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Non-crimp fabric composites in wind turbines
21 Non-crimp fabric composites in wind turbines G. ADOLPHS and C. SKINNER, OCV Technical Fabrics, Belgium
Abstract: Historical development and modern use of non-crimp fabric (NCF) in wind energy applications such as blade and nacelle are given. The influence of fabric processing such as skewing, resin infusion and pre-impregnation (prepreg) are also shown. Key words: wind energy, blade design, specific strength, resin infusion, nacelle design.
21.1
Introduction
21.1.1 The oil crisis as the initiator for wind energy The trigger for the development of the modern wind power industry is often described as the energy crisis of 1973. The ‘oil price shock’ of 1973 initiated a public debate about the dependence of Western economies on oil imports so, in addition to energy saving measures, politicians turned their attention to the search for alternative energy sources. This led to the development of many programmes, such as the those sponsored under the US Federal Wind Energy Programme (1973),1 the formation of the National Swedish Board for Energy Source Development (1975), a range of experimental turbines in Denmark and a range of government subsidised programmes in Germany led by the Bundesministerium fur Forschung und Technologie, which ultimately led to the construction of the ‘Growian’ (Gross Windkraft-Anlage) which gained much notoriety.2,3 In many cases, these extensive government-funded programmes resulted in little tangible development of the industry and, after the crisis, only one country demonstrated the consistently successful operation of wind turbines: Denmark. The basic technical concepts of the turbines employed had been developed in the beginning of the 20th century by Poul La Cour (Askov, Denmark), Albert Betz (Göttingen, Germany) or Palmer Cosslett Putnam (Vermont, USA) and had found relatively widespread adoption due to the superiority of the design due to the following characteristics. • •
Sleek, fast-running propeller designs which produce low thrust at high torque and can more easily withstand high wind speeds. Rotor speed and power output can be controlled by pitching the blades and this also provides effective protection against extreme wind speeds. 481 © Woodhead Publishing Limited, 2011
482 •
Non-crimp fabric composites
Fibre-reinforced composite materials allow aerodynamically optimised shapes and enabling highest efficiency of the unit while being lightweight, fatigue and weather resistant.
In this area some small- and medium-sized manufacturing firms in Denmark, for example, VESTAS, seized an opportunity of constructing these three-blade rotors and grid connections and started to sell these wind turbine concepts to private owners or farmers. These were typically small units (60 kw) with rotors of 15–16 m. In 1986, these units contributed ∼1% of Denmark′s power requirements, but from this point began to grow exponentially based on clearly defined permits and the availability of appropriate testing stations.
21.1.2 The evolution material choice in wind rotor blade technology Rotor blade technology for these wind turbines has evolved in the 40 years since the ‘energy crisis’ of the 1970s and now can be associated more with lightweight aeronautical engineering than with conventional mechanical engineering.4,5 In contrast to all other components of the turbine which can be ‘borrowed’ for existing fields of engineering, the rotor blades must be developed to match the load spectrum of a given turbine. However, in contrast to aircraft engineering the cost structure of wind energy operation prohibits traditional aircraft manufacturing methods and production technology and was adopted from other fields. In the case of rotor blades the transfer primarily comes from the modern boat building and industrial engineering where fibreglass composites predominate.6 The rotor blades of older Danish wind turbines were almost always manufactured by former boat builders. In the past, the starting point for rotor blade design was the question of which material would be the most suitable. Design and manufacturing methods are in reality determined by which material is the most suitable. In other words, it is impossible to separate material and manufacturing process. Analysis of materials common in aerospace engineering highlights the following materials as ‘suitable in principle’ for rotor blade construction.7 • • • •
Aluminium. Titanium. Steel. Fibre composite material (glass, carbon, aramide).
The most important properties by which a first assessment can be made are • •
specific strength (strength/density); modulus of elasticity;
© Woodhead Publishing Limited, 2011
Non-crimp fabric composites in wind turbines
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21.1 Comparison of unit material cost per unit of strength for a beam in bending under a cyclic load analogous to a wind rotor blade.9
• •
specific modulus (modulus of elasticity/density); ∧ ∧ fatigue strength after 10 7 to 10 8 load cycles.8
Comparison of the material properties highlights that the excellent balance of properties exhibited by fiber glass/epoxy composites making them ideal choices for delivering cost effective strength in engineering configurations as found in wind rotor blades. Figure 21.1 shows different material options in terms of their specific strength and specific cost performances. Fibre glass/epoxy has both a low mass and cost per unit of strength which explains its adoption as the material of choice for rotor blades.
21.2
Development of non-crimp fabric (NCF) composites in wind energy
21.2.1 The impact of infusion technology on NCFs
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21.2 Blade length over time, main years of build of different blade lengths.
21.2.2 Traditional woven structures in wind rotor blades From the inception of the wind energy industry in Denmark, the materials chosen for the construction of the wind rotor blades for fibre-reinforced composites was based on reinforcing structures taken from the boat building industry. From the mid-1970s, the reinforcement materials typical used in the construction of composite craft were: •
woven roving using plain ligament and equal amount in warp and weft fibre in 300, 500, 600 and 800 gr/m2 total area weight;
21.3 Typical blade design, middle section, two shear webs, source: presentation M. Zvanik, Owens Corning and DIAB, CFA Show 2001, Tampa, Florida.
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combination of these typical woven materials produced in combination with chopped fibre mat of 300 and 450 gr/m2, creating combination products, mostly used were woven roving of 500 and 600 gr/m2 and mat of 300 gr/m2; and unidirectional woven roving of 600 gr/m2 and 800 gr/m2, later as well of 1200 gr/m2 having a percentage of warp reinforcement of usually 90%.
Instead of using +/−45° biaxial non-crimp fabric (NCF) woven fabrics were laid up at an angle of 45° converting warp in +45° and weft in −45° or vice versa. A typical blade design during this period was using the unidirectional (UD) fabrics to form spar caps, woven fabrics for the skin structure and shear web and complexes for the coupling area.
21.2.3 Introduction of skewed fabrics in the late 1980s An evolution in the construction of reinforcements for the wind rotor blades was the use of skewed fabrics in the late 1980s. In this process a woven weft UD is skewed to a 45° fabric during winding or unwinding and at the same time another woven fabric is skewed to −45°. During the assembly process the individual fabrics are combined by a warp-knitting operation similar to the combination structures described earlier. Figure 21.4 shows this process schematically. The process also allowed the addition of chopped fibre to modify the infusion
21.4 Schematic of a typical skewing process using a Malimo or Liba knitting machine.
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characteristics of the complexes. The materials from this type of process are still in use today in the construction of wind rotor blades. Another option was during the 1980s and 1990s to use sequential weft insertion warp-knitting machines which produce a weft UD fabric and skew this fabric to the desired +45° or −45° angle and assemble two skewed fabrics with or without chopped. As the knitting operation inserts the weft yarns one by one, they were already NCF by today's standards. However, the described skewing and assembly process causes fibre misalignment and distortion of the selected fibre orientation. Both effects have shown inferior mechanical properties.10 During the 1980s it was as already possible to produce +/−45° structures in a direct process by early weft-laying NCF machines, but they lacked a suitable adjustment to produce true parallel weft. The weft structure was crosslaid and the resulting mechanical laminate properties were inferior to the true parallel weft insertion materials. As these materials were expensive, they were used to little extent. Production methods used by the vast majority of blade producers were still hand lay-up.
21.2.4 Vacuum-assisted resin transfer and the growth of NCFs To make the blade longer and aerodynamically more effective, the materials needed to improve. For a longer blade an increase in material stiffness is desirable and more valued. Appropriate scaling factors which relate properties to rotor diameter have been published.11 Importantly, for the development of the wind rotor blade technology a new process technology was developing. The vacuum-assisted resin infusion process or vacuumassisted resin transfer moulding (VARTM) or vacuum infusion process (VIP) became increasingly popular. This methodology allowed an increment in material stiffness by incrementing the fibre volume fraction from 35% to 40% fibre volume fraction (FVF) typical of a hand lay-up to 50% to 55% FVF by the VARTM process. The effect on laminate tensile stiffness made out of UD NCF fabrics is an increase from approximately 25–30 GPa to over 37 GPa, which means an increase of around 25%. In woven structures, a higher area weight causes a dense structure of reinforcement material which needs to be interlaced, which causes increased crimp of the reinforcement fibre. In an NCF this effect is drastically reduced as reinforcement fibre is laid parallel and the formed ply of reinforcement is laid one over the other. The new vacuum processes not only delivered an increment in mechanical properties. Heavyweight NCF could still be impregnated and a high fibre volume fraction could be achieved. The combination of both the NCF reinforcements and the VARTM process allowed wind blade producers to benefit from the improved cost and reduced production time which offered this combination. Reduced costs and higher stiffness facilitated the development of longer and more efficient blades.
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Table 21.1 Possible and frequently used warp weight (in bold) in unidirectional materials Warp roving Gauge [tex] [yarn/inch]
[yarn/cm]
1200 1200 1200 1200 1200 1200 1200 1200 2400 2400 2400 2400 2400 2400 2400 2400
1.97 2.36 2.76 3.15 3.54 3.94 4.72 5.51 1.97 2.36 2.76 3.15 3.54 3.94 4.72 5.51
5 6 7 8 9 10 12 14 5 6 7 8 9 10 12 14
Area weight [gr/cm2] 236 283 331 378 425 472 567 661 472 567 661 756 850 945 1134 1323
Many of these early NCF materials used in the VARTM processes were UD materials. The main glass roving available were 1200 tex and 2400 tex, and rarely 1500 tex and 2200 tex. Taking usually available knitting machine elements the following area weights shown in Table 21.1 could be produced, with the highlighted combinations being the more frequent versions observed in the industry. Apart from the UD NCF, multiaxial fabric as +/−45° biaxials are needed in the laminate design of modern windmill blades. During the 1980s the first parallel weftproducing multiaxial warp-knitting machines were presented and allowed the production of true parallel weft at adjustable angles ranging from 90° to around 30°. As well as with UD fabrics, a range of biaxial fabrics established itself as a function of available glass roving and cost efficiency in production.12 Early version of these fabrics have been produced using 68 tex to 134 tex glass yarn and +/−45° fabrics of around 300 gr/m2 were produced. The laminate thickness in longer blades increased. Heavier area weight could be used. Consequently 200 tex direct roving was used to produce biaxial fabrics of 450 gr/m2 and today 300, 600 and 1200 tex direct roving is used to produce +/−45° biaxial fabrics of 800 gr/m2, 1000 gr/m2 and above.
21.2.5 Development of weft technologies in NCF reinforcement structures Over time the machine technology also allowed a more precise insertion of weft yarns. While in many older machine configurations the weft lay-up was not
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21.5 Description of typical weft insertion possibilities using NCF production machine.
synchronised with the exact production speed, cross weft or substantial parallel weft was inserted as shown in Figure 21.5. Modern-day machines for the production of NCF structures with numerical control allow for the construction of true parallel weft. As consequence of the irregular crossover of weft reinforcement local small defects are common in the types of fabrics resulting in lower mechanical properties. A more tailored used of multiaxial material can be found in the production of substructures of wind rotor blades as, for example, flanging rings. These are designed to allow the blade to be screwed to the cone and are coupled with the main blade laminate. This construction can be prepared by infusion together with the main shell or potentially produced as a separate part and are assembled in a final production stage by structural adhesives. The screws can be of a T-bolt type or carrot type, glued in screws. The T-bolt type needs in general a laminate design adequate to absorb the shear loads and pressure caused by the T-bolt and its design is similar to laminate designs for riveting with an approximate amount of 50% UD and 50% of a +/−45° biaxial oriented fiber.13 An example can be named a flanging ring build-up by winding 45°/90°/−45° in 200/400/200 gr/m2 triaxial fabric. A glued-in screw can in general terms build up by much higher UD-oriented designs. An example is a flanging ring build-up by +80°/0°/−80° 300/60/300 gr/m2 oriented NCF multiaxial fabric produced in a fabric winding operation. Generally, it is possible to describe the construction of a typical wind rotor blade using NCF reinforcement structures using a range of typical styles as in Figure 21.5.14 As it can be seen in Plate XIII (see between pages 396 and 397), a substantial part of the laminate is build-up by UD and triaxial NCF fabrics. In common designs, the UD NCF portion in a modern wind rotor blade is 50–60% of the NCF fabrics. Triaxials are reduced in favor of UD and biaxial +/−45° fabrics in bigger blades. Shear webs are build mainly using NCF constructions of +/−45° materials. In such designs a critical area in a blade is the large flat surface area towards the root. In this area the loads are more complex caused by combinations of air pressure, flap-wise deflection and edge-wise deflections. Local stress concentrations can occur and buckling can be caused in the compression zone under deflection. An image from possible buckling is described in Fig. 21.6.
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21.6 Buckling simulation of a blade (Locke J., Valencia U., ‘Design Studies for Twist Coupled Wind Turbine Blades’, SAND 2004-0522, Albuquerque/Wichita, USA).
21.2.6 The use of UD prepregs in wind rotor blade design An alternative design option developed by Danish wind rotor blade manufacturers is shown in Fig. 21.7. The figure highlights that in this construction the shear web construction commonly associated with the VARTM process to produce a full blade have been replaced by a central ‘wingbox’ structure. The top and bottom of these constructions are built up using UD reinforcements and both sides have the function of the shear web. The box structure holds approximately 85% of the mechanical load and is covered by a fairing that is glued on. The UD structure is composed of UD prepreg and biaxial dry fabrics. Other biaxial prepregs build up the sidewalls together with core material, forming said shear web. The fairing is designed using a combination of triaxial and biaxial prepreged NCF fabrics and core materials. Pre-impregnated reinforcement fibre and fabrics, a material type and related vacuum-bagging process used mainly for aerospace part manufacture, show, as the infusion process, a clear improvement in mechanical properties versus the hand lay-up process. As the materials used are pre-impregnated, their commercial use requires a refrigerated supply chain, a precise cut, lay-up and debulking process to remove trapped air in between the material layers. A common step in the process in aerospace applications is to use an autoclave to compact the material under higher
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21.7 Central spar design typical of those used commercially by VESTAS and Gamesa.
than atmospheric pressure. Because of the immense size of wind turbine blades and their components or subparts this cannot be done for economic reasons. However, tools must be heated up to around 80°C to cure the used epoxy resin system. The UD prepreg approach, however, holds some advantages with respect to NCF, as it can be made of 100% oriented fibre structures in an UD direction, no weft is necessary and the amount of resin can be precisely adjusted to the required tolerances and the resins can have higher viscosity than resin systems used for the infusion process. Triaxial and biaxial prepreg show no or little differences to material used in the infusion process and demonstrate very similar mechanical properties. The reason is that those prepregs are made entirely out of the same NCF. With respect to the use of carbon fibre for UD structures as spar cap (or girder) the UP prepreg process offer some advantages. • •
The impregnation process can be controlled and adjusted to produce a full impregnation of the carbon fibre prior to their use in the moulding application. The impregnation process can be adjusted to increment the amount of resin in the UD prepreg vs. VARTM thus reducing the fibre volume fraction. This reduces overall material cost and reduces the carbon prepreg laminate modulus. The reduction in modulus improves the compatibility with glass fibre prepreg in hybrid carbon–glass designs too.
The major loads that wind rotor blades are subjected to aerodynamic loads as thrust and lift on the blade. Thrust is causing a flap-wise deflection, aerodynamic lift causes a mixed flap and edgewise bending and the torque to spin the rotor and produce energy. Torsion loads twisting the blade along their axis are caused by thrust and lift. The blade mass is subject to both gravity and inertia loads. Gravity loads pull the blade down and act as an edgewise load. Inertia loads are caused by the rotation and by vibrations. Lower masses and reduced vibrations reduce substantially these loads (Plate XIV).15 All these loads cause multiple static and dynamic stresses which any material used on the construction of a wind rotor blade must be able to withstand over the designed lifetime which is commonly between 20 and 25 years. To determine the suitability of a reinforcement material for this operating environment a range of tests are therefore carried out to show the suitability of the used materials. These
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are described by standards edited by the main certification bodies.16 All NCF materials to be used in a wind rotor blade have to be tested according to these standards. The main parameters are • • •
tensile strength and modulus in reinforcement direction and transversal direction; shear strength and modulus; and fatigue strength in three different load ratios up to two million load cycles.
Another important parameter is the thickness of the fabric used. As wind rotor blades are built by lay-up of individual NCF or prepreg layers, at each start and end of a new layer, a drop in laminate thickness is generated. This causes a sudden difference in laminate properties and a shear tension at the cut edge of the new layer in the boundary between new layer and existing laminate. The material design must withstand the deformation at these boundaries over time. Another specific requirement is caused by frequent lightning strikes to wind energy converters. NCF materials that are used in wind turbine blades must, in addition, show a uniform dielectric behavior and must not offer undesired local conductivity.
21.3
NCF materials used in nacelle construction
In almost all horizontal wind energy converters, the key components of the machine are housed in a closed nacelle on top of the tower as a prolongation of the turbine axis. In reality, the design concept of the nacelle is defined by the arrangement of the drive turbine and the generator. Ease of assembly and total cost are also key defining factors in the nacelle construction. The most common design is of either a welded bedplate or a cast bedplate, on which the non-load bearing structure is attached. Various materials have been used for this non-load bearing structure such as aluminium or steel structures. Because of their light weight, facility to be designed in distinctive free-form designed shapes, very good corrosion and weather resistance, these shells have been made using glass fibre reinforced composite materials for the past decades. The nacelle design and production methods have followed closely the evolution of rotor blades. At one hand, most of the designers used in dimensioning and layout of blades worked in the design and laminate layout of nacelles, and on the other, the requirements with regard to lifetime, weight and cost are similar. So it is no wonder that when hand lay-up dominated the blade manufacture, nacelle covers, as shown in Plate XV, were initially produced in a hand lay-up processes, but when blade manufacture went to infusion processes, the nacelle production went too. Today there are two basic processes used. One is VARTM, as used in blade manufacture, but using the more convenient 0/90 biaxial NCFs instead of more costly biaxial +/−45° laminates, and, to build up the required
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thickness as well, a combination products of NCF with mat, sometime with enhanced resin flow properties. The other process applied is RTM light where a rigid mould and semi-flexible counter (male) mould is used. Advantages of this system are both outside and inside smoother finishes, but the resin amount needed is higher. A compressible reinforcement material is needed too which increases cost.
21.4
Future trends
Currently there are several issues related to the future use of NCF in the efficient construction of wind rotor blades. Production consistency One of the major concerns is the high irregularity of mechanical properties as a consequence of fibre misalignment, local imperfections as missing or displaced reinforcement fibre and wrinkles of the reinforcement structure formed in the laminate. Other issues can be attributed to material and processes variations. Foaming of the resin, incomplete wet-out, or an excessive exothermic reaction or ‘heat generation’ during the reaction of the used resins cause a further deterioration of the laminate properties. Even though the required security factors against failure are estimated on statistical requirements lower than in aerospace applications the level of inherent over design due to these factors has an increasing impact as blades become longer and heavier in the drive to lower of cost of electricity generation through wind power. Manufacture of larger and longer blades To be able to produce bigger and bigger blades and due to the challenge of transporting structures higher than 5 m in a cost-effective manner, new concepts have been evolved. One possibility is to build a blade in segments or sub-assemblies which are assembled or produced as preassembled dry structure, so called preforms. These parts are either infused as a whole or, if produced independently, assembled using structural adhesives. Examples of this type of approach are pioneered by companies such as Blade Dynamics (Plate XVI). Optimisation of the aerodynamic performance of wind rotor blades In the drive to constantly reduce the cost of wind energy, there is a tendency to the production of sleeker and aerodynamically more efficient blades. To realise these types of designs, the requirement for sophisticated fibre and NCF fabric properties increases, providing another challenge for the producers of NCF reinforcement structures.17
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21.5
493
References
1. Goodmann Gr., F. R; Vachon W.A.; United States Electricity Utility Activities in Wind Power, Fourth International Symposium on Wind Energy Systems, Stockholm, Sweden, Sept. 1–24, 1982; Hydro-Quebec: Project Eole, 4-Mw Vertical Axis Aerogenerator, Montreal, 1985. 2. Ministry of Energy (Danish Energy Agency): Wind Energy in Denmark, Research and technological Development, 1990. 3. Friis, P., Large Scale Wind Turbines, Operating Hours and Energy Production, Elsam Project A/S. Internal Report, 1993. 4. Thomas Ackermann, Lennart Söder, Wind energy technology and current status: a review Renewable and Sustainable Energy Reviews, Volume 4, Issue 4, December 2000, Pages 315–374. 5. C. Soutis, Fibre reinforced composites in aircraft construction Progress in Aerospace Sciences, Volume 41, Issue 2, February 2005, Pages 143–151. 6. Isao Kimpara, Use of advanced composite materials in marine vehicles, Marine Structures, Volume 4, Issue 2, 1991, Pages 117–127. 7. L.M. Wyatt : Materials for MW sized aerogenerators Part 2. Materials characteristics Materials & Design, Volume 4, Issue 5, October–November 1983, Pages 880–884. 8. Christoph W. Kensche: Fatigue of composites for wind turbines, International Journal of Fatigue, Volume 28, Issue 10, October 2006, Pages 1363–1374. 9. Data from internal analysis by Owens Corning, 2010. 10. Mandell, J.F., Samborsky, D.D., and Cairns, D.S., ‘Fatigue of Composite Materials and Substructures for Wind Turbine Blades’, Contractor Report SAND2002-0771, Sandia National Laboratories, Albuquerque, NM 2002. 11. Manwell J.F, McGowan J.G, Rogers A.L., Wind Energy Explained, University of Massachussetts, Amherst, USA. 12. P.J. Hogg, A. Ahmadnia, F.J. Guild: The mechanical properties of non-crimped fabricbased composites, Composites, Volume 24, Issue 5, July 1993, Pages 423–432. 13. Michaeli/Huybrechts/Wegener, Dimensionieren mit Faserverbundkunststoffen, Hanser, 1995. 14. Generic IEC Class II blade construction – Owens Corning internal information 2010. 15. Det Norske Veritas, Guidelines for Design of Wind Turbines, DNV/Risoe, Copenhagen, Denmark, 2nd Edition. 16. Germanischer Lloyd, Guidelines for the Certification of Wind Turbines, Edition 2010, Hamburg, Germany, 2010]. 17. Locke J., Valencia U., ‘Design Studies for Twist Coupled Wind Turbine Blades’, SAND 2004-0522, Albuquerque/Wichita, USA]
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Plate XI Parameters for the delamination interface model for materials NCF/LY3505 (* estimated) (Chapter 16).
Plate XII C-scans of impacted non-crimp fabric and prepreg specimens (30J) (Chapter 18).
Plate XIII Typical blade layout IEC class II 40m blade detailing the laminate constructions based on non-crimp fabric reinforcement materials (Chapter 21).
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Plate XIV Blade deflecting under loads, coloured areas indicate highest stresses (Chapter 21).
Plate XV Nacelle cover under load (wind, snow). (Source M. Zvanik, OC Presentation CFA 2001, Tampa, FL). (Chapter 21).
Plate XVI Production in segments: Shell, spar cap, and root joint/ flanging reinforcements (Chapter 21).
© Woodhead Publishing Limited, 2011
Abstract: Historical development and modern use of non-crimp fabric (NCF) in wind energy applications such as blade and nacelle are given. The influence of fabric processing such as skewing, resin infusion and pre-impregnation (prepreg) are also shown. Key words: wind energy, blade design, specific strength, resin infusion, nacelle design.
21.1
Introduction
21.1.1 The oil crisis as the initiator for wind energy The trigger for the development of the modern wind power industry is often described as the energy crisis of 1973. The ‘oil price shock’ of 1973 initiated a public debate about the dependence of Western economies on oil imports so, in addition to energy saving measures, politicians turned their attention to the search for alternative energy sources. This led to the development of many programmes, such as the those sponsored under the US Federal Wind Energy Programme (1973),1 the formation of the National Swedish Board for Energy Source Development (1975), a range of experimental turbines in Denmark and a range of government subsidised programmes in Germany led by the Bundesministerium fur Forschung und Technologie, which ultimately led to the construction of the ‘Growian’ (Gross Windkraft-Anlage) which gained much notoriety.2,3 In many cases, these extensive government-funded programmes resulted in little tangible development of the industry and, after the crisis, only one country demonstrated the consistently successful operation of wind turbines: Denmark. The basic technical concepts of the turbines employed had been developed in the beginning of the 20th century by Poul La Cour (Askov, Denmark), Albert Betz (Göttingen, Germany) or Palmer Cosslett Putnam (Vermont, USA) and had found relatively widespread adoption due to the superiority of the design due to the following characteristics. • •
Sleek, fast-running propeller designs which produce low thrust at high torque and can more easily withstand high wind speeds. Rotor speed and power output can be controlled by pitching the blades and this also provides effective protection against extreme wind speeds. 481 © Woodhead Publishing Limited, 2011
482 •
Non-crimp fabric composites
Fibre-reinforced composite materials allow aerodynamically optimised shapes and enabling highest efficiency of the unit while being lightweight, fatigue and weather resistant.
In this area some small- and medium-sized manufacturing firms in Denmark, for example, VESTAS, seized an opportunity of constructing these three-blade rotors and grid connections and started to sell these wind turbine concepts to private owners or farmers. These were typically small units (60 kw) with rotors of 15–16 m. In 1986, these units contributed ∼1% of Denmark′s power requirements, but from this point began to grow exponentially based on clearly defined permits and the availability of appropriate testing stations.
21.1.2 The evolution material choice in wind rotor blade technology Rotor blade technology for these wind turbines has evolved in the 40 years since the ‘energy crisis’ of the 1970s and now can be associated more with lightweight aeronautical engineering than with conventional mechanical engineering.4,5 In contrast to all other components of the turbine which can be ‘borrowed’ for existing fields of engineering, the rotor blades must be developed to match the load spectrum of a given turbine. However, in contrast to aircraft engineering the cost structure of wind energy operation prohibits traditional aircraft manufacturing methods and production technology and was adopted from other fields. In the case of rotor blades the transfer primarily comes from the modern boat building and industrial engineering where fibreglass composites predominate.6 The rotor blades of older Danish wind turbines were almost always manufactured by former boat builders. In the past, the starting point for rotor blade design was the question of which material would be the most suitable. Design and manufacturing methods are in reality determined by which material is the most suitable. In other words, it is impossible to separate material and manufacturing process. Analysis of materials common in aerospace engineering highlights the following materials as ‘suitable in principle’ for rotor blade construction.7 • • • •
Aluminium. Titanium. Steel. Fibre composite material (glass, carbon, aramide).
The most important properties by which a first assessment can be made are • •
specific strength (strength/density); modulus of elasticity;
© Woodhead Publishing Limited, 2011
Non-crimp fabric composites in wind turbines
483
21.1 Comparison of unit material cost per unit of strength for a beam in bending under a cyclic load analogous to a wind rotor blade.9
• •
specific modulus (modulus of elasticity/density); ∧ ∧ fatigue strength after 10 7 to 10 8 load cycles.8
Comparison of the material properties highlights that the excellent balance of properties exhibited by fiber glass/epoxy composites making them ideal choices for delivering cost effective strength in engineering configurations as found in wind rotor blades. Figure 21.1 shows different material options in terms of their specific strength and specific cost performances. Fibre glass/epoxy has both a low mass and cost per unit of strength which explains its adoption as the material of choice for rotor blades.
21.2
Development of non-crimp fabric (NCF) composites in wind energy
21.2.1 The impact of infusion technology on NCFs
© Woodhead Publishing Limited, 2011
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Non-crimp fabric composites
21.2 Blade length over time, main years of build of different blade lengths.
21.2.2 Traditional woven structures in wind rotor blades From the inception of the wind energy industry in Denmark, the materials chosen for the construction of the wind rotor blades for fibre-reinforced composites was based on reinforcing structures taken from the boat building industry. From the mid-1970s, the reinforcement materials typical used in the construction of composite craft were: •
woven roving using plain ligament and equal amount in warp and weft fibre in 300, 500, 600 and 800 gr/m2 total area weight;
21.3 Typical blade design, middle section, two shear webs, source: presentation M. Zvanik, Owens Corning and DIAB, CFA Show 2001, Tampa, Florida.
© Woodhead Publishing Limited, 2011
Non-crimp fabric composites in wind turbines • •
485
combination of these typical woven materials produced in combination with chopped fibre mat of 300 and 450 gr/m2, creating combination products, mostly used were woven roving of 500 and 600 gr/m2 and mat of 300 gr/m2; and unidirectional woven roving of 600 gr/m2 and 800 gr/m2, later as well of 1200 gr/m2 having a percentage of warp reinforcement of usually 90%.
Instead of using +/−45° biaxial non-crimp fabric (NCF) woven fabrics were laid up at an angle of 45° converting warp in +45° and weft in −45° or vice versa. A typical blade design during this period was using the unidirectional (UD) fabrics to form spar caps, woven fabrics for the skin structure and shear web and complexes for the coupling area.
21.2.3 Introduction of skewed fabrics in the late 1980s An evolution in the construction of reinforcements for the wind rotor blades was the use of skewed fabrics in the late 1980s. In this process a woven weft UD is skewed to a 45° fabric during winding or unwinding and at the same time another woven fabric is skewed to −45°. During the assembly process the individual fabrics are combined by a warp-knitting operation similar to the combination structures described earlier. Figure 21.4 shows this process schematically. The process also allowed the addition of chopped fibre to modify the infusion
21.4 Schematic of a typical skewing process using a Malimo or Liba knitting machine.
© Woodhead Publishing Limited, 2011
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characteristics of the complexes. The materials from this type of process are still in use today in the construction of wind rotor blades. Another option was during the 1980s and 1990s to use sequential weft insertion warp-knitting machines which produce a weft UD fabric and skew this fabric to the desired +45° or −45° angle and assemble two skewed fabrics with or without chopped. As the knitting operation inserts the weft yarns one by one, they were already NCF by today's standards. However, the described skewing and assembly process causes fibre misalignment and distortion of the selected fibre orientation. Both effects have shown inferior mechanical properties.10 During the 1980s it was as already possible to produce +/−45° structures in a direct process by early weft-laying NCF machines, but they lacked a suitable adjustment to produce true parallel weft. The weft structure was crosslaid and the resulting mechanical laminate properties were inferior to the true parallel weft insertion materials. As these materials were expensive, they were used to little extent. Production methods used by the vast majority of blade producers were still hand lay-up.
21.2.4 Vacuum-assisted resin transfer and the growth of NCFs To make the blade longer and aerodynamically more effective, the materials needed to improve. For a longer blade an increase in material stiffness is desirable and more valued. Appropriate scaling factors which relate properties to rotor diameter have been published.11 Importantly, for the development of the wind rotor blade technology a new process technology was developing. The vacuum-assisted resin infusion process or vacuumassisted resin transfer moulding (VARTM) or vacuum infusion process (VIP) became increasingly popular. This methodology allowed an increment in material stiffness by incrementing the fibre volume fraction from 35% to 40% fibre volume fraction (FVF) typical of a hand lay-up to 50% to 55% FVF by the VARTM process. The effect on laminate tensile stiffness made out of UD NCF fabrics is an increase from approximately 25–30 GPa to over 37 GPa, which means an increase of around 25%. In woven structures, a higher area weight causes a dense structure of reinforcement material which needs to be interlaced, which causes increased crimp of the reinforcement fibre. In an NCF this effect is drastically reduced as reinforcement fibre is laid parallel and the formed ply of reinforcement is laid one over the other. The new vacuum processes not only delivered an increment in mechanical properties. Heavyweight NCF could still be impregnated and a high fibre volume fraction could be achieved. The combination of both the NCF reinforcements and the VARTM process allowed wind blade producers to benefit from the improved cost and reduced production time which offered this combination. Reduced costs and higher stiffness facilitated the development of longer and more efficient blades.
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Table 21.1 Possible and frequently used warp weight (in bold) in unidirectional materials Warp roving Gauge [tex] [yarn/inch]
[yarn/cm]
1200 1200 1200 1200 1200 1200 1200 1200 2400 2400 2400 2400 2400 2400 2400 2400
1.97 2.36 2.76 3.15 3.54 3.94 4.72 5.51 1.97 2.36 2.76 3.15 3.54 3.94 4.72 5.51
5 6 7 8 9 10 12 14 5 6 7 8 9 10 12 14
Area weight [gr/cm2] 236 283 331 378 425 472 567 661 472 567 661 756 850 945 1134 1323
Many of these early NCF materials used in the VARTM processes were UD materials. The main glass roving available were 1200 tex and 2400 tex, and rarely 1500 tex and 2200 tex. Taking usually available knitting machine elements the following area weights shown in Table 21.1 could be produced, with the highlighted combinations being the more frequent versions observed in the industry. Apart from the UD NCF, multiaxial fabric as +/−45° biaxials are needed in the laminate design of modern windmill blades. During the 1980s the first parallel weftproducing multiaxial warp-knitting machines were presented and allowed the production of true parallel weft at adjustable angles ranging from 90° to around 30°. As well as with UD fabrics, a range of biaxial fabrics established itself as a function of available glass roving and cost efficiency in production.12 Early version of these fabrics have been produced using 68 tex to 134 tex glass yarn and +/−45° fabrics of around 300 gr/m2 were produced. The laminate thickness in longer blades increased. Heavier area weight could be used. Consequently 200 tex direct roving was used to produce biaxial fabrics of 450 gr/m2 and today 300, 600 and 1200 tex direct roving is used to produce +/−45° biaxial fabrics of 800 gr/m2, 1000 gr/m2 and above.
21.2.5 Development of weft technologies in NCF reinforcement structures Over time the machine technology also allowed a more precise insertion of weft yarns. While in many older machine configurations the weft lay-up was not
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21.5 Description of typical weft insertion possibilities using NCF production machine.
synchronised with the exact production speed, cross weft or substantial parallel weft was inserted as shown in Figure 21.5. Modern-day machines for the production of NCF structures with numerical control allow for the construction of true parallel weft. As consequence of the irregular crossover of weft reinforcement local small defects are common in the types of fabrics resulting in lower mechanical properties. A more tailored used of multiaxial material can be found in the production of substructures of wind rotor blades as, for example, flanging rings. These are designed to allow the blade to be screwed to the cone and are coupled with the main blade laminate. This construction can be prepared by infusion together with the main shell or potentially produced as a separate part and are assembled in a final production stage by structural adhesives. The screws can be of a T-bolt type or carrot type, glued in screws. The T-bolt type needs in general a laminate design adequate to absorb the shear loads and pressure caused by the T-bolt and its design is similar to laminate designs for riveting with an approximate amount of 50% UD and 50% of a +/−45° biaxial oriented fiber.13 An example can be named a flanging ring build-up by winding 45°/90°/−45° in 200/400/200 gr/m2 triaxial fabric. A glued-in screw can in general terms build up by much higher UD-oriented designs. An example is a flanging ring build-up by +80°/0°/−80° 300/60/300 gr/m2 oriented NCF multiaxial fabric produced in a fabric winding operation. Generally, it is possible to describe the construction of a typical wind rotor blade using NCF reinforcement structures using a range of typical styles as in Figure 21.5.14 As it can be seen in Plate XIII (see between pages 396 and 397), a substantial part of the laminate is build-up by UD and triaxial NCF fabrics. In common designs, the UD NCF portion in a modern wind rotor blade is 50–60% of the NCF fabrics. Triaxials are reduced in favor of UD and biaxial +/−45° fabrics in bigger blades. Shear webs are build mainly using NCF constructions of +/−45° materials. In such designs a critical area in a blade is the large flat surface area towards the root. In this area the loads are more complex caused by combinations of air pressure, flap-wise deflection and edge-wise deflections. Local stress concentrations can occur and buckling can be caused in the compression zone under deflection. An image from possible buckling is described in Fig. 21.6.
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21.6 Buckling simulation of a blade (Locke J., Valencia U., ‘Design Studies for Twist Coupled Wind Turbine Blades’, SAND 2004-0522, Albuquerque/Wichita, USA).
21.2.6 The use of UD prepregs in wind rotor blade design An alternative design option developed by Danish wind rotor blade manufacturers is shown in Fig. 21.7. The figure highlights that in this construction the shear web construction commonly associated with the VARTM process to produce a full blade have been replaced by a central ‘wingbox’ structure. The top and bottom of these constructions are built up using UD reinforcements and both sides have the function of the shear web. The box structure holds approximately 85% of the mechanical load and is covered by a fairing that is glued on. The UD structure is composed of UD prepreg and biaxial dry fabrics. Other biaxial prepregs build up the sidewalls together with core material, forming said shear web. The fairing is designed using a combination of triaxial and biaxial prepreged NCF fabrics and core materials. Pre-impregnated reinforcement fibre and fabrics, a material type and related vacuum-bagging process used mainly for aerospace part manufacture, show, as the infusion process, a clear improvement in mechanical properties versus the hand lay-up process. As the materials used are pre-impregnated, their commercial use requires a refrigerated supply chain, a precise cut, lay-up and debulking process to remove trapped air in between the material layers. A common step in the process in aerospace applications is to use an autoclave to compact the material under higher
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21.7 Central spar design typical of those used commercially by VESTAS and Gamesa.
than atmospheric pressure. Because of the immense size of wind turbine blades and their components or subparts this cannot be done for economic reasons. However, tools must be heated up to around 80°C to cure the used epoxy resin system. The UD prepreg approach, however, holds some advantages with respect to NCF, as it can be made of 100% oriented fibre structures in an UD direction, no weft is necessary and the amount of resin can be precisely adjusted to the required tolerances and the resins can have higher viscosity than resin systems used for the infusion process. Triaxial and biaxial prepreg show no or little differences to material used in the infusion process and demonstrate very similar mechanical properties. The reason is that those prepregs are made entirely out of the same NCF. With respect to the use of carbon fibre for UD structures as spar cap (or girder) the UP prepreg process offer some advantages. • •
The impregnation process can be controlled and adjusted to produce a full impregnation of the carbon fibre prior to their use in the moulding application. The impregnation process can be adjusted to increment the amount of resin in the UD prepreg vs. VARTM thus reducing the fibre volume fraction. This reduces overall material cost and reduces the carbon prepreg laminate modulus. The reduction in modulus improves the compatibility with glass fibre prepreg in hybrid carbon–glass designs too.
The major loads that wind rotor blades are subjected to aerodynamic loads as thrust and lift on the blade. Thrust is causing a flap-wise deflection, aerodynamic lift causes a mixed flap and edgewise bending and the torque to spin the rotor and produce energy. Torsion loads twisting the blade along their axis are caused by thrust and lift. The blade mass is subject to both gravity and inertia loads. Gravity loads pull the blade down and act as an edgewise load. Inertia loads are caused by the rotation and by vibrations. Lower masses and reduced vibrations reduce substantially these loads (Plate XIV).15 All these loads cause multiple static and dynamic stresses which any material used on the construction of a wind rotor blade must be able to withstand over the designed lifetime which is commonly between 20 and 25 years. To determine the suitability of a reinforcement material for this operating environment a range of tests are therefore carried out to show the suitability of the used materials. These
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are described by standards edited by the main certification bodies.16 All NCF materials to be used in a wind rotor blade have to be tested according to these standards. The main parameters are • • •
tensile strength and modulus in reinforcement direction and transversal direction; shear strength and modulus; and fatigue strength in three different load ratios up to two million load cycles.
Another important parameter is the thickness of the fabric used. As wind rotor blades are built by lay-up of individual NCF or prepreg layers, at each start and end of a new layer, a drop in laminate thickness is generated. This causes a sudden difference in laminate properties and a shear tension at the cut edge of the new layer in the boundary between new layer and existing laminate. The material design must withstand the deformation at these boundaries over time. Another specific requirement is caused by frequent lightning strikes to wind energy converters. NCF materials that are used in wind turbine blades must, in addition, show a uniform dielectric behavior and must not offer undesired local conductivity.
21.3
NCF materials used in nacelle construction
In almost all horizontal wind energy converters, the key components of the machine are housed in a closed nacelle on top of the tower as a prolongation of the turbine axis. In reality, the design concept of the nacelle is defined by the arrangement of the drive turbine and the generator. Ease of assembly and total cost are also key defining factors in the nacelle construction. The most common design is of either a welded bedplate or a cast bedplate, on which the non-load bearing structure is attached. Various materials have been used for this non-load bearing structure such as aluminium or steel structures. Because of their light weight, facility to be designed in distinctive free-form designed shapes, very good corrosion and weather resistance, these shells have been made using glass fibre reinforced composite materials for the past decades. The nacelle design and production methods have followed closely the evolution of rotor blades. At one hand, most of the designers used in dimensioning and layout of blades worked in the design and laminate layout of nacelles, and on the other, the requirements with regard to lifetime, weight and cost are similar. So it is no wonder that when hand lay-up dominated the blade manufacture, nacelle covers, as shown in Plate XV, were initially produced in a hand lay-up processes, but when blade manufacture went to infusion processes, the nacelle production went too. Today there are two basic processes used. One is VARTM, as used in blade manufacture, but using the more convenient 0/90 biaxial NCFs instead of more costly biaxial +/−45° laminates, and, to build up the required
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thickness as well, a combination products of NCF with mat, sometime with enhanced resin flow properties. The other process applied is RTM light where a rigid mould and semi-flexible counter (male) mould is used. Advantages of this system are both outside and inside smoother finishes, but the resin amount needed is higher. A compressible reinforcement material is needed too which increases cost.
21.4
Future trends
Currently there are several issues related to the future use of NCF in the efficient construction of wind rotor blades. Production consistency One of the major concerns is the high irregularity of mechanical properties as a consequence of fibre misalignment, local imperfections as missing or displaced reinforcement fibre and wrinkles of the reinforcement structure formed in the laminate. Other issues can be attributed to material and processes variations. Foaming of the resin, incomplete wet-out, or an excessive exothermic reaction or ‘heat generation’ during the reaction of the used resins cause a further deterioration of the laminate properties. Even though the required security factors against failure are estimated on statistical requirements lower than in aerospace applications the level of inherent over design due to these factors has an increasing impact as blades become longer and heavier in the drive to lower of cost of electricity generation through wind power. Manufacture of larger and longer blades To be able to produce bigger and bigger blades and due to the challenge of transporting structures higher than 5 m in a cost-effective manner, new concepts have been evolved. One possibility is to build a blade in segments or sub-assemblies which are assembled or produced as preassembled dry structure, so called preforms. These parts are either infused as a whole or, if produced independently, assembled using structural adhesives. Examples of this type of approach are pioneered by companies such as Blade Dynamics (Plate XVI). Optimisation of the aerodynamic performance of wind rotor blades In the drive to constantly reduce the cost of wind energy, there is a tendency to the production of sleeker and aerodynamically more efficient blades. To realise these types of designs, the requirement for sophisticated fibre and NCF fabric properties increases, providing another challenge for the producers of NCF reinforcement structures.17
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References
1. Goodmann Gr., F. R; Vachon W.A.; United States Electricity Utility Activities in Wind Power, Fourth International Symposium on Wind Energy Systems, Stockholm, Sweden, Sept. 1–24, 1982; Hydro-Quebec: Project Eole, 4-Mw Vertical Axis Aerogenerator, Montreal, 1985. 2. Ministry of Energy (Danish Energy Agency): Wind Energy in Denmark, Research and technological Development, 1990. 3. Friis, P., Large Scale Wind Turbines, Operating Hours and Energy Production, Elsam Project A/S. Internal Report, 1993. 4. Thomas Ackermann, Lennart Söder, Wind energy technology and current status: a review Renewable and Sustainable Energy Reviews, Volume 4, Issue 4, December 2000, Pages 315–374. 5. C. Soutis, Fibre reinforced composites in aircraft construction Progress in Aerospace Sciences, Volume 41, Issue 2, February 2005, Pages 143–151. 6. Isao Kimpara, Use of advanced composite materials in marine vehicles, Marine Structures, Volume 4, Issue 2, 1991, Pages 117–127. 7. L.M. Wyatt : Materials for MW sized aerogenerators Part 2. Materials characteristics Materials & Design, Volume 4, Issue 5, October–November 1983, Pages 880–884. 8. Christoph W. Kensche: Fatigue of composites for wind turbines, International Journal of Fatigue, Volume 28, Issue 10, October 2006, Pages 1363–1374. 9. Data from internal analysis by Owens Corning, 2010. 10. Mandell, J.F., Samborsky, D.D., and Cairns, D.S., ‘Fatigue of Composite Materials and Substructures for Wind Turbine Blades’, Contractor Report SAND2002-0771, Sandia National Laboratories, Albuquerque, NM 2002. 11. Manwell J.F, McGowan J.G, Rogers A.L., Wind Energy Explained, University of Massachussetts, Amherst, USA. 12. P.J. Hogg, A. Ahmadnia, F.J. Guild: The mechanical properties of non-crimped fabricbased composites, Composites, Volume 24, Issue 5, July 1993, Pages 423–432. 13. Michaeli/Huybrechts/Wegener, Dimensionieren mit Faserverbundkunststoffen, Hanser, 1995. 14. Generic IEC Class II blade construction – Owens Corning internal information 2010. 15. Det Norske Veritas, Guidelines for Design of Wind Turbines, DNV/Risoe, Copenhagen, Denmark, 2nd Edition. 16. Germanischer Lloyd, Guidelines for the Certification of Wind Turbines, Edition 2010, Hamburg, Germany, 2010]. 17. Locke J., Valencia U., ‘Design Studies for Twist Coupled Wind Turbine Blades’, SAND 2004-0522, Albuquerque/Wichita, USA]
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Plate XI Parameters for the delamination interface model for materials NCF/LY3505 (* estimated) (Chapter 16).
Plate XII C-scans of impacted non-crimp fabric and prepreg specimens (30J) (Chapter 18).
Plate XIII Typical blade layout IEC class II 40m blade detailing the laminate constructions based on non-crimp fabric reinforcement materials (Chapter 21).
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Plate XIV Blade deflecting under loads, coloured areas indicate highest stresses (Chapter 21).
Plate XV Nacelle cover under load (wind, snow). (Source M. Zvanik, OC Presentation CFA 2001, Tampa, FL). (Chapter 21).
Plate XVI Production in segments: Shell, spar cap, and root joint/ flanging reinforcements (Chapter 21).
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