Wind turbine blades: how much longer?

Siemens wind turbines, Vestas installation
Siemens wind turbines, Vestas installation

The wind power industry is facing increasing pressure to cement its products and services as a viable alternative to non-renewable generation. And, as manufacturers look to make turbines more and more efficient, long turbine blades are emerging as an interesting area of research.

The need for the wind power industry to innovate is now greater than ever, despite the steady expansion of wind power, which has attained a total world capacity of nearly 400GW, accounting for about 3 per cent of the world’s electricity, according to the Global Wind Energy Council. The industry needs to compete with other sources of energy, both renewable and non-renewable. It must deal adequately with many challenges, including the requirement for wind turbines to perform well, and remain in service in a wide variety of weather conditions.

To address those challenges, the wind energy industry has come up with many technological improvements and refinements, and has developed several alternatives to the standard horizontal axis, three-blade wind turbine systems.

One trend has been to manufacture ever-longer turbine blades. The power in the wind that impinges upon the turbine is proportional to the square of the radius of the area swept by the turbine blades, and proportional to the cube of the wind speed. Therefore, apart from wind speed, increasing the radius increases the power available from the wind as it blows through the turbine. Another rationale for longer blades is that the longer the blade, the greater its ‘capacity factor’, i.e. the more independent the turbine will be on the wind conditions, and the more continuously the turbine will operate.

Using larger wind turbines reduces the number of wind turbines needed, decreasing installation and maintenance costs. This is especially true in the case of offshore wind farms, which require large, specialised ships for the transportation of the turbines.

Also, developing larger wind blades will allow harnessing wind energy into new regions with moderate wind speeds.

However, increasing the blade length brings many problems with it. Usually, the larger the blades, the heavier they are and the more difficult they are to transport, and this increases costs. The challenges presented by those long blades also include panel buckling, gravitational fatigue loading, and the increased importance of the aeroelastic effects on the blades. With longer blades comes the potential for significant deflections of the structure due to both aerodynamic and gravitational forces. There is also the chance for the blade to experience flutter – a phenomenon involving the interaction of a fluid and structural frequencies leading to large amplitude periodic motion.

R&D issues to enable large blades include: air foil architecture, material choices, design optimisation, blade joints, load alleviation, and designs for flutter suppression.

However, the fact remains that, as you increase the length of the blades, there is a point of diminishing returns, where the accompanying benefits no longer outweigh the drawbacks – and nobody knows precisely where that point is.

Presently, there are several wind turbines in operation with blades of more than 70m length.

The turbines with the longest blades are probably those of SSP Technology (Stenstrup, Denmark), which has installed a 7MW offshore wind turbine with three 83.5m-long blades off the Scottish coast. The root diameter of the blades is only 4.2m, which facilitates road transportation. The design and installation resulted from a close co-operation of SSP Technology and Samsung Heavy Industries.

The Fukushima floating offshore wind farm, completed last year, uses downwind-type blades, located leeward. Downwind-type blades were used for the project in order to make the most of the upward wind blowing from the surface of the sea. The project includes two 7MW Mitsubishi floating wind turbines on floating platforms. Each turbine has a rotor diameter of 164m and three 82m-long blades.

The blades of the SWT-6.0-154 wind turbine from Siemens AG (of Erlangen, Germany) are 75m long, while the entire rotor assembly measures 154m in diameter. As they spin, the blades sweep an area of 18,600sq m. The seamless blades are fabricated as a single cast part comprising glass, epoxy and balsa wood, and molded via the company’s IntegralBlade process. Weight savings of 20 per cent compare with conventionally produced blades is achieved through a specially designed blade profile, shaped to maximise the rotor capacity factor. The turbine has a cut-in wind speed of 3-5m/s, produces nominal power at 12-24m/s and has a cut-out wind speed of 25m/s.

The longest turbine blades of Vestas Wind Systems A/S (Aarhus, Denmark) are 80m for offshore installations and 67m for onshore installations. In common with other companies, Vestas’ efforts are directed at producing more energy and lowering costs. The company’s V136-3.45MW turbines have a rotor diameter of 136m and blade length of 66.7m, with a maximum chord of 4.1m. The blades’ swept area is 14,527sq m. Cut-in wind speed is 3m/s, and cut-out wind speed is 22.5m/s. According to the company the aerofoil design enables a 12 per cent increase in annual energy production while at the same time minimising structural loads.

LM Wind Power (of Kolding, Denmark) has installed its 73.5m blades at a 6MW wind turbine in Carnet, France, for Alstom (of Levallois-Perret, France) The glass/polyester blades feature the company’s SuperRoot design with blades that are 20 per cent longer without an increase in rotor diameter.

Blade Dynamics Ltd (Southampton, UK) has been appointed by the UK Energy Technologies Institute to develop long wind turbine blades. The blades would measure between 80m and 100m in length and would be used on the next generation of large offshore wind turbines generating 8MW to 10MW. The blades would be made of carbon-reinforced fibreglass and would weigh up to 40 per cent less than the traditional fibreglass blades. Blade Dynamics was recently acquired by GE Wind Energy (of Fairfield, Connecticut), a subsidiary of General Electric.

One of the major projects to develop super-turbines with blades up to 200m long, aiming to eventually install offshore 50MW wind turbines is being carried out in the US by Sandia National Laboratories (Albuquerque, New Mexico). The project is funded by the US Department of Energy’s Advanced Research Projects Agency-Energy program.

The project team is led by the University of Virginia and includes researchers from the University of Illinois, the University of Colorado, the Colorado School of Mines, and the National Renewable Energy Laboratory. Corporate advisory partners include Dominion Resources, General Electric Co, Siemens AG and Vestas Wind Systems.

Dr Todd Griffith, head blade designer on the project, said, “Exascale turbines take advantage of economies of scale. The US has great offshore wind energy potential, but offshore installations are expensive, so larger turbines are needed to capture that energy at an affordable cost.”

Work on the project is based on Sandia’s previous work on 13MW, horizontal axis systems, using 100m blades. A major part of the project is a blade design inspired by the trunk of palm trees and the way they adapt to the wind. The blades spread out to catch the wind in favourable conditions, to maximise energy production, and fold together like palm fronds in stormy, dangerous weather conditions, to minimise the risk of damage. The design is called a Segmented Ultralight Morphing Rotor (SUMR). This allows the blades to be assembled on site from segments. The blades’ adaptability to the wind will allow them to have less structural mass and to be ultralight.

The SUMR design will allow the blades to withstand wind gusts of up to 320km/h.

The project’s 13MW baseline model with 100m-long blades, termed SNL100-00, incorporates conventional geometry, all-glass materials, and traditional manufacturing methods. The SNL100-00 is to be used as a research tool for evaluating new design options to overcome challenging large blade design issues.

A few major issues have been observed in the project. Firstly, panel buckling is a significant concern in the case of large blades. Secondly, the growth in gravitational loading for the larger rotor required significant trailing edge reinforcement to reduce edge-wise strains. Gravitational loading is a particular concern for fatigue life. The project demonstrated that the edge-wise fatigue life is paramount for the SNL100-00 blade over the traditionally aerodynamically-drive, flap-wise fatigue. Thirdly, although no additional reinforcements were needed to satisfy deflection requirements beyond those needed for buckling and fatigue, the margins were smallest for an operating case corresponding to an extreme coherent gust with direction change.

Flutter was identified as a potentially significant issue. A fatigue analysis showed that flap-wise fatigue at an inboard location was the critical failure – as opposed to the dominant edge-wise gravitational fatigue of the baseline model.

Testing is at the heart of blade innovation. It is one of the largest cost factors in certification and there is, therefore, a need for faster validation and certification of designs. Longer blade lengths are placing a special burden on testing.

Fatigue testing of whole blades can be exceedingly time consuming. It involves testing a blade’s resilience to material stresses across its 20-year lifespan. It is done using hydraulic cylinders to push and pull the blades through large deflections, creating oscillations at the blade’s natural frequencies.

A German institute, which engages in blade testing, the Fraunhofer Institute for Wind Energy System Technology (IWES), located in the port city of Bremen, has announced a project to develop novel testing methods, partly in response to the issues created by long blades. One of the new methods is substructure testing – where smaller sections of blade are tested in place of whole blades. Natural frequencies increase with shorter blade segments and, thus, testing time is greatly reduced.

IWES and other organisations, including the Research Alliance Wind Energy (FVWE), the German Aerospace Center, the Universities of Oldenburg, Hanover and Bremen are engaged in the Smart Blades project, also partly a response to the development of long wind turbine blades. They developed new concepts for intelligent rotor blades that can adapt their geometry to suit the local wind conditions. For example, they examined both flexible and rigid trailing edge flaps, inspired by aviation. Both options reduce the load on the blades.

It is yet to be seen to what extent those exciting developments will widen the use of wind energy and strengthen its competition with other sources of energy.

By Paul Grad, engineering writer