Wind turbines, fully in motion

Wind turbines in the sea stand on constructions on enormous steel piles that are driven 35 metres deep into the seabed. ECN part of TNO has a separate research line for, among other things, the improvement of the design of these so-called monopiles.

Since the 1970s there has been a lot of experimentation with the generation of wind energy. The conventional horizontal axis wind turbine, which came out best, did not stand still from a design point of view either. ECN part of TNO is constantly working on innovations that make the turbine better and more cost-efficient.

The design and further development of wind turbines involves a great deal of work and requires a lot of knowledge from various disciplines, in particular aerodynamics, aero-elasticity and control. All three are core activities of ECN part of TNO.

One of the main challenges at the moment is to increase the size of the rotor of the wind turbines. The blades have to be designed longer, as it were. That may seem simple, but it is not. If the blade is scaled up - that is to say, it becomes larger - it becomes heavier to the power of three according to the square cube law. And mass has an impact on costs, with a larger blade more expensive in almost all cases.

Heavier yet cheaper

However, this is particularly worthwhile in maritime applications, because the foundations and the construction of these foundations are expensive parts of the total cost. Wind turbines can be fixed on a pile or float in the sea, but these are very expensive options. It therefore pays to place them as much as possible on a foundation in order to ultimately - despite increased costs on parts - generate cheaper energy.

However, there are mechanical limits to the mass: the rotor cannot be made heavier indefinitely. If the blades become too heavy, this will affect the entire turbine, even the mast and the foundation will become more expensive. One solution is clever aerodynamic and control innovations that reduce load and thus mass. An example is the use of special measuring equipment (LiDARs), which detects gusts of wind in advance and allows the rotor to anticipate them. Options also include the development of other concepts for generating wind energy, such as a vertical axis wind turbine or a kite.

Core disciplines

Important disciplines are aerodynamics as well as aero-elasticity. In aerodynamics, the force exerted by the wind on the blades of the wind turbine is predicted. Aero-elasticity refers to the interaction of aerodynamics with blade deformation. The forces that the wind exerts on the rotor have an effect on the shape: the blades can bend and twist, and start to vibrate. If too much vibration occurs, the wind turbine breaks down.

A third important discipline is the so-called controllers, or regulators, that adjust the blade angle and rotation speed in order to optimally utilise the wind and reduce loads. Because the onsite wind varies greatly at different times during field measurements, ECN part of TNO, together with TU Delft and others, carried out studies in wind tunnels where the operation of the wind and the air forces can be examined in a controlled way.

Towards large-scale generation of wind energy

Dr Koen Boorsma

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