Characterizing tilt effects on wind plants

Abstract

Tilting the nacelle of a wind turbine modifies entrainment into the wind plant and impacts total efficiency. Wakes are deflected vertically by tilt and in the case of large angles can disrupt entertainment from the undisturbed flow or dissipate on the ground. The effect of nacelle tilt on wake behavior is investigated in a series of wind tunnel experiments for the first time. Scale model turbines with a hub height and a diameter of 12 cm are arranged in a Cartesian array composed of four rows of three turbines each. The tilt angle was varied in the third turbine row from −15° to 15° in chosen 5° increments. Stereo particle image velocimetry measurements of the instantaneous velocity field were recorded at four locations for each angle. Tilted wakes are described in terms of the average streamwise velocity, vertical velocity, and Reynolds stresses. Mean kinetic energy quantities are presented, and conditional sampling is employed to quantify the importance of sweep to ejection events in vertical momentum transfer. Additionally, the effect of nacelle tilt on net power production is presented and compared to existing models. Numerical simulations accurately predict losses in net efficiency for positive angles but diverge for negative tilt angles. The results demonstrate that the tilt angle influences wake magnitude, displacement, and recovery. Positive angles deflect wakes above the wind plant, while negative angles encourage entrainment into the wind plant and exhibit rapid recovery.

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

“…Unless a downwind rotor is used or the tower design is corrected, it is almost inevitable that the rotor will be slightly lowered by forwards tilting. It should also be recognized that tower interactions can change for positive and negative tilt (Santoni et al 2017;Scott et al 2020a). 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.…”

“…For negative tilt, the wake is directed towards the ground such that it is affected by blockage of the ground, and interacts with lower-momentum flow with a higher level of turbulence. This downwards displacement aids in directing high-momentum flow from above the turbine to the rotor region of the next turbine, thus enhancing downwards transfer of kinetic energy (Scott et al 2020a); a relevant quantity as indicated by Calaf et al (2010) and Cal et al (2010). As a result, the wake shape spreads wider as it is pushed into the ground.…”

“…So far, wake-steering by yaw misalignment has received the most interest, as it can be fitted retroactively to existing wind farms. Yet, several large eddy simulations (LES) studies (Fleming et al 2014;Cossu 2020) and recent experimental results (Scott, Bossuyt & Cal 2020a) have indicated that tilt-misalignment could allow for even larger gains. Tilt-misalignment of a wind turbine deflects the wake vertically, such that a wake can be steered upwards into higher momentum flow, or downwards into the ground (Fleming et al 2014;Rockel et al 2014Rockel et al , 2016Rockel et al , 2017Annoni et al 2017;Bay et al 2019;Kadum et al 2019;Su & Bliss 2020;Scott et al 2020a,b;Wang, Liao & Ma 2020).…”

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

“…In addition to yaw control, where wakes are deflected horizontally, it has been more recently proposed to deflect wakes in the vertical direction by acting on the rotor-tilt angle (see e.g. Guntur et al, 2012;Fleming et al, 2014Fleming et al, , 2015Annoni et al, 2017;Bay et al, 2019;Cossu, 2020;Scott et al, 2020;Nanos et al, 2020). Best performances are obtained for positive tilt angles, where the wake is deflected towards the ground (or the sea surface).…”

“…Global power gains are achievable also for tilt control despite the power losses experienced by tilted-rotors upstream turbines (Fleming et al, 2015;Annoni et al, 2017;Bay et al, 2019;Cossu, 2020;Scott et al, 2020;Nanos et al, 2020). In a companion paper (Cossu, 2020, referred to as C2020 in the following), we have recently shown that by tilting upstream rotors of spanwiseperiodic turbine rows, turbine wakes (low-speed fluid) can be replaced with high-speed streaks, i.e.…”