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SUPERGEN Wind Wind Energy Technology Using novel materials for wind turbine Chi Zhang 1st Training Seminar Glasgow, 23rd -24th September 2010
SUPERGEN WindWind Energy Technology
Using novel materials for wind turbine
Chi Zhang
1st Training Seminar
Glasgow, 23rd -24th September 2010
Summary
• Background of wind turbine and blades
• Introduction of composite materials
• Fibre preforms and matrix
• Manufacturing of wind turbine blades
• Driving forces and challenges
• Questions
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Globe wind industry statistics
World wind energy report 2009 (WWEA)
• Worldwide capacity reached 159,213 MW, out of which 38,312 MW were added in 2009.
• Wind power showed a growth rate of 31.7%, the highest rate since 2001. The trend continued that wind capacity doubles every 3 years.
• The wind sector in 2009 had a turnover of 42 billion GBP .
• China continued its role as the locomotive of the international wind industry and added 13,800 MW within one year – as the biggest market for new turbines –, more than doubling the installations for the fourth year in a row.
• The USA maintained its number one position in terms of total installed capacity and China became number two in total capacity, only slightly ahead of Germany, both of them with around 26’000 Megawatt of wind capacity installed.
• Asia accounted for the largest share of new installations (40.4 %), followed by North America (28.4 %) and Europe fell back to the third place (27.3 %).
A total wind capacity of 200,000 Megawatt will be exceeded within the year 2010.
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Globe wind industry statistics
World wind energy report 2009 (WWEA)
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Globe wind industry statistics
• Offshore wind turbines
Global offshore wind build (figures from BWEA)
(WWEA)
- By the end of last, wind farms installed in the sea could be found in 12 countries, 10 of them in Europe and some minor installations in China and Japan.
- Total installed capacity amounted to almost 2 GW, 1.2 % of the total wind capacity worldwide. Wherein, 454MW were added in 2009.
- In 2009, most offshore wind turbines were installed by UK and Denmark. And UK still maintained its number position on total offshore capacity.
- China installed the first major offshorewind farm outside of Europe – a 21MW, near Shanghai.
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Impact for composites
• The value of the composite blade market in 2007 was $4.3 billion
• Based on the expected industry-wide growth, the value of manufactured blades will grow up to approximately $34 billionper year by 2017
Figure. Estimated Composite Wind Turbine Blade Unit Production, 2005-2017. (figures from Composites Technology)
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What are the requirements?
• Blades
• Towers
• Nacelles Weight
Turbine weight
26 Tons 30 Tons
Generator case
39.7 Tons 50 Tons
Tower weight 78.6 Tons 93.6 Tons
Total weight 144.3 Tons 173.6 Tons
1 MW 1.5MW
Diameter of rotor 60m 70m
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What are the requirements?
• TOWERS
• The towers are required to be strong enough to support the weight of the blades and generator/nacelles.
• They must also withstand fluctuating wind loading and loading resulting from turbine blade rotation.
• This means that the towers must be stiff and strong. They do not need to be particularly light if positioned on-shore, although all foundations will be cheaper if the overall turbine weight is low.
• Transport to location is an issue and it will help if the tower can be made from sections and assembled to ease transport issues. Again weight will be an issue but it is secondary to other considerations.
• If a turbine is placed off-shore, then the weight becomes of greater importance in terms of the costs of foundations and the cranes necessary to install the turbine.
• Corrosion is an issue particularly off-shore.
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Towers = Steel
• Cheap, stiff and strong. They are getting difficult to weld at the larger thicknesses.
• Corrosion is also an issue
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Alternative to Steel
Pre stressed concrete
Composite lattice(small turbines)
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Future Idea?
• Hybrid steel – concrete
• Glass fibre composite monoliths
• Glass fibre composite/ concrete duplex
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Nacelles
•Weight is important, materials aren’t.•Common structures are made from glass fibre composite.•Easy to mould, light, corrosion resistant•No Supergen activity in this area
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Blades
Shear webs Spar Caps
Balsa cores Skins
Typical Blade Profile
Cross Section of Blade
•Blades are required to preserve an optimum cross section for aerodynamic efficiency to generate the maximum torque to drive the generators.
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Loading Situations
The blades will be subject to a wide range of loads, including flapping, tension and compression, twisting – all induced by the central movement and the variable winds loadings
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Loading Situations
The blades must be stiff enough to avoid hitting the towers when deflected by the wind loads.
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Blade Design
Structural design of a blade is optimised by adopting a shell structure with a long stiff central spar. The spar caps provide stiffness and strength in bending and extension, while the spar webs provide shear stiffness
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Blade Design
Shear webs Spar Caps
Balsa cores Skins
Typical Blade Profile
Cross Section of Blade
Twist ( shear)
Flap
Extension
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Blade Design
• The exact shape of the internal structure will determine the stiffness and strength of the blade under each loading mode for any given materials. In general terms however we need a material that is as light as possible for a given stiffness in order to satisfy the blade design criteria and to minimise the weight induced fatigue loads.
• Reducing the weight of the blades also will directly reduce the loads on the tower and foundations.
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Blade Design
What materials give us low weight with maximum stiffness and strength?
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Why composites?
• Composites are materials that combine high strength and high stiffness fibres with a polymeric resin matrix.
• Fibres are extremely stiff and strong and can have a very low density. Fibres alone can only exhibit tensile properties along the fibre’s length (Need resin to bind them together)
• Composites can be engineered for
– high strengths and stiffnesses
– ease of moulding complex shapes
– high environmental (corrosion) resistance
– low densities, etc.
The resultant material is superior to metals for many applications!
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What are composites?
• Dictionary definition:
“A composite refers to something made up of various parts and elements”
• The different constituents must have two prime characteristics
– Chemically different
– Insoluble in each other.
• In almost all cases, there is a
– Strong and stiff component forming the reinforcement
– Soft constituent (matrix) that binds the reinforcement
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An example of composites: Wood
Wood is a natural compositeRequirements:
• It has to be tall and straight
• It must be strong and light and resist bending forces
• It is composed of multiple fibre bundles (lamellae) each of which contains multiple layers of cellulose fibres in a lignin matrix
Effects of Anisotropy:• Material has high stiffness and strength along fibres, but
cracks can also easily propagate along it
• Cracks very difficult to propagate across the fibres
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E-glass: Most common glass fibreUseful balance of mechanical, chemical & electrical properties
Reinforcements (Fibre)
• Glass fibres– Original structural reinforcement
& most common
– Competitively priced & widely available
– Ease of processing & good handleability.
• Carbon fibres– Best known & most widely used
high performance fibre
– Wide range of mechanical properties
– Linear stress-strain behaviour
Typical properties of carbon fibres
Strength 3.5-6.4 GPaStiffness 240-310 GPaDensity ~1.85 g/cm3
Diameter 5-10 µmCost ~20-85 GBP/kg
Strength 3.45 GPaStiffness 75.8 GPaDensity 2.56 g/cm3
Diameter 8-15 µmCost ~1-3 GBP/kg
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Reinforcements (Fibre)
Kevlar-49: Most common aramid reinforcement fibre
• Aramid fibres– Organic fibre– High tensile stiffness & strength
• Low stiffness = ballistic grade• High stiffness = reinforcement grade
– Very poor compressive properties (similar to that of glass fibres)
– Most commonly known as Kevlar
• Silicon carbide (SiC) & Alumina– Used in metal matrix composites and ceramic
matrix composites
– Good thermal stability
• Boron fibres– Monofilament wires
– Excellent strength and stiffness
– More expensive than carbon fibres
– Used in polymer matrix composites and metal matrix composites
• High performance thermoplastics– Highly drawn UHMWPE
• Natural fibres– Derived from plants, i.e. eco-friendly
Strength 3.45 GPaStiffness 180 GPaDensity ~1.4 g/cm3
Diameter ~12 µmCost ~20-50 GBP/kg
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Fibre Preforms
Fibres need to be preformed before they can be used in composites manufacturing
– 1D preforms
• Yarns & rovings
– 2D preforms
• Technical textiles incl. woven fabrics, uniweave
• Mats
• 2D braids
• Prepregs
• Moulding compounds
– 3D preforms
• 3D braided & woven structures
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1D preforms
1D preforms are fibres that have been bundled into rovings or yarns
– Yarns are linear assemblies of fibres characterized by a substantial length and relatively small cross-section
– Rovings are preferred since they have minimum twist – makes them suitable for high tensile applications
– Combining two fibre types creates a hybrid roving
– Used in applications like pultrusion, filament winding and manufacture of pre-pregs
Property Description
Count linear density (tex=grams/km)Diameter thickness/bulk (microns)Twist direction and no. of turns per meterFiber type (polymer) compositionSurface finish chemical or mechanical finishSingle (1) or ply (>1) number of individual strandsContinuous/ discontinuous
filaments run essentially the whole length of the yarn/short filaments
Blend/hybrid mixture of two or more fiber typesTexture heat set, fibrillated
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2D fibre preforms
• 2D Preforms are most commonly used in manufacture:
– UD & woven prepreg made by laying several resin-impregnated rovings together to form sheets or tapes
• Tend to be used for highly loaded/high performance structures due to high raw material and processing cost
• Thermoset resin is B-staged producing a short shelf life, but provides tackiness (stickiness) for laying fibres onto tool and onto themselves
– Multiple layer preforms are multilayer UD preforms similar to stacked prepregs but use dry rovings that are stitched together
• Combines advantage of straight rovings and easy handleability since reinforcement is dry
• Multiple axes stitched together reduces cutting and lay-up time
• Stitching keeps the rovings in place
• Gaps between stitches allow for resin to permeate fabrics
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Prepreg manufacture
• Solution-dip impregnation– Used for impregnating
woven fabric– Solvent added to resin to
reduce viscosity– Drying tower to remove
solvents
• Hot-melt impregnation– Fibre tows are aligned through comb –
prevents tow crossing and fibre damage
– Resin sheared onto backing– Fibres contacted and covered with
release paper– System heated in impregnation zone
and passed through nip-rollers to control thickness
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Non Crimp Fabrics (NCF)
• NCFs also known as Multi-axial weft-inserted Warp Knit fabrics (MWK)
– Warp knitting machine used to stitch multiple layers of fibres (up to 8 layers from -20° to + 20° orientation (source Liba Machines, de)
– Absence of crimp means fibres perform to structural capacity in compression
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Woven fabrics
• Woven fabrics are the most commonly used materials in composites processing– More balanced properties in a single layer than UD fabrics since they combine two
orientations
– Low fabrication cost: Technology is well known & mature
– In the plain weave, crimp of the yarn is a significant problem leading to reduced strength of final product
• Variations on plain weave reduce the crimp in fibres, but stability of woven fabric is reduced
– Basket weave is a variation on the plain weave with better drapability
– Twill weave is a further improvement but further reduces stability
– 4, 5 & 8-harness satin weave are most common in composites applications
• Have good drapability, smooth surface and minimal thickness
• Due to the open nature of the weave it tends to come apart very easily
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2D Braiding
• 2D-Braiding– Two or more yarn systems are interlaced by following intersecting circular paths in
opposite directions. The resulting crossover sequence has a tubular shape (called a bias braid)
– Flat braids possible by use of modified paths of yarn carriers
– Ability of braids to deform radially allows them to conform to complex tubular shapes
Braiding vs. wovens
– Near-net shapes can be produced:
• Simply over-braiding a part until the desired geometry or desired thickness is obtained
– Braiding angle can vary from 5° to almost 85°
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Other 2D preforms
• Knitting is defined as the process in which loops of yarns are meshed to form a fabric. Knitting allows complex shapes with 3D structures. They have higher fracture toughness since loops join all layers together.
• Random fibre preforms (or mats) are characterised by almost random distribution of fibre orientations in the textile
- Chopped Strand Mat (CSM)- Continuous filament random mats- Surface veils
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3D preforms: 3D weaving
• 3D woven structures– Created on multi-warp looms allowing up
to ~20 layers to be woven at once
– Various complex architectures have been produced using the multiwarp approach:
• Panels with variable cross-sectional thickness, T and I beams, trusses etc.
– Process is slow and expensive
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3D preforms: 3D braiding
• 3D braided structures– Can produce neat-net shapes with multi-
directional reinforcement
– Slow production rates
– Traditionally, the market for these reinforcement architectures has been limited to aerospace applications
• E.g. rocket motor components
– Attempts are being made to expand into other application areas
• E.g. sea and ground transport
– Part dimensions are limited by size of machines
– Complex patterns are required
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3D preforms: Stitching
• Stitching– Simple through-thickness reinforcement
• Originally a modification of textile stitching, developments have allowed the introduction of special composite stitching heads
– Allows mechanical joining of preforms into a complete structure before processing:
• This eases fabrication as multiple layers are bonded together prior to impregnation with resin
• Joining alleviates delamination problems
Tufting is similar to carpet production•Can work with preforms up to 40mm thick
One sided stitching is a modification of the single chain stitch and provides maximal reinforcement, but requires tools to have space for the needles to penetrate the preform
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Matrices – thermosetting resins
Typical properties of unsaturated polyesters
• Polyester resins– Most commonly used resins & wide
range of formulations, curing agents, etc.
– Acceptable mechanical properties & acceptable environmental durability
– Very good adhesion to glass fibre
– High styrene emissions & high shrinkage on cure
• Vinyl ester resins– Similar processing to polyesters
– Very high chemical and environmental resistance
– Better overall properties to polyesters
– Higher cost
Strength 55-90 MPaStiffness 3.4-4.4 GPaStrain to failure 1.6-4.5 %Density 1.1-1.5 g/cm3
Cost ~1-3 GBP/kg
Typical properties of vinyl esters
Strength 60-93 MPaStiffness 2.9-3.9 GPaStrain to failure 3.0-16 %Density 1.0-1.3 g/cm3
Cost ~3-6 GBP/kg
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Matrices – thermosetting resins
Typical properties of epoxies
• Epoxy resins– Most used resin for advanced
composites
– Very good mechanical and thermal properties
– Good water resistance
– Low shrinkage on cure
– Needs proper mixing formulation
– Expensive
• Phenolics– High fire resistance & excellent thermal
properties
– Cure by condensation reaction resulting in voidy laminate
• Cyanate esters– Superb electrical properties & low
moisture absorbance
– Used in radomes, antennas, etc
– Very expensive (~20 GBP/kg)
• Bismalemides (BMI)– Superior to epoxies for hot/wet use &
suitable for high operational temps.
– >75 GBP/kg
• Polyimides– Higher operational temps. than BMI
– Cures similar to phenolics
– Extremely expensive (>110 GBP/kg)
Strength 55-130 MPaStiffness 2.5-6.0 GPaStrain to failure 3.1-15 %Density 1.1-1.4 g/cm3
Cost ~4-20 GBP/kg
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Matrices – thermoplastic resins
• Thermoplastic resinsOffer– Increased toughness
• Higher strain to failure than thermosets
• Improved impact resistance
– Improved hot/wet resistance:
• They do not absorb any significant amount of water but are subject to chemical attack
– Indefinite shelf life
• Can be molten and moulded as needed
Problem– Operational temperature must be
below the Tg
– Subject to creep at high temperatures
Engineering thermoplastics• PA (Polyamide)
– Self-lubricating & exhibit good abrasion resistance
– Good chemical resistance but high water absorption
• PP (Polypropylene)– Low density & low cost– High impact properties
• PET (Polyester terephtalate)– Comparable processing to PP– Higher service temperature &– Stiffer than PP
• PEEK (Polyether ether ketone)– Highest performing engineering
thermoplastic– High cost & cost of processing
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Manufacturing of blades
Figure. Lay-up sequence of a 750kw wind turbine blade [C.Kong et al, Energy 2005]
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Manufacturing of blades
• Hand-lay up
• Vacuum Assisted Resin Transfer Moulding (VARTM)
• Prepreg
• VARTM and hand layup of prepreg in open molds are today the dominant processes.
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Hand lay-up process
• Usually is used for manufacturing small blades
• The simplest process requiring only a single sided tool
• Liquid resins that cure at room temperature are typically used
– Gel coat achieves Class A surface finish – can be pigmented
– Skin coat of CSM helps improve corrosion and chemical resistance
– Fabric layers impregnated on top
• De-mould
• Post cure
• Clean tool and apply release coat
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VARTM process
• Basically an extension of the hand lay-up process where pressure is applied to the laminate once laid-up:
– Improves consolidation.
• This is achieved by
– Covering the laminate with peel-ply, release film, breather/bleeder fabric
• Peel ply is a porous fabric that permits resin to bleed from the laminate
• Breather/bleeder fabric allows uniform pressure over the whole laminate by giving air paths & soaks up excess resin bled from the laminate
– Sealing a plastic film over the lay-up and onto the tool.
– The air under the bag is extracted by a vacuum pump
• Up to 1 atmosphere of pressure can be applied to the laminate to consolidate it.
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Source: Huntsman
Manufacturing process for pregregs
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All prepregs contain all the elements required for full cure- Triggered by high temperatures - Cures slowly at room temperatures – i.e. store in freezer
This introduces cost:- Storage- Time involved in unfreezing prepreg before use- Waste when material exceeds life- Tracking materials (in/out etc)
Major cost of composite part produced using prepregs is lay down time.
Manufacturing process for pregregs
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Advantages
• Allows production of high fibre volume fraction composites (>60%)
• Simple to complex parts can be easily produced
• Process suitable for prototypes as tooling costs are low (though the autoclave requires considerable investment)
• Very strong and stiff parts can be fabricated with this process
Limitations• Process is labour intensive• Process is not suitable for high-volume production• Tooling needs to be able to withstand the process temperatures
• Autoclaves once involved, becomes expensive, slow to operate and limited in size• Parts produced by prepreg lay-up are expensive
Cutting-edge technologies
1) Robot-controlled glass application cartsReduce time for lay down by 25%
2) Hydraulic “Power Hinges” also greatly reduces assembly time and inproves accuracy
3) Laser projection systems for realy time tool based projection of ply locations, bonding adhesive outlines and shear web location. (ref: TPI)
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1) 2)
3)
4)
4) Automated machines can cut several layers at once. Thus it can minimise cutting time and material wasting.
Driving forces and challenges
Criteria on which composites are selected depend on the industry in which they will be used:
– Aerospace: mainly weight reduction with increased stiffness/strength• High scrap levels are (were?) tolerated
• There is a preference for high performance materials in order to reach the weight savings
• Fibres need to be continuous and volume fractions need to be high
– Transportation: emphasis is on decreasing cost
• Return on investment, complex shapes, recycling, etc.
• Need to reduce weight as increased safety requirements = heavier vehicles = worse fuel economy
• Manufacturing routes need to be low-cost and high speed: fibre volume fractions not so much of an issue
Aerospace:Strength, stiffness,weight, quality control
Mechanical Industry:Design, strength, quality
Automotive:Automated fabrication
Perf
orm
ance
1/Cost
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Driving forces and challenges
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Reliable and cost-effectivewind turbine blades
Design criteria of wind turbine
blades
WeightBendingstiffness
Cost
FatigueResistance Manufacturing
processes
Labour, Material,Equipment
Production rate
Maintenance and repair
Environmental effects:UV, radar,
Corrosions, Lightening
– Wind Power: Increasing of stiffness/strength + weight reduction + cost/kg reduction
As the blades are getting longer and longer, the challenge faced by structural designers is that it is becoming increasingly difficult to satisfy the design criteria of wind turbine blades where it is required to reduce the weight of structures without losing stiffness and fatigue resistant performance.
Driving forces and challenges
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• Scale and economics will influence choice of materials, process routes.
• Recycling, sustainability issues will become more important
• Manufacturing rate will dominate choices.
