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.
http://www.renewableenergyworld.com/articles/2015/07/materials-are-blowing-in-the-wind.html