  The intentional yaw misalignment of leading, upwind turbines in a wind farm,
termed wake steering, has demonstrated potential as a collective control
approach for wind farm power maximization. The optimal control strategy, and
resulting effect of wake steering on wind farm power production, are in part
dictated by the power degradation of the upwind yaw misaligned wind turbines.
In the atmospheric boundary layer, the wind speed and direction may vary
significantly over the wind turbine rotor area, depending on atmospheric
conditions and stability, resulting in freestream turbine power production
which is asymmetric as a function of the direction of yaw misalignment and
which varies during the diurnal cycle. In this study, we propose a model for
the power production of a wind turbine in yaw misalignment based on aerodynamic
blade elements which incorporates the effects of wind speed and direction
changes over the turbine rotor area in yaw misalignment. A field experiment is
performed using multiple utility-scale wind turbines to characterize the power
production of yawed freestream operating turbines depending on the wind
conditions, and the model is validated using the experimental data. The
resulting power production of a yaw misaligned variable speed wind turbine
depends on a nonlinear interaction between the yaw misalignment, the
atmospheric conditions, and the wind turbine control system.
