Wind turbine performance and control in complex terrain

Abstract. Wind energy projects in complex terrain are often associated with high uncertainties regarding the expected power performance. These uncertainties are mostly attributed to difficulties in obtaining reliable wind speed estimates. However, an additional factor is that the physical limits of energy extraction vary in these cases, and that the employed wind turbine might operate differently than expected in these conditions. Reynolds-averaged Navier-Stokes (RANS) simulations of a wind turbine modeled as an actuator disk (AD) subject to a neutral atmospheric inflow are performed. The influence of the turbine position relative to a quasi-two-dimensional Gaussian hill on the maximum power performance and on the response of a torque controller in region two of the power curve is investigated. When the turbine is located at the foot of the hill, the maximum power coefficient increases by 3.5 %. At the top of the hill, the maximum power decreases by 20.0 %. A consequence of this is that, when placing wind turbines on elevated locations, the power does not scale with the cube of the increase of wind speed. It is furthermore found, that the torque controller operates in a way, that local flow angles remain constant, irrespectively of the location of the turbine. Also, the power coefficient based on the disturbed wind speed in the rotor plane remains constant, which does not necessarily coincide with the maximum power coefficient based on the undisturbed wind speed. As a consequence, a torque controller does not track maximum performance in complex terrain. Overall, this study sheds light on the interpretation of performance results of wind turbines in complex terrain and helps to shape efforts to decrease prediction uncertainties for future onshore wind projects.