The document discusses potential benefits of downwind force-aligned rotors for extreme-scale offshore wind turbines. It describes how load alignment through concepts like morphing blades and pre-aligned blade segments could help reduce structural loads and mass. Steady-state analysis shows morphing could allow major reductions in blade and tower mass. Pre-alignment offers load savings with less complexity than morphing. Aerodynamic shrouds around the tower also help reduce wake effects. More analysis is still needed to determine the feasibility and benefits of these force-aligned downwind concepts.

1. Downwind Force-Aligned Rotors for
Extreme-Scale Off-Shore Wind
1
Eric Loth
University of Virginia

2. 2
Early evaluations of new wind turbine concept
Presentation outline
Extreme-scale Issues & Force alignment
Morphing
Pre-Alignment
Tower Shadow Issues

3. 3
Extreme-Scale Issues &
Force Alignment

4. 4
Integral blades (single piece) become more
and more difficult to fabricate, transport, and
install as extreme scales are approached (R =
120 m)

5. 5
Increasing sizes lead to increasing mass & increasing
exponents (2.5+) once gravity loads start to dominate
Can load alignment help reduce mass?

6. Bio-Inspiration
6
Palm trees have a light segmented trunk
that bends downstream – load-alignment
which reduce cantilever aerodynamic
loads (vs. “stiff” strategy of an oak
tree)
6

7. 7
Wind Turbine Forces
Conventional vs. Load-
Aligned
Load combination of centrifugal (C), gravity (G), and thrust (T)
aligned along the blade path via downwind coning

8. 8
Load Path Angle
Conventional scales accommodate with pre-cone
& shaft-tilt but angles are high at
extreme scale, indicating potential benefits
of downwind alignment (and perhaps of
teetering)

9. 9
Morphing

10. Morphing in Nature

11. 11
Load Path Angle as a function of Speed
Small benefit for small turbines but
significant load-path angles at larger scale,
indicating potential benefits of alignment
(and perhaps importance of teetering for
downwind)

12. 12
First-Order Finite Element Analysis
NREL 5 MW baseline upscaled to 10 MW with a
radius of 82 m Used to compare to morphing & pre-aligned
concepts
Blade mass = 35,800 kg
Material = based on E-LT-5500 fiberglass (
Modulus of elasticity = 41.8 GPa
Poisson’s ratio = 0.28
Density = 1920 kg/m3

13. 13
Von-Mises Stresses
Conventional (35.6 Mg)
Fixed-Mass (35.6 Mg)
Reduced-Mass (17.8 Mg) & Increased
Length

14. 14
Increasing Length Benefits
Morphing increases capture area below rated; makes up
for losses due to coning; advantage will increase as size
increases

15. 15
Conclusions for Steady-State Morphing Analysis
But …
Dynamics, Gusts and Fatigue tend to drive structural mass
and so the above savings my be lost or only partially
rHeianlgizinegd & Actuating Rotor Blades also introduces
additional control complexity and hub design complexity

16. 16
Pre-Aligned

17. 1177
Pre-Aligned in Nature
Fix load alignment to reduce cantilever
moments (some aero-elastic flexing to
adapt even further but no hinges)

18. 18
Fixed Mass Pre-Aligned Blade
Zero moment nodes induced over 4 segments by setting proper
alignment angles such that resultant force of each segment is
tension aligned

19. 19
Tower Shadow Issues

20. Aerodynamically faired shroud around tower to
substantially reduce wake effects and thus blade
fatigue, and uses passive shroud alignment for
yaw
20

21. Previous Studies
Cylinder drag reduction w/ flow control:
Lee et al. (2004)
Hwang and Yang (2007)
Sosa et al. (2009)
Triyogi et al. (2009)
Mashud et al. (2010)
Blade active load control:
Baker et al. (2007)
Cooperman et al. (2013)
Faired wind turbine tower:
Hand et al. (2001)
Janajreh et al. (2010)
21

22. Drag Reduction for an Inscribed Diameter
1
0.8
0.6
0.4
0.2
0
0 0.2 0.4 0.6 0.8 1
Cd,D
D/c
(ReD=1x105)
Ellipse 1e5)
NACA Airfoil (ReD=1x105)
1e5)
NACA Airfoil (ReD=8.3e6)
Cylinder with flow control
8.3x106)
Cylinder with
no flow control
Airfoils have much lower drag than ellipses or cylinder
22
Cd,D º Fd '
1
2rV2D
Minimum drag for ~ 3:1 chord-to-thickness ratio

23. Select Pressure Distributions
α = 0°, 3% Fixed Transition
-1.5
-1
-0.5
Cp x/c
0
0.5
1
0 0.2 0.4 0.6 0.8 1
NACA0033
E862
E863
C30u
C31u
23

24. PIV Setup
24
Flow Direction
Y
X Interrogation
Window
Laser Sheet
Spherical and
Cylindrical Lenses
Mirror
Model
Laser Source

25. u Velocity Profile (X = 2D)
25
E863
C30u
C30u
but at 10°
Cylinder

26. 26
or Self-yawing
tripod for off-shore
Downwind rotor
on a floating tripod
with cabling for
yaw so downwind
force aligns
turbine to the wind
and truss
structure reduces
net tower wake
Cabled Pad

27. 27
CONCLUSIONS
Steady-state analysis for morphing concept may
allow substantial reductions in structural blade and
tower mass at extreme-scales; but, hinge system
may be heavy & complex
Pre-alignment reduces complexity while retaining
most of load savings; however, still have shadow
effects
Aerodynamic shrouds can dramatically reduces
wake effects; however, wind angle changes effects
and downwind tower loads may be problematic
Much more work needed to determine relative
feasibility for force-aligned downwind systems