Konstantinos Poulis defends his PhD Thursday 16th January. The defence takes place ind building 306, room 37 at DTU from 13.00 - 17.00.
A common goal among many countries worldwide is to increase the share of renewable energy in the overall energy supply. As response to such an aspiration, wind energy is becoming more and more cost effective through improved technology and increased size of wind turbines. One of the subsystems that are affected by the tendency for building larger units, is the yaw system of horizontal axis wind turbines.
State of the art wind turbine yaw systems consist of either a large roller element bearing or a corresponding segmented sliding bearing that connects the wind turbine nacelle and tower. An additional disc brake is typically included as an independent system. However, the increasing size of wind turbines makes roller element bearings an economically costly option. Moreover, the additional brake system increases com- plexity and consequently adds further production and maintenance costs. One of the innovations aiming at reducing complexity in the yaw system consists in combining a segmented sliding bearing and a brake into a single system.
This thesis studies the tribological implications of such a hybrid sliding bearing and brake for the yaw system of wind turbines. Based to a large extent on experimental testing, it aims at providing designers with friction coefficient and wear rate values for different material candidates, to serve as a basis for appropriate material selection and proper dimensioning of the system. Moreover, the experimentally studied cases are put into a theoretical framework by identifying known friction and wear mechanisms, supported by topography measurements and micrographs. Finally, a numerical model for contact between rough surfaces was developed and used for studying friction phenomena in a more quantitative manner.