materials and manufacturing techniques for wind turbine blades

Materials and Manufacturing Techniques for

Wind Turbine Blades Wind Turbine Concepts and Applications

Haseeb Ahmad

[email protected]

Submitted to

Dr. Bahri Uzunoglu

Wind Power Project Management 2012

Högskolan på Gotland, Sweden

mailto:[email protected]

Wind Turbine Concepts and Applications

2

Project Summary

Project Title Materials and Manufacturing Techniques for Wind Turbine Blades

Investigator Haseeb Ahmad

This project describes the comparison between different materials and techniques used for wind

turbine blade manufacturing. Blades are the integral part of wind power projects so the focus is

on the characteristics of materials and manufacturing techniques for the blades. The objective is

to provide basic information and understanding about the materials and manufacturing

techniques for wind turbine blades. This document will cover the process methodology,

advantages and disadvantages of different manufacturing techniques.

Key Words: Wind Turbine Blades, Blade Geometry, Composite Materials, Manufacturing

Technique Characteristics.


3

Contents

1. Introduction

4

2. Wind Turbine Blades 5

2.1 Blade Geometry

5

3. Wind Turbine Blade Materials 7

3.1 Wood 7

3.2 Steel and Nickel Alloy 7

3.3 Aluminum 7

3.4 Fiber 7

3.4.1 Glass fiber 8

3.4.2 Carbon fiber 10

3.5 Matrix materials 11

3.5.1 Thermosetting Polymers 11

3.5.2 Thermoplastic Polymers 13

3.6 Composite materials

13

4. Manufacturing Technologies 16

4.1 Wet hand lay up 16

4.2 Filament winding 17

4.3 Prepreg technology 18

4.4 Resin Infusion Technology 18

4.4.1 Resin Transfer Molding (RTM) 19

4.4.2 Seeman Composite Resin Infusion Molding Process (SCRIMP) 20

4.4.3 Resin Film Infusion (RFI)

20

5. Conclusions 21

References


4

1. Introduction

Blades are one of the main components of wind turbines. They have shape like aero-plane

propeller, and attached to the rotor hub. They posses almost the same aerodynamic and structural

properties like aero plane wings. But there are some modifications which have to be made

because the blades are used in a wind power generation system. The wind turbine blades

(airfoils) convert the wind energy into mechanical energy of shaft through their rotation. In the

modern era, where wind turbines are operated with different speeds, the strength of blades plays

an important role in the success of wind power project because, they not only transform wind

energy but also regulate the power production through their orientation.

Wind turbines blades, during their normal operating cycle i.e. 20 years, pass through severe

environmental conditions e.g. wide range of temperatures, hails, ultraviolet and bird collisions

etc. They bear static and dynamic lift, drag and inertial loads so it is important to select those

blades which can withstand these challenges otherwise they can face structural damages and

fatigue related issues due to cyclic loading. These problems of fatigue could be resolved by

improving the characteristics of materials for the blade manufacturing, production processes and

modification in the design and configuration of the blades. It is of utmost importance that the

blades should be highly rigid having low weight and must possess rotational inertia, and above

all they can resist wear and fatigue.

If we look into the history of wind turbines, the early wind turbines used the paddle like

wooden blades and thin sheets of wood until 1870, when the first steel bladed wind turbine put

into operation for water pumping.1 In 1968, the airfoil type fiberglass and plastic blades were

used by German and Danish scientists.2 After 80s, the power production stepped up from 1 or 2

megawatts to 10 megawatts per single turbine which required larger blades to extract the

requisite amount of energy. The advancement put enormous pressure on the manufacturing

industry to come up with right materials for turbine blades. As a result of that, glass-fiber

reinforced polymers composites, epoxy based composites and carbon fiber reinforced materials

were developed to achieve the set goals. More detail about these materials and their

manufacturing techniques will come in the following text.


5

2. Wind Turbine Blades

With an increase in the size of modern utility scale wind turbines, the size of blades has

increased tremendously over the past 25 years thus created a plenty of challenges for the

manufactures to create durable as well as low cost materials for blade manufacturing. There are

some issues related with the larger blades such as

Larger blades produce more stress on the mechanical and gear components.

The manufacturing cost would increase because of the complex and expansive molding

The issues of logistics and transportation will be more complex.

Larger blades have high tip speed ratio (the ratio of rotational velocity of blade tip and

actual wind speed) so they create more sound.

As we go offshore, the blade size gets even bigger and it clearly seems, within next two

years, the market for blades larger than 50 meters would increase greatly.

In the early years of wind power development, wood and metals were used to build wind

turbine blades but these materials pose some limitations from megawatt industry’s perspective.

As discussed earlier, wind turbine blades have been manufactured by metals e.g. steel and

aluminum, composite materials e.g. wood, fiber glass and carbon fibers. The inertial and

gyroscopic loads cause fatigue in the blades so they must be stiff, strong and at the same time

lighter to curtail the effect of these loads. The steel and aluminum may not be the right choice for

large blade manufacturing because of their high density, high cost and low fatigue life compared

to modern composites.3 During the last few years, composite materials mainly fiberglass has

been used for manufacturing of blades. Some of the resins which are used in composites

materials are polyester, vinyl-esters and epoxy. The carbon fibers are also being used because of

their high strength to weight ratio.4

2.1. Blade Geometry

The design of blades is incredibly important to figure out the whole methodology behind

wind turbine blades’ materials. Blades experience a wide range of loads from almost all

directions. These loads include flapping, tension, compression and twisting. These loads can be


6

generated either by the movement of blades or by variable winds. In the following Figure 1,

different loads are shown which act on wind turbine blades in general.

Today, the blades are optimized on the basis of low energy cost. The maximum annual

energy production is not the main target for the developers now. In fact the low cost of wind

power is the main goal, and to achieve this goal, efficient and cost effective blade geometry plays

a key role. In the Figure 2, the inside structure of a blade is shown. The design of blade is

optimized by making it a shell like structure. The central spar is used to make it stronger. The

upper and lower spar caps are used to give strength and stiffness during bending and extension.

The spar webs provide shear stiffness during the operation of blades.

Figure 1. The loads acting on wind turbine blades

(http://www.supergen-wind.org.uk/docs/presentations)

Figure 2. Cross-Section of the wind turbine blade

(http://web.mit.edu/windenergy/windweek/Presentations/Nolet_Blades.pdf)


7

3. Wind Turbine Blade Materials

The design life of modern wind turbine blades is generally taken as 20 years and they

normally rotate 109 times during their life.

5The basic design aspect for the wind turbine rotor

blades is the selection of materials. The materials must be strong, stiff and light to achieve cost-

effective production.

The selection of correct material is a tough task and is based on different factors such as

properties of materials, performance, reliability, safety, affects on environment, availability,

recyclability and most important, economics. The wind turbine rotor blades must be selected on

the basis of following properties.

The engineering design of rotor blade is dependent on the properties of materials. These

properties are chemical, mechanical and thermal.

The material of wind turbine blade must allow the blade to perform its function properly

during its design life without failure.

The material must be reliable and must be able to perform its function safely.

The material should be easily accessible and must be available in large quantities.

The cost of manufacturing or processing must be low to make it suitable for profitable

wind energy production.

The material must be able to withstand environmental influences during operational life

of blades.

The design of wind turbine blades is a complex task and it requires the optimization of

properties, performance and economy. In wind turbine rotor blade, stiffness of material is

required to maintain the optimal shape of performance, low density is essential to lessen the

effect of gravity forces, extensive fatigue life is necessary to trim down the material degradation.

Following are the different materials used for wind turbine blade manufacturing.

3.1. Wood

Wood remained a common construction material for wind turbine blades for many years.

Wood is a composite material of cellulose and lignin. Wood could be potentially very important

manufacturing material due to its low density and environmental friendliness. But its low


8

stiffness confines it from being a right choice for large rotor blades because large blades suffer

from high elastic deflections. Moreover the availability, in terms of large quantities, is another

issue so it is difficult to conduct large scale economical manufacturing with wood.

3.2. Steel and Nickel Alloy

Steel was once thought to be a perfect and optimum choice for blade manufacturing. Steel is

an alloy of iron and carbon. Nickel alloy steel has also been used for blade manufacturing due

good thermal and chemical properties like low corrosion etc. but during the last 20 years it was

discarded due to its high weight high cost and low fatigue level.

3.3. Aluminum

Aluminum has a lower density and lower cost than steel. It is a silvery white metal with good

thermal conductivity and ductility. But it cannot be used for commercial blades due to its low

fatigue level and less stiffness. However it can be used for testing purposes.

3.4. Fiber

Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces,

similar to lengths of threads. These materials are stiff and strong. Apart from their stiffness and

strength, these cannot be solely used in the manufacturing of wind turbine blades. However they

are combined in composite materials.6 The fibers are divided into different sub classes e.g. glass

fiber, carbon fiber, aramid, polyethylene and cellulose. In the following text, we will focus more

on glass fiber and carbon fiber.

3.4.1. Glass Fiber

The constituents of glass fiber are the oxides of silicon and aluminum and some other oxides

as well. The glass fiber is an amorphous solid and having properties like stiffness and thermal

expansion. The glass fibers are made with different chemical compositions according to the

specific requirements. The glass fibers are prepared by pulling the fibers from molten glass by

the spinnerets and kept into huge bundles. A simplified picture of spinneret is shown in Figure 3

below7. The diameters of glass fiber is in the range of 10 to 20 m.

8 Their surfaces are coated


9

immediately with a polymer sizing which is used to guard them against cracking and enhance the

bonding force with polymer matrix.9

The E-Glass Fiber is the most widely used fiber in composites. The E-Glass or Electrical

Glass has excellent reinforcing capability. It has the composition SiO2 54wt%, Al2O3 14wt%,

CaO+MgO 22wt%, B2O3 10wt% and Na2O+K2O less then 2wt%.10

It has low cost.

It possesses higher strength and high stiffness.

It’s density is relatively low

Non-flammable, resistant to heat and has good chemical resistance.

It has good electrical insulation

It shows same properties over a wide range of variation in conditions.

Some of the disadvantages are low fatigue resistance, low elastic modulus and higher density

than carbon fibers. The glass fibers are used in different forms in Figure 3, some common forms

are shown.

Figure 3. Spinnerets


10

3.4.2. Carbon Fiber

The carbon fibers are composed of carbon which forms a crystallographic lattice with a

hexagonal shape called graphite. The atoms are bounded by strong covalent bond. The carbon

fibers provide higher strength than glass fibers and they are more useful in handling the fatigue.

But they are more costly than glass fiber. Another issue is the electrical conductivity; their

contact with metal can cause corrosion problems.

Carbon fibers are produced by two methods. The first is Polyacrylonitrile (PAN) method and

the second is Natural Tar method.9

In the first method, PAN fibers are oxidized, heat treated up

E-Glass Fiber Carbon Fiber

Figure 4. Fibers


11

to 1500 oC to 2500

oC and the original C-C backbone of PAN is coupled with graphite hexagon

planes. The atomic structure of PAN is shown in the Figure 5 below.11

In the second method, the natural tar mixtures containing graphite are processed through

various steps and fiber is obtained from spinnerets. The first method is widely used for

commercial purposes. However, research is in progress to find the cheaper raw material for

carbon fiber manufacturing. In Table 1, properties of fibers are shown.

3.5. Matrix Materials

The matrix materials are divided into two main classes’ i.e. thermosetting polymers and

thermoplastic polymers.12

These matrix materials are combined with fibers to make composite

materials. The function of matrix materials is to bind the fibers in the composite materials and

give structural stability to them. The stability and failure strain is moderate for thermosetting

polymers, it is in the range of 5 to 8 % and for thermoplastics it’s usually 50 to 100 %9 that’s

how matrices give high strength to composite materials. Following is the description of

thermosetting and thermoplastic materials.

3.5.1. Thermosetting Polymers

The thermosetting polymers are also divided into three subclasses i.e. unsaturated polyesters,

epoxy resin and vinyl esters.13

The thermosetting polymers have stiffness value in the range of 3

to 4 GPa and densities in the range of 1.1 to 1.3 g/cm3.

They provide internal strength in the

composites through their irreversible curing action.13

The irreversible curing action means they

could not adopt the same form as was before the curing.

Unsaturated Polyesters The unsaturated polyesters are mainly used due to their low cost and

short cure time. They are based on orthophthalic acid or isophthalic acid. The orthophthalic acid

Figure 5. C-C backbone of PAN


12

based polyester resins are less expansive but at the same time they are chemically less stable and

more brittle than isophthalic acid based polyesters. Unsaturated means they have c-c double

bonds in the polyester chain backbone and provide free locations for cross linkages while

polyester means the recurring esters are linked together in a polyester chain back bone. The cross

linking agent in c-c chain is mostly styrene.10

It gives low viscosity which helps the processing

of polyester resin. They can be cured at room temperature but it takes some times e.g. 5 to 6

hours however quick curing could be done at elevated temperature in short time.

Epoxy Resins In these resins, the epoxide ring structure serves as cross-linking site. Different

epoxy resins are being used in market e.g. di-functional epoxy resins like diglycidyl ether of

bisphenol-A (DGEBPA) and tetra-functional epoxy resins like tetraglycidyl methylene dianiline

(TGMDA). The strength at elevated temperatures could be shown in epoxidized phenolic

novolacs resins and tetraglycidyl ether based resins however (DGEBPA) have high fracture

toughness than others. The curing of epoxy resins are done at 150 oC for 3 hours under 1.4 MPa

pressure. Different catalyst and curing agents such as Lewis acids or amines are also used.14

Vinyl Esters There are different kinds of vinyl esters; either they are based on epoxy resins or

non-epoxy resins. But in the composite manufacturing, methacrylate ester is mostly used.15

Their

physical properties are quite similar to epoxy resins while they are could be cured in short time

like unsaturated polyesters. In Table.1 different properties of thermosetting polymer are shown.14


13

3.5.2 Thermoplastic Polymers

These polymers are of plastic that can be softened by heat, hardened by cooling, and then

softened by heat over and over again. Thermoplastics are not cross-linked.The thermoplastic

polymers are also available now for composite manufacturing. But most of them are under

development. They have low values of density i.e. 0.9 g/cm3 and stiffness i.e. 1 to 3 GPa.

16 Most

of the research is being carried out on the recycling of thermoplastics.

The difference between thermosetting polymers and thermoplastic polymer is that

thermosetting polymers do not soften, and will only char and break down at high temperatures

whereas thermoplastics remain permanently fusible so that they will soften and eventually melt

when heat is applied. Thermosetting polymers are heavily cross-linked unlike thermoplastics.

Thermoplastics can be recycled while thermosetting polymers cannot be recycled.

3.6 Composite Materials

The fibers and matrix materials are combined to form a composite material. The fibers act as

reinforcing agents and matrix materials act as binder. In some advanced composites, (polymer

matrix composites) high-strength and high-modulus fibers are enclosed in polymer resin

matrix.17

The most common composite materials are glass fiber composites. These have been used

for many years in manufacturing applications. The glass fiber (70 to 75 % by weight) is bounded

with the epoxy or unsaturated polyester resins. The glass fiber composites have moderate

properties and these are mainly used because of their simple processing technology. The carbon

fiber composites are better than glass fiber composite in terms of weight and stiffness. The use of

carbon fiber composites has increased in large wind turbine rotor blades.18

Due to the high cost

of carbon fiber composites, glass fibers and carbon fibers are combined to make a hybrid

composite. These are combined on layer basis and are widely used to manufacture wind turbine

blades.

The properties of composite materials are dependent on the amount (volume fraction),

orientation of fiber and bond between fiber and matrix materials. The physical properties of these

two materials also affect the properties of the resultant composite material. Stiffness of

composite material is an important property which can be calculated as follows. 19

Ec = Vf*Ef + Vm+Em


14

Where,

Ec = Stiffness (elastic modulus) of composite material

= orientation factor for fiber;

For aligned parallel fibers loaded along the fiber direction=1

For a randomly oriented fiber assembly in two dimensions= 0.334

Vf , Vm = Volume fraction of fiber and matrix material respectively; for perfect composite with

no porosity the sum will be equal 1.

Ef , Em = Stiffness of fiber and matrix material respectively

The composite materials are generally divided into two classes i.e. laminates and sandwiches.

Laminates In laminates, piles or layers of composite materials are bonded together. This is

achieved by compiling the individual layers consist of high-modulus, high-strength fibers

and matrix material. Typical fibers used include graphite, glass, boron, and silicon carbide, and

some matrix materials are epoxies, polyimides, aluminum, titanium, and alumina. In laminates

number of sheets, each having the fibers oriented in different directions is stacked and welded

together to obtain high strength and stiffness within a plane. The arrangement is shown in the

Figure below.


15

Sandwiches are the special forms of a laminated composite comprising a combination of

different materials that are bonded to each other so as to utilize the properties of each separate

component to the structural advantage of the whole assembly. Typically a sandwich composite

consists of three main parts; two thin, stiff and strong faces separated by a thick, light and

weaker core. The faces are adhesively bonded to the core to obtain a load transfer between the

components. Polyvinyl chloride, polyurethane, polystyrene foams, balsa wood, synthetic foams

and honeycombs are used as core materials. An adhesive is used to bind core material with the

layers of composite material

Figure 6. Stiffness versus density graph of different materials

Sandwich Composite

Laminate Composite


16

4 Manufacturing Technologies

After the energy crisis in 1970, the wind turbine manufacturing techniques took a revolution.

Especially the blades increased in size and quantity, the manufacturing processes also shifted

from inefficient, wet and open towards more sophisticated ones. Following is the details of the

processes and techniques.

4.1 Wet Hand Lay Up

It is the oldest manufacturing technique for making wind turbine blades. In this process,

fibers are impregnated by resins in an open mold and allowed to cure under standard atmospheric

conditions. Usually fiber glass and polyester resins are used in this technology. The fibers could

have any specific orientation in the composite. 20

The upper and lower shells are adhesively bound together to form an airfoil structure. For

large blades, webs are inserted into the foils to withstand bending and shear loads. In order to

improve the stiffness of blades, the fiber orientation is changed and unidirectional woven fiber in

the longitudinal direction is used. In some cases fiber roving are laid up parallel with Chopped

strand mats. The fiber roving is also used to make the root end of rotor blade. They are wound

around the steel bushes or tubes and continued back into the blade. These steel bushes form the

holes which become flange of the blade. This flange is used to fix the blade with rotor hub.

Advantages

The process is quite simple.

Cost of tooling is incredibly low

Wide range of fibers and resins can be combined

Disadvantages

Since it is an open process, so the Health, Safety and Environment (HSE) pose lot of

limitations

The quality of final product and also the intermediate steps are dependent on human

skills.

The process only requires low viscosity resins because they are easily operated.

The labor cost is much higher for this process.


17

4.2 Filament Winding

In filament winding process, the fiber material and resin are wound together around a shape,

called mandrel, to make composite material. The filament winding process can employ many

different fibers and resins to get the required characteristics for the finished product. The end

result is an extremely efficient process to create low cost, lightweight, and strong composite

material. This technique was developed in 1970s after hand lay ups techniques. Kaman

Aerospace Corporation and Structural Composites Industries in United States developed 45 m

long blade using this technology.21

Advantages

Low weight and stiff composites can be manufactured by this technology

The filament winding technique can be done by high speed automation

The labor cost is very low so overall cost of this process is low compared to hand lay ups.

Disadvantages

The finished product from filament winding technique has rough external surface which

is not acceptable for upper side of an airfoil.

The viscosity and pot life of resin must be carefully chosen

Some shapes are not easily made by this technology

Wet Hand up Technique26 Filament Winding Technique

26

Figure. 7


18

4.3 Prepreg Technique

In prepreg (pre-impregnated) technology, the fiber material is pre-impregnated with resin at

room temperature to form an intermediate product. This intermediate product (prepregs) is then

stacked and subjected to high temperature and pressure where resin melts and consolidates the

fiber fabrics. The product is now subjected to be cured. This technique is widely used in

aerospace industry. For large wind turbine blades the prepregs are cured at 80 oC which

decreases the process and product cost. 22

Vestas Wind Systems, one of the wind turbine

manufacturers, uses the prepreg technology for their blade production. Vestas has used

glass/epoxy prepreg technology for many years, and they have now introduced carbon fibers in

their 45-m-long blades.23

Advantages

It is easier to control and obtain constant material properties and higher fiber content

gives stiffness and lower weight to final product.

The process is clean so no limitations from Health, Safety and Environment (HSE)

Automation could be installed to enhance the efficiency of overall process

Disadvantages

Vacuum bagging is required for this process which increase the cost of process

Expansive oven system is required

It is difficult to bag complex shaped parts of composite materials

4.4 Resin Infusion Technology

In this manufacturing technology, the dry fibers are sited in a mold, which seals and

encapsulate the dry fibers from all sides. The liquid resin is then injected into these molds, and

the components are kept to cure for some time. The technology was developed in 1950s but after

1990s it has been widely adopted by the industry. The fibers, resins and all the accessories have

been developed now. One of the important issues related with Resin Infusion Technology is the

proper wetness of fibers during the process.24

This problem has vastly been addressed by the

researchers but still more work is need in this field. However following are some parameters

which can solve this issue.


19

The fiber material should be designed and oriented in a special way to control the flow

regimes of resin.

The viscosity of the resin should be lower to enhance the wetability of fiber.

The control of volatile release from resins under vacuum because it creates voids in flow.

Proper accessories must be used for resin flow i.e. resin distribution mesh etc.

Continues mixing of resin without introducing air because the air bubbles affect the

proper layer mixing of resin

The designs of mold should allow proper flow of resins through fiber materials.

The instruments, computer modeling is required for flow measurements and flow

prediction.

LM Glass fiber in Denmark, the leading blade manufacturing company, uses resin infusion

technology and recently it has manufactured 61.5 m long blade using this technology.25

There are

three methods to perform resin infusion technology.

4.4.1 Resin Transfer Molding (RTM)

In RMT technology, dry and pressed stack of fiber material is laid into a mould,

sometimes the mold shape is given to the stack for better fitting. After that, upper mould tool is

kept on the stack and resin is injected from the inlet cavity. Once all the fiber material is wet, the

resin inlets are closed and the laminate is allowed to cure under room temperature or elevated

temperatures.

Vacuum can also be applied to the mould for better flow of resin. This technique is called

Vacuum Assisted Resin Transfer Molding (VARTM). Any combination of fiber and matrix

material can be used for RTM however bismaleimides resins work well at high temperatures

while stitched fibers provide good flow patterns for resins. 26

Advantages

Closed processing technology provides good health, safety and environment (HSE)

control

High fiber volume laminates could be attained with low void contents.

The final component is molded from both sides so it gives more strength.

The labor cost is low for this process


20

Disadvantages

Sometimes un-wetted fractions of fiber fabric ruined the whole mass and it is very

expansive to discard the scrap.

It works very fine with smaller components.

4.4.2 Seeman Composite Resin Infusion Molding Process (SCRIMP)

In this process, dry fiber material is stacked in an open mold and is covered with a non-

structural knitted type fabric and a solid layer. The whole structure is kept in vacuum to remove

the voids within the materials. After vacuum treatment, the resin is transferred trough non-

structural fiber in to the fiber material wets the whole laminate.27

Epoxy, polyester and vinyl

esters are generally used resins with any fiber materials. The difference between (SCRIMP) and

(VARTM) is that initial step of (SCRIMP) is carried out in an open mold while in (VARTM)

every step is done in covered molds. Another difference is that a non-structural fiber is used in

the first step of SCRIMP while this is not the case with (VARTM)

Advantages

The cored structures can be produced in one operation.

Tooling cost is lower than RTM

Large components could be manufactured.

The standard tools for wet layout could be modified for this process

Disadvantages

It is a complex process

Resin should be low in viscosity

Scrap problem is same as with RTM

4.4.3 Resin Film Infusion (RFI)

In this process, the dry fiber fabric is stacked with semi-solid layers of resins. The assembly

of fiber and resin is subjected to vacuum to remove any air entrapped. Then the whole structure

is heated and cured to make a stiff composite.28

Generally epoxy resins are used with any kind of

fiber material while polyvinylchloride is usually taken as core material.

Advantages

The void contents are generally very low in the composites prepared by this technology.

Lower cost than Prepreg technology.


21

The Health, Safety and Environment (HSE) are well controlled

Disadvantages

This process is not well proven outside the aerospace industry

Tooling cost is generally higher because the treatment includes variable temperature

ranges

Costly oven and vacuum system is needed

Core materials suffer from high temperature and pressure

5 Conclusion

As fuel prices rise and the climate warms everyone is looking for alternative sources of

energy to power our lives and wind is an obvious source. Wind turbines have been used to

capture energy from the wind for many years. The success of wind turbine depends heavily on

the correct blade design and its life. The blade life and its sustainable operation both are

dependent on the material of construction. The material selection is very important task because

the planner has to make a very delicate optimization of strength and economics in order to keep

the wind power project potentially and economically viable.

Resin Transfer Molding26

SCRIMP28 Resin Film Infusion

27

Figure. 8


22

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24

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