Some scientific challenges in aerodynamics for wind turbines

Some scientific challenges in aerodynamics for wind turbines Christian Bak Senior Scientist Team Leader: Aerodynamics, aeroacoustics, airfoil and blade design Technical University of Denmark DTU Wind Energy Risø Campus Aeroelastic Design chba@dtu.dk

Content Upscaling Modeling BEM vs advanced models Aerodynamic devices e.g. vortex generators Measurements Full scale measurements Wind tunnel measurements Leading edge erosion New designs Thick airfoils Aerodynamic devices Coupling to noise Low induction rotors 2

Upscaling A key challenge LM 61.5 blade Blade mass [tons] 50 45 40 35 30 25 20 15 10 5 73.5m blade upscaled with x^3 73.5m blade upscaled with x^2.16 DTU-10MW-RWT blade Glasfiber Carbonfiber Upscale from 40m blades with x^3 Power (Glasfiber) Power (Carbonfiber) Mass glass = 0.0023*Length 2.17 Mass carbon = 9E-05*Length 2.95 Upscaling rated power by x 100 in 30 years Upscaling: Square-cube law Power increases by blade length squared Mass increases by blade length cubed 0 30 50 70 90 Blade length[m]

Upscaling An example: DTU 10 MW Reference WT Reference blade designed with existing technology for use in Light rotor project with Vestas and INWIND.EU project Nominal power Rotor configuration Control Rotor diameter Hub height Rated tip speed Blade pre-bend Tower mass Nacelle mass Rotor mass Blade mass 10.0 MW Upwind, 3 blades Variable-speed, collective pitch 178.3 m 119.0 m 90 m/s 3.3 m 628.4 tons 446.0 tons 230.7 tons 41.7 tons http://dtu-10mw-rwt.vindenergi.dtu.dk 4

Modeling BEM vs advanced models Emergency Shutdown Standstill Vibrations 5

Modeling Thick airfoils and 3D correction of airfoil data Test section 2D LM wind tunnel 3D

Modeling Small scale turbulence Tested in LM Wind Power LSWT Effect on e.g.: - Transition correlating to Reynolds number - Inflow correlating to noise - Boundary layer development correlating to noise

Modeling Aerodynamic devices: Vortex generators (1) Engineering model 8

Computational Fluid Dynamics model Modeling Aerodynamic devices: Vortex generators (2) 9

Measurements DANAERO: Measuring full scale quantities 10

Measurements DANAERO 3D/2D polars: Normal to chord, c n

Measurements National wind tunnel in establishment Aerodynamic and aeroacoustic tests Test section: HxWxL=2.2m x 3.3m x 10.0m Maximum flow speed: U max =105m/s Turbulence intensity: TI<0.1% Max Reynolds number on 1.1m chord: Re max =7.7x10 6

Measurements Leading edge roughness/erosion 1.6 c L 1.2 1.6 c L 1.2 0.8 0.4 NACA 64 3-618 Re = 6.0x10 6 0.8 0.4 0.0 0.00 0.01 0.02 c d 0.03-0.4-0.8 0.0-20 -10 0 10 20 30 α ( o ) -0.4 Wrap-around rough Clean -0.8-1.2-1.2 Timmer, W.A.; Bak, Christian; Aerodynamic characteristics of wind turbine blade airfoils. Advances in wind turbine blade design and materials. ed. / Povl Brøndsted; Rogier Nijssen. Woodhead Publishing, 2013. (Woodhead Publishing Series in Energy; No. 47). http://windsystemsmag.com/ 13

New designs Airfoil design: Thick airfoils Light Rotor: The LRP2-30 airfoil Airfoil designed in the EUDP-2010 project Light Rotor in a cooperation between DTU Wind Energy and Vestas

New designs Airfoil design: Thick airfoils (with slats) Test in LM Wind Power wind tunnel A multi-element airfoil was designed and tested The slat was designed using an optimization tool coupled with EllipSys2D. 2D CFD succeeded to a large extent in predicting the correct characteristics.

New Designs Aerodynamic devices: Multi-element airfoils DTU 10 MW RWT: 1-2% increase in power

New designs DTU 10 MW RWT: Winglet, flat-back airfoils, GF 17

New designs Active aerodynamic devices Elastomeric controllable flap activated by pressure in voids 20-30% reduction in bladeand tower fatigue loads Variable trailing edge flap 18

New designs Low induction rotors 12 Two rotors designed by simple means and maintaining thrust at 10m/s and rotational speed: Radius=89.166m, a=0.333, TSR=8.0, CP=49.5% Radius=98.083m, a=0.243, TSR=8.8, CP=46.7% Area increase=21%, CP reduction=5.6% Total power increase at low wind speed=14% AEP increase ~7% 12 Power [MW] 10 8 6 4 2 0 a=1/3 a=0.243 4 6 8 10 12 14 Wind speed [m/s] Chord [m] 10 8 6 4 10MW rotor with a=0.333 10MW rotor with a=0.243 2 0 19 0 20 40 60 80 100 120 Radius [m]

Thank you for your attention 20