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Wind Turbine Blades Push Size Limits

Using sophisticated modelling techniques and comparing the results with existing turbine technology, a 2011 report set out to see if 20-MW offshore turbines could become a reality. The report, entitled Upwind: Design Limits and Solutions for Very Large Wind Turbines, compared theoretical 20-MW designs with present technology. 'Yes, we have the technology but the economics will decide. Costs models will tell us the way forward,' says Bernard Bulder of the Energy Research Centre of the Netherlands (ECN).

The goal of a larger turbine — to increase efficiency by capturing more wind energy using longer blades — was achievable, the report said. The US$33 million UpWind project determined to what extent such turbines are feasible, and whether they make financial sense.

A look into the near future was offered early last year, when Danish company Vestas debuted its 7-MW offshore giant. In October 2011 DONG Energy announced it would test the turbine, with plans to install six at a demonstration site in 2013. Also in 2011, GE Global Research, the technology development arm of General Electric, announced it was to partner with the Oak Ridge National Laboratory to develop a generator to support large-scale wind turbines in the 10-15 MW range. Work has begun on the first phase of the two-year, $3 million project funded by the US Department of Energy.

But the EU-backed Upwind research project is a step forward in assessing the challenges ahead. Made up of 48 partners, half from the private sector and half from the research and academic sector, UpWind is the largest public/private partnership designed for the wind energy sector.

UpWind demonstrates that a 20 MW design is feasible. No significant problems were found when upscaling wind turbines to that size, provided some key innovations are developed and integrated. These innovations come with extra cost, and the cost:benefit ratio depends on a complex set of parameters. The project resulted, for instance, in the specification of mass:strength ratios for future very large blades securing the same load levels as the present generation of wind turbines. Thus, in principle at least, future large rotors and other turbine components could be realised without cost increases, assuming the new materials are within certain set cost limits.

For its assessment of the differences between the parameters of the upscaled wind turbine, UpWind adopted a reference 5-MW wind turbine. This reference was based on the IEA reference turbine developed by the National Renewable Energy Laboratory (NREL). As a first step, this reference design was extrapolated (or upscaled) to 10 MW. The 20-MW goal emerged progressively during the project, while in the meantime the industry worked on larger machines. The largest concepts which are now on the drawing board measure close to 150 metres in rotor diameter and have an installed power capacity of 10 MW. While a 10 MW concept progressively took shape, UpWind set its mind to a larger wind turbine, a turbine of about 250 metres in rotor diameter and a rated power of 20 MW.

Pushing the Boundaries

New concepts, components and materials are an essential part of the equation when it comes to upping the wind game. Research by Risø Denmark’s National Laboratory for Sustainable Energy at the Technical University of Denmark shows that new technological possibilities arise as a consequence of the development of new materials with improved properties. For a long time the wind turbine sector has focused on the reduction of weight and an increase in the strength of blades. However, components such as the gear train and generator will also benefit from the use of new materials and advanced simulations.

In 2011 Vestas took its first steps towards these mega-turbines, revealing details of its next-generation dedicated offshore turbine: the V164 7 MW. Designed to ensure the lowest possible cost of energy, with a rotor diameter of 164 metres, it represents a dedicated offshore turbine, able to cope in rough North Sea conditions.

Lowering the cost of energy in relation to offshore wind is essential. Some of the major stepping stones in achieving this are size and subsequent increased energy capture, which means a need for much bigger turbines, specifically designed for the challenging offshore environment.

Vestas CEO Ditlev Engel says of the new turbine: ‘Seeing the positive indications from governments worldwide, and especially from the UK, to increase the utilisation of wind energy is indeed very promising. We look forward to this new turbine doing its part in making these political targets a reality.’

According to Anders Søe-Jensen, president of Vestas Offshore, the offshore wind market is set to really take off over the coming years. ‘We expect the major part of offshore wind development to happen in the northern part of Europe, where the conditions at sea are particularly rough. Based on our broad true offshore experience, we have specifically designed the V164 7 MW to provide the highest energy capture and the highest reliability in this rough environment.’

One of the innovative parts is the medium speed drive-train. ‘Offshore wind customers do not want new and untested solutions,’ says Finn Strøm Madsen, president of Vestas technology R&D. ‘They want reliability and business case certainty.’ Construction of the first prototypes is expected in Q4 of this year.

Bigger Means Better Design

But, as the Upwind report points out, growing turbines to 20 MW will require even greater innovation in a number of areas. The 20 MW concept provides values and behaviour used as model entries for optimisation — it’s a virtual turbine, which could be designed with the existing tools, without including the UpWind innovations. This extrapolated virtual 20 MW design was found to be almost impossible to manufacture, and uneconomic. The extrapolated 20 MW would weigh 880 tonnes on top of a tower, making it impossible to store at a standard dockside, or install offshore with the current vessels and cranes. The support structures able to carry such mass placed at 153 metres height aren’t possible to mass manufacture. The blade length would exceed 120 metres, making it the world’s largest manufactured composite element, which cannot be produced as a single piece with today’s technologies. The blade length would also require new types of fibres to resist the loads. However, the UpWind project developed innovations to enable this basic design to be significantly improved, and become a potentially economically sound design.

Key weaknesses of the extrapolated virtual 20 MW design are the weight on top of the tower, the corresponding loads on the entire structure and the aerodynamic rotor blade control. The future large-scale wind turbine system drawn up by the UpWind project, however, is smart, reliable, accessible, efficient and lightweight.                                              

After reducing fatigue loads and applying materials with a lower mass:strength ratio, a third essential step is needed. The application of distributed aerodynamic blade control, requiring advanced blade concepts with integrated control features and aerodynamic devices, is also a significant departure from current technology. Fatigue loads could be reduced 20-40 percent. Various devices can achieve this, such as trailing edge flaps, (continuous) camber control, synthetic jets, micro tabs, or flexible, controllable blade-root coupling.

Further reducing the load requires advanced rotor control strategies for ‘smart’ turbines. The UpWind project demonstrated that individual pitching of the blades could lower fatigue loads by 20-30 percent. Dual pitch (pitching the blade in two sections) as the first step towards a more continuous distributed blade control could lead to load reductions of 15 percent.

A Streamlined System

Advanced control strategies are important for large offshore arrays, where UpWind demonstrated that 20 percent of power output can be lost due to wake effects between turbines. Optimised wind-farm layouts were proposed, and innovative control strategies developed, for instance lowering the power output of the first row (thus making these wind turbines more transparent for air flow), facing the undisturbed wind, allowing for higher overall wind farm efficiency.

‘The more your system is optimised, the more your wind measurement must be reliable and accurate,’ says Peter Eecen, work package leader at the Energy Research Centre of the Netherlands (ECN). ‘Wind-measurement techniques for wind energy are progressing quickly. The UpWind project acted as a node to narrow down wind measurement uncertainties. It helped translate innovation into IEC standards, with support of the whole measurement community,’ says Eecen.

After five years of research, the engineers at Risø concluded that the mega turbines would come with a 20 percent higher price tag than their smaller 5-MW siblings if these larger turbines were built in the same fashion. From the tip of the turbine blade to its base, and further to the grid, the project examined several areas needing further exploration to make mega wind turbines cost competitive.

Researchers hope that the increased power, combined with higher efficiency, will achieve greater economies of scale, reducing the cost of wind generation.

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