Integrated bi-directional vibration control and energy harvesting of monopile offshore wind turbines

Offshore wind turbines (OWTs) subjected to combined wind and wave loadings experience excessive bi-directional vibrations that adversely influence the system performance and the structural integrity. The present paper utilizes a three-dimensional pendulum tuned mass damper (3d-PTMD) to mitigate the bi-directional vibrations as well as harvest the kinetic energy using a linear electromagnetic energy harvester. The proposed energy harvester consists of magnets and coil assemblies which are connected with the pendulum to convert the kinetic energy of the pendulum into electricity. An analytical model of the offshore wind turbine coupled with the 3d-PTMD is established using Euler-Lagrangian equation. The mathematical model of the linear electromagnetic energy harvester is established and integrated with the wind turbine model. The optimum design of the electromagnetic energy harvester to minimize the nacelle displacement RMS as well as to maximize the energy output is determined via a numerical search method. The NREL 5 MW baseline wind turbine model is utilized to evaluate the performance of the 3d-PTMD and the energy harvester. Results show that the 3d-PTMD can reduce the bi-directional vibrations induced by misaligned wind and wave loadings. Additionally, electrical energy in orders of magnitude of kilowatts can be harnessed via using the energy harvester.