Annu.Rev.Mater.Res.2005.35:505–38
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doi:10.1146/annurev.matsci.35.100303.110641
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2005by Annual Reviews.All rights rerved First published online as a Review in Advance on April 4,2005
C OMPOSITE M ATERIALS FOR W IN
D P OWER T URBIN
E B LADES
Povl Brøndsted,Hans Lilholt,and Aage Lystrup
Materials Rearch Department,Risoe National Laboratory,DK 4000Roskilde,Denmark;email:povl.brondsted@risoe.dk,hans.lilholt@risoe.dk,aage.lystrup@risoe.dk Key Words composites,properties,processing,damage,fatigue ■Abstract Renewable energy resources,
of which wind energy is prominent,are part of the solution to the global energy problem.Wind turbine and the rotorblade concepts are reviewed,and loadings by wind and gravity as important factors for the fatigue performance of the materials are considered.Wood and composites are discusd as candidates for rotorblades.The fibers and matrices for composites are described,and their high stiffness,low density,and good fatigue performance are em-phasized.Manufacturing technologies for composites are prented and evaluated with respect to advantages,problems,and industrial potential.The important technologies of today are prepreg (pre-impregnated)technology and resin infusion technology.The mechanical properties of fiber composite materials are discusd,with a focus on fa-tigue performance.Damage and materials degradation during fatigue are described.Testing procedures for documentation of properties are reviewed,and fatigue loading histories are discusd,together with methods for data handling and statistical analysis of (large)amounts of test data.Future challenges for materials in the field of wind turbines are prented,with a focus on thermoplastic composites,new structural ma-terials concepts,new structural design aspects,structural health monitoring,and the coming trends and markets for wind energy.INTRODUCTION The international wind energy market showed a new record in 2003with a growth
rate of 15%.Globally,a total power of 8.3GW was installed.The total installed wind energy power has now reached more than 40GW,and the average growth in the market during the past five years has been 26%per year.The figures were reported in the annual report on the status for wind energy in March 2004from the consulting company BTM (1).This illustrates how during the past 25to 30years the u of wind turbines for electricity generation has grown from a grass-root initiative to an efficient alternative energy resource.The capacity of a commercial wind turbine has today reached the range of 2–5MW,with large plants being built world-wide on land and offshore.
秤杆提米0084-6600/05/0804-0505$20.00505
A n n u . R e v . M a t e r . R e s . 2005.35:505-538. D o w n l o a d e d f r o m a r j o u r n a l s .a n n u a l r e v i e w s .o r g b y F O R S K N I N G S C E N T E R R I S O o n 09/06/05. F o r p e r s o n a l u s e o n l y .
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This status illustrates the increasing concern in the world,and perhaps in Eu-rope in particular,about the supply and consumption of energy in a modern and civilized society.Over the centuries,energy has been supplied by wood,coke,coal,oil,and natural gas,as well as by uranium (nuclear energy).The
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early sources were natural in the n that they renewed themlves over a short time period (years),while the later energy supplies were tapped from sources established millions of years ago.The increasing energy demands in the world today,owing to improved and expanded civilizations and to increasing popula-tions,have led to concerns over the limited energy resources in storage on the earth.At the same time there is an increasing concern about the pollution of the world/environment (generation of waste).This has led to the focus on a sus-tainable energy supply,which (probably)implies optimized u of energy,mini-mized pollution and,implicitly,reduction in energy consumption.The aspects have led to an increasing focus on the short-time stored energy resources;among the the most developed types today are wind energy and biomass (in various forms).WIND ENERGY For wind energy a converter is needed to turn the kinetic wind energy into opera-tional ,electricity and/or heat.The converter is bad on a rotor driven by the wind,thereby extracting a power of P =αρA v 3, 1.where αis an aerodynamic efficiency constant,ρthe density of air,A the area of rotor-plane,and v the wind velocity.The rotor needs some sort of aerodynamic ,a wing or rotorblade with an aerodynamic shape,to be able to rotate.The rotor is typically placed on a tower,and this converter is usually called a wind turbine (in the past,a wind mill).In the early years the United States ud the des-ignation wind energy conversion system (WECS).Today nearly all wind turbines have rotors with three blades,where the rotor is mounted in an approximately
vertical plane,on a horizontal axis,facing into the wind (Figure 1).
ROTORBLADES
The prent review is focud on the rotorblades,which probably prent the most challenging materials,design,and engineering problems.The rotor and its three rotorblades constitute a rather flimsy structure,consisting of cantilever-mounted blades on a central hub.The basic design aspects for a rotorblade are the lection of material and shape.The material should be stiff,strong,and light.The shape should be aerodynamic,similar to that of an airplane wing.
A n n u . R e v . M a t e r . R e s . 2005.35:505-538. D o w n l o a d e d f r o m a r j o u r n a l s .a n n u a l r e v i e w s .o r g b y F O R S K N I N G S C E N T E R R I S O o n 09/06/05. F o r p e r s o n a l u s e o n l y .
MATERIALS FOR WIND TURBINE BLADES
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北京四合院特点Figure 2Cross-ction principle of a rotorblade giving the nomenclature of the different blade construction elements.The shape of a rotorblade in cross ction is shown in Figure 2.The aerody-namic contours are formed by the (outer)relatively thin shells.They are supported structurally by a longitudinal beam or by webs,which carry a substantial part of the load on the blade.In the longitudinal direction,the rotorblades are tapered and twisted.The tapering is needed to economize with weight of the material becau of the increasing loads from tip to root of a cantilever structure.The tapering,both in external shape and in thickness of the shells/beams/webs,is usually designed to ensure the same materials ,a maximum strain as design allowable.It is inter
esting to note,as the pioneering work in the United States in 1970(and earlier)obrved,that a simple plank of constant cross-ctional area,rounded along the forward edge and sharpened along the rear edge,and with no twist of the (rectangular)profile,would have an aerodynamic efficiency of 75–80%,where 100%accounts for a rotorblade optimized according to the principles described above.The challenge for the designers is thus to go beyond the simple plank and integrate the aerodynamic shape,the tapering and the twist,into a design of the blade structure that is optimized with respect to materials lection and cost-effective production.
LOADS ON ROTORBLADES
The rotor and the rotorblades are expod to external loads.The originate from the wind and from gravity.The general rotorblade geometry shown in Figure 2has the blades arranged with their flat dimension in the plane of the rotor.This is so becau the linear velocity of the outer part of the blade is high with blade tip speeds of 75–85m/s,which are much higher than the wind speeds,even at storm conditions (∼25m/s).Therefore the relative wind direction,as en by the blade,is nearly in the plane of the rotor,although the real wind direction is at a right angle to the rotor plane.
A n n u . R e v . M a t e r . R e s . 2005.35:505-538. D o w n l o a d e d f r o m a r j o u r n a l s .a n
n u a l r e v i e w s .o r g b y F O R S K N I N G S C E N T E R R I S O o n 09/06/05. F o r p e r s o n a l u s e o n l y .
508BRØNDSTED LILHOLT LYSTRUP
世味年来薄似纱The blades are expod to the wind that through the lift on the aerodynamic profile caus loads at right angle to the blades,which therefore react by bending flap-wi.The loads are both static,causing a permanent bending of the blades,and dynamic,causing a fatigue flap-wi bending becau of the natural variations in wind speed.In addition,the static and fatigue load spectra vary during rotation,as en by a given blade,when the blade points upward and downward,respectively;this is caud by the natural wind shear,which is the increa of average wind speed with increasing height over the terrain.Normally,the maximum wind speed for operation of the wind turbine is about 25m/,storm conditions,beyond which the rotor is brought to a standstill by turning the rotorblades out of the wind by rotation about the longitudinal axis of the blade.This position expos the rotorblades to the wind,coming in a right angle to blade planar surface,and caus the blade to bend as under a steady load on its surface.The natural variations in wind speed will cau dynamic flap-wi fatigue load spectra,as en by the blade.The absolute magnitudes of the loads are comparable to the aerodynamic lift on the blades,as described above.
Therefore,the rotorblades are expod to flap-wi bending fatigue loads in varying spectra distributions under all conditions during rvice.The blades are also expod to gravity,and this is most pronounced when they are in their horizontal position.The loads cau bending in an edge-wi mode,and a given blade bends one way on the right-hand side and the opposite way on the left-hand side of rotor plane.This explains why the edge-wi bending also caus fatigue of the blade material and structure during rotation.Furthermore,the blades are expod to centrifugal forces during the rotation of the rotor.Due to the limits of the linear blade velocity,the rotational speed is relatively low,typically from 20rpm down to 10rpm for large rotors/long rotorblades.Therefore,the longitudinal tensile loads in the blades are relatively low and normally are not taken into account as a design parameter.The design lifetime of modern wind turbines is normally thought to be 20years,and the corresponding number of rotations is of the order 108to 109.MATERIALS REQUIREMENTS
The operational parameters and conditions lead to the following requirements focud on stiffness,density,and long-time fatigue:
high material stiffness is needed to maintain optimal aerodynamic perfor-mance,
low density is needed to reduce gravity forces,
long-fatigue life is needed to reduce material degradation.The optimal design of the rotorblades is today a complex and multifaceted task and requires optimization of properties,performance,and economy.It is not the objective of this review to discuss the design process in detail,but rather to focus
A n n u . R e v . M a t e r . R e s . 2005.35:505-538. D o w n l o a d e d f r o m a r j o u r n a l s .a n n u a l r e v i e w s .o r g b y F O R S K N I N G S C E N T E R R I S O o n 09/06/05. F o r p e r s o n a l u s e o n l y .
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MATERIALS FOR WIND TURBINE BLADES
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表白信怎么写Figure 3Development in rotorblade weight versus length.Symbols indicate different manufacturers and the materials and how they meet the requirements of wind turbine performance,as discusd above.The combined result of the design process and the materials can be illustrated in the form of rotorblade weight as a function of rotorblade length,as prented in Figure 3.The lower end of the curve reprents the relatively short blades of length 12–15m,as were common in the early years of wind turbine design (1980s).The points reprent rotorblades of increasing length,as developed until the prent.An empirical curve is shown for the points reprenting blades with lengths below 40m,giving a power law with an exponent of about 2.6;this is lower than expected for a simple up-scaling of the design on a volume basis (exponent 3),but it corresponds more cloly to up-scaling of only two dimensions (length and thickness,exponent 2).This rather low exponent is a good indication of the high quality of the design process.Three recent and very long rotorblades of 54to 61.5m (2),plotted in Figure 3,show the further improvement in the design process by the points being below the extrapolated empirical curve,although it should be
父母教noted that they might refer to different wind load classifications.An additional discussion of the recent design improvements is given below.
The materials properties requirements of high stiffness,low weight,and long-fatigue life can be ud to perform a materials lection,initially looking at all materials.The property combinations prented in Reference (3)are illustrative for a first lection of potentially usable materials class.In a simplified form,the diagram of stiffness versus density in Figure 4shows the procedure to be ud (4).For details of materials lection,e Reference (3).The mechanical design of a rotorblade corresponds nominally to a beam,and the merit index is for this ca
M b =E 1/2/ρ, 2.A n n u . R e v . M a t e r . R e s . 2005.35:505-538. D o w n l o a d e d f r o m a r j o u r n a l s .a n n u a l r e v i e w s .o r g b y F O R S K N I N G S C E N T E R R I S O o n 09/06/05. F o r p e r s o n a l u s e o n l y .
