softeners?

Cambridge has hard water and it scales up our appliances. Some of them we can clean with a bit of effort, but some are not so easy. You can clean the heating element in your kettle just by soaking in hot dilute vinegar – but getting at the heat exchanger in your hot water cylinder is a tricky job. The obvious answer is a water softener. 

There are two problems.

1. Water softeners use extra water and we are already water stressed in this area.
2. Water softeners use a lot of salt and this is bad for the environment. It has to go somewhere: usually it ends up in our rivers.

How bad is this and what can we do about it?

Learnings from monitoring the solar hot water system

For reasons too complicated to explain (best summarised – if I was you I would not start from here) my beloved has implemented a controller for our solar thermal pump. This is for a solar thermal panel, rather than a solar electricity panel. The pump circulates hot water (with glycol) from the panel to the hot water cylinder and back. Knowing my love of data he added some monitoring into this system and we have been poring over the charts. Here is one from a reasonably sunny day.

Monitored temperatures on a sunny day (times in GMT)