Michael Howland

Wake structure in actuator disk models of wind turbines in yaw under uniform inflow conditions

Reducing wake losses in wind farms by deflecting the wakes through turbine yawing has been shown to be a feasible wind farm controls approach. Nonetheless, the effectiveness of yawing depends not only on the degree of wake deflection but also on the resulting shape of the wake. In this work, the deflection and morphology of wakes behind a porous disk model of a wind turbine operating in yawed conditions are studied using wind tunnel experiments and uniform inflow. First, by measuring velocity distributions at various downstream positions and comparing with prior studies, we confirm that the non-rotating porous disk wind turbine model in yaw generates realistic wake deflections. Second, we characterize the wake shape and make observations of what is termed as curled wake, displaying significant spanwise asymmetry. The wake curling observed in the experiments is also reproduced qualitatively in Large Eddy Simulations using both actuator disk and actuator line models. Results suggest that when a wind turbine is yawed for the benefit of downstream turbines, the curled shape of the wake and its asymmetry must be taken into account since it affects how much of it intersects the downstream turbines.

Wind tunnel study of the power output spectrum in a micro wind farm

Instrumented small-scale porous disk models are used to study the spectrum of a surrogate for the power output in a micro wind farm with 100 models of wind turbines. The power spectra of individual porous disk models in the first row of the wind farm show the expected -5/3 power law at higher frequencies. Downstream models measure an increased variance due to wake effects. Conversely, the power spectrum of the sum of the power over the entire wind farm shows a peak at the turbine-to-turbine travel frequency between the model turbines, and a near -5/3 power law region at a much wider range of lower frequencies, confirming previous LES results. Comparison with the spectrum that would result when assuming that the signals are uncorrelated, highlights the strong effects of correlations and anti-correlations in the fluctuations at various frequencies.

Measuring power output intermittency and unsteady loading in a micro wind farm model

In this study porous disc models are used as a turbine model for a wind-tunnel wind farm experiment, allowing the measurement of the power output, thrust force and spatially averaged incoming velocity for every turbine. The model’s capabilities for studying the unsteady turbine loading, wind farm power output intermittency and spatio temporal correlations between wind turbines are demonstrated on an aligned wind farm, consisting of 100 wind turbine models.

Assessing the impact of power rate limitation based wind control strategy

With the DOE 20% by 2030 scenario for wind capacity, wind penetration is on the rise, and intermittency of this energy resource must be addressed since it hurts grid reliability and increases ancillary service needs. To cope with the rising variability, this study analyzed the effects of wind control, specifically absolute power rate limitation (ramping control). A ramping constraint was modeled to cap the increase in energy output of a wind generation unit for a minute interval, simulated using the IEEE 24 bus system. Results of this study include a decrease in variability of the wind output, a decrease in the ancillary service regulation requirement, a decrease in the production cost of the system, a decrease in de-commitments of coal-fired power plants, and a decrease in cycling cost of the system.