A floating offshore wind turbine is a structure that allows the turbine to generate electricity in water depths where fixed-foundation turbines are not feasible. Floating wind farms have the potential to significantly increase the sea area available for offshore wind farms, especially in countries with limited shallow waters. Locating wind farms further offshore can also reduce visual pollution, provide better accommodation for fishing and shipping lanes, and reach stronger and more consistent winds. However, the design of floating offshore wind turbines is challenging because of the strong coupling between the aerodynamic forces on the wind turbine rotor and the platform response to hydrodynamic forces and the mooring system.
In a recent article published in IEEE Control Systems, researchers have highlighted the significant challenges in experimental methodologies for floating wind turbines. They have also introduced innovative solutions, such as the implementation of filters and a controller with a tower-top velocity feedback loop, to address these challenges. These insights are invaluable for researchers, practitioners, and stakeholders in the field, providing a deeper understanding of the complexities and potential solutions in floating offshore wind turbine control.
The Design of Scaled Wind Turbine Systems
Constructing scaled models of floating wind turbines for experimental studies in a wave basin requires fulfilling Froude similitude conditions. However, this approach substantially reduces the Reynolds number under lab conditions, consequently altering the aerodynamic performance of the turbine. The rotor was explicitly redesigned for low Reynolds number conditions to address this issue, ensuring the correct aerodynamic thrust coefficient in subsequent tests. The design of the scaled turbines used in these campaigns utilized Froude similitude, ensuring proper scaling of the timescale and structural masses. The rotor was designed to generate the necessary thrust forces under the lower Reynolds number conditions in the laboratory.
The 1:60 scaled DTU 10MW wind turbine on the tested design variant of the TetraSub floater in the DHI wave basin.
Setting Up the Test Environment
A key argument used by the researchers is that building a wave basin within a wind tunnel is both challenging and expensive. Therefore, in most experiments, a wind field is generated by an open-jet wind generator system positioned above the wave tank. The wind generator consists of multiple identical fan units that propel the air through guide vanes and honeycomb screens to create the desired wind field. Consequently, as the researchers note, the precision of the measured wind speed is reduced compared to that for closed wind tunnel experiments.
A wind generator system
Further, the surrounding environment can influence the open-jet airflow, introducing changes in the measured wind speed. These flow effects demonstrate the need to monitor the wind field and its replicability during the tests—for example, by frequent examination of wind probes and comparison to reference data.
Control System Implementation
In the experimental study, researchers modified the turbine controller into a simplified code programmed in the C language. Two challenges are related to the control experiments: start-up/shutdown procedures and signal processing for tower-top velocity data. These challenges require careful planning and execution to ensure the safety and effectiveness of the experimental testing.
Upon reaching above-rated rotor speeds, the pitch controller increases the pitch angle. In the event of instability, an emergency stop procedure is promptly initiated using a manual stop button, pitching the turbine blades to 90°. These procedures ensure the safety of the experimental testing. The floating wind turbine controller incorporates a tower-top velocity measurement derived from acceleration data through low-pass filtering and integration. However, integrating acceleration signals often introduces velocity drift, exacerbated by low-frequency white noise corruption. A multistep process optimizes the accuracy and reliability of tower-top velocity measurements, which is essential for effective floating wind turbine control.
Software-in-the-Loop Testing
Hybrid test methods can be applied as an alternative to tests with a physical rotor to avoid the challenges in aerodynamic scaling and wind field generation. Small propellers or a wire system replace the wind generator setup and rotor, which generates the required aerodynamic forces calculated via an aeroelastic simulation code. The physical motions of the wind turbine and platform are measured and fed into the aeroelastic simulation tool in real-time.
Diagram of a proposed real-time SIL test.
The hybrid test approach offers several benefits for testing multi-physical systems, though it also has limitations, such as challenges in scaling, a more complex setup, and cost.
Key Takeaways
The researchers have introduced a hybrid testing method that incorporates the software-in-the-loop concept. They have discussed its advantages and disadvantages, highlighting its potential to advance experimental studies in floating wind turbine control. These critical perspectives serve as a valuable reference for future research, offering practical guidance and lessons learned. The innovative solutions presented by the hybrid testing method push the boundaries of what's possible in renewable energy research.
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