Wind turbines in the sea stand on constructions on enormous steel piles that are driven 35 metres deep into the seabed. TNO has a separate research line for, among other things, the improvement of the design of these so-called monopiles.
The wind turbine comprises the mast, nacelle and rotor, including the blades. On land, the turbines rest on concrete foundations. Offshore wind turbines require a different construction: the standard turbines are mounted on 90-metre-long steel monopiles. This length will increase even further. In addition, floating support structuresare being developed for a new market: floating wind farms.
The wind turbines are getting bigger and bigger. This also requires adjustments to the support structures. The steel support constructions are therefore becoming heavier and heavier and contain important details that determine the design: for example, the cables must be routed through the wall of steel construction to the outside.
The monopiles can only be efficiently designed as an integral part of the wind turbines. This is one of the research areas of TNO.
Unlike the oil and gas platforms, the most important force is not gravity, perpendicularly downwards. In wind turbines, the forces are exerted much higher on the rotor, and especially horizontally. That changing force works very differently than gravity alone. Knowing exactly how that force is exerted on the wind turbines, and where it is applied, is necessary to make safe and cost-effective designs.
TNO develops monitoring programmes by placing measuring instruments on the wind turbines in the wind farm in order to measure the forces. The forces are not only accurately measured to make the most efficient design, but also, for example, to calculate the life span of the support structures as accurately as possible.
These sensors provide important information that can be used to optimally adjust the individual wind turbines. By adjusting the optimum position of the rotor and the angle of the blades to the wind, the energy production can be optimally controlled. But not only that. The optimum setting also reduces the impact of the forces on the supporting structure. This extends the service life of the supporting structure.
The horizontal shaft wind turbines in which the blades rotate are the classical form as in the case of Dutch mills. The vertical mast turbine, where long blades rotate in a circle around a vertical mast, is not yet common. That was a choice made in the last century. However, this vertical axis concept may well become a winning concept in the near future. A vertical axis construction is very suitable for the floating wind farms. The centre of gravity of a vertical axis turbine is much lower and therefore the floating construction is potentially lighter and therefore cheaper. It is not inconceivable that in five to ten years' time there will be many more vertical axis wind turbines offshore.