design of wind turbines - uniroma1.it. fmec wind turbin… · ideal wind turbine [1] real wind...

Design of Wind Turbines

Alessio Castorrini(i), Alessandro Corsini

Department of Mechanical & Aerospace EngineeringSapienza University of Rome, Italy

(i) now School of Engineering

University of Basilicata, Potenza, Italy

DESIGN OF WIND TURBINE BLADES

Index

• Basic conceptso Ideal blade and Real blade

o Main aspects and parameters

o Design constraints

o Design Strategy

• Aerodynamic designo Methods

o Constraints and practical considerations

o Optimization

• Structure designo Loads

o Materials

o Structures

• Testing and certificationo Standards

o Tools

o Testing

• New concepts

19/12/2019Titolo Presentazione Pagina 3

BASIC CONCEPTS

Wind turbine blades design


19/12/2019Co- and tri-generation: basic concepts, technologies, and applications

Pagina 5

Introduction

Ideal wind turbine

[1]

Real wind turbine design issues

An ideal wind turbine can be seen as an

actuator disk which captures the maximum

possible amount of energy from the wind flow

crossing its section

The efficiency of an ideal wind turbine is given

by the Betz limit.

Physical limit, no losses

= 0.593 …

Aerodynamic issues:

• Finite number of blades

• Inflow not constant in space (boundary layer) and time (turbulence)

• Rotational flow forces (wake rotation)

• Viscous flow (aerodynamic drag)

• Tip losses

Structural issues:

• Weight

• Ultimate and Cyclic loads

• Connection to other parts and load transferring

• Protection and aging

Control issues:

• Variable velocity

• Variable geometry

• Variable pitch

Logistic issues:

• Transport

• Erection

• O&M

Technological issues:

• Manufacturing techniques

• Monitoring

…and the most important: the economic sustainability


Design input parameters: rotor nominal power

=

The power density defines the geometrical

size of the blade and the nominal speed

=1

2!"

#$

• 0.3 < Cpmax < 0.5, O(100)

• 1.0 < ! < 1.225 [kg/m3], O(100)

• A ~ L2 , 1 < L < 100 [m], O(101 – 104)

• Vn3, 3 < Vn < 20 [m/s], O(102 – 104)

If we fix a nominal power target, we can range on these four parameters, however, Cp needs to be the best possible for a good design, therefore, in the preliminary dimensioning, we only focus on Vn and on the power density (L)


Design input parameters: nominal tip speed ratio

Tip Speed Ratio Low High

Utilisation Windmill and pumps Electricity generation

Torque High Low

Efficiency Low High

Centrifugal Stress Low High

Aerodynamic Stress Low High

Area of Solidity High Low

Blade Profile Large Narrow

Aerodynamics Simple Critical

Noise The noise increase with the 6th power of TSR

[3]

The TSR parameter identifies the type of machine we are going to design

In practice, the choice of TSR defines the nominal speed and the number of bladesΩ

λ=%&

'(


Design input parameters: solidity (& blade number)

c

tσ = B: is the blade number

c: is the blade chord

• Higher solidity means more power density and more torque (especially at V<Vn) but less efficiency at the design point

• Too many blades block the flow, too few produce less energy

• Blades are expensive (10% of the turbine costs) after a certain number the advantage in energy generation does not

justify the costs

• 1 and 2 blades rotors have cyclic load problem which reduce the maximum possible size

2 rt

B

π=( )


Design input parameters: solidity (& blade number)

c

tσ = B: is the blade number

c: is the blade chord

σ

[2][3]

Solidity is the ratio between the total area of

the blades and the disk area. The parameter

is related to the number of blade and to the

max chord of the blades

2 rt

B

π=( )


Design parameters: axis & aerodynamic driver

[2]

[2][3]

Name Dutch Windmill American farm windmill Modern 3-blades 2-blades 1-blade

Propulsion Lift Lift Lift Lift Lift

Efficiency 27% 31% 50% 47% 43%

Name Cup Savonius Darrieus Delta Giromill Darrieus

Propulsion Drag Drag Lift Lift Lift

Efficiency 8% 16% 40% 30% 30%

HAWT – Horizontal axis

VAWT – Vertical axis


Main design variables

• Chord law, c(r)

• Twist law, θ(r)

• Airfoils

• Section structural layout

• Blade structural layout

• Materials and thicknesses

• Coating and protection

[4]

Losses

[https://physics.stackexchange.com/questions/143155/do-wind-turbines-convert-the-kinetic-energy-of-air]


Constraints and practical considerations

Noise constraint:

• Limitation on the peripheral speed, typically:

Ω* < 90 /

• Blade tip optimized by noise reduction strategy

https://www.letsgosolar.com/consumer-education/solar-power-wind-power/



Structural constraint:

The root side of the blade is critical from a structural point of view, the sections must be thick

[3]



Manufacturing constraint:

The increasing of cost for build a non-linear chord shape and a high variation twist law, typically is not justified by the increase of energy produced -> linear chord function, reduced variation of twist law

https://nawindpower.com/new-turbine-blade-

manufacturing-method-granted-dnv-gl-certification

http://www.eng.cam.ac.uk/news/wind-energy-turns-bamboo[3]


Other aspects to keep into account

• Application:

o Offshore, onshore

o Class

• Configuration

o Downwind or upwind HAWT

o Coning or pre-bend

• Control strategy:

o Pitch control Pitch to feather

Pitch to stall

o Rotation control Fixed

Variable

o Active and passive devices on blade Flaps

Morphing


Other aspects to keep into account

• O&M:

o Direct inspection

o Monitoring systems

• Protection systems:

o Lightening

o Impacts

o Erosion

o Ice

http://www.kellyaerospace.com/wind_turbine_deice.html

https://www.exponent.com/knowledge/alerts/2017/06/lightning-protection-for-wind-

turbines/?pageSize=NaN&pageNum=0&loadAllByPageSize=true

http://www.metrolaserinc.com/technologies/aerodynamic-modeling-over-rough-

surfaces/

Design strategy:

1. Minimize the LCOE (levelized cost of electricity)

2. Maximize the IRR (Internal Rate of Return)


Main design strategy

Aerodynamic design:

• Variables: chord, twist, airfoils

• Objectives: Maximum Cp, Maximum

Energy

• Constraints: Max chord, max tip

speed, geometry

Structural design:

• Variables: blade components

thickness, material, internal structure

• Constraints: Ultimate and cyclic loads

safe resistance, Maximum load

transmitted, Costs

The LCOE can also be regarded as the average minimum

cost at which electricity must be sold in order to break-even

over the lifetime of the project.

The internal rate of return on an investment or project is the

"annualized effective compounded return rate" or rate of

return that sets the net present value of all cash flows (both

positive and negative) from the investment equal to zero.


Main design strategy

Initial data

Pn, Vn, R, cmax, TSR,

B, …

Constraints

Aerodynamic design Structural design

Performance tests Structure tests

Economics Certifiability

Pass the output to the next step

Something went wrong, change and repeat

Blade shape plan

Bla

de

str

uctu

ral m

od

el

Energy production

Bla

de

ae

rod

yn

am

ic s

ha

pe

Strength and estimated life

AERODYNAMIC DESIGN

Wind turbine blade design



Aerodynamic design of lift based WT blades

HAWT blade

• Find the chord, twist and airfoil

distributions that matches the design

objectives

VAWT blades

• Find the chord and airfoil that

matches the design objectives

[3]

[siemens.com]

[Author: W.Wacker][http://www.reuk.co.uk/wordpress/wi

nd/giromill-darrieus-wind-turbines/]


Aerodynamic airfoils for wind turbines

Guidelines for airfoil selection (or design):

Input data• Range of AoA

• Reynolds number

• Turbulence intensity

• Blade wearing and soiling

• Thickness

Main factors

• Aerodynamic efficiency: - =./

.0

o Lift: power and thrust

o Drag: power reduction and losses, power control

• Sensitivity to inflow unsteadiness

• Sensitivity to wearing, roughness and irregular surface

• Stall and off-design behavior

-100

-50

0

50

100

150

200

-15 -10 -5 0 5 10 15

Cl/Cd

NACA 64-618

DU 93-W-210

DU 91-W2-250

UTC-W-180

UTC-W-210

UTC-W-250

[5]



• The use of a single aerofoil for the entire blade length would result in inefficient design.

• Each section of the blade has a differing relative air velocity and structural requirement and therefore should have its aerofoil section tailored accordingly.

• At the root, the blade sections have large minimum thickness which is essential for the intensive loads carried resulting in thick profiles.

• Approaching the tip blades blend into thinner sections with reduced load, higher linear velocity and increasingly critical aerodynamic performance.

• Thick aerofoil sections generally have a lower lift to drag ratio. Special consideration is therefore made for increasing the lift of thick aerofoil sections for use in wind turbine blade designs



Design and test of airfoils

1. Generate the airfoil shape: 1. Generate a NACA shape

2. Obtain from literature

3. Design a new spline foil

2. Test the airfoil and find the complete aerodynamic polar

1. XFOIL:

1. Get the software (http://web.mit.edu/drela/Public/web/xfoil/) or a GUI which uses it (XLFR)

2. Generate a coordinate file and run the simulation setting Re, Ncrit (Turbulence) and boundary layer transition point and AoA range (with |AoA|<30 deg)

3. Check the polars and extend with Viterna method

2. CFD 1. Computational domain:

– 2D, C, O, H or unstructured hybrid grid

– First cell distance from the wall measured using the y+ suggested for the wall treatment, Reynolds

number and turbulence model (typically, y+ = 1 for LES and RANS with no wall functions,

30<y+<100 for RANS with wall function)

– Domain extension around 30 times the airfoil cord in every direction

2. Boundary condition:

- Inllet surface: fixed inflow velocity free pressure,

- Wall: no-slip condition or wall function for velocity and turbulence quantities

- zero gradient elsewhere

3. Wind tunnel

Blade plan design: Blade element momentum method

1'

2a

=

Ω

ω

2

1

11

2

= −

Va

V

dL cosΦ

Centro di

pressione

∞

[2]

( ) ( ) 2 3

1' 2 '2 4 ' 1 ' (1)dT p p rdr a a r drπ πρ= − = + Ω

( )3

31

24 ' 1

r r r r

VdP dM A a a X dX

X= Ω = −ρ

Axial

Tangential

Thrust law:

Power law:

( ) 3

23

01

8' 1

1

2

X

rr r

PCp a a X dX

XAVρ

= = −∫

General momentum theory:

Induction factors

After applying the conservation laws at the disk:

Blade plan design: Blade element momentum method

The model of the blade uses actuator disk theory (infinite number of blades). To

take into account the effect of the finite number of blades using two models:

Constant circulation along the blades B with helical vortexes that detach from the tip and a vortex to the hub comprising B

concatenated vortices

Helicoid vortical sheet that rigidly translates with constant velocity. This generates a circulation and then a lift on the blades

These models require a complex mathematical solution. We prefer to use the strip

theory by adding a correction factor proposed by Prandtl for tip losses

(a) Joukowsky (b) Betz


Aerodynamic design of drag based turbines

Parameters:

• Height

• Eccentricity

• Blade radius

• Turbine diameter

Design method and tools:The aerodynamics of a wind turbine based on drag is very complex and requires the simulation of the flow field equations. 2D and 3D CFD are the main tools for design and test

[Author: Nitin Bagre] [http://www.reuk.co.uk/wordpress/wind/savonius-wind-turbines/,

http://cleangreenenergyzone.com/cardboard-savonius-wind-turbine/ ]

http://www.savonius.net/project-background.html

STRUCTURAL DESIGN

Wind turbine blade design



Blade frame of reference

[6]

In the dynamic analysis of the rotor,

four systems of reference are usually

introduced to define the load systems

acting on the blade:

• Nacelle frame (I), fixed

• Hub frame (h), rotating

• Blade frame (b), rotating

• Section frame (t), local

Blade elastic motions:

• Flapwise bending

• Edgewise bending

• Axial traction/compression/twist


Loads

The main loads acting on a blade can be divided in:

• Aerodynamic loads:

o Average, maximum at event, high-cycles and/or chaotic

o Principally oriented out-of-plane (flap-wise)

• Inertial loads

o Average, maximum at event, low-cycles

o Principally axial

• Gravity loads

o Cyclic (high number of cycles)

o Principally in-plane (edgewise)


Aerodynamic loads: definition and estimation

Definition and evaluation:

https://www.3ders.org/articles/201603

22-3d-printed-wind-turbine-blades-

could-provide-cheaper-more-effective-

clean-energy.html

In first approximation, the flapwise load can be

considered the dominant in the aerodynamic loads

system as:

• Low-twist, low-angle of attack configuration

are usually preferred for the most part of the

operating conditions

• In modern turbines, where pitch to feather

control is applied, the blade stall is limited and

the lift is the dominant force for most of the

time

• The airfoil shape implies a lower sectional

stiffness (and thus lower moment of inertia) in

flapwise direction


Aerodynamic loads: definition and estimation

Definition and evaluation:

122,4 = 5 678 − :*

5 =1

2!;8

<6=

122,4 =>

<!

?@'A@BCDEF

DG@EF − : · * 6=

122,4 = 4J! K'A

@?(>M?)

OP

&

QR − :* Rz

From the blade element theory it is possible to

obtain an estimation of the aerodynamic moment

in flapwise direction

- wind

- rotation


Aerodynamic loads: design strategy

Structural solution to hold the flapwise moment:

[3]

The blade structure can be seen as a beam

structure. To hold the bending load, a couple of

flanges is usually adopted (spar caps).

The spar caps are located and centered in the

maximum thickness area of the airfoil, in order to

use the maximum possible distance from the

neutral axis.


Inertial loads: Definition and evaluation

Main inertial load sources

• Axial centrifugal force

• Coning angle and pre-bend

• Yaw motion

• Emergency maneuvers

• Transport

[8]


Inertial loads: Definition and evaluation

Axial Inertial force first estimation:

. = K !$RΩ< R R&

Q

The centrifugal loads generated by the rotation of the blade are span wise. When

considering the rotor coning angle β, the centrifugal loads per unit length of the blade are

given by centrifugal tensile force

And centrifugal shear force

If there is no coning angle, we can write the centrifugal traction as


Inertial loads: design strategy

As a normal practice in rotating machine, it is important to keep the

local center of gravity of all the section, aligned with the blade axis.

The relative positions of center of pressure, center of gravity and

elastic center of the section are fundamental parameters to keep

under control


Gravity loads: definition and evaluation

Edgewise bending moment due to gravity:

• Because of its weight and configuration, the

blade is subjected to a cyclic load

• Maximum load when the blade is horizontal

• negligible for smaller blades with negligible

blade mass

• cubic rise in blade mass with increasing

turbine size -> critical when R>70m

1SS,T = ± K V!$ R R − :* R&

Q

[3]


Gravity loads: design strategy

Bending stress diagram oriented

with the longest side of the

section (larger sectional stiffness)

Main spar caps (load carrying

material) normal to the neutral

axis

Reinforcement far from the

bending plane[3]


Typical section structure

• Spar caps (bending moments, axial

forces)

• Shear webs (torsion, shear forces)

• Reinforcement and thicknesses

(buckling and edgewise moment)

• Adhesion reinforcement (trailing edge

and macro layers detachment

[11]

http://www.wirz.seas.ucla.edu/research/wind-

energy/aero-structural-design-investigations-for-biplane-

wind-turbine-blades

[12]


Typical section structure

mathwoks.com

Depending on sizes, material and manufacturing, the number of shear webs can vary along the span

Typical solution is to use a closed section internal structure, glued inside two aerodynamic shells


Materials

Load carrying materials:

• Metals and wood (small sizes, old turbines)

• Fiber reinforced plastic composite laminates

• Sandwich structures with low density fillings and foams

• Adhesives

• LE, TE reinforcements

Protection:

• Coating layer

• Tapes and reinforcements

o impact

o erosion

• Paintings for

o appearence and UV protection

o Segnalazione

[13]


Fibre reinforced plastic laminates

UD laminate:

The layers (ply) present parallel fibers distribute in rows.

• High strength in the fibers direction

• High ortogonal anisotropy of the ply mechanical

properties

Mainly used for spar caps

Fabric and Prepreg laminate:

The fibers are woven in patterns and textures forming a

“tissue” structure

• Less strength

• Higher symmetry degree of the mechanical properties

Mainly used for shear webs

The best performance/cost ratio is given most of the times

using glass fibers in epoxy resin

http://www.fibre-reinforced-plastic.com/2010/12/lamina-and-laminate-

what-is-that.html


Sandwich structure composites

A sandwich-structured composite is a special class of

composite materials that is fabricated by attaching two

thin but stiff skins to a lightweight but thick core. The

core material is normally low strength material, but its

higher thickness provides the sandwich composite

with high bending stiffness with overall low density

[George William Herbert - Own work, produced by

George William Herbert]

[14]

https://www.mt.com/my/en/home/supportive_content

/matchar_apps/MatChar_UC314.html


Final blade structure

https://www.windpowermonthly.com/article/1137943/service-maintain-wind-turbine-blade

TESTING AND CERTIFICATION




International standards

• IEC 61400-1 Design requirements

• IEC 61400-2 Small wind turbines

• IEC 61400-3 Design requirements for offshore wind turbines

• IEC 61400-4 Gears for wind turbines

• IEC 61400-(5) Wind Turbine Rotor Blades

• IEC 61400-11, Acoustic noise measurement techniques

• IEC 61400-12-1 Power performance measurements

• IEC 61400-13 Measurement of mechanical loads

• IEC 61400-14 Declaration of sound power level and tonality

• IEC 61400-21 Measurement of power quality characteristics

• IEC 61400-22 Conformity Testing and Certification of wind turbines

• IEC 61400-23 TR Full scale structural blade testing

• IEC 61400-24 TR Lightning protection

• IEC 61400-25-(1-6) Communication

• IEC 61400-26 TS Availability

• IEC 61400-27 Electrical simulation models for wind power generation

• IEC 60076-16: Transformers for wind turbines applications


IEC61400 Parts 1, 2, 3 – Blade design

Principles

“specifies essential design requirements to ensure the engineering integrity

of wind turbines. Its purpose is to provide an appropriate level of protection

against damage from all hazards during the planned lifetime”

Content

• External conditions (e.g. wind) –

• Wind turbine classes

• Structural design (e.g. load cases and methods)

• Control and protection system (what to consider)

• Mechanical system (e.g. yaw, brakes)

• Electrical system (e.g. lightning)

• Site assessment

• Assembly, installation, erection

• Commisioning, operation, maintenance



Wind turbine classes

Class I II III IV S

Reference wind velocity, Vrif (m/s) 50 42,5 37,5 30 To specify

Annual average velocity, Vmed (m/s) 10 8,5 7,5 6 To specify

50 years maximum, Vrif x 1,4 (m/s) 70 59,5 52,5 42 To specify

1 year maximum, Vrif x 1,05 (m/s) 52,5 44,6 39,4 31,5 To specify

Turbulence class A (Iref) 0,16

To specifyTurbulence class B (Iref) 0,14

Turbulence class C (Iref) 0,12



Load cases

The table 2 of IEC61400 – 1 collect all the load cases to test for design certification

Design situation:

• Power production

• Power production plus fault

• Start up

• Shut down (norma and emergency)

• Parking and fault

• Transport, assembly O&M

Wind condition:

• NTM: normal turbulence model

• ETM: extreme turbulence model

• EOG: Extreme operating gust

• EDG: extreme direction change

• ECD: Exrteme coherent gust with direction change

• EWS: Extreme wind shear

Type of analysis:

• Ultimate load

• Fatigue load

All models and quantities are defined, the final framework will be a combination of all these aspects for the range of velocities defined

for the design


Numerical testing of prototypes

• CFD

– RANS and LES simulation of the single blade or entire machine

– Software examples: Fluent, Abaqus, … (commercial), OpenFOAM

(opensource)

• FEM

– Linear, non-linear simulations

– Static, modal and dynamic

– Single blade or multibody

– Software example: Ansys, Nastran, Abaqus, ... (commercial)

• Dedicated software for full multibody dynamic simulation, design and certification of WT:

– FAST: opensource

– Focus, GH Bladed, …: commercial


Testing of real prototypes

Fatigue testing of rotor blades“The video shows how a 32 m rotor blade is fatigue tested by being bent cyclically in a flapwise direction for 5 million full cycles. A full flapwise test takes about three months. The blades are bent using a cycle close to the natural frequency of the blade.The blade tends to be much stiffer in the edgewise direction, thus it has a higher natural frequency for edgewise bending.Each blade is set in motion by an electric motor mounted on the blade which swings a weight up and down. The foundations which carrythe blade socket have to be very solid: the foundation for the large blade socket consists of 2,000 tonnes of concrete.This video was shot at the rotor blade test facility of the Risoe National Laboratory Sparkær Test Centre in Jutland, Denmark. Type approval requirements for rotor blades are very strict in Denmark, requiring physical testing of rotor blades for both fatigue properties (fatigue testing) and strength properties (static testing). Other countries usually have less stringent requirements for type approval of rotor blades.The purpose of rotor blade testing is to verify that laminations in the blade are, safe, i.e. that the layers of the rotor blade do not separate (delamination). Also, the test verifies that the fibres do not break under repeated stress. “

http://mstudioblackboard.tudelft.nl/duwind/Wind%20e

nergy%20online%20reader/Static_pages/manufactur

e_test_blades.htm

http://eng.lzfrp.com/news_Detail.aspx?n_id=715

Ultimate strength testThe full size blade is constrained on a massive base and statically loaded in several points to reproduce the maximum load computed and extrapolated from the IEC tests

NEW CONCEPTS




Smart blades and new control concepts

Smart control of performance:

• Vortex generators

• Microtabs

• Jets and suction flows

• Plasma actuators

• Flaps

• Winglets

Smart control of the load:

• Section morphing

• Blade morphing

• Blade prebending

• Prediction and control

of twist-bending coupling[3]


New control concepts

Some effects

[15]

References

1. Nick Jenkins Tony Burton, David Sharpe and Ervin Bossanyi. Wind Energy Handbook. J. Wiley & Sons, 2001

2. R.Pallabazzer, Sistemi Eolici

3. Schubel, Peter J., and Richard J. Crossley. Wind turbine blade design. Energies 5.9 (2012): 3425-3449.

4. Habali, S.M.; Saleh, I.A. Local design, testing and manufacturing of small mixed airfoil wind turbine blades of glass fiber reinforced plastics Part I: Design of the blade and root. Energy Convers. Manag. 2000, 41, 249–280.

5. Bianchini, Alessandro, et al. "An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines—Part II: Post-stall Data Extrapolation Methods." Journal of Engineering for Gas Turbines and Power 138.3 (2016): 032603.

6. Ju, Dayuan, and Qiao Sun. "Modeling of a Wind Turbine Rotor Blade System." Journal of Vibration and Acoustics 139.5 (2017): 051013.

7. Mishnaevsky, Leon, et al. "Materials for Wind Turbine Blades: An Overview." Materials 10.11 (2017): 1285.

8. Koh, J. H., and E. Y. K. Ng. "Downwind offshore wind turbines: Opportunities, trends and technical challenges." Renewable and Sustainable Energy Reviews 54 (2016): 797-808

9. Xiong, Liu, et al. "Dynamic response analysis of the rotating blade of horizontal axis wind turbine." Wind Engineering 34.5 (2010): 543-559.

10. Bir, Gunjit S. "Computerized method for preliminary structural design of composite wind turbine blades." Journal of solar energy engineering 123.4 (2001): 372-381.

11. Zhu, Jie, Xin Cai, and Rongrong Gu. "Multi-Objective Aerodynamic and Structural Optimization of Horizontal-Axis Wind Turbine Blades." Energies 10.1 (2017): 101

12. Quispitupa, Amilcar, Bente Vestergaard, and Tomasz Sieradzan. "Certification of wind turbine blades–The DNV Procedure." (2013)

13. Bir, Gunjit S., Michael J. Lawson, and Ye Li. "Structural design of a horizontal-axis tidal current turbine composite blade." ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering. American Society of Mechanical Engineers, 2011.

14. Thomsen, Ole Thybo. "Sandwich materials for wind turbine blades—present and future." Journal of Sandwich Structures & Materials 11.1 (2009): 7-26.

15. Barlas T.K.; van Kuik, G.A.M. Review of state of the art in smart rotor control research for wind turbines. Prog. Aerosp. Sci. 2010, 46, 1–27.


design of wind turbines - uniroma1.it. fmec wind turbin… · ideal wind turbine [1] real wind...

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