Renewable energy provides nearly 20% of the UK’s electricity, and half of this comes from wind, according to industry association RenewableUK. The Committee on Climate Change predicts that by 2030 renewable electricity should provide 50% of the UK’s power needs, most of which will be wind power.
Offshore wind in particular is showing huge potential. The current UK offshore wind pipeline, if fully built out, would deliver around 20% of the UK’s electricity needs. Next generation turbines averaging an installed capacity of around 7MW each, produce enough electricity on an annual basis for 5000 homes.
Increased production has come largely from a move to larger offshore turbines, whose longer blades produce more energy. In addition to the political, economic and load management issues that are regularly discussed in the media, there are also important opportunities and challenges in the material design of the blades themselves. If we are to get the best out of turbines, and allow wind to reach its full potential as an energy provider, these need to be taken as seriously as the other challenges.
Changing designs, new challenges
Changes in design and increased use in more hostile locations, especially offshore, create new challenges both in terms of aerodynamics and durability.
A major change is being driven by a desire for longer turbine blades. The longer the blade, the more power it can produce. Today’s blades are up to 80m in length, but the next generation are likely to reach over 100m.
This creates a huge challenge for those working in materials development. Lengthening a blade hugely increases its mass, by almost a factor of three. This means that today’s blades are now so large that gravity has overtaken aerodynamics as the dominant load on a turbine. Reducing weight has become a top priority in blade design.
Environment also affects design, a big issue as more turbines are built for offshore. Offshore turbines experience an average wind speed of around 14 m/s off the coast of the UK, much higher that onshore. The high wind speed combined with rain in the high humidity salty sea air seriously affects durability.
Durability is an even bigger issue for offshore than onshore. The remote location of offshore turbines means that repair and maintenance of blades is hugely expensive. Finding a way to extend the lifetime of materials is therefore a priority for the industry.
So the two big materials challenges relate to efficiently increasing blade length and creating greater durability, especially in offshore environments.
Lighter weight blades: The story so far
Longer blades add weight, which creates a greater need for lighter weight materials to offset this. Fibre-reinforced composite materials have tended to come out on top, and various types of composites are used for key structural components in modern wind turbines, such as blades.
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 in the internal spar cap. 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 and polyester, 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.
The materials challenges of longer blades
For blades that exceed 80m in length, turbine blades will need to be made using a combination of several composite materials, making production even more complex, as each material will behave differently at each temperature. Even with careful control, you run the risk of exceeding the heat capability of the core material, destroying the blade’s structural integrity before it even goes into use.
Solving this will be down to mastering the chemistry behind these composites – a key focus of the Knowledge Centre for Materials Chemistry (KCMC). An important, but often-overlooked, aspect of composite is curing – this is the process by which the polymer matrix component hardens, and its effectiveness has a huge impact on the strength of the final composite. Better computational modelling of such processes offers big potential to enhance materials performance.
Recent research efforts in the area of turbine blades include resin systems that delay the start of the curing process, but which rapidly cure towards the end of the process, optimising the formation of chemical bonds. The addition of nano-components, such as carbon nanotubes and graphene, especially as a strengthening agent at the fibre-matrix interface, is also an active area of research.
Coping with tough environments
Composites don’t offer all the answers. For offshore turbines, mechanically-tough glass fibres are, like other materials, susceptible to damage by the windy, salty environment of the seas. Given that offshore blade life needs to be a minimum of 25 years, durable coating materials are a key aspect to ensuring erosion-resistance in wind turbines. Coating materials currently used include epoxy and polyurethane gel-coats, polyurethane paint systems and tapes.
Most of these coatings started off in the aerospace industry, and so their performance tends to be optimised for those applications. However, the UV combined with the high humidity of the salty sea air, extreme wind conditions and high-speed spinning blades of an offshore turbine, combine to form a highly erosive environment that far surpasses anything experienced by an aircraft. Specialist coatings are being produced by manufacturers such as AkzoNobel and 3M, but many turbines still use the epoxy and polyester resins developed decades ago.
There is therefore a need for companies to develop new formulations both for coatings and composite resins – ones that are designed specifically to withstand the harsh environments in which offshore turbines must operate. At a recent KCMC industry meeting, chemical company, Scott Bader, talked about using novel polymers to help enable more efficient manufacturing and recyclability of components.
One of the biggest issues in turbine performance is to predict the behaviour of both the composite and its coating once in use. A key research area for the Offshore Renewable Energy (ORE) Catapult is blade leading edge erosion. The leading edge of a turbine blade is that which cuts through the air (shown in diagram), and it is the region of the blade that experiences the highest level of erosion.
In order to better understand this erosion, a group of UK and Ireland-based universities, wind turbine developers and operators are now collaborating on a huge research activity called BLEEP (blade leading edge erosion programme). Led by ORE Catapult, BLEEP will design and build an erosion test rig that has been fully validated against actual erosion measured in service. The rig will be used to understand the erosion failure mechanisms of various coating materials and blade structures in the offshore environment.
In order to go beyond what is currently possible with traditional composites, we’ll need to see a step change in both blade design and materials choice. As we have outlined, there are some exciting areas of research pursuing solutions to the challenges or durability and blade length, but more work is still needed to provide complete solutions. Companies are playing an important part too. Blade Dynamics, for example, are beginning to change the way we look at turbine development – they are using novel thermoplastics to increase the mechanical performance of the blade’s leading edge.
Innovative companies and research groups are certainly helping to push the industry forward. But a more-joined up approach will be the only way to make large, lightweight blades that can cope with the harsh environments they must operate in.
Wind turbines are set to play an important part in future energy production, but they are still costly to build and even more costly to maintain. The materials developments from organisations such as the KCMC and ORE Catapult and their partners in business and academia will have wide implications and play a key part in the industry’s success.
Understanding fundamental chemistry will help to reduce the cost of the wind industry. And the knowledge gained in programmes such as BLEEP will drive standardisation and the development of new coatings and structural solutions for a new generation of high-efficiency wind turbines.
ABOUT THE AUTHORS
Dr John Conti-Ramsden is Director of the Knowledge Centre for Materials Chemistry (KCMC) and
Dr Kirsten Dyer is Research Materials Engineer at the Offshore Renewable Energy (ORE) Catapult.
Knowledge Centre for Materials Chemistry (KCMC)- http://materialschemistry.org.uk/
Offshore Renewable Energy (ORE) Catapult - https://ore.catapult.org.uk/