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Downwind Force-Aligned Rotors for

Extreme-Scale Off-Shore Wind

Eric LothUniversity of Virginia

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Early evaluations of new wind turbine concept

Presentation outlineExtreme-scale Issues & Force alignmentMorphing Pre-AlignmentTower Shadow Issues

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Extreme-Scale Issues & Force Alignment

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

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

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Increasing sizes lead to increasing mass & increasing exponents (2.5+) once gravity

loads start to dominate

Can load alignment help reduce mass?

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

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

Bio-InspirationBio-Inspiration

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Wind Turbine Forces

Load combination of centrifugal (C), gravity (G), and thrust (T) aligned along the blade path via downwind coning

Conventional vs. Load-Aligned

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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)

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Morphing

Morphing in NatureMorphing in Nature

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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)

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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 kgMaterial = based on E-LT-5500 fiberglass (Modulus of elasticity = 41.8 GPaPoisson’s ratio = 0.28Density = 1920 kg/m3

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Fixed-Mass (35.6 Mg)

Reduced-Mass (17.8 Mg) & Increased Length

Conventional (35.6 Mg)

Von-Mises Stresses

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Increasing Length Benefits

Morphing increases capture area below rated; makes up for losses due to coning; advantage will increase as size increases

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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 realizedHinging & Actuating Rotor Blades also introduces additional control complexity and hub design complexity

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Pre-Aligned

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Pre-Aligned in NaturePre-Aligned in Nature

Fix load alignment to reduce cantilever moments (some aero-elastic flexing to adapt even further but no hinges)

Fix load alignment to reduce cantilever moments (some aero-elastic flexing to adapt even further but no hinges)

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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

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Tower Shadow Issues

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

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

Previous StudiesCylinder 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)

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Drag Reduction for an Inscribed Diameter

Airfoils have much lower drag than ellipses or cylinder

22Minimum drag for ~ 3:1 chord-to-thickness ratio

Select Pressure Distributionsα = 0°, 3% Fixed Transition

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PIV Setup

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Flow Direction

Y

X Interrogation Window

Laser Sheet

Spherical and Cylindrical Lenses

Mirror

Laser Source

Model

u Velocity Profile (X = 2D)

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E863but at 10°

C30u

C30u

Cylinder

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or Self-yawing tripod for off-shoreDownwind 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 PadCabled Pad

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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