Manipulating The Flow Through Wind Farms To Increase Their Efficiency

Wind farms are adversely affected by the very wakes that are a necessary by-product of their energy extraction process. While turbines that are most upstream produce at or near their maximum power production, those downstream suffer from the reduction in available energy and increase in turbulence created by the upstream turbines. Methods to combat these effects generally either attempt to increase the available energy to the downstream turbines or mitigate the effects of the upstream turbines' wakes. Wake dissipation also requires entrainment of energy from the ambient flow into the wake, so, in fact, wake mitigation and increasing available energy are two sides of the same coin. To explore methods that would improve wind farm efficiency, we performed model-scale wind turbine experiments while recording their power production and measuring the flow. Accurately measuring power production is the subject of the first paper in which a high-accuracy torque transducer was designed and validated to facilitate mechanical power measurements with very low uncertainty. This transducer was necessary to obtain non-intrusive measurements of torque via a calibration to a turbine's current output. In the second paper, a wake mitigation technique, called dynamic induction control, was studied. With this technique, the set point of the turbine is periodically varied in an effort to trigger or accelerate instabilities in its wake. Contrary to existing literature that indicated that the oscillation frequency would be critical, results showed that the amplitude of oscillation, which corresponds to the turbine's rotation rate, had the largest effect. We theorize that switching to higher rotation rates reduces the pitch of the tip vortex helices promoting more destructive interactions among them. Accelerating the decay of the tip vortices allows for greater mixing with the ambient flow and ultimately accelerated wake decay compared to conventional steady operation. Finally, in the last paper, static axial induction control was studied with an array of five turbines while measuring the flow and turbine power in an effort to understand the fluid dynamics associated with the increase in power. In static axial induction control (AIC), upstream turbines are derated in an effort to maximize the total power production of the array, though not all turbines operate at their individual optima. While data from the power production of individual turbines and flow measurements are difficult to interpret, the total power production of the treatment using AIC was successfully increased. Flow measurements indicate that unharvested energy in the array was redistributed to the edges of the wakes as hypothesized.