Speaker
Description
The accelerated switch to renewable energy makes wind energy harvesting more necessary. Germany and other European countries announced ambitious goals for future energy generations from renewable sources – especially wind. However, for onshore wind energy generation there is often a conflict of interest between wind turbines and third parties. For offshore wind generations faster and more constant wind flows promise higher gains. Here, the available space which provides shallow water depth for ground-based wind turbines but does not be in conflict with ship traffic and nature conservation requirements is limited and nearly exhausted. Thus, future wind turbines must move to larger water depth which makes ground-based mounting challenging and floating structures more interesting.
One concept for a floating wind turbine consists of a hexagonal foundation providing the buoyancy, and a centrally placed tower carrying the wind turbine. Several tubes connect the foundation and the tower and stiffen the structure. The hexagonal foundation has a length of 240 m and a breadth of 208 m. The nacelle caries a NREL 5 MW turbine having a three-blade rotor which hub is placed approximately 125 m above the water level and has a diameter of 130 m [1]. Froude scaling was applied to down-scaled the floating wind turbine by a factor of 150 to allow model tests.
The model was equipped with an Inertia Measurement Unit (IMU) which measured pitch, roll, and yaw motions as well as vertical, longitudinal, and lateral accelerations. Accelerations were integrated twice to get the translational motions. An electric engine was mounted in the model nacelle to run the rotor at a constant speed. Model natural frequencies were obtained by performing decay tests with and without rotor motion. Additionally, the influence of the mooring system was analysed.
In the present study, we investigated floatation stability in long-crested harmonic waves and irregular seas. For harmonic waves, Response Amplitude Operators (RAOs) were determined for various wavelengths and wave heights. Additionally, test repeatability and the influence of a rotating rotor were evaluated. For irregular seas, short-term statistics were determined based on a full-scale test duration of three hours. Exceedance probabilities of roll and pitch angles and translational accelerations were calculated for operational conditions as well as severe storm conditions. Resulting angles and accelerations indicate that the floating wind turbine floats stable and safely during operational and extreme weather conditions.
References
[1] Jonkman, J.; Butterfield, S.; Musial, W.; Scott, G.: Definition of a 5-MW Reference Wind Turbine for Offshore System Development, National Renewable Energy Laboratory 2009.
| Keywords | Floating Wind Turbine, Renewable Energy, Model Tests. |
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