Besides the above mentioned benefits, upscaling of a VAWT is not driven by gravity forces (as for HAWT) and the VAWT does not require a yaw system. However, there are also challenges to be faced, for which the consortium initiated the project S4VAWT: Semi-Submersible Support Structure for Vertical Axis Wind Turbine. The project focused on the following main activities: 1) aerodynamic and structural modelling of the VAWT to generate the expected loads acting on the floating support, 2) design of the rotor and controller for a floating VAWT with blade pitch, 3) optimization of the floating support structure for VAWT and 4) requirements for a wave basin model test campaign.
Due to vertical orientation and rotation of the blades, the blades see large angle of attack variations and blade-wake interaction. TNO developed an accurate but sufficiently fast aerodynamic model for the VAWT to predict the loads on the floating support. The model is based on the free-vortex-wake method and implemented in the AeroModule. This is a library with aerodynamic models that can be used for standalone analysis or coupled to structural analysis tool. The model is computationally expensive, but several methods to reduce calculation time have been investigated and implemented to reach acceptable speed for load case calculations. The model has been validated against high-fidelity numerical methods.
As second main task, TNO developed a VAWT controller that increases power capture and reduces loads for the design driving cases during both normal production and parked survival operation.
Typically, small-scale VAWTs are designed for simplicity, omitting blade pitch (rotation round its axis) control. In the S4AWT concept however, blade pitch control is added for its potential to increase power capture in partial load. The large angle of attack variation seen by the blades reduces their aerodynamic efficiency and this can be mitigated by pitching the blade along with the rotation. This leads to an increase in power capture of up to 5%.
The presence of blade pitch actuators in the S4VAWT concept also allowed for a novel strategy of generator speed and power control. This approach makes use of our extensive knowledge of horizontal axis wind turbine control, but applied in a VAWT setting. Regulation of the generator speed and power using blade pitch control alleviates the loads on the support structure and the generator.
Initial investigation revealed the parked survival case to be design driving. An active pitch control method has been developed by TNO, that largely reduced the loads for this case and mitigated its impact on the design of the floating platform and mooring.
In short, the work of TNO contributed to the optimization of the floating support, leading to a steel mass reduction of 25% compared to a horizontal axis system. Please contact us in case you would like to hear more about this floating VAWT development.