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Much is involved in designing a wind turbine blade, including taking note of changing industry conditions and the need to address persistent issues of reliability and resilience. Tildy Bayar spoke with blade designers and manufacturers to see how the industry is addressing the issues and designing the blades of the future

Blades are a crucial wind turbine component, but they are also one of the most vulnerable. A Google search for “wind turbine blade break” produces a series of news reports from the past few years, involving many of the major global wind OEMs. The breakages have been attributed to improper maintenance, structural failures, manufacturing defects, cracks, tower vibration, storms and high winds, lightning strikes, and often to unknown causes that require in-depth investigation.

A September 2014 report from insurer GCube, titled Breaking blades: global trends in wind turbine downtime events, estimated that there are 700,000 wind turbine blades in operation in 85 countries, with around 3800 blade failure incidents per year, or a 1:184 failure rate. According to Jatin Sharma, head of business development at GCube, blades have the highest failure rate of any wind turbine component. In GCube’s Top Five Wind Energy Claims report for 2012, blade damage was responsible for 41 per cent of claims.

While blade failure is not a new issue, GCube has found it to be increasing as wind power capacity grows globally. This growth has led to rapid expansion of the supply chain, moves to more remote regions, and growing pressure to produce cost-competitive electricity with minimum downtime.

“The continuing downward pressure on reducing the levelized cost of energy has taken its toll,” the Breaking Blades report found, with the result that “the relative profit margins of the turbine supply chain have, on occasion, been compromised. Such compromises are made at the expense of sustaining quality output while increasing turbine size, but reducing manufacturing costs in terms of people, processes and production locations have all played a part. So too has the attitude taken to minimize operational expenditure by slashing spares inventories.”

Jose Antonio Riveiro, director general at wind technology repair firm IMFuture, was interviewed for the report. He confirmed that, as wind turbine demand has grown over the past 10 years, his firm has noted a corresponding decline in blade quality. “We believe that the stress on the manufacturing process to generate a supply to match the growing demand has led to an increase in structural damage to the blades,” he said. “This case seems to be more prevalent with some of the larger blades being manufactured.”

In its report, GCube breaks down blade failure insurance claims in terms of manufacturers, but declines to name them. One major manufacturer, referred to as Manufacturer A, was responsible for 42 per cent of claims between 2008 and 2013, while so-called Manufacturer B was responsible for 35 per cent of claims.

How blades fail

While some industry insiders agree that aging can be a cause of blade failure, Gcube’s Sharma disagrees, saying that while there are multiple causes, “GCube’s data infers that age is not statistically significant as a variable”. Instead, he says, it is important to note that blades – especially today’s longer and lighter ones – are more prone to the effects of wear and tear than other wind turbine components, and this includes additional factors such as the harshness of the environment and damage during transportation. “A turbine could be 20 years old,” he says, “but if the blade fails it’s not because of its age.” (However, age is a factor in the failure of all other wind turbine components.)

Carlo Durante, chief executive of Italy’s Eta Blades, says: “The first cause of blade failure is nature. The length of time spent by a blade on top of a turbine for 15 years has a major impact on the surface and structure of the blade. All the surface of the turbine is hit by sun, rain, ice, snow and so on, and in the lifecycle of a turbine this creates major damages.” Sharma agrees, saying that environmental factors can contribute to small cracks and increased delamination.

“Even if the visible damage is not that much,” Durante adds, “surface deterioration also brings deterioration of efficiency.”

An interesting insight into the causes of blade failure, singly or in combination, is given by GCube’s claims reports. There are a lot of things that can, and do, go wrong with blades, including damage during transport, installation, maintenance or repair; manufacturing defects and inadequate quality control; the wrong design for the operating conditions or incorrect operation for the design; failure of the control system to detect problems or imbalances; environmental damage including lightning strikes, and human error during operation or maintenance.

The wind industry has collectively taken steps to reduce the most significant blade breakage risks. For example, “in terms of real damages,” Durante says, “lightning is one of the most important as it breaks the wing in a number of different little parts”, making it more difficult to repair. However, LM Wind Power’s vice-president of engineering, Dr Roel Schuring, notes that “in the past, lightning strikes were an important source of damage but these have been reduced to insignificant levels even offshore, due to design improvements like the integrated lightning protection system.” LM says its own system captures lightning through receptors on the blade, transporting it down the conductor system without creating too much heat which could damage components and laminates.

Erosion is another problem, reducing a blade’s aerodynamic performance through pitting, gouging and delamination and, if water enters the blade, making it fail. Various technologies are used to reduce blade erosion due to environmental factors such as sand, rain, hail and salt spray, including paint-on polyurethane coatings and protective tapes. In addition, minor damage can be repaired using epoxy and polyurethane fillers.

GCube recommends inspecting blades at least every two years, and after any storms, for visual evidence of “cracks, air pockets, delamination, drainage, protective film and erosion of the leading edge, lightning protection system, spark gap, documentation, blade pitch angle [and] documentation of blade moment balance”, as well as ultrasonic or infrared testing. Steven Hughes, operations director with renewable energy developer RES, says: “The most common types of problems we see on blades are cracks, chips and minor delaminations. These can be fixed easily if they are picked up quickly.” IMFuture’s Riveiro concurs that “an operator’s reaction time to blade damage” can be a factor in how easy it is to fix.

While risk can be mitigated through protective measures, protection must also be designed into the structure of the blade. However, avoiding breakage is only one consideration involved in today’s blade design; costs and the demands placed on modern wind turbines must also be considered. Craig Langford, blade engineer with Danish blade developer SSP Technology, says: “It is a fine line between building blades that are strong enough to withstand damage from the elements and general fatigue, and producing blades as light and relatively flexible as possible – blades cannot be too heavy due to the limitations of the load capacity of the hub or tower connection.

“It is generally accepted in the industry that on-site repairs due to lifecycle fatigue or weather damage are to be expected,” he adds.

Blade design and materials

Cost and efficiency are significant factors in designers’ decisions about which materials to use for today’s longer blades. Ben Hendriks, head of mechanical engineering at DNV GL, believes optimization is “more and more important because, in the old days with smaller turbines, blade design could focus on maximum energy performance. For larger blades, it’s increasingly important to balance aerodynamic performance with blade costs, but also with blade loading. From optimizing to maximize power performance, design has changed towards optimizing for the minimum cost of energy. We see this across the industry.

“If you scale up a turbine,” he explains, “this means that if you double the diameter of the turbine the area goes up with the square, and the mass of a structure goes up with the cube of the scaling factor. With a double diameter, the rotor area is four times more, which equals four times more performance energy. The mass may go up with the cube, leading to eight times more mass. So the ratio of mass to performance is, in effect, going down – the increase in mass is more rapid than the increase in power performance.”

To address this trend, designers need to change design concepts and materials, he says. For example, “one of the reasons why you see carbon being used more in larger blades than in smaller ones is because it’s lighter than a full glass-fibre blade.” Ultimately, he believes that the current trend toward optimization will lead to “more revolutionary changes” in blade design and materials.

SSP’s Langford says the choice of materials depends first on where a turbine will be situated (onshore or offshore), and then on the size of the turbine and blades. Designers begin with “a pretty basic package” of proven materials, he explains, and then they can “play around” with different blade material combinations such as “carbon fibre and glass fibre and different core materials, balsa wood and different types of PVC foam with different densities and strength characteristics”. He notes that some companies are currently trying to find alternatives – such as high-density PVC foam – to the balsa wood which has traditionally been used as a blade core but is increasingly costly. However, new materials can take a while to be adopted.

“In areas where the customer wants to ensure that the blade has good strength or reinforcement, manufacturers are sticking to a balsa core,” he says. “Maybe a foam core supplier could say in testing that they think it’s strong enough, but blade manufacturers are kind of dubious to risk using these products until someone else does it. It might be an issue to get some of these products tried and proven – there’s a time factor before manufacturers will take the step and start using a material, but then word will spread and everyone will be using it.”

Factors driving research

According to Eta’s Durante, today’s designs are based on “materials that are able to stand time and weather for a longer period” and on “shapes which are more consistent with longer exposure to natural elements.” In modern blade design, he explains, “we can do thinner than before, with every twisting and binding created by wind and torsion and controlled by the materials themselves. We can have larger rotors and longer blades on the same turbine without disturbing any other ingredient of the turbine and its installed surface”.

The Netherlands’ University of Delft is researching the application of thermoplastic material as a matrix for thick laminates in blade manufacturing. According to Hendriks, the driving factor behind this research is that the blades produced would be recyclable, unlike today’s fibreglass blades. A thermoplastic blade can be melted down, with the plastic melting out and the glass recycled for use in new blades. Another advantage is that “you can be much more creative in blade design,” says Hendriks, “because, similar to welding steel, you can weld two components together”.

Eta’s Durante adds that R&D on materials and composites has “completely reshaped” how blades are made. “In the past,” he says, “there was a lot of wood in the actual blade, but this is no longer used. The glue is completely different: the previous glue seemed solid, while modern resin is much more flexible. All of the structure of the blade has been completely redesigned. The previous fashion was for a lot more resin and material used to glue together the components, but this is no longer useful today as components are fit together in a completely different fashion. We now use composites and also a lot of carbon fibre, which was not used in the past. The blade itself looks similar to past blades, but actually the interior components are completely different.”

Looking to the future

LM’s Schuring predicts that “the demand for higher reliability and proven technology will only increase. [LM’s] 73.5-metre blade [used on Alstom’s Haliade 150 6 MW offshore turbine] is a good example of a product that meets those demands. As the industry matures and the offshore segment continues to grow, the challenge and focus remains on how to ensure the highest performing and cost-effective product throughout its lifetime.”

One promising incipient trend involves so-called ‘smart blades’, which feature local aerodynamic control in the form of tabs or flaps – called ‘gustbusters’ by US firm Frontier Wind.

The technology will be most effective for larger blades, DNV’s Hendriks says: “If you cross the 200-metre diameter border then it may be cost-effective to apply smart blade technology. But it’s a question of how cost-effective the design can be with high reliability and low maintenance needs,” he adds.

SSP’s Langford believes that “the exciting thing at the moment in the industry is that there is no limit for the size of blades yet. Big offshore turbines are being built, and in some years’ time we may see 100-metre blades on offshore turbines. This could, in itself, lead to the industry needing to find different materials or different material combinations.”

Hendriks says that, until now, he had “always thought turbines would continue to upscale” and the size of rotor diameters would continue to increase – but before any further upscaling, he would like to see the offshore wind sector become “a little bit more stable” and more of a global market. “We don’t know how the technology will develop in the coming years in terms of cost-effectiveness and reliability,” he explains, “and on the other hand we don’t know how the market will develop.”

Technology trends will follow market trends, he predicts: “If the offshore market trend towards larger machines continues, market demands for ‘smart’ blades will increase.”

Going forward, newer and bigger turbines are only part of the blade technology picture. Eta’s Durante is hoping for a market trend in the opposite direction, toward growth in the small wind segment. He expects to see a 200-300 per cent increase within the next year.

“Of course, to follow this market,” he says, “we have created a blade for the 60 kW turbine, which needs to have a blade in the range of 9-11 metres, maybe 12 metres with a rotor between 19 and 22 metres.”

Meanwhile, as turbines age and reach the end of their guaranteed life, not every wind farm operator is going to want to abandon their project and move on. One solution to performance challenges for older turbines is re-blading, which Durante says can achieve a 20 per cent efficiency improvement. Re-blading can address the efficiency losses caused by natural factors during 20 years of operation and improve aerodynamics through reshaping the blade and applying new materials.

“Traditional manufacturers seem to not have the patience and space and manufacturing competencies to do this because, of course, they are focusing on new products and selling new turbines: larger rotors, multi-MW, etc,” he says. “Old assets are no longer on their radar. But wind farm operators don’t just want return on [their initial] investment – they want equity.”

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