Abstract
In this study, horizontally periodic large eddy simulations (LES) are utilized to study turbulent atmospheric boundary-layer flow over wind turbines in the far-downstream portion of a large wind farm where the wakes have merged and the flow is fully developed. In an attempt to increase power generation by enhancing the mean kinetic energy (MKE) entrainment to the wind turbines, hypothetical synthetic forcing is applied to the flow at the turbine rotor locations. The synthetic forcing is not meant to represent any existing devices or control schemes, but rather acts as a proof of concept to inform future designs. The turbines are modeled using traditional actuator disks, and the unconventional synthetic forcing is applied in the vertical direction with the magnitude and direction dependent on the instantaneous velocity fluctuation at the rotor disk; in one set of LES meant to enhance the vertical entrainment of MKE, a downward force is prescribed in conjunction with a positive axial velocity fluctuation, whereas a negative axial velocity fluctuation results in an upward force. The magnitude of the forcing is proportional to the instantaneous thrust force with prefactors ranging from 0.1 to 1. The synthetic vertical forcing is found to have a significant effect on the power generated by the wind farm. Consistent with previous findings, the MKE flux to the level of the turbines is found to vary along with the total power produced by the wind turbine array. The reverse strategy of downward forcing of slow axial velocity flow is found to have almost no effect on the power output or entrainment. Several of the scenarios tested, e.g., where the vertical force is of similar magnitude to the horizontal thrust, would be very difficult to implement in practice, but the simulations serve the purpose of identifying trends and bounds on possible power increases from flow modifications through action at the turbine rotor.
Highlights
To meet the worldâs growing energy demands while minimizing the emission of greenhouse gases, developers around the world have installed over 35 GW of wind energy capacity in the year 2013 [1].These new wind turbines are often installed in clusters or arrays to take advantage of available land and infrastructure, but when wind turbines are placed in proximity to one another, the wakes of upstream turbines can reduce the available energy to nearby downstream turbines, while simultaneously increasing undesirable small-scale turbulent fluctuations
Regardless, at this stage, the investigation is primarily intended as a proof of concept to identify whether such synthetic forcing near the turbine disk could have an appreciable effect on wind farm performance and the flowâs rate of vertical entrainment of mean kinetic energy (MKE), while maintaining other control parameters fixed
We examine the kinetic energy balance to determine if the wind farm power extraction corresponds to an expected increase in kinetic energy entrainment and the extent to which the turbulent dissipation negates these effects
Summary
To meet the worldâs growing energy demands while minimizing the emission of greenhouse gases, developers around the world have installed over 35 GW of wind energy capacity in the year 2013 [1]. Tilting, yawing, turning the blades; see [22]) to induce a downward force when encountering high-speed streamwise flow, for example, has the potential to enhance the already existing turbulent transfer This effect would be especially pronounced for very large wind farms in which the atmospheric boundary layer has found an equilibrium with the wind turbines; in this state, the power tends to level off (as seen in [2]) as the flow becomes well mixed, and the turbine wakes are less able to recover by horizontal expansion. The results are intended to provide information about the possibilities and inherent limitations of using modified rotor designs or control schemes to affect the entrainment of kinetic energy in the wind turbine array boundary layer. Since the motivation for this operational strategy is to enhance the kinetic energy entrainment, we measure the degree to which the entrainment changes and whether this change is coincident with a change in the turbulent kinetic energy dissipation (or the production of small-scale turbulence) in the flow
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