A friction-based passive control technique to mitigate wind induced structural demand to wind turbines

Wind turbines are growing in size in order to reach stronger winds and to produce more energy, even thanks to longer blades. This leads to bigger and bigger cross dimensions for the towers, with high construction costs. On the other hand, in the countries that have long been engaged in the production of wind energy, the need for repowering of existing plants is increasingly strong. In the latter case, a solution is that of replacing old towers and blades with new even longer elements for greater energy production, however taking into account the limited capacity of the existing foundation, so expensive to be replaced. Reducing the demand for stress on wind towers and, therefore, on the foundation can be definitely useful both for new installations and for upgrading existing plants. Herein a passive control technique is proposed to do that, based on the use of a rotational friction damper (RFD) installed at the base of the tower in parallel with a rotational spring. While the RFD dissipates energy thanks to the base, the spring has the task of favoring the re-centering of the tower. The design parameters for such a system have been identified and a procedure for their optimal calibration is also proposed and tested with reference to a case-study structure. The NREL 5 MW wind turbine has been subjected to three different wind loads and the results show that the proposed technique can be a suitable solution, cheap and practical to be used on wind turbines, especially for extreme winds. Actually it may lead to a reduction of base moment demand to the tower up to 40%, at the same time limiting top displacement demand and related undesired second order effects.