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Stiftungslehrstuhl Windenergieam Institut für Flugzeugbau
Design Challenges for Bend Twist Coupled Blades for Wind Turbines: and application to standard blades2012 Sandia Wind Turbine Blade Workshop
Mark Capellaro
Phd Researcher
Chair of Wind Energy (SWE)
University of Stuttgart, Germany
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Table of Contents
• Introduction
• Bend twist coupled wind turbine blades• Load reduction potential• background research
• Improving blade performance through better models
2
Design Challenges for Bend Twist Coupled Blades for Wind Turbines: and application to standard blades
Rotor Plane
Vo
Rotor Plane
Vo
Vo + V
oV
o + V
oV
o - V
oV
o - V
o
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• Visiting researcher Sandia National Labs
• Doctoral Candidate from the University of Stuttgart Faculty of Aerospace Engineering and Geodesy (Diss. Submittal 3/12).
• Topic: BTC Blades
• Master of Science in Wind Energy from the Danish Technical University/Risoe
• Bachelor of Science Mechanical Engineering, University of Washington
• Bachelor of Arts, Economics and South Asian Studies, University of Wisconsin – Madison.
Who (1): Mark Capellaro
3
• Design Engineer, Wind Turbine Company, Seattle, WA (2000-2002)
• Vestas Intern/Test Engineer, IOW UK (2005)
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Bend Twist Coupling: Explained
9
The coupling between the bending and torsional deflection of a wind turbine blade.
Also known as ‘aero-elastic tailoring‘.
In wind turbine applications, we want the flap or out of plane bending to force the blade to twist along the long axis of the blade (torsion).
The torsional deflection towards feather, can be used to lower the loads.
Blade bend twist coupling towards feather – lowering the angle of attack
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The rotor will twist or pitch itself with a change in wind conditions.
+ Positive Gust (more wind) blade twists to lower the angle of attack.
- Negative Gust (less wind) blade twists to increase angle of attack.
10
Rotor Plane
Vo
Rotor Plane
Vo
Vo + V
oV
o + V
oV
o - V
oV
o - V
o
Bend Twist Coupling: Explained
Equilibrium Condition Vo + Vo (+ Gust) Vo - Vo (- Gust)
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An airfoil generates Lift and Drag forces based on the relative wind speed (VREL), length of airfoil (c), air density () and lift and drag coefficients (CL, CD).
Formulas:
Bend Twist Coupling: Explained
11
2
2
12
12
L REL
D REL
Lift C c v
Drag C c v
Aero loads directions along blade.
Vo
U=R
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Formulas:
The coefficients of Lift and Dragare dependent upon the angle of attack () the airfoil chord line makes with the wind (VREL).
Bend Twist Coupling: Explained
12
2
2
12
12
L REL
D REL
Lift C c v
Drag C c v
-45 -30 -15 0 15 30 45 60 75 90-1
-0.5
0
0.5
1
1.5
2
Co
effi
cien
ts [
-]
A - Angle of Attack [deg]
CL - Lift
CD
- Drag
Typical aero properties for wind turbine blade tip section profile
(~18% thick NACA)
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-15 -10 -5 0 5 10 15 20 25 30 35 40 45-1
-0.5
0
0.5
1
1.5
2
Co
effi
cien
ts [
-]
A - Angle of Attack [deg]
CL - Lift
CD
- Drag
Bend Twist Coupling: Explained
13
Typical aero properties for wind turbine blade tip section profile
(~18% thick NACA)
A wind turbine blade operates in this region of angles of attack under normal operating conditions.
Lowering the angle of attack lowers the lift, lowering the blade loading.
Formulas: 2
2
12
12
L REL
D REL
Lift C c v
Drag C c v
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• First suggested by Karaolis – UK in 1988 as a method for control of a small turbine.
• Stoddard et al. - USA found bend twist coupling in existing blades (1989).
• ECN – Netherlands studied the use of twist to stall blades for increasing power production (ca. 1996).
• Sandia – USA various models for pitch and stall turbines (1996-2003)
• aero instability check (flutter and divergence)
• Use on modern turbines with pitch systems
Background Research:
14
• Sandia/Wetzel Engineering/GEC –USA finite element calculation of potential twisting based on composite bend twist coupled blades (2002-2005)
Sou
rce:
Kar
aolis
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The Sandia papers determined that coupling to stall was not a very good idea and that towards feather could increase power production (if the blade was lengthened) without increasing loads.
The FE modeling determined that the lay up and twisting was limited based on theoretical limits and material limits.
Wetzel/Griffin did the most work in designing composite lay ups for bend twist coupled blades.
Background Research:
15
Recommendations:
• Twist towards feather
• Load reduction instead of turbine control
• Only outer portion of blade to be coupled.
FE blade model (FAROB WMC/ECN)
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Classical Laminate TheoryThe anisotropic properties of fiber composite materials can be used to create coupled lamina materials.
The amount of coupling in a lamina or simple laminate can be calculated with Classic Laminate Theory.
Coupling is inversely correlated with bending stiffness in a lamina*.
*Above a certain angle, the coupling decreases, along with the bending stiffness.
Bend Twist Coupling: How
16
-40 -30 -20 -10 0 10 20 30 40-100
-50
0
50
100
Fiber Angle [deg]S
tiffn
ess
and
Cou
plin
g [\%
max
]
BendingCoupling
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The more anisotropic the material, the more potential for coupling.
The limit of coupling () is based on the bending stiffness and torsional stiffness.
Early simulations used to estimate the coupling.
Bend Twist Coupling: How
17
-60 -40 -20 0 20 40 60-80
-60
-40
-20
0
20
40
60
80
Fiber Angle [deg]
Alp
ha V
alue
[\%
max
]
CarbonGlass
1 1
gEI GJ
g EI GJ
b
t
MEI g bMg GJ t
Lamina Analysis (simple UD material)
Here is reverse engineered from the g value in the stiffness matrix.
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Blade models were created with various fiber angles, based on recommendations from previous research.
Blades created in VABS and converted to Phatas wind turbine aerodynamic simulation code from ECN.
Bend Twist Coupling: Turbine Modelling
18
Model Name Spar Cap UD Fiber AngleM0C2 P00 0 degree (baseline blade)M0C2 N05 -5 degreesM0C2 N10 -10 degreesM0C2 N15 -15 degreesM0C2 N20 -20 degrees
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Coupling in a wind turbine blade is
created by creating an asymmetric
lay up in the fibers.
The spar cap uni directional material
is used in these models to create the
asymmetry and coupling.
Using the recommendations from previous research, only the outer section was coupled.
Bend Twist Coupling: How
19
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
GF UDTriaxBiaxFoam1Foam2CF UDGlue
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Bend Twist Coupling: How
20
The potential coupling is dependent upon the material (anisotropic properties), geometry of section and fiber placement.
Better potential near the tip (thinner profiles are used).
R=60m R=35m
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Wind Turbine Blade Structural Analysis
Variational Asymptotic Beam Solver (VABS)FE code developed in the US originally tomodel helicopter rotor blades.VABS Inputs:• Geometry• Material data (E1, E2, Angles..)• Thicknesses• LocationsVABS Outputs (Beam Element inputs)• Stiffness (Flap EI2, Edge EI1 and
Torsion GJ) • Couplings (full matrices for each
section1)• Mass/m• Shear center, neutral axes, mass
properties
3.93E+09 0.00E+00 3.56E+08 -5.26E+080.00E+00 3.50E+07 0.00E+00 0.00E+003.56E+08 0.00E+00 2.27E+08 -4.87E+07-5.26E+08 0.00E+00 -4.87E+07 1.13E+09
1
1
2
0 0 00 00 0 00 0
AEGJ g
EIg EI
VABS Output Example – classic stiffness matrix
*Note that the PHATAS code uses axial, flap andedge twist couplings only.
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1
1
2
0 0 00 0 0
/0 0 00 0 0
AEGJ
kg mEI
EI
Wind Turbine Blade Model
Wind turbine aero-elastic simulation codes use beam elements (usually modally reduced) to model the blades.
Phatas allows for the coupled beam elements for the blade model.
Beam input comes directly from VABS structural analysis.
1
1
2
0 0 00 0
/0 0 00 0
AEGJ g
kg mEI
g EI
1
1
2
0 0 00 0
/0 0 00 0
AEGJ g
kg mEI
g EI
1
1
2
0 0 00 0
/0 0 00 0
AEGJ g
kg mEI
g EI
1
1
2
0 0 00 0 0
/0 0 00 0 0
AEGJ
kg mEI
EI
1
1
2
0 0 00 0 0
/0 0 00 0 0
AEGJ
kg mEI
EI
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The NREL 5MW reference turbine was used as the baseline turbine model for the blade properties.
The lay-up was estimated from the blade properties to create the reference lay-up blade.
Bend Twist Coupling: Turbine Modelling
23
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Blade models were created with various fiber angles, based on recommendations from previous research.
Blades created in VABS and converted to Phatas wind turbine aerodynamic simulation code from ECN.
Bend Twist Coupling: Turbine Modelling
24
Model Name Spar Cap UD Fiber AngleM0C2 P00 0 degree (baseline blade)M0C2 N05 -5 degreesM0C2 N10 -10 degreesM0C2 N15 -15 degreesM0C2 N20 -20 degrees
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Results: Power Production (Stationary Analysis)
When the blade twists under loading, the angle of attack changes.
This lowers the CP and power if the blade is not re-tuned for the dynamic twist angle.
Here a standard Weibull wind speed distribution is used to calculate the AEP.
A = 8, k = 1.9
25
98
99
100
101
M0C
2 P
00
M1C
2 N
05
M1C
2 N
10
M1C
2 N
15
M1C
2 N
20
AE
P [%
Bas
elin
e]
Annual Energy Production Comparison
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Results: Tip Deflection (Stationary Analysis)
The rotation of the fiber angle away from the blade axis lowers the bending stiffness (if not reinforced).
The loading decreases, but not at the same rate as the bending stiffness.
Blades with larger fiber angles (> 5 degrees) have more tip deflection.
26
95
100
105
110
115
120
M0C
2 P
00
M1C
2 N
05
M1C
2 N
10
M1C
2 N
15
M1C
2 N
20
Fla
p T
ip D
efle
ctio
n [%
Bas
elin
e]
Tip Deflection Comparison
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Results: Load Reduction (Dynamic Analysis)
The damage equivalent loading is reduced for the increasing fiber angle.
The flap load reduction is reduced by ~4% for the M1C2N15 model.The stress reduction would be greater due to the superposition of both the torsion and bending loads.
27
Edge Torsion Flap 80
85
90
95
100
105
Bla
de 1
DE
L [%
Bas
elin
e]
M0C2 P00M1C2 N05M1C2 N10M1C2 N15M1C2 N20
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Most of the previous results were confirmed (re-learned).
However, the detailed composite modeling (possible with VABS) and aero-elastic simulation (PHATAS) showed limits that were not clarified with earlier research.1.Coupling reduces bending stiffness* increasing tip deflection
2.Bend Twist DOF in simulation reduces expected CP unless blade is retuned.
3.Load reduction is not strictly correlated with fiber angle.
Discussion of Results
28
Proposed Solutions1. Stiffening the blade increases the cost
and changes the coupling parameters(and benefit is hard to calculate).
2. Blade can be re-tuned (twist angle) for induced deflection (ongoing workwith FE-Aeroelasticsimulation/optimization)
3. Coupling is only beneficial in therange where tip deflection isdecreased and/or overall blade rootloading with minimal power decrease(AEP).
4. Increase blade length to meet DEL from uncouped blade (‚grow therotor‘)
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• The small angles provided load reduction, reduced or equivalent tip deflection and approximately the same AEP.
• Larger angles had more load reduction, but increased tip deflection and decreased AEP.
• Ignoring the changing structural properties with changing fiber angle can lead to overly optimistic results.
29
Discussion of Results
98
99
100
101
AE
P [%
Bas
elin
e]
Annual Energy Production Comparison
95
100
105
110
115
120
M0C
2 P
00
M1C
2 N
05
M1C
2 N
10
M1C
2 N
15
M1C
2 N
20
Fla
p T
ip D
efle
ctio
n [%
Bas
elin
e]
Tip Deflection Comparison
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Lessons learned and applications for wind energy.
98
99
100
101
AE
P [%
Bas
elin
e]
Annual Energy Production Comparison
The decrease in annual energy production is due to the torsional deflection of the BTC blade.
Some improvement in the Cp curve is possible by dynamically calculting the structural twist for the blade.
9.5 10 10.5 11 11.5 12 12.5 13
11
11.5
12
12.5
13
Windspeed
Pow
er [M
W]
Standard Twist, Rigid BladeStandard Twist, 3DOF BladeNew Twist, 3DOF Blade
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Preliminary:• The blade shape (twist angle) should be at the optimal angle of attack
under load (when deflected).
• The best wind speed or point of optimization is the peak power density(Weibull distribution x power curve).
• Simple loading (BEM) can be used since the blade twist angle is to beoptimized for steady inflow.
• Method should be valid for any blade with torsional deflection (ie, all blades).
Challenges:• If you generate a BEM loading, load a blade model and observe torsional
deflection, then your preliminary load was wrong.
• Your deformed shape is also wrong (but probably closer to reality).
Blade Shape Improvement: Proposed Method
31
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Numad (Sandia) used to modify S100 blade
ABIT example, swept blade to emphasize twist
32
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Step 1: Generate a more accurate deformed shape.• Load blade model (NuMAD) and data (airfoil lift/drag...)
• Generate BEM load (ANSYS APDL)
• Load blade model
• Determine induced twisting along blade (LE and TE nodes)
• Correct load by altering the blade twist angle but only in theBEM calculation (for speed).
Blade Shape Improvement: Methodology
33
Twist 1 (Betz, Schmitz..) Induced twisting Corrected twist (iteration 2)
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Step 2: Iterate towards a optimal deformed blade shape (twist).• Generate BEM load (ANSYS APDL) with
• Load blade model
• Determine induced twisting along blade
• Correct load by altering the blade twist angle but only in the BEM calculation (for speed).
• Correct blade shape (iterated) should be same as first (undeflected) shape.
Blade Shape Improvement: Methodology
34
Induced twisting back to optimal
Corrected twist (from deformatoin iteration)
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Corrected twist (iteration 2)
ABIT: Aero-elastic Blade Improvement Tool
35
Induced twisting
Corrected twist (from deformation iteration)
Induced twisting back to optimal
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The blade can be optimized for power production if the deformed shape is optimal for the peak of the power density (power curve x wind distribution).
This becomes a site specific turbine.
ABIT: Tune the blade shape for the max power density.
36
4 6 8 10 120
2
4
6
8
10
12
Windspeed
Pow
er [M
W]
Standard Twist, Rigid BladeStandard Twist, 3DOF BladeNew Twist 10ms, 3DOF BladeNew Twist 8ms, 3DOF Blade
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Thank you for your attention!
Mark Capellaro
Stiftungslehrstuhl Windenergie
University of Stuttgart
GermanySpecial thanks to the Sandia team!
37