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In a recent article, we profiled Vestas' approach to scaling up technology for its massive new generation of wind turbine blades, including a look at the company's new facility on the Isle of Wight in the UK.
Staying (partly) in the same location, but for this installment of our series on blades, a startlingly different approach to scaling up wind turbine rotors is advocated by Blade Dynamics Ltd, a young company located on the Isle of Wight and in New Orleans, U.S. In fact Blade Dynamics claims to deliver a “paradigm shift in performance and reliability from existing rotor blade technology”.
A bold insertion indeed, but the company insists that instead of developing ever larger monolithic fabrications, a better option is to go smaller.
Explaining this paradox, company founder and chairman Paul Rudling begins, “we're turning conventional wisdom on its head. Instead of making larger components to produce bigger wind turbine blades, we're scaling down. We believe that the present approach to up-sizing, as you get towards the 6MW-10MW machines now in prospect, will bring horrendous issues of repeatable quality, and that the cost of overcoming these will be prohibitive. There comes a point at which it makes no sense to produce blades in ever-bigger units”. Thicker composite sections are hard to cure and prone to voids, mould tools become vast and the logistics become impossible. Bonding together outsize parts, such as the shells that form the blade envelope, is fraught with difficulty.
“Therefore,” Rudling expounds, “instead of taking a conventional approach to blade production, why not make a number of blade components of manageable size, using well-proven technologies, sensible tooling, established quality systems and average factory floor space? You could produce high-quality modules in normal factories and transport them by normal means to a final assembly area close to where the wind turbines will be deployed. There you'd bond them together into the complete blade.”
This is counter-intuitive to the prevailing mind set which considers that, to have integrity, a structure has to be monolithic.
But Rudling doesn't think it has to be that way.
“Formula 1 cars,” he says, “have high-tech race boats, and aircraft are made by bonding together dozens if not hundreds of parts. The technology for producing good joints is well understood and, if they are carefully made under controlled conditions using quality materials, there is little risk. We can build quality into our joints far more reliably than can the builders of large monolithic structures, by applying well-proven bonding and testing processes that are not out of their depth in terms of scale.”
Re-examining the trend
Though this ‘scaling up to scale down’ philosophy is the most attention-grabbing aspect of Blade Dynamics' approach, it is far from being the whole story. It is just one part of a fundamental re-examination of design and manufacturing processes aimed at making blades longer. Conventional technology, says Rudling, is running into the buffers presented by the empirical rule that energy capture increases with the square of rotor radius whereas blade mass increases by a cube factor.
So, therefore, he says, “if you could make rotors larger for the same weight and inertia, that would extend present limits, increasing the amount of energy that can be harvested.”
From boats to blades
According to sales director Theo Botha, current composite fabrication is flawed because much of it is rooted in decades-old technology – first used in mass market boat building. Such methods are, he argues, relatively crude and not suited for large, high-performance structures such as wind turbine blades. Nor are they particularly effective, he argues, because carbon composites are increasingly used in the largest blades for their properties of stiffness and low weight. As a result, quality is “highly variable”. The measures designers must apply to allow for this variability result in structures that are bulky and overweight, he claims.
By going back to basics, Blade Dynamics says it is providing a “slimming treatment”. Blade design and manufacturing processes are equally under review.
Clean sheet approach
Despite being only four years old, Blade Dynamics has well-rooted antecedents. Chairman Paul Rudling is a composites professional, having previously founded SP Systems, which was later acquired by Swiss composites house Gurit AG. To hasten its evolution, the fledgling company then teamed up with American Superconductor Corporation [now AMSC, Ed] and the Dow Chemical Corporation. This brought in additional finance as well as a fruitful partnership with the wind energy interests of AMSC, and backward integration into the composites supply chain through Dow.
Rudling says the company's brief is to take nothing for granted and re-examine every aspect of blade design and manufacture. By considering afresh everything from basic configuration to the specifics of roots, spars, skins etc., the company hopes they will deliver a “game changing” wind turbine technology that could increase annual energy production by up to 10%, with commensurate reliability and durability improvements.
Partnering with American corporations has also facilitated access to a substantial manufacturing site in Michoud, near New Orleans. A 100,000 ft. factory on a 43-acre site, leased from NASA (which once used it to produce propellant tanks for the space shuttle) has extensive quayside access to the Mississippi, so that blades, jigs and tools can be transported by river and sea, free from the size and form limitations of road and rail travel.
Explaining the relationship between the UK and US establishments, Rudling says, “the Isle of Wight is our ‘skunk works’ [a reference to the iconic R&D centre responsible for many ground-breaking innovations for the Lockheed Martin Corporation, Ed.], whereas Michoud is our centre for advanced manufacturing, especially for the American market. We may also in the future manufacture in the UK for the European market. These are the two markets we are targeting initially.”
Re-defining blade design
While the company has found a useful revenue stream in consultancy, its first tangible product is the Dynamic 49 blade, manufacture of which is due to start about now at Michoud. The company claims that this new product, with its several patented technologies, redefines blade design and manufacture to provide a step improvement in performance versus cost.
The blade is made up of a number of major sub-components that are subsequently bonded together. All the components are transportable in 40 ft. containers. Carbon-epoxy composite is used for the hollow main spar and other skeletal elements, where stiffness and strength are key, while the envelope comprises low-weight glass reinforced plastic (GRP)/foam sandwich skins. Novel techniques are used to assemble the carbon spar, and it has not been necessary to incorporate pre-bend.
At this stage, the company is sparing with serious details. It claims that, in general, it is not wedded to any particular materials or processes, explaining that these will vary according to the component, manufacturing location and the required combination of performance, quality, manufacturability and affordability. Clearly it will seek material combinations that provide strength, stiffness and low weight together with high fatigue endurance and durability.
Blade Dynamics says it ran hundreds of specimen tests before alighting on the material and process combinations it has selected for the Dynamic 49. It expects the 49 m long blade, intended for 2 MW wind turbines such as that produced by AMSC's licencees, will weigh some 5900 kg, compared with about 8 tonnes for some competing rotors of similar size. It can therefore provide a larger rotor for existing 2 MW machines, without over-loading bearing mechanisms, shafts and other drive train components.
Carrots and skins
While most of the innovations emerging from the skunk works are still under wraps, two are now sufficiently protected to be cited as examples. These involve the critical root attachment area and blade surface protection.
A fundamental re-think of the way the blade-hub attachment is normally engineered convinced the design team that present arrangements are archaic, heavy and inherently unreliable:
“Installing large constant-diameter threaded bolts into thick composite sections by drilling and bonding did not seem the way to go,” explains Rudling. “Loads exerted on the bolts get concentrated onto bolt ends and adjacent threaded portions, so that load bearing and transfer are uneven. Added to which, load is transmitted from the bolt into the laminate via an interface layer which, inevitably, is resin rich and fibre deficient so that is another weakness. It's like trying to secure something by driving a screw into end-grain wood.”
Engineers have had to compensate for these weaknesses by over-specifying the whole joint – laminate, bolt and all – if joints are to bear, over a 20-year lifespan with high gust loads, while still resisting long-term fatigue effects. This results in excessive weight, inertia and material usage.
Blade Dynamics engineers have addressed this attachment difficulty with a composite root insert. This looks a bit like a carrot, the name often given in the industry to the metallic insert normally used for the purpose. The new ‘carrot’ insert tapers from a bulbous cylindrical outboard end to a thin wedge inner, this shape being designed to avoid thick edges and hence stress concentrations so that load can instead be transmitted progressively into the rotor hub laminate.
The new carrot root insert comprises a composite outer jacket which envelopes a threaded metal socket that will receive the blade attachment bolt. The metal socket is spiral-threaded on the outside as well as appropriately threaded on the inside to receive the specified securing bolt.
The unit is manufactured in such a way that resin-impregnated composite fibres are wrapped round the spiral-threaded socket, ensuring a metal-to-composite interface having properties that are fibre dominated rather than resin dominated. Each fibre helps transmit load from the attachment bolt through the socket and into the laminate; this should ensure that the joint is both strong and durable.
The complete root insert can be coated with adhesive and bonded into the hub laminate, or inserted as part of a secondary infusion process. This results in a firm bonded fit which, thanks to the large bonding surface area afforded by the bulbous end and the clever distribution of fibres, will ensure a highly secure composite-to-composite joint once the resin has cured.
Thanks to the higher effectiveness and security it confers, the novel root insert enables designers to engineer hub and root structures that are substantially slimmer and lighter than they would conventionally need to be. Benefits from this accrue throughout the drive train, since a lighter root and hub enable designers also to specify lighter drive shafts, bearings and associated components. Under test, the root insert for the Dynamic 49 has proved able to withstand loadings above 200 kN, well in excess of extreme gust requirements, and offers vastly improved fatigue performance over more conventional designs.
The second revealed improvement is a blade finishing process patented under the name BladeSkyn. This involves the application of a thin (130 micron) surface film derived from aerospace practice and based on thermoplastic nanotechnology.
The material adheres well to lightly sanded glass-epoxy composite and has complete opacity in the range of colours used in the wind turbine market. By protecting against pitting and abrasion, it prevents the progressive degradation of aerodynamic performance that affects blades having standard gel coat and paint finishes. It resists rain, ice, UV, chemicals and solvents; can endure service temperatures of −40 to +80°C, has very low reflectivity and repels dirt.
Coating blades in this way means that surface properties are pre-assured, with much lower variation than with conventional finishes. It also avoids the need for flatting back and other finishing operations that can degrade surface properties.
BladeSkyn's additional cost is said to be modest compared with the O&M savings that it secures through life.
Other innovations and new products can be expected as Blade Dynamics continues with its drive to re-think and update wind turbine blade design and manufacture. As Paul Rudling summarises:
“It's the kilowatt hours a wind turbine produces that matters, not its nameplate rating. Increasing the area swept by the rotor for the same weight and inertia can increase kilowatt hours by 10%. That's a prize worth having and that is what we are in the process of delivering.”
About George Marsh: Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.