Is building bigger, better?

As the demand for wind power continues to rise, manufacturers are having to build ever larger wind turbine blades. But can the materials cope, or does it require a rethink?

When it comes to offshore wind turbine blades, bigger is definitely better. The wind energy industry is ferociously competitive, with producers fighting other forms of electric power generation and one another other for a share of the market. Larger turbine blades capture more energy, improving the economic viability of offshore wind turbines.

Accordingly, the average capacity of turbines installed in European waters has doubled in the last 15 years or so, from 2 MW in 2000 to 4 MW in 2014. The largest turbine blades to be produced are now in excess of 80m in length.

However, increasing the size of these blades ­– typically manufactured using glass fibre-reinforced plastic (GFRP) – can present significant challenges. They require stronger support structures and must withstand greater physical loads. Further, the blades become more susceptible to erosion.

As blades get longer, manufacturers are turning to stiffer and lighter, but expensive, carbon fibre to produce structural parts – such as the spar caps – to keep their weight in check. Indeed, the likes of Vestas and Gamesa have been using carbon fibre to manufacture 40m blades for a number of years now.

In response to these demands, glass fibre manufacturers are working hard to develop products that bridge the performance gap between glass and carbon fibres. Luc Peters, technical service manager, wind energy and performance composites market at 3B says: "As a glass fibre manufacturer, we understand that as blades get longer [their manufacturers] need better materials in terms of specific stiffness and strength, and that fatigue requirements are getting more and more stringent."

To this end, the company has launched a range of reinforcements to meet these demands. For example, its HiPer-tex W2020 roving possesses significantly greater strength and strain-to-failure properties in comparison with conventional E-glass. A unidirectional laminate fabricated using 60% by volume HiPer-tex W2020 demonstrates a modulus of elasticity of 54–56 GPa, a transverse tensile strength of 55–60 MPa, and increased fatigue resistance in comparison with components produced using traditional E-glass.

Turbine blades manufactured using HiPer-tex W2020 can be up to 10% lighter than those manufactured using traditional E-glass, according to 3B. Alternatively, the length of the blade can be increased by 6%, while maintaining the same weight—increasing energy output by 12%.

Weight can also be shed through the use of intelligent design. For example, Siemens' 75-m glass, epoxy and balsa wood B75 blade features specially designed blade profiles – called QuantumBlade – shaped in a way that delivers high rotor performance at a range of different wind speeds.As a result, the blades are 20% lighter than they would be otherwise.

Erosion is particular problem for these longer offshore blades, particularly at their leading edges. Kirsten Dyer, a Research Materials Engineer at the UK's Offshore Renewable Energy Catapult (ORE), says: "The higher tip speeds and the increased wind speed means that the impact of the rain on the blade is much higher than you see on [shorter] onshore blades, which is accelerating the erosion issue."

Conventional coatings developed for onshore turbine blades based on materials such as polyurethane (PU) do not stand up well to these harsh conditions, according to Dyer.

Erosion affects the aerodynamic performance of the blade – its flow, lift, and power – and its structural integrity as water ingress and exposure to ultraviolet (UV) light can lead to damage.The costs of this leading edge erosion cannot be overestimated. In January of this year, Dong Energy announced that it has to repair and upgrade all 273 blades at its six-year-old, 209-MW Horns Rev 2 offshore wind farm in Denmark. Each blade will need to be removed from its turbine and returned to shore, a colossal undertaking.

To combat the problem of leading edge erosion, ORE has set up the The Blade Leading Edge Erosion Programme (BLEEP), which comprises a number of projects. The first project will determine the cost of leading edge erosion and its impact on performance. The second project will involve the development of a test rig and will run alongside the third project, a set of research packages being set-up with academic partners who have expertise in erosion. These packages will examine offshore conditions and then determine how these coatings degrade

Dyer says: "The idea is that we do the whole set of research and use it to inform industry and standards."

So how could the problem of leading edge erosion be solved? Dyer believes that the solution could lie in tailoring the properties of the coating to the blade it protects. She says: "At the moment you have a very compliant and elastic coating bonded to a very stiff, brittle epoxy or polyester structure, so there's a mismatch. Stress passing through the coating hits a brick wall and then travels back. I think you need more compliant blade materials and the coatings need to be developed in line with these structures."

Offshore wind energy is in its infancy relative to industries such as automotive and aerospace. As such, the technology surrounding it is constantly evolving.

Both Peters and Dyer believe that glass fibres will remain an important part of the material mix. Indeed, as the mechanical properties of glass are improved, so current exponents of carbon fibre could switch as they seek to reduce the costs of their blades.

There could also be significant changes in the way these blades are manufactured. Currently, these structures are fabricated in one piece or as two half-shells and a spar. However, this makes their transport a logistical challenge and means that production facilities must be close to the point of final installation. Companies such as Blade Dynamics are investigating the manufacture of these blades in more manageable sections.

However, any joins that would be necessary for these multi-segment blades could be possible points of failure. Dyer's colleague at ORE, Senior Engineering Specialist Raul Prieto, says: "In an offshore environment reliability is of the upmost importance. Even though [turbine manufacturers] have been engaged in the development of split blades, it is telling that they still rely on full-size blades."

James Bakewell

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The author mentions the use of glass fibre-reinforced plastic (GFRP). What about using Continouos Fiber Reinforced Thermoplastics (aka tapes)? Endless long, using a winding technology to produce the blades, non-corrosive, with either glass fiber or carbon fiber?

Comment Mark, 29/06/2015

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