Sustainability of composite wind turbines.

Wind energy has been present in the market for decades and is being perceived as a green source of energy. They do generate energy with low CO2 emissions compared to fossil fuels, however, materials used to build blades and recyclability of those blades are still a hot topic in the wind industry. In 2017 there were over 341 000 working wind turbines in the world, steadily increasing in the years after, with 2020 being a record year for the global wind power industry. Moreover, the forecast is to double the current capacity by 2030 (Global Wind Enery Council report). To give an overview of the numbers, the longest currently manufactured blade counts 107 metres long, and weighs around 55 tonnes. Combining all of the above and considering the forecasts for the following years, the world will end up with an extreme amount of waste, once the lifecycle of the blades finishes.

Due to continuous fatigue loads, weight and weather conditions, composite materials are the most suitable to manufacture turbine blades, regardless of their size. The positive side of that is the fact, that lifespan of a single blade has been extended to 25 years. The negative is that currently there is no scalable technology to recycle blades that are no longer safe to use on a turbine. The vast majority of the blades are being built using thermoset resins as matrices. The main characteristic of thermoset polymers is irreversible hardening. Once hardened, a thermoset cannot be melted for reshaping or recycling. This makes recycling difficult and expensive, thus economically unattractive for the companies. There have been multiple ideas to cut down the blades into pieces creating bike sheds, park benches or pedestrian bridges but this is not a valid solution from a long-term perspective. Still, the majority of the blades end up being buried in the landfills or burned, creating emissions they were supposed to limit.

Siemens Gamesa, being one of the leading blade manufacturers in the world is already building blades made out of thermoplastic resin, which unlike the thermoset can be reshaped after hardening. Currently, their capacity is at 81 meters long blade, which is significantly smaller compared to market-leading solutions. The reason for that is that the blade is being made in a single shot, unlike the other designs. This approach, although innovative and efficient, is creating a lot of challenges with bigger blades. Due to increasing demand, the turbines are getting bigger and more often installed off-shore meaning that one-shot technology will not be sufficient anymore. The traditional approach is to create two shells separately and bond them together afterwards. However, the bonding paste used for connecting the element is again- thermoset, and there is no thermoplastic replacement for the adhesive. It is estimated, that the bonding paste alone stands for 2-3% of total mass, resulting in hundreds of kilograms of epoxy adhesive per blade.

The alternative solution which I have a chance to be working on is thermoplastic welding. The overall concept is not new, just has not been used in the wind industry on a bigger scale yet. The main principles of this technology are to heat the resin locally and apply pressure on the bonded area. Heat can be provided in many ways, however, there are two most common methods induction and resistance welding. Both require metal insert between welded surfaces, but the heat is generated differently. The goal of both, however, is to increase the temperature of resin up to the melting point and apply pressure over the bonding area. The outlook of this technology is to enable having longer blades built, which will compete with the ones made using thermoset resin, and is entirely recyclable. If the project succeeds, the thermoplastic resin could be used on a larger scale in the wind industry, finally becoming a standard.

Although the project is still on the laboratory scale, recent tests resulted in the shear strength in the bonding of around 25 MPa, which becomes reasonably close to industry-grade epoxy bonding pastes used in turbine blades. At the time of publishing this entry, the technology is still facing challenges in power supply. From the two aforementioned methods (induction and resistive), I work to develop resistive welding, meaning that there has to be a sufficient amount of electric energy flowing through the medium. Power consumption comes from the required temperature to melt the resin. The metallic insert used to heat up the resin is charged with direct current, where the power reaches values of 2-3 kilowatts to weld a surface of 400 square centimetres. This area is far from what the bond-line area in a turbine blade looks like, thus the focus is placed to increase the welding surface and come up with sequential welding, one section at a time. The second challenge that has to be faced in the near future is quality control. For adhesive bonding, the process is quite straightforward. Usually, the amount of bonding paste applied is excessive and after the elements are positioned and pressurised the squeeze-out is being scrapped and proves that the bond line has a proper quality and is void-free. With thermoplastic welding, this topic is not that simple, because the bond line thickness is much lower in the first place. Secondly, prior the welding the surfaces are solid, unlike bonding paste hence the quality of the weld cannot be briefly analysed visually, without specialised tools. Nevertheless, considering the progress made so far in this development, the further results are rather positive.

Fortunately, governments started to notice the growing number of blades in landfills, let alone all those blades that will become out of order in the coming years. The Netherlands, Austria, Finland and Germany already have landfill bans for wind turbine blades in place, and France is introducing requirements for the recyclability of turbines in upcoming offshore wind tenders. Meaning that companies will be placed under higher pressure to push technologies to build more sustainable turbines forward. Hopefully, government regulations together with innovative technologies will be a sufficient reason for companies to follow a more sustainable approach in designing and building turbines that can be called green without any exceptions.

This creates a solid foundation for technologies like thermoplastic welding, mostly because there is no feasible alternative at the moment. If companies will be required to provide more sustainable blades, they are likely to increase the funding for researching new technologies which may help overcome obstacles we have been facing so far and increase the pace of the development. Another problem which remains, however, is where to apply all the material retrieved from the blades. Due to demanding working conditions and constant loads, the recycled material will unlikely be used in any structurally loaded construction. Nevertheless, the most important element is to make the blades entirely recyclable in the first place, and the rest will follow.