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Feature

Composites help improve wind turbine breed


George Marsh

Blade technology is key to the effective harnessing of wind energy. Current efforts are focussing on developing blades that have higher wind energy capture and which are smarter and more durable than their predecessors.

Rotors and blade technology are central to the effective harnessing of wind energy. Current efforts are focused on development of blades that have higher ‘yield’ (wind energy capture) and are smarter and more durable than their predecessors. The state of the art in materials usage is advancing to keep pace.

An example of technology evolving in line with the market's requirements is the FutureBlade concept, a comprehensive approach developed over the last several years by leading blade specialist LM Glasfiber of Denmark. First exploited with the 54 m blades that LM introduced last year, FutureBlade is actually a palette of materials, processes, structural concepts and design tools from which designers can pick the most appropriate for meeting the needs of particular turbine manufacturers.

“Blades today are much more turbine-specific than they used to be, which explains why we now produce four to six new blade types each year,” LM's marketing manager Steen Broust Nielsen told Reinforced Plastics. “FutureBlade gives us the flexibility to select the best material combinations and fabrication processes to meet particular customer specifications at the lowest viable cost. We are not tied in to a single inflexible technology - prepregs for example.”

Thus LM Glasfiber can utilise polyester, vinyl ester or epoxy resin, along with the most appropriate reinforcement (whether E-glass, S-glass or carbon) in various unidirectional or multiaxial tape or fabric forms, in the best configurations for meeting specified load cases. As its name suggests, the company is rooted in glass fibre and still prefers to use this material whenever possible because it is ‘about one tenth the cost of carbon fibre.’ It constantly seeks, therefore, to ‘push the envelope’ on the size of blades for which glass remains suitable. (Maximum blade size continues to trend upwards as turbine producers seek to minimise cost per kilowatt hour of generated electricity, both onshore and offshore). LM Glasfiber has stretched the limits of glass through careful design and by using higher grade materials, for example epoxy resins in place of polyesters.

Nevertheless, for the very largest blades it is now developing, LM is beginning to use carbon, albeit in carefully selected areas where the material's tensile and stiffness properties are most beneficial. So far, this is chiefly in the blade trailing edge. Thanks to FutureBlade, the tools and knowledge necessary to support carbon application were already available. This technological tool kit also provides for resin infusion, a process which is progressively displacing traditional labour intensive wet lay-up techniques throughout the blade industry.

Additionally, FutureBlade encompasses software support for whatever material/process combination is decided upon. Computer simulations, for instance, enable LM to ‘fine tune’ blade structures and laminate designs and then determine the process variables best able to deliver them. For the 54 m blades, they facilitated investigations into the use of the robot technology for laying out the dry fibre layers and for applying the glue in the bonding process. Another application of IT is the data-logging of process histories, providing the traceability that end customers require as part of their reliability assurance.

Again, the change from predominantly metal moulds to composite moulds is allowed for. Composite tooling is increasingly preferred for large blade fabrication because, for the lengths concerned, such moulds can be made lighter and more rigid than metal counterparts. Less aggressive forming and curing processes mean that the tooling composites can last longer than would have been the case a few years ago.

Giant

According to Steen Broust Nielsen, the FutureBlade approach is proving highly successful having, for instance, enabled the company to produce a 54 m blade that weighs only 13.4 tonnes (about half the weight of some of its competitors), without using carbon fibres at all. Moreover, the methodology is being employed in the production of the industry's latest giant, a 61.5 m blade LM is producing for the 5 MW turbine launched by REpower Systems AG for offshore wind farms.

In November a prototype turbine located next to a 27 year old nuclear plant in northern Germany (appropriately symbolic in view of that country's intended phasing out of nuclear power in favour of wind power), received its 126 m diameter rotor. Each blade weighs just under 18 tons, a creditable figure which means that of a total rotor weight of 120 tonnes, less than half is accounted for by the blades. This is likely to set a bar for weight in blades of around this size that competitors will find hard to beat. Lighter rotors mean that drive trains and shafts can be less massive and less highly engineered than they would otherwise have to be.

Under a test programme now in progress, the turbine first achieved its maximum rated capacity of 5 MW just before the end of 2004 and started operating automatically on 2 February this year. Trials on a number of turbines placed onshore will follow, before installation commences offshore, probably in 2006-2007. LM expects the initial test period to validate some of the technol-ogical enhancements implemented in the new giant. A special focus will be the LM BladeMonitoring, an embedded optical fibre system that monitors structural integrity and senses the loads exerted on the blades so that the turbine controller can act quickly to reduce them. Problem conditions such as lightning strikes and incipient cracking are also sensed. The company expects such monitoring to help reduce wind power costs by enhancing reliability.

Another FutureBlade refinement on the 61.5 m model is LM's SuperRoot concept. Essentially, this has involved careful attention to areas surrounding the bolts and bolt holes, with the result that roots of existing diameter can support blades 20% longer than was possible previously. A patented system for pre-bending the blades allows for weight reduction and increased energy production. A sophisticated lightning protection system exploits the ability of carbon fibre to conduct electricity.

Betz barrier

For German wind turbine manufacturer Enercon, a radical revision of its blade design and construction has led to an improvement in the yield of its blades by several percent. In fact, with a measured aerodynamic efficiency of 56%, its latest 33 m blades (the first of a new generation based on fresh research findings) are within striking distance of the 59.3% figure calculated by German physicist Albert Betz as being the maximum amount of the wind's energy that a turbine could ever capture.

But Betz's calculations date from the mid-1920s and Enercon decided to question some of the assumptions made at that time and carried forward since. After making numerous in-field measurements of load, sound and power over the last decade, researchers noted that theoret-ical predictions deviated significantly from values recorded on actual turbines. This, they surmised, was because there had been excessive reliance on wind tunnel tests, which insufficiently reflect the highly dynamic and turbulent conditions that blades are likely to encounter in service.

Advances in computational fluid dynamics and software enabled the research team to model with better accuracy the conditions blades actually experience. As a result they were able to improve blade aerodynamics to such an extent that turbine rotational speed can be reduced by 5% even while yield is improved, also by 5%. Reducing rotational speed cuts the acoustic signature by 3 dB - effectively halving the perceived noise. At the same time, operating loads are reduced, enabling rotor diameter to be increased from 30 m to 33 m for the same drive train and hub. The resulting greater area swept by the rotor translates into a 25% improvement in yield. Planform improvements include ‘winglets’ at the blade tips to inhibit turbulent flow and vortex formation, a more optimum profile between tip and root, slimmer outer blade sections and a major reshaping and deepening of the blade root to improve energy capture near the turbine's sizeable nacelle. Rotors based on the new E33 blade were first deployed two years ago.

For reinforced plastics specialists, the point is that the distinctively shaped new blades could hardly have been achieved without the use of composites. Glass fibre is retained for the E33 blades, along with balsa wood core, but Enercon took the opportunity with the new generation to transition from wet lay-up to vacuum infusion. This process is better suited, says the company, to volume production as well as being cleaner and avoiding operatives having to come into direct contact with the materials used.

Design and constructional principles underpinning the E33 blades are also being applied to the E70 (71 m rotor diameter) blades designed for Enercon's current 2 MW turbine. New-generation rotors installed at Westdorf in East Frisia near older E-66/20.70 (70 m rotor diameter) turbines permitted a direct performance comparison, which verified a significant improvement. Enercon expects a 10-12% higher yield for the E70, depending on location. Company sources say that, in particular, challenging mid-1920s findings that little yield could be expected from the blade root area has paid dividends. Decades of neglect of this zone have been redressed in the new designs.

Enercon's innovations have also influenced the company's new ‘ultra-blade,’ the E112 (112 m rotor diameter) designed for 4.5 MW turbines. Tilted ‘winglet’ blade tips help account for the fact that a single E112 is said to make less noise than three smaller E66 models. In common with most later-generation Enercon turbines, the glass/carbon/epoxy-bladed system delivers hundreds of electronic status messages, many of which can precipitate automatic control measures. Monthly computer-generated reports enable any problematic trends to be identified so that remedial action can be taken. Last summer, regional power company EWE AG was proudly taking visitors to Emden, north Germany, to see what was then the ‘largest wind turbine worldwide,’ a 4.5 MW E112, at work.

Partners in the E112 project included Abeking & Rasmussen Rotec GmbH, which constructed moulds and tooling for the blades, along with two prototype blade sets, and helped refine the production process.

Greenblade

Although new ultra-blades should not reach the ends of their service lives for 20-30 years, disposing of them will eventually become an issue. If predictions that Germany will install 7500 very large turbines to provide 25% of its electrical power prove correct, at least 22 500 ultra-blades will need to be disposed of from this country alone. In addition, many thousands of smaller blades will have served out their useful lives within years rather than decades and, though some of the early models were primarily wooden, most are composite. Overall, those keen to ensure that power should be generated as sustainably as possible would be wise to start thinking about the disposal issue now. Ideally, recycling would be the solution.

That's why a fresh initiative to produce blades in thermoplastic materials is of interest. The concept is not new, but wind energy company Gaoth Tec Teo (Gaoth is Irish for wind) is revisiting it, along with collaborators Mitsubishi Heavy Industries and Cyclics Corp (USA). Explaining that thermoplastic blades have so far been ruled out by their higher costs, Gaoth Tec argues that those costs are based on present, imperfect, means of production. It claims to have pioneered a new manufacturing method (though it is understandably reticent on details) that will enable thermoplastic blades to be produced less expensively than thermoset equivalents. Shifting the economics still further in favour of thermoplastics, it says, will be lower aerofoil weights which will bring a knock-on reduction in turret and drive train weights, as well as transport and erection costs.

Fundamentals of the processing technology to be used have been developed within the Advanced Technology Research Programme (ATRP), funded by Enterprise Ireland and carried out at the University of Limerick and the National University of Galway. This technology takes full advantage of the water-like processing viscosities of Cyclics Corp's engineering thermoplastic materials and the productivity improvements they offer for manufacturing large structural composites. Blades will be made using Cyclics' CBT® resin and a variety of reinforcing additives. Cyclics says the ability to re-use the 19 tonnes of blade material per average wind turbine at the end of its useful life is unprecedented, and will enhance the sustainability of wind power as a viable energy option.

“Faster and safer manufacturing, increased blade performance and the ability to recycle blades and manufacturing scrap are all significant improvements over the way blades are currently made,” according to Roman Eder, managing director of Cyclics Europe GmbH.

An initial project phase involves producing a series of 12.6 m blades at Gaoth Tec's Galway, western Ireland, facility. Mitsubishi is to run a test of the new ‘Greenblade’ on an existing wind turbine in the Nagasaki area. The Japanese company has also provided sample blades to assist with the design of the aerofoil profile. Dr Conchur O'Bradiagh, joint managing director of Gaoth Tec Teo, a young sister company of Irish Composites, says that successful tests should be followed by development of a production-standard blade of 30 m or more. Green-blade performance goals include a cost reduction, compared with current glass/epoxy blades, of at least a quarter; a production cycle time at least a third less, blades a tenth lighter with half as much again impact resistance, and complete recyclability.

With wind turbine blade business growing at over 10% per annum on a base already close to €1 billion globally, and given the environmental issues surrounding thermosets, a bright future should await any organisation that can truly deliver economically viable thermoplastic blades. Gaoth Tec Teo and its partners aim to be in the vanguard.

Thermoset blade manufacturers are, in the main, tackling the disposal issue by developing improved processes for shredding composite material before it goes to landfill, and for incinerating material and utilising the energy so released. Some, however, go further. Vestas, for example, says that much of its prepreg waste is now recycled rather than being sent to landfill, while consumables involved in the manufacturing process are disposed of by combustion. Prepreg recycling involves sending material to a company that process it into panels for the construction industry. Other manufacturers, including LM Glasfiber, are experimenting with controlled thermal processes to separate out fibres from matrices so that both can be recycled more readily.

From the foregoing, and the many other examples we could have cited, it seems that innovative use of reinforced plastic materials continues to support the evolution of wind power as a renewable form of energy able to compete with energy derived conventionally from fossil fuels.

 

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