Evangelos Ploumakis MSc Student Delft University of Technology Faculty of Aerospace Engineering
Wind Energy Section Kluyverweg 1 2629 HS Delft The Netherlands eploumakis@yahoo.gr
lr.tudelft.nl
Enhanced Kinetic Energy Entrainment in Wind Farm Wakes – LES Study
of a Wind Turbine Array with Tethered Kites
Evangelos Ploumakis1, Dhruv Mehta1,2, Lorenzo Lignarolo1, Wim Bierbooms1 1Faculty of Aerospace Engineering, Delft University of Technology
2Energy Research Centre of the Netherlands
Wake effects in wind farms are a major source of power production losses and fatigue loads on the rotors. It has been demonstrated that in large wind farms the only source of kinetic energy to balance that extracted by the turbines is the vertical transport of the free-stream flow kinetic energy from above the wind-turbine canopy [1]. In the present study, the possibility to enhance such pro-cess by introducing kites in steady flight within the wind-turbine array is studied with numerical simulations. An aligned array of four wind turbines is simulated within the LES framework available in the computational fluid dynamics (CFD) code FLUENT. The turbines are placed in a pre-generated turbulent atmospheric boundary layer (ABL) and modelled as actuator discs with both axial and tangential inductions, to account for the wake rotation [2]. The inter-turbine spacing is six rotor diameters while a kite is suspended three diameters downwind each wind turbine, 0.25 diameters above the top-tip height. The kite is modelled as a body force on the flow, equal in magni-tude and opposite in direction to the vector sum of the lift and drag forces acting on the kite. The introduction of the kite system is found to have a significant effect on the power generated by the wind turbine array primarily
due to the enhanced kinetic energy entrainment in the lower part of the ABL. The simulation results, consistent with previous findings [3], clearly suggest that the applied forcing is capable of redirecting the free-stream flow and assist wake recovery. Follow-up research on kites placed in a "fully developed" [4] wind turbine array boundary layer, as well as coupled CFD/RBD (rigid-body dynamics) simulations on kite flight paths (i.e. crosswind) in terms of kinetic energy entrainment in the wake is suggested. References:
[1] Cal R. B., Lebrón J., Castillo L., Kang H. S., Meneveau C.: Experi-mental study of the horizontally averaged flow structure in a model wind-turbine array boundary layer. Journal of Renewable and Sus-tainable Energy, Vol. 2, No. 1, (2010)
[2] Porté-Agel F., Wu Y. T., Lu H., Conzemius R. J.: Large-eddy sim-ulation of atmospheric boundary layer flow through wind turbines and wind farms. Journal of Wind Engineering and Industrial Aero-dynamics, Vol. 99, No. 4, pp. 154-168 (2011)
[3] VerHulst C., Meneveau C.: Altering Kinetic Energy Entrainment in Large Eddy Simulations of Large Wind Farms Using Unconventional Wind Turbine Actuator Forcing. Energies, Vol. 8, No. 1, pp. 370–386 (2015)
[4] Calaf M., Meneveau C., Meyers J.: Large eddy simulation study of fully developed wind-turbine array boundary layers. Physics of Fluids, Vol. 22, No. 1 (2010)
