The offshore sector promises the next significant increment in materials uptake because of the larger wind turbines it will call for and the more extensive wind farms that can be placed at sea. A single blade for a large offshore turbine can weigh 20 tonnes, with most of that material being composite – mainly glass but increasingly with carbon stiffening in the spars that support the blade shells. Many hundreds of new-generation large blades will be called for.

Consider the size of some of the projects now being pursued. Work could yet start later this year on the London Array, a wind farm in the Thames Estuary that could eventually generate up to 1000 MW, enough to meet the electricity needs of 750 000 homes, or a quarter of London's domestic load. It would also avoid the need to emit, from conventional power generators, 1.9 million tonnes of carbon dioxide per year. London Array Ltd, the consortium that has applied for consent to build the farm and comprises Shell Wind Energy, EON UK Renewables and CORE Ltd, expects the £1.5 billion project to commission up to 270 turbines, calling for 810 blades plus spares. London Array alone could potentially meet 10% of the UK government's target for 2010 of power available from renewable energies.

That is scale indeed, but there could be more to come.

“This is the first of a number of similar-sized wind farm schemes that will place the UK market at the forefront of offshore renewable energy workdwide,” according to Eric Kjaer Sorensen, director of CORE.

Meanwhile, Shell Wind Energy, with partner Nuon, is starting to construct the first Dutch offshore wind project, the 108 MW Egmond aan Zee wind park, which is due to come on stream late this year. SWE is a business unit of Shell Renewables, Hydrogen and CO2 whose CEO, Graeme Sweeney, believes that, to be viable, offshore wind parks will have to be of 100 MW plus.

“These give you the best prospects for profitability and will be necessary if reliance on government incentives is to be phased out,” he says. “We are aiming ultimately for commercial sustainability with no govern-ment incentives.”

Turbines for the London Array would start at about 3 MW power, but those installed in the later phases of the four-year project could be more powerful – up to 7 MW if such machines are sufficiently mature by then. They could well be, given that machines as large as 5 MW are being fielded right now. REpower Systems AG is supplying Talisman Energy's Beatrice wind farm off the east coast of Scotland with two of its new 5 MW machines, following the installation and operation of a prototype onshore in Germany last year.

Rotors for REpower's machine are 126 m diameter giants for which LM Glasfiber produces the blades. Each of the 61.5 m long blades weighs almost 18 tonnes. The fact that most of this is accounted for by glass/epoxy structure illustrates the scale of material usage that powerful offshore machines will represent. It is worth noting that the 18-tonne weight is considered modest for a predominantly glass/epoxy structure of this size, since LM Glasfiber has used its Future-Blade technology tools to good effect in terms of weight reduction. A 54 m blade developed using the same concepts and weighing 13.4 tonnes is said to be half the weight of some of its competitors.

Clouds

But there are clouds on the horizon and the promised crock of gold might not materialise as soon as some in the industry expect. Issues include the rising costs of offshore install-ation, the sector's current mixed appeal for the investor community, and the availability of suitable qualified materials. Associated with the last named is continued volatility in the availability and pricing of carbon, likely to be needed for the largest rotors.

According to Adam Westwood of energy analysts Douglas Westwood Ltd, the economies of scale offshore that were once expected to be evident by now, have not yet materialised. Writing in Reinforced Plastics' sister magazine REfocus, Westwood points to failing contract negotiations and growing reluctance to invest up to $3 million per MW of installed capacity when costs targeted a couple of years ago were in the order of $2 million/MW. He puts the rise down to unworkable contracting systems, rising turbine prices, insufficient market competition and a previous under-estimate of the risks involved in taming the capricious offshore environment. Recent faults with turbines offshore have not helped percept-ions of risk.

Westwood adds that the present strength of the turbine market onshore is doing nothing to stabilise prices. The advent of larger machines of 5 MW plus will drive prices up still further. He argues that cost savings of up to 20% are needed in the supply chain to allow momentum in the offshore sector to build. Another useful confidence booster for investors would, he says, be a number of offshore contracts that run relatively smoothly.

Tarnished allure of the offshore sector is serious given the pickings that are still available onshore. Blade constructors are augmenting their production capacity, particularly in regions where significant wind farm development can be expected (some offshore but mostly onshore). As turbines become larger, it becomes less feasible to transport blades long distances from the point of manufacture to where they will be used.

Thus Vestas Wind Systems, for example, expects to deliver the first blades from a new factory in China, later this year. They will be 39 m blades for Vestas V80-2.0 MW machines ordered by Jiangsu Unipower Wind Power Ltd for a 100 MW wind farm – one of 20 the Chinese authorities plan to establish by 2010. LM Glasfiber has its plant at Gaspe, Quebec, as a blade production centre for the North American market. This facility, along with another in North Dakota, gives it an annual production capacity of over 900 MW in that continent. The Danish company is also investing some DKr100 mil-lion in blade production facilities in India, another area where high wind energy growth is projected. Gamesa Eolica is expanding its North American footprint, having inaugurated a new $18.5 million facility in Ebensberg, Pennsylvania, as one of several US manufacturing units for its Fiberblade SA subsidiary. Despite a recent downturn in expected blade orders from certain parts of Europe where the market is maturing, resulting in redundancies at some blade plants there, constructors remain bullish about prospects overall.

Most of this added capacity is associated with the onshore market, though doubtless some could also service offshore needs as required. In serving their clients' needs, material suppliers have likewise been preoccupied with onshore-related activity. In their case, however, there is less differentiation between products required for the onshore and offshore markets so added capacity will be applicable to both. Many suppliers have been hard pressed to meet current demand levels.

Glass reinforced plastic generally poses less of a supply issue than carbon, where the availability of carbon tow and its precursors continues to teeter on the familiar demand/supply knife edge. In this respect capacity increases now being implemented, especially for affordable commercial-grade carbon, are encouraging. Zoltek, for instance, which has targeted wind blade constructors with its Panex 35 product in particular, is expanding carbon manufacturing facilities at its plants in Abilene, Texas, and in Hungary. As a result, it expects to double capacity this year and probably double it again in 2007. Mitsubishi Rayon Co will, thanks to an alliance made last year with SGL Carbon of Germany, have access to a quality-grade carbon due to start emerging from an SGL facility in Scotland about now. MRC is also growing its carbon capacity in Japan, as is competitor Toray. Hexcel Corp is investing $100 million to achieve a 40% hike in its capacity to produce high-grade carbon, and though most of the output from its plants in California and Madrid will be earmarked for aerospace, leading-edge wind turbine blade manu-facture could benefit too.

Material response

Material technologists are contributing to cost reduction efforts with new materials designed to facilitate the task of blade fabrication. Adrian Williams, wind energy business manager for Gurit Wind Energy (formerly SP Systems), reports that recent customer trials have demonstrated the cost effectiveness of two new materials developed primarily for turbine blade producers. Both had a public airing at this year's JEC Show, France. The first material is SPRINT Triax, a triaxial form of this established semi-preg material which was previously absent from the company's product portfolio.

Williams explains that the material comprises stitched triaxial E-glass fabric pre-impregnated with the company's WT93 epoxy resin system. Its chief benefit over conventional prepreg material, he says, is its high breathability, which results in a void-free cured composite with a superior surface finish. This reduces the time blade shell producers have to spend in finishing operations. They can simply apply a protective paint coat or gel-coat straight onto the moulded blade.

Since the SPRINT concept results in minimal resin flow during cure, resin content is closely controlled, making it easier to control the weight of the final product. Cure is a simple process that takes place at 93°C. The material promises faster and less labour intensive fabrication than competing methods. Gurit has been careful to match the resulting fabrication cost reductions with an efficient system for producing the material itself. Costs are said to compare well with those of prepregs.

“We're experiencing strong demand right across our range of wind energy products at this time,” according to Williams. “However, usage of our SPRINT materials appears to be growing more strongly than prepreg.”

Gurit is expanding production capacity for the new products, as well as existing materials, at all its main production centres – Newport, Isle of Wight; Magog, Quebec; and Spain. An Asia Pacific presence is also in prospect.

Gurit's second material innovation is Airstream, a material system that addresses the difficulty of fully infusing thick carbon laminates. Gurit's answer, which could prove a blessing to fabricators of carbon blade spar caps and other primary structure, is a form made up of stacked prepreg plies pre-consolidated to customer-defined geometries.

“Providing per-cut, pre-kitted parts like this avoids cutting and wastage at customers' premises, as well as the necessity to handle carbon fibre,” declares Williams. “Customers have only to assemble, cure and finish these semi-manufactured forms in order to produce finished parts; it's fast, clean and simple.”

“We believe this is the only well proven and robust manufacturing solution for producing thick carbon fibre laminates,” he adds. “Typically, we can make laminates up to 20 mm thick – up to 100 mm thick if we superimpose layers together. The system assures a high quality of laminate with less than 2% void content in the manufactured part. This is especially important with carbon which is strength critical, compared with glass which is stiffness critical and less sensitive to void content.”

The fibre is mainly unidirectional, held within a standard SP wind energy resin, curing in the range 90–120°C (depending on the precise material chosen). To date, production-standard parts have been supplied from the Isle of Wight manufact-uring facility to two customers, with a third pending. Following positive feedback from those customers, consideration is now being given to expanding Airstream production capacity, in Spain and North America as well as Newport.

There are many other examples of cost-reducing material advances. A number of suppliers, including Johns Manville, Saertex, Metyx and Owens Corning, continue to develop materials that that can add strength and dimensional stability to turbine blades via engineered fabrics. Johns Manville's StarRov 806, for instance, is a fatigue-resistant glass roving optimised for use in woven and multiaxial fabrics used in wind turbine blade fabrication. Owens Corning chose the recent European Wind Energy Conference and Exhibition in Athens, Greece, as the opportunity to announce a new single-end roving and knitted fabric that is says can help reduce wind energy costs.

WindStrand™ will, the company claims, allow turbine manufacturers to increase blade lengths by 6% and deliver up to 12% more power, for up to 20% less cost than competing carbon-glass hybrid solutions. The product is said to offer up to 35% higher tensile strength and 17% higher modulus than conventional E-glass reinforcement, along with enhanced fatigue, impact, ageing, corrosion, and thermal resistance character-istics. Scheduled to become commercially available later this year, it is the first application of Owen Corning's HiPertex™ high-performance reinforcement, resulting from advances in glass melting and sizing technology.

The Netherlands Composite Technology Centre (CTC) has noted positive benefits achieved by replacing traditional E-glass with WindStrand for several components in a 44-m long blade, suitable for a 2.5 MW wind turbine.

Molded Fiber Glass Composites of Ohio has worked with the US Department of Energy to develop materials and processes that can help reduce blade costs. Results have ranged from proprietary gel-coats able to enhance blade weathering and ultraviolet (UV) resistance, to an infusion system applicable to large blades, including those incorporating carbon fibre. Core producers have optimised internal core products for the wind energy market. Research interest is still evident in some quarters (eg US and Ireland) in possibilities for fibre reinforced thermoplastic blades. These would have processing advantages over thermosets and offer the additional advantage of being recyclable, a factor that will gain significance as first-generation blades approach the ends of their useful lives. Further examples of material progress will be evident at various wind energy shows this year.

Finally, one more factor that can help reduce future blade costs is the ability to reduce design allowables – safety factors allowing for uncertainties that still surround the design and durability of composites. Design codes that can be applied more universally may help here. To this end, the US Department of Energy's National Renewable Energy Laboratory (NREL) has collaborated with leading European classification society Germanischer Lloyd in the formulation of new design codes that can be used for worldwide turbine certification. These codes, FAST and ADAMS, will now help designers in Europe and America to estimate turbine design loads on a uniform basis. They are part of a growing body of codes and qualification criteria that can help those charged with the responsibility of designing reinforced plastic turbine blades.

Material advances, reduced supply chain costs, faster certification, design codes, early project successes – all can help get the offshore wind energy sector rolling. Nevertheless, knowing when it is likely to take off remains, for the moment, a puzzle.