Low-Level Jet — Wind Turbine Interactions and their Impacts on Performance, Power Production and Mechanical Responses: A Numerical Approach

Nocturnal low-level jets (LLJs) are defined as relative maxima in the vertical profile of the horizontal wind speed at the top of the stable boundary layer. Such peaks constitute major power resources and will be very important for the future of wind turbines. However, they have also been associated with an increase in wind tur-bine damages. Harnessing the power of LLJs will play a key role in satisfying the US’s energy demands. Wind energy is by far the best alternative to protect the increasingly strategic water reserves, as it has near-zero water consumption. It is currently the only option that is at the same time pollutant-free, cost-effective, water-preserving, unlim-ited and compatible with other land uses (cattle, agriculture). High-frequency (50Hz) observational data from the 200-m meteorological tow-er data (Reese Technology Center, Texas) have been prescribed as inflow conditions into the NREL FAST code in order to evaluate the structural impacts of LLJs on a typical commercial wind turbine. A vertical region of interest for the analysis of the interaction LLJ - wind turbine has been delimited and the LLJ length scales have been calculated. It has been observed that during a LLJ event the turbulence intensity and turbulence kinetic energy (TKE) were significantly lower than those during unstable conditions, and cyclical aerodynamic loads on the turbine blades produced a negative impact on the wind turbine, mainly due to the enhanced wind shear. Dominant fre-quencies present in the power spectra of the incoming wind were also observed in fre-quencies related to the dynamic loads of the turbines. It was found that the wind tur-bine can mimic the signals from the approaching inlet flow, potentially exciting specif-ic modes. This research also performed a methodical approach to the study of the influ-ence of the negative wind shear above the LLJ’s peak on wind turbines. Up to now, the presence of negative shears inside the turbine's rotor has been largely ignored. A parameter has been proposed to quantify that presence in future studies of LLJs – wind turbines interactions. It was found that the presence of negative wind shears at the heights of the turbine's rotor appeared to exert a positive impact in reducing the motions of the nacelle and the tower in every direction, with oscillations reaching a minimum when negative shears covered completely the turbine sweep area. The forces and moments were also reduced by the negative shears. The aforementioned impacts were less beneficial in the rotating parts, such as the blades and the shafts. Finally, the variance of power production was also reduced. Those findings can be very important for the next generation of wind turbines as they reach deeper into LLJ’s typical heights. The study demonstrated that the presence of negative shears is significant in reducing the loading on wind turbines. A major conclusion of this study is that the wind turbines of the future should probably be designed with the aim of reaching more often the top of the nightly boundary layer, and therefore the altitudes where negative shears are more frequent. Doing so will help to reduce the positive shear's associated damages and to capture the significant LLJ energy. It has been detected that the signals from the incoming wind entered into the wind turbine’s responses through the interactions with the blades and then travelled almost linearly through the turbine structure. This may confirm the reason why charac-teristic frequencies detected in the LLJ were not dissipated and instead appeared in the responses at several turbine parts. The responses in each blade tended to have a cardioid-like shape in an azimuth plot due to the modification of the surrounding wind field as each blade approached the tower. Therefore, the presence of the tower breaks the symmetry and shapes the responses at the blades. The interaction between tower and blades creates a perturba-tion in the wind field near the tower.