Structural Design of Composite Blades for Wind and Hydrokinetic Turbines

Danny Sale and Alberto Aliseda

Northwest National Marine Renewable Energy Center

Dept. of Mechanical Engineering

University of Washington

Feb. 13, 2012

Outline

2

•Previous Work

–coupled aero-structural optimization (HARP_Opt code)

–simple structural model

•Newly Developed Structural Analysis Tool (CoBlade)

–methodology & applications

•Structural Optimization

–problem formulation

–design of composite blade for tidal turbine

•Recommended Future Work

Previous Work: HARP_Opt

3

flow speed (m/s)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

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5

10

15

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30

35

0

5

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0

5

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20

25

30

35

40

45

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Rotor SpeedBlade Pitch

Prated CPmax

ωmin

ωmax

Horizontal Axis Rotor Performance Optimization • given: turbine & environmental specifications

• optimizes: blade shape, rotor speed & blade pitch control

• satisfying: maximum Annual Energy Production (AEP)

performance constraints (power, cavitation, etc.)

Previous Work: HARP_Opt

4

Bending Strain:

Simple Structural Model

• Thin-shelled cantilever beam

•One material w/ isotropic properties

• Bending strain is only constraint

• Shell thickness is only design variable

Previous Work: HARP_Opt

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Coupled Aerodynamic-Structural Optimization

•maximize energy production & minimize blade mass

• genetic algorithm identifies set of Pareto-efficient designs

Moving Forward: Structural Design

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Develop a tool capable of modeling realistic composite blades

Image: www.Gurit.com

Overview of CoBlade software

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Structural Analysis and Design of Composite Blades

• realistic modeling of composite blades –arbitrary topology & material properties

• computes structural properties –stiffnesses: bending, torsional, axial

– inertias: mass, mass moments of inertia

–principal axes: inertial/centroidal/elastic principal axes

–offsets: center-of-mass, tension-center, shear-center

• structural analysis tool –arbitrary applied loads & body forces

– recovery of 2D lamina-level strains & stresses

–blade deflection & modal analysis

– linear buckling analysis

• optimization of composite layup

Image: replica of Sandia SNL100-00 wind turbine blade using CoBlade

Methodology

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Classical Lamination Theory (CLT)

• describes mechanical response of laminated plates

Image: G.S. Bir. “PreComp User’ Guide”

Classical Lamination Theory + Euler-Bernoulli beam model + shear flow

extensional stiffness

coupling stiffness

bending stiffness

Methodology

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Composite Euler-Bernoulli Beam and shear flow approach

• describes global mechanical behavior of composite beam

Convert Beam Stresses into Equivalent Plate Loads

• recover 2D strains & stress at lamina level

s

f3

f1

f4

f2

Methodology

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[1] G.S. Bir, 2005. “User’s Guide to BModes: Software for Computing Rotating Beam Coupled Modes,” NREL TP-500-38976, Golden, CO: National Renewable Energy Laboratory.

Linear Buckling Analysis

• pinned boundary conditions (conservative)

• contributions from panel stiffness, curvature, thickness, & width

shear web panels top/bottom surface panels

Modal Analysis

• BModes: Rotating Beam Coupled Modes (NREL code)

beam divided into finite elements

Optimization: Composite Layup

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blade-root

blade-shell

spar-uni

spar-core

LEP-core

TEP-core

web-shell

web-core

Material Legend:

E11

E22

G12

ν12

ρ

σ11,fT

σ11,fC

σ22,yT

σ22,yC

τ12,y

root build-up

blade tip

LEP TEP spar

transition

to LEP

transition

to TEP

transition

to spar

shear webs

Material Properties: Failure Stresses:

Composite Layup

• all laminates balanced & symmetric

• high & low pressure surfaces symmetric

• identical shear web laminates

Optimization: Design Variables

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Design Variables

• spar-cap width at inboard & outboard stations

• lamina thicknesses along blade length

Optimization: Objectives & Constraints

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= BladeMass * minimize:

subject to:

penalty factors for maximum stress

penalty factors for buckling under compression & shear

penalty factor for tip deflection

penalty factor for separation of blade freqs. & rotor freq.

constraints ensure feasible geometry

Optimization: Example Design

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0

5

10

15

20

25

30

0

400

800

1200

1600

2000

0.0 1.0 2.0 3.0

Bla

de P

itch (

deg)

Root

Bend

ing M

om

ent

(kN

-m)

Flow Speed (m/s)

Root Moment

Blade Pitch

pre

ssure

(P

a)

turbulent eddy

Image: ref [2]

[1] M.J. Lawson, Y. Li, and D.C. Sale, 2011. “Development and Verification of a Computational Fluid Dynamics Model of a Horizontal Axis Tidal Current Turbine.” The 30th International Conference on Ocean, Offshore and Arctic Engineering.

[2] G.S. Bir, M.J. Lawson, and Y. Li, 2011. “Structural Design of a Horizontal-Axis Tidal Current Turbine Composite Blade.” The 30th International Conference on Ocean, Offshore and Arctic Engineering.

Composite Blade Design for Tidal Turbine

• hydrodynamic design: Department of Energy Reference Tidal Current Turbine, ref. [1]

• design loads: extreme operating conditions in Puget Sound, WA., ref. [2]

Optimization: Results

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CoBlade is fast: single evaluation: ~1 sec, total optimization: ~40 min

Blade mass is minimized, final iteration satisfies all constraints (no penalties)

Optimization: Results

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normal stress, σzz (MPa)

shear stress, | τzs | (MPa)

buckling criteria, R

-400 -300 -200 -100 0 100 200 300

5 10 15 20 25 30

0 0.2 0.4 0.6 0.8 1

root build-up material

spar-cap material

core material

blade-shell material

Top Surface Lamina Stress Failure Criteria

Optimization: Results

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0

50

100

150

200

0

50

100

150

200

0

500

1,000

1,500

2,000

Axial Stiffness

Torsional Stiffness

0

100

200

300

flapwise

edgewise

-0.14

-0.09

-0.04

0.01

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

1st mode: 8.72 Hz

2nd mode: 18.32 Hz

3rd mode: 27.73 Hz

mas

s (k

g/m

) ax

ial

stif

fne

ss (

N)

x10

6

tors

ion

al

stif

fne

ss (

N-m

2)

x10

6

be

nd

ing

sti

ffn

ess

(N

-m2)

x10

6

flap

d

isp

lace

me

nt

blade length (m)

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Conclusions • Capable structural design tool, modeling of complex layups possible with CoBlade

•NOT a replacement for higher-fidelity FEM, but very effective for preliminary design work

• Limited validation studies

–excellent agreement for analytically obtainable results

–good agreement with ANSYS FEM model of tapered composite beam (collaboration w/ Penn. State)

Future Work • Preliminary results seem reasonable, but require further validation

–anisotropic layups

–buckling

–lamina-level strains/stresses

• Repeat coupled aero-structural optimization (HARP_Opt) with structural capabilities of CoBlade

• Include cross-coupled terms from CLT into beam equations

• Public release of CoBlade code & documentation

Thank you! Questions?

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Acknowledgements

This work has also been made possible by

• National Science Foundation Graduate Research Fellowship under Grant No. DGE-0718124

• Department of Energy, National Renewable Energy Laboratory

• University of Washington, Northwest National Marine Renewable Energy Center

Dr. Mark Tuttle (University of Washington)

Matt Trudeau (Pennsylvania State University)

Extra

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XR

YR

SC

CM

TC

xtc ytc

xcm

ycm xsc

ysc

θaero θelastic θinertial θcentroidal

reference plane (blade coordinate system x-axis)

chord line

centroidal principal axes

inertial principal axes

elastic principal axes

section

reference axes

R