Vertical wake deflection for offshore floating wind turbines by differential ballast control

Abstract

This paper discusses the idea of vertical wake deflection for offshore wind turbines using active ballast control. First, the concept of active ballast control for changing the attitude of an offshore wind energy semi-submersible platform is presented and briefly explored. Next, the influence of vertical wake deflection on the wake behavior and power production of a cluster of two wind turbines is studied through wind tunnel experiments and LES simulations. Results show that vertical deflection changes substantially the wake characteristics and affects the inflow at a downstream wind turbine in terms of available kinetic energy and turbulence intensity. For the most favorable case considered here, the cluster power was increased by approximately 8%.

Quantification of wake shape modulation and deflection for tilt and yaw misaligned wind turbines

“…Downwards deflecting tilt results in the highest f AP of 80 %, significantly higher than for all other cases. The higher measured f AP is in good agreement with power measurements of two scaled wind turbine models with tilt misalignment by Nanos et al (2020), and indicates the higher potential power gains that can be made with downwards deflecting tilt. It is interesting to note that if the second turbine is placed at z/D ≈ −0.75, negative yaw steering can provide an even higher f AP than downwards deflecting tilt.…”

“…However, in this study the tower is tilted with the rotor, thereby keeping the tower-blade distance constant, such that a minimal impact on performance is expected. It is important to note that even despite a potential ground effect, arrays of downwards deflecting turbines have been found to show superior gains for the overall power output of a farm, thanks to a major reduction of wake losses (Annoni et al 2017;Cossu 2020;Nanos et al 2020), and the improvement of vertical entrainment of mean kinetic energy (Scott et al 2020a). For these measurement results, the circulation of the vortex cores can be approximated reasonably well by a power-law decay, as indicated by the plotted fits of y = ax b .…”

Evaluation of tilt control for wind-turbine arrays in the atmospheric boundary layer

“…Following C2020, the simulations are repeated operating tilted-rotor turbines at higher C T . To clearly isolate the effect of operating tilted turbines at higher C T on tilt control, results will be shown for the large value C T = 3 (the same value considered by C2020) that is near the maximum of the range attainable by real turbines (see, e.g., Goit and Meyers, 2015;Munters and Meyers, 2017;Cossu, 2021). Quantitatively intermediate whilst qualitatively similar results are obtained for intermediate values of C T , as shown in Appendix B.…”

“…This model has been shown to provide reliable results for the characteristics of turbine wakes except in the wake formation region (Wu and Porté-Agel, 2011). To obtain general results, not depending on the specific control law assumed for the considered turbines, the total force exerted by each turbine on the fluid is assumed to be of the form F = −C T ρu 2 n Ae n /2, as done by, e.g., Calaf et al (2010), Goit and Meyers (2015), and Munters and Meyers (2017), where C T is the disk-based thrust coefficient, e n is the unit vector normal to the rotor disk, u n is the wind velocity component normal to the rotor averaged over the disk surface of area A = π D 2 /4 and D is the rotor diameter. The force is assumed to be uniformly distributed on the rotor, and wake rotation effects are neglected because their modeling would reintroduce a dependence on specific turbine design and control.…”

“…Power gains that will be computed in the following, however, being ratios of powers computed with the same χ, do not depend on the specific value chosen for χ. The value C T = 2 corresponds to the optimal Betz value maximizing the power output of an isolated ideal turbine (Burton et al, 2001;Munters and Meyers, 2017).…”