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1. The use of Composites in Rotor Blade Design
The wind power industry in Europe is expected to grow significantly in the coming
years to coincide with EU targets for the reduction of carbon emissions by 2020 and
beyond. The development of technology towards deep water installations of wind
farms is the vital step towards achieving the target of 20% of Europe’s energy
demands by 2020. Harnessing stronger and more consistent winds in deeper water
with larger turbines rated at 8-10MW and higher, is key to providing cost-efficient
electricity. To realise the production of energy at prices competitive with energy
from fossil fuels, turbines will need to be cheaper to manufacture, to install, and to
maintain.
Turbine blades conventionally have been constructed with glass-fibre or carbon-fibre
composite materials, and are designed to have a lifetime of twenty years or more.
One of the important aspects of turbines installed on deep water farms is the cost of
servicing and maintenance. Due to their location and the difficulty and costs of
working at sea, it is essential that the larger turbines designed for offshore farms
are highly reliable to reduce downtime for servicing and repairs.
The industry as a whole and blade manufacturers in particular, are looking for
composite materials with a variety of characteristics to suit this purpose. The
materials must be strong enough to cope with the excess load caused by high winds
on much longer blades. The reduction of the weight of the blades is also important,
as the extra weight and fierce weather conditions could cause failures in other
components of the turbine, thus creating extra downtime. Cost-efficiency is another
driving factor; manufacturing and production costs should not adversely affect the
ultimate target of producing cheaper electricity en mass.
Research
In 2010, The Danish Council for Strategic Research’s Programme Commission on
Energy and Environment awarded funding of DKK 38 million for the establishment of
The Danish Centre for Composite Structures and Materials for Wind Turbines
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2. (DCCSM) (1). The research centre involves several industry partners including Risø
DTU (National Laboratory for Sustainable Energy), The Department of Mechanical
Engineering at Aalborg University, Siemens Wind Power, LM Glasfibre, Fiberline
Composites, and Bach Composite Industry.
The project is intended to carry out extensive research into the development of
experimental methods of producing and monitoring blades at various lengths. It is
hoped the research will give a greater insight into the characteristics required of
composite materials in terms of controlling damage evolution and crack growth. It is
also aiming to develop new methods of analysing and predicting the extent of
damage and crack growth.
Wind turbine blades for offshore installation are expected to be 60 metres or more,
and it is vital that the turbines are able to withstand small amounts of damage
without coming to a complete standstill and causing downtime. It is also anticipated
that the turbines of the future will incorporate a range of sensors to detect and
assess damage as and when it occurs. This research will be invaluable for blade
manufacturers as it will enable designers to develop larger, lighter blades, while the
integration of sensors will facilitate damage detection and analysis so that decisions
can be made quickly and efficiently as to what type of action is required.
Manufacturing
Most turbine blades are manufactured from some type of glass-fibre material, and
the development of these composites as well as research into the implementation of
carbon-fibre composites is on-going throughout the industry. The aim of developing
these composites is to create blades with high tensile strength to cope with the
conditions offshore, while retaining a light-weight and cost-efficient design. One
manufacturer embracing both technologies in the design of its blades is Gamesa,
who have introduced carbon-fibre composites into the construction of their latest
blades, including the G10X-4.5 MW sectional blade (2).
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3. Source: Gamesa
Manufacturing of the internal beam of the blade uses both glass-fibre and carbon-
fibre materials which have been impregnated with epoxy resin as a base. Several
pieces of cloth are cut and placed into the mould before undergoing a curing process
(shown above). The shell of the blade is constructed via the same method, with
glass-fibre placed into a mould after a protective coating of paint. Once both shells
have been produced, the beam is bonded between the two, and the blade is passed
though a kiln to form a compact unit. Finally the blade assembly is completed by
the finishing of the leading and trailing edges to meet specification.
The challenge for designers and engineers is one of cost versus benefit of using
carbon-fibre for blade manufacture. While it has the potential to be stronger and
lighter than glass-fibre materials, the technology and manufacturing processes are
not in place yet to deliver cost-efficiency and reliability in mass production for the
entire blade structure. Many manufacturers are using carbon-fibre composites for
component parts of the blade to increase strength in specific areas, but it remains
to be seen what the ideal composite material will be for large turbine blade
construction.
Composites for the Wind Energy Industry
3B Fibreglass Company is one of Europe’s leaders in the development and supply of
fibreglass products for the wind industry, and produces several composite materials
which can be used for blade manufacture (3). One of their products which may be
particularly well suited to the manufacture of longer blades for offshore applications
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4. is Advantex®. It is a boron-free E-CR glass with corrosion resistance properties
which could be ideal for the aggressive weather conditions experienced in deep
water installations. It offers several benefits over traditional E-glass, including a 9%
higher tensile strength, optimised sizing for improved processing and enhanced
fatigue performance, and a cleaner environmental footprint.
The removal of boron from the glass composite stops the creation of dust particles,
which are associated with partial volatilisation when exposed to high temperatures.
The removal of added fluorides also helps to reduce the creation of dust particles.
The use of modern melting technologies results in a significant reduction in
greenhouse gas emissions, and increased energy efficiency also aids the reduction
of carbon emissions.
Further developments by 3B have seen them introduce another composite, HiPer-
texTM, resulting from manufacturing developments in their own technology. The fibre
is based on a new patented glass formulation, which 3B say respects the
environment with optimised melting and sizing technologies. Key to the wind energy
industry is that this material may facilitate the production of longer blades with up
to 30% higher tensile strength than E-glass, up to 17% higher tensile modulus, and
up to 40% increased fatigue strength, and crucially up to 8-10% weight savings.
The potential for this new type of composite is that engineers may be able to design
longer blades without any increase in weight, which are able to cope with higher
loads from wind speed. The increased fatigue strength may also prolong a blade’s
lifetime and reduce the frequency of service and maintenance visits; something
which is essential to the development of deep water offshore wind farms. The
material also retains the anti-corrosive properties which will be required for offshore
installations.
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5. Summary
With EC regulations dictating that renewable energy should supply 20% of Europe’s
electricity by 2020, the wind power industry is undergoing continual development at
a rapid pace. The most viable way of achieving the target is for the wind industry to
develop deep water offshore wind farms with future turbines to be rated at 10MW or
more. There is substantial investment in all aspects of wind farm design, from
foundations to turbine development to grid integration; and there is a need for
innovation in all areas to design turbines which can be mass produced cost-
effectively.
Blades can account for 20-25% of the overall cost of a turbine, and are therefore a
significant component in terms of cost reduction. Despite the requirements to
reduce overall cost, blades will need to be produced which are in excess of 60
metres to be able to supply enough power to meet demand. The contradiction of
increasing size while reducing cost is a complex design issue for blade and turbine
manufacturers, and one which must be assessed at component level. Manufacturing
processes are being continually developed to streamline the production of such
blades, and material selection is a key issue for manufacturers as they look to
improve strength and durability, while lowering the cost of production.
The use of composites in the construction of turbine blades is commonplace already,
but the development of newer glass-fibre and carbon-fibre materials offers the
potential to improve upon existing designs as manufacturers look towards larger
and larger turbines. Some manufacturers are already integrating carbon-fibre
composites into the construction of their blades, while others are further improving
the characteristics of glass-fibre materials. At this stage there is no ‘ideal’ composite
material for the construction of turbine blades, and it is likely that several years of
research and development will be required, along with experience of turbines under
real installation conditions, before the best possible types of materials become
universally accepted.
Colin Pawsey
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6. References:
(1) http://www.risoe.dtu.dk/Research/sustainable_energy/wind_energy/projects/
AFM_DCCSM.aspx
(2) http://www.gamesacorp.com/en/manufacturing-and-assembly-process.html
(3) http://www.3b-fibreglass.com/wp-
content/themes/3b/pdf/brochures/Brochure-Wind_UK.pdf
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