Wind energy is big business, and with 1,184 offshore turbines, the UK is a world leader. According to RenewableUK, wind energy’s contribution to UK energy needs has skyrocketed in less than 15 years, from less than 400 MW to over 11,000 MW today. In 2013/14 alone, wind capacity grew by almost 15 percent, achieved by increasing the number of both offshore and onshore systems available to the grid.
The properties of wind turbine materials are often over-looked by the general public, but they have a huge impact on their performance. Just consider their operating environments — offshore turbines must cope with a dynamic wind environment, with speeds upwards of 14 m/s in many locations. They must also withstand the high humidity of the salty sea air, combined with strong UV doses from the sun. And they need to do all of this for a minimum of 25 years. So there is a considerable research effort into finding the optimal combination of mechanically-strong materials and durable coatings, to produce erosion- and corrosion-resistant wind turbines.
But there is an additional trend in wind power which has a materials chemistry dimension — turbines continue to grow. It’s expected that turbine blades upwards of 100 meters in length will soon make their way to market. Because the power output of a turbine is proportional to the area swept by the blades, longer blades mean more power. But they also mean increased mass — so much so that turbine manufacturers are having to consider a whole new generation of materials in blade design.
Because of their high strength-to-weight ratio, composites have long been used in the wind energy sector — according to the University of Cambridge, wind turbine manufacturers use ten times more composite materials than the car and aerospace industries combined. Many composites are made up of just two materials — high-strength reinforcing fibres and the matrix, which binds and surrounds them. Glass fibre is by far the dominant reinforcement, but its high density means that for future large blades, it will be too heavy. Given that, carbon fibre is often used in selected areas, including key load-bearing components. But because of its high cost, carbon is not the only answer to the question of strong, yet lightweight blades. The choice of matrix material is also important and varies widely across the sector — from thermoset polymers like epoxy, to more novel materials like thermoplastics. A team at the University of Bristol, is developing self-healing polymers for use in composites — they release a high-performance adhesive into any cracks that form during use.
For those “super-blades” that exceed 80 meters in length, the only sustainable solution is likely to be a combination of several composite materials. Like all compromises, this option won’t be without its problems — given that each material will behave differently to changes in temperature, production will be more complex than for single composite structures. Solving this will be down to mastering the chemistry behind these composites — a key focus of our team at the Knowledge Centre for Materials Chemistry (KCMC). Recent research efforts in this area include the addition of nano-components, such as carbon nanotubes and graphene, to act as strengthening agents within the matrix, and resins that cure in a highly-controlled fashion, minimising the risk of manufacturing errors.
But, however impressive composite materials are, like many other materials, they are susceptible to damage by their surrounding environment. Because of this, manufacturers of coating materials are key players in today’s wind sector. Originating from the aerospace sector, these epoxy and polyurethane gel-coats, paint systems and tapes are widely used on offshore turbines. While manufacturers such as AzkoNobel and 3M are developing specialist coatings, there are still opportunities to develop further. At a recent KCMC industry meeting, Scott Bader talked about using novel polymers to help enable more efficient manufacturing and recyclability of components.
One of the biggest remaining challenge in turbine performance is also materials-related – to better understand the failure mechanisms of turbine blades and to predict the material behaviour. Led by Dr Kirsten Dyer at the Offshore Renewable Energy (ORE) Catapult, a large research activity called BLEEP (blade leading edge erosion programme) aims to do just that.
The leading edge of a turbine blade experiences the highest level of erosion because it ‘cuts’ through the air. The aim of BLEEP is to understand the erosion failure mechanisms of the various coating materials and blade structures in the offshore environment. According to Dyer, “Current systems can only give a qualitative assessment of material performance. This programme will work at the interface between industry and academia, combining modelling and measurement to create a quantitative system that can characterise fibre-composites and their specialist coatings.”
It’s clear that a step-change in both blade design and materials choice will be needed if we are to go beyond what is currently possible with traditional composites. Understanding fundamental chemistry will be key to that, as will the drive towards standardisation, through programmes such as BLEEP. For the KCMC, open, cross-industry collaborations will be the only way to realistically make large, lightweight blades that can cope with anything that Mother Nature can throw at them.
Lead image: Wind turbine. Credit: Shutterstock