And the repairs and replacements are expensive - EUR5,000-10,000 for a non-structural repair of an onshore blade, at least three times that much offshore, depending on the weather and vessel availability. A structural repair or swap costs EUR50,000-100,000 onshore - not including the cost of the blade itself - but developers are unlikely to see any change from EUR500,000 for a similar process offshore, even if they have the appropriate vessels to hand and good weather on their side. Blade failures are also the primary cause of insurance claims from US onshore wind farms, accounting for more than 40% of claims in 2013 with an average cost of EUR215,000.
These sobering figures, provided by insurance underwriters GCube and delivered by Andrew Bellamy, former head of Areva's 8MW blade programme, in his opening address to May's Windpower Monthly's blade manufacturing and composites conference in London. They set the scene for talks ranging from the need for more automated production and the shortage of skilled workers, to the price and availability of composite materials and the time and cost of developing, testing and certifying them for manufacture.
Bellamy, who co-founded new renewables advisory firm Aarufield with former Areva Wind UK chief Julian Brown, reflected on industry forecasts from ten years ago. "The predictions on turbine size and blade length - 10MW machines and 100-metre blades - were a bit strong," he said. "We've settled down at about 3MW onshore and 6-8MW offshore. But project sizes have grown considerably. Ten years ago, onshore, they were generally in the 10-20MW range. Now they're more likely to be about 70MW, while offshore is going through enormous growth and still accelerating."
The principal effect of the expansion in project size has been that owners and developers have had to look beyond their own balance sheets for the necessary finance. The good news, said Bellamy, is that wind energy has become "investable" - financial institutions are increasingly aware of the long-term profitability of wind projects and are able and willing to stump up the money - but he warned that outside investors have different priorities to the specialist wind companies. They are entirely focused on returns. "Before they put money into a project they will spend a great deal of time and effort to satisfy themselves that developers have done everything possible to maximise their returns," he said. That could well mean relatively conservative choices in turbines and blades to satisfy their aversion to risk, slowing down the rate of technological progress.
Christian Vogl, rotor blades project manager at Nordex, made the point that technological progress, particularly with regard to composite materials, was quite slow already. "Getting a new material into production takes a very long time - years; and it costs millions in developing, testing and certifying," he said. Fatigue tests of the material rather than the blade design was proving to be the main bottleneck.
Vogl pointed to four criteria that new materials have to satisfy to make them viable for use in blade design: their properties, the process of incorporating them into manufacturing, their cost, and their availability. All were problematic to some extent, especially carbon fibre. Its properties are hard to test accurately and faults are difficult to locate, and its conductive properties can cause short-circuits in electrical systems, while also making lightning protection essential. Incorporating carbon fibre into the manufacturing process requires workers skilled in handling the material, rather than expensive automation. "Blades are not built in large enough numbers to make full-scale production feasible," he said.
In terms of cost, Vogl was quick to point out that a new material might be more expensive to buy than the one it was replacing - certainly true with carbon fibre - but the true measure was the material's effect on the levelised cost of energy (LCOE), not its purchase price. Securing supply is also a cause for concern, he said, echoing Bellamy's earlier remarks that the wind industry faced tough competition from the automotive and aerospace industries for industrial-quality carbon fibre. "Carbon fibre will become harder to get. The automotive and aircraft sectors are able - and willing - to spend more than we are," said Bellamy.
The extent to which blade manufacture could, or should, be automated was one of the conference's hottest topics. There was widespread agreement that current, largely hand-made, production processes, designed for 30-40-metre blades a decade or more ago, are no longer "fit for purpose" with the generation of exra-long blades. But it was a very important factor in providing jobs for skilled workers to meet local content requirements in countries such as Brazil. Without those jobs, the wind industry would have a much harder time establishing operations in emerging markets.
Will modular construction of rotor blades provide at least part of the answer? Theo Botha, co-founder of Blade Dynamics, which is preparing its first prototype 78-metre blade for fatigue testing later this year, made the case. "Cheap energy and cheap blades are not the same thing, and processes cannot be scaled-up indefinitely," he said. Different parts of a modular blade can be made in different places, before coming together for final assembly, easing pressure on the supply chain and reducing transport and logistics constraints, he said.
Leading-edge erosion continues to be a high priority for further research and development. According to Andrew Kay, renewable technology engineer at ORE Catapult, blades that lack sufficient leading-edge protection are showing serious signs of erosion after only two years, causing significant power production losses. "But erosion resistance is not consistent - it depends on a lot of things. It's a very complex issue," he said.
Kay is leading ORE Catapult's blade leading-edge erosion programme (Bleep), which is aiming to improve the quality and speed of testing and diagnostics in this field. "We need tests that are representative and repeatable," he said.
What can wind energy learn from aerospace in terms of technology and manufacturing? Not much, argued Chris Payne, technology programme manager for the National Composites Centre. The relationship between the two sectors could be summed up in two words: arrogance and ignorance, he said. Arrogance on the part of the aerospace industry which is long established, well funded and tends to talk down to other industries. Ignorance on the part of the wind business, which needs to develop its own, sector-specific solutions rather than adopt systems and methods from industries with different priorities.
"Wind has low margins compared to aerospace, and aircraft are not growing in size in anything like the same way or rate as rotor blades," Payne said. The key drivers for aerospace are reducing weight, and improving quality and safety, while wind has to focus on cost, durability and solving the transport and logistical difficulties of moving massive components. If a blade breaks on a turbine, it is unlikely to mean more than an, admittedly expensive, repair or replacement. But if a wing breaks on an aeroplane ...
"The industries are too different for synergy," said Payne. But there could be some crossover in terms of new ideas and capabilities, and some of the technology might be transferable, if we can overcome the arrogance-ignorance block.