A new generation of superblades would require a breakthrough in materials and controller expertise. But several factors are constraining the next leap forward.

By Karl Harder

Given that roughly one third of the cost of offshore wind turbine installation is sunk into the turbine's foundations, maximizing the amount of energy harvested by each installation is crucial.  As Helle Anderson from LM glasfibre explains, “The longer the blades the better. Longer blades equal more energy production and thus greater return on investment for the turbine owner.” However, turbine manufacturer REpower cautions that this can only be achieved within reason.

Leading the way

The race for scale is being led by REpower Systems and LM glasfibre, who collaborated in 2002 to produce the largest blade on the market, the LM61.5.

REpower has since modified the LM61.5 blade by including aerodynamic add-ons for use on its 5MW and 6MW Turbines. “This enabled us to boost the rated output from 5MW to 6MW on our most recent turbine,” explains Marc Petsche, Team Leader of Aerodynamic Development at Repower, a German turbine manufacturer.

However, Petsche says it is unlikely that blade sizes will be pushed much higher in the near future. “Though it is technically feasible to produce 70-80 metre-long blades we do not believe they would be cost effective. To allow a new generation of super blades there will have to be significant breakthroughs in both materials and controller expertise,” he explains.  

Big Blades Challenges 

Balancing cost and durability represents a huge technical challenge with big blade design.  With 5MW turbines having only been operational for the last 5 years, it is also by no means clear whether this challenge has been overcome.  

“Designing a big blade is not as simple as taking a small blade and doubling everything,” says Petsche. “If you double the blade length, the blade mass increases eight times, but the stiffness increases just four times. When designing a big blade to ensure durability you effectively need to start from scratch and design a totally new geometric.” 

Glasfibres’ Anderson adds: “The larger the blade, the more important it is to focus on the weight, in order to reduce the stresses on the turbine during operation.”    

The controller is the other central element when scaling up turbines. The controller is the brain of the turbine and can alter or monitor over 500 parameters.  This allows the turbine to adjust itself to take advantage of changes in wind conditions. Critically, it can also allow the machine to position itself in order to minimize blade stress. 

“Currently, controllers are not sophisticated enough to manage the stress on blades longer than 61 meters,” explains Dr Ganapathisubramani, Senior Lecturer in Aerodynamics at Imperial College London.

Pushing the limits

Nevertheless, efforts are being made to push back the boundaries. LM glasfibre have plans to develop larger blades while Clipper Wind recently won a £4.4m UK Government grant to start developing a 7.5MW turbine with an expected blade length of about 70m. 

A notable absentee from the world of big blades is Vestas, the world’s number one producer of wind turbines. While Vestas has been slow to get out of the starting blocks, sources say that they may be considering developing their own 5MW turbine.

Logistics and finance hurdles

The final challenges are the logistics and financing hurdles. In scaling-up turbines, the logistics of construction become more complicated and expensive.  

“Greater automation in blade manufacturing processes will lower maintenance costs and research and development will continue to drive progress in the creation of lighter, stronger composite materials,’ says Charlie Hodges, an analyst at New Energy Finance.

He adds: “Similarly, cost-effective transport from the factory to the site will lower installation costs and we are seeing industry respond to this challenge by investing in facilities and services much closer to harbours.”

However, uncertainty about the durability of these offshore technologies will continue to constrain the financing of offshore wind farms by limiting the pool of lenders. The issue is relevant to all size of turbines but because larger turbines have a shorter track record, the perceived risk is higher. 

 “Globally, only 3 out of the last 33 offshore projects received non-recourse debt finance. When the first Repower 5MW turbines were installed at Thornton Bank, off Belgium, REpower’s manufacturer warranties had to be particularly strong – providing an availability guarantee for the first ten years of production - before enough banks were satisfied with the risk profile” Hodges explains. 

He concludes, “Until there is greater evidence that these turbines can stand up to the harsh sea conditions and generate reasonable returns, debt finance will be the limiting factor in the growth of the sector towards 2015.”