Conceptual research of a downwind turbine, based on a Suzlon 2 Conceptual research of a downwind turbine, based on a Suzlon 2.1MW onshore turbine Gesine Wanke* & **, Torben J. Larsen**, Morten Hansen***, Thomas Buhl*, Jens I. Madsen*, Leonardo Bergami* e-mail: gesine.wanke@suzlon.com or: geswan@dtu.dk * Suzlon Blade Science Center, Havneparken 1, 7100 Vejle ** DTU Wind Energy, Frederiksborgvej 399, 4000 Roskilde *** SDU Mads Clausen Institute, Alsion 2, 6400 Sønderborg

Why downwind? Potential Risk: Blade – tower clearance  More flexible blades  Less material Passive yaw alignment  Reduction in LCOE Risk: Tower passage  Noise  Fatigue loads Alignment center of gravity to thrust  Extreme load Power reduction

Research Project How much can be saved on LCOE by a change of conept? Industrial PhD-project Start Dec. 2016, End Dec. 2019 Suzlon Blade Science Center & DTU Conceptual research Baseline: existing 2.1MW onshore turbine Changes in design driving loads Stability of a free yawing turbine Change of the rotor design Tower shadow (noise and fatigue load) alleviation Control strategy Industrial PhD IEA Wind task 40 Cooperation between Company & University founded by Innovation Fund We would like to join the IEA task with our project  Jump to slide 9 if there is only 10 minutes

Change in design driving load cases wind direction upwind turbine S111 class IIIA 2.1MW turbine 1 turbine – 2 configurations Design load bases (DTU)[1] HAWC2 simulations Basic DTU controller [2] downwind turbine wind direction Both Configurations: Cone and tilt away from the tower Difference: Prebend towards Suctions side: more initial tower clearance uw, less initial tower clearance dw According to IEC: Inclination angle = 8°, mass and aerodynamic imbalance DTU controller: adjusted routine for stuck pitch angle 2.2y

Tower clearance – Design driving load cases Normalized tower clearance Upwind DLC 2.2y 0.21 Downwind DLC 2.1 0.13 Decrease to 62% wind direction upwind turbine downwind turbine Normalized with the unloaded tower clearance of upwind configuration Design driving load cases Highly loaded blades upwind Unloading situations downwind DLC 2.2y abnormal yaw error DLC 2.1 grid loss during operation

Extreme load – Changes in design driving load cases Tower bottom flange moment Longitudinal DLC 4.2 to 6.2 +20% Lateral DLC 6.2 +22% Torsion DLC 1.3 -27% Blade root moment Flapwise DLC 1.3 -18% Edgewise DLC 2.2y - 5% Torsion DLC 2.2y - 5% Main bearing moment Tilt DLC 1.3 to 6.2 +74% Yaw DLC 7.1 to 2.2y -22% DLC 1.3, DLC 2.2y, DLC6.2 are cases with highest loads from yaw error  how about a free yawing turbine? Tower: effect of alignment of thrust and gravity force Blades: effect of coning downwind DLC 1.3 extreme turbulence DLC 2.2y abnormal yaw error DLC 4.2 shut down during gust DLC 6.2 idling at abnormal yaw error DLC 7.1 parked in 1y extreme wind

DLC 6.2 - Idling at large yaw errors after grid loss No rotation: Blade gets stuck behind tower Yaw range -40° to -50° Balance of blade forces Idling: Change of tower shadow model Maximum pitch angle Feasibility? Tried to : Take out imbalances mass and aerodynamic Change the turbulence seed Change the yaw error slightly Feasability: Aoa in ranges where we do not trust the airfoil data Blade aerodnamically approximated as a line, while tower shadow model with pure reduction in velocity

Fatigue load – Change in life time equivalent load Tower bottom flange moment Longitudinal -3% Lateral +4% Torsion +0% Blade root moment Flapwise +5% Edgewise +7% Torsion +8% Main bearing moment Tilt +7% Yaw -2% 98% to 99% of life time equivalent load from DLC 1.2 – normal operation Effect from tower shadow Effect from alignment of inclination angle and tilt angle

Free yawing concept Would a free yawing concept give a cost advantage? Highest loads from cases with yaw error A yaw drive costs about 1 – 2% of capex Four research questions: Are all modes positively damped within the operational range? Do the models agree on the damping? What would give a stability boundary at lower wind speeds? Does the system align itself with the wind direction? Yaw error on DLC 1.3, DLC 2.2y and DLC 6.2 You never get rid of the full yaw drive due to the unwindidng mechanism Maybe delete question 2 and just mention it, presentation might get too long.

Stability of a free yawing turbine Free yawing model without bearing friction 2 DOF analytical model for parameter study HAWCStab2 for comparison and extension of DOFs HAWC2 for alignment study wind direction Motivation for 2DOF model: Analytical expressions in the 2 by 2 matrices ”See” the parameters influencing the solution Study the change in damping due to varyation of parameter

Yaw stability - analytical 2DOF model Ls Lcg Wind direction gc y x q(t) Side view Top view 2DOFs: ux(t) and q(t) BEM for Aerodynamic forces Linearization around 0° yaw Parameter study gc +/-10° Ls +/- 50% Lcg + 7 /-0.5 Damping from eigenanalysis ux(t) Assumption: No tilt (-> (+stiff in tilting) no gravity influence) No structural damping No bearing friction Aerodynamically straight blades  Quasi steady aerodynamics  The steady state is at 0° Limitations: It does NOT predict the correct values of the damping for the full turbine It DOES show the influence of parameter variation on damping Straight blades No tilt No structural damping

Yaw frequency and damping - Comparison with HAWCStab2 Comparison between analytical 2DOF model and HAWCStab2 Reasonable agreement in frequency and eigenvalue Stable yaw mode Yaw frequency for v>44m/s This slight could as well be deleted for time reasons and the Questions could be reduced to 3 Yes, the models agree on the damping and I can use it for the parameter study. If f=0Hz than damping = 100% or -100%

Stability boundary for yaw mode - Parameter study Cone angle Stabilizing effect Instability for negative cone angles Length parameter Minor influence from Lcg and Ls Minimum damping 75% No instability Positive cone away from the tower Negative cone towards the tower Sharp change in damping because f=0Hz A negative or too low cone makes the yaw mode instable

Stability boundary- HAWCStab2 extension of 2DOF model Including blade geometry and rotor flexibility Instable yaw mode v>23m/s Including full turbine flexibility and structural damping Instable yaw mode at v>18m/s To calculate the stability boundary you NEED to include the rotor flexibility And better: Full turbine flexibility  The flexibility of the rotor will give a stability boundary at low wind speeds

Aligning with the wind direction HAWC2 Released yaw in constant wind field after 300 seconds Misalignment due to tilt Minor change with full model Controller reduces misalignment Low yaw frequency v>17m/s Power loss below rated wind speed Stability vs alignment Stable= positively damped motion, not necessary at 0° Without controller = predescribed pitch and rotational speed No load assesment done for these high yaw misalignments But we would expect very high loads for the operation at large yaw errors

Summary- free yawing turbine Stabilizing effect from cone 2DOF model is insufficient Instability for high wind speeds Misalignment with wind direction Controller can reduce misalignment Loss in power Yaw mechanism for cable unwind Future work with yaw controlled turbine in normal operation.

Future work and cooperation with IEA wind task 40 Cost out on existing rotor New rotor design Tower shadow alleviation Control strategy Cooperation proposal Tower shadow/ noise modeling Testing models in simulation environment PhD-exchange What are our next steps? And how would we like to cooperate with the IEA task? – Let’s read that document about what they will do...  And let’s discuss it BEFORE I go  They might ask if we would test on a real turbine. And the answer is: maybe.

Thank you, for your attention.

References [1] Hansen, M. H., Thomsen, K., Natarajan, A., & Barlas, A. (2015). Design Load Basis for onshore turbines -Revision 00 [2] BasicDTUController, https://github.com/DTUWindEnergy/BasicDTUController