FUTURE MATERIALS FOR WIND TURBINE BLADES –
A CRITICAL REVIEW
R.T. Durai Prabhakaran
Section of Composites and Materials Mechanics, Department of Wind Energy,
Technical University of Denmark, Risø Campus, Frederiksborgvej 399,
4000 Roskilde, Denmark
ABSTRACT
Wind turbine industry is continuously evaluating material systems to replace the current thermoset composite
technologies. Since turbine blades are the key component in the wind turbines and the size of the blade is increasing in
today’s wind design, the material selection has become crucial focusing several factors like less weight, less price, higher
performance, longer life, ease of processing, and capability of recycling. In the present market scenario, wind industry
needs to improve their business for onshore and for off-shore applications demonstrating the new blade designs and
stating higher performance under severe environmental conditions. The current article reviews various material
alternatives and demonstrates the advantageous and disadvantageous for future wind turbine blade developments.
Keywords: Thermoplastics, Thermosets, Glass fibres, Natural fibres, Hybrid composites, Compression properties.
1. INTRODUCTION
In the present market scenario, wind industry needs to improve their business both for onshore and off-shore applications
demonstrating the new blade designs and stating higher performances of alternative material systems for the
development of future wind turbine blade structures. Blade designers are continuously trying for new innovative blade
design solutions, which can open up options to utilize the benefits of the new material systems. Wind industry is finding
out alternate material systems for turbine blades which can compete the current thermoset technology. Since turbine
blades are the key component in the wind turbines and the size of the blade is increasing in today’s wind turbines, the
blade designer should focus material systems with less expensive, higher performance, low density, longer life, ease of
processing, and capability of recycling. These factors can lead to a blade structure with optimal aerodynamic
performance, reduce gravity forces and reduce material degradation during its operational stage leading better life-cycle.
Several research articles reported in literature, demonstrate the material performance and its suitability for future
wind turbine blades. Composite materials like thermosets [1], thermoplastics [2-5], natural fibre systems [6], and
upcoming hybrid material technologies are the possible options for future blade materials. Carbon-/glass fibre hybrid
reinforcements can offer an interesting solution by combining the performance of carbon fibre with the good
processability of glass fibre [7]. Similar to carbon/glass fibre, combining glass fibre or carbon fibre with natural fibre as
hybrid reinforcements also give better performance compared to current thermoset composites [8]. Thomsen [9]
discussed the potential advantages and challenges of the use of sandwich materials for the load-carrying parts of wind
turbine blades such as spar flanges (or spar caps). It would be advantageous to use flanges on the suction side of the
airfoil, since sandwich design will provide additional buckling capacity and/or provide a more lightweight design with
similar buckling capacity [9-10].
Processing large composite structures like turbine blades (with an overall length of 80m long) is a challenging task
for blade manufacturers. In general, all materials have a great influence from the processing parameters on the structural
performance, but composite materials have a much more intimate materials/processing/structural performance
connection due to their macroscopic in-homogeneities. Prior to implementation of any new material systems, industry
need to investigate the issues, challenges, process and material limitations associated with it in order to utilize the
benefits associated with the materials.
*[email protected], phone (+45) 46 77 57 17
The current article reviews the pros and cons of various material systems i.e. thermoset, thermoplastic, natural and
hybrid composites. With noted issues of processing and performance characteristics, these materials are evaluated with
respect to their suitability for wind turbine blade designs. Assessment of these material systems mainly focuses on
properties like static and fatigue performance of composites (at laminate level). The material systems used for blade
construction requires very high static compression and fatigue properties with a minimum compression strength equal to
compression strength of glass/polyester system.
Figure 1 Schematic diagram for a blade profile
2. FUTURE MATERIALS
The wind turbine is a complex system consisting of several components where turbine blades form key structural
elements for achieving higher power generation. Due to their size, wind turbines are costly to manufacture and since they
should have a long lifetime such as a life cycle of 20-25 years, it is of high importance that the turbine blades are well
designed with innovative blade design concepts (see figure 1). The biggest challenge for blade designers include finding
out suitable material systems which can give of huge cost saving as well can deliver both performance and weight
reductions. Thus the possible candidate materials and their pros and cons are explained in the upcoming sections.
2.1 Fibre reinforcements
When the composite structure is loaded, fibres are the key constituent carries the loads along their longitudinal
directions. It is essentially the reinforcement ‘architecture’ that determines the load-bearing characteristics of a fibre
reinforced polymer composite. Fibres can be long (continuous), short, chopped, or in the form of elongated single
crystals. Continuous fibres come in the form of untwisted bundles as strands, or twisted bundles as yarns, and also as a
collection of parallel continuous strands, which are referred to as roving. The reinforcements readily available in the
markets for composite product developments are glass fibres (E- and S-glass), carbon fibres (IM- and HS-carbon),
aramid fibres, steel fibres, natural fibres (jute fibre, hemp fibres, flax fibres, etc), hybrid fibres (glass/carbon), and
thermoplastic polymer fibres (polyamide fibres, PET fibres, PP fibres, etc). Current turbine blades are made by either
glass fibre or carbon fibre reinforcements. The stress-strain curves for few standard fibre reinforcements published in
literature [11] are shown in Figure 2. The significant benefit associated with glass fibres are higher strain to failure with
acceptable strengths. Whereas the carbon fibres demonstrate very high tensile and compression strengths compared to
other fibre reinforcements, but they have a drawback of very low values of strain to failure (Figure 2). A resin
compatible fibre surface treatment is another important requirement for reinforcements. Sizing is the treatment of fibre
surface with coupling agents (that couple resin to fibres), to protect the fibre against moisture and reactive fluid attacks.
Sizing chemicals improves wettability of the fibre surface against the polymers used for processing, therefore creates
stronger bond between the fibre and resin. It is necessary for effective transfer of stresses between the fibres and the
matrix. The better the interface bond between polymer (resin) and fibre reinforcements can deliver higher compression
and fatigue strengths. For thermosets polymers both glass and carbon fibres has compatible sizing, but with
thermoplastic polymer the sizing chemicals for glass and carbon fibres are still under development stage. Latest
developments in reinforcement industry are hybrid reinforcements such as glass/carbon hybrid fabrics [7, 12].
Combining glass fibres with carbon fibre with optimal mix ratios for composite applications are expected to deliver
better strength and strain to failures.
Figure 2 Stress-Strain curves for standard fibre reinforcements [11]
2.2 Thermoset composites
Wind industry currently produces turbine blades using thermoset technologies such as glass or carbon fibres reinforced
with thermoset polymers (polyester or epoxy resins). The drawbacks with the current thermoset blades include styrene
emissions (polyester resins) while processing (by vacuum infusion techniques), and cost issues with epoxy resins. The
big issue is after life cycle of the blade, recycling is a big issue with thermoset blades. Another disadvantage include
repair or joining of thermoset composites which is possible only with special adhesives. Selection of polymer for blade
manufacturing has become an important economical factor in today’s wind industry. Blade lengths are increasing and the
cost of the blade also increasing a lot, in this critical situation the industry needs to find a polymer which costs less and
give similar processability like thermoset polymers as well perform better than current blade materials. For example, the
largest blade with 80 m long weighs around 23 tons out of that 30% of the weight is the polymer. For a turbine with 3
blades this means polymer used in turbine blades production takes space of 21 tons. This demonstrates significance of
the polymer in terms of weight and cost savings for a wind turbine blade development. Among the wind industries, there
is huge competition to develop the new blades with entirely different material systems compared to current blade
materials. The above reasons initiate special interests among material researchers for finding out an alternative material
system which can benefit wind industry to overcome the current issues.
Several literature reports demonstrate that the new materials such as polyurethane (PU), and dicyclopentadiene
(DCPD) are suitable for future blade materials. Younes and Bradish [13] discussed the development of new polyurethane
class of resins and its implication in the manufacture of large wind turbine blades, and compare its performance in
composite systems to systems made from epoxy and vinyl ester. The author also demonstrated polyurethane systems
which are adapted to current blade manufacturing processes and could be retrofitted into existing designs at minimal
cost. These urethane systems showed much improved fatigue and fracture toughness properties as well as faster demold
than resins currently used in wind turbine blade manufacture. Gardiner [14] and McCarthy [15] demonstrated a new resin
system aimed at infusion of wind turbine blades, “new proxima polydicyclopentadiene (pDCPD) resin” takes viscosity to
a low value of 10 to 20 cps at 23°C under processing. Materia, USA claims, the resin enables one-shot infusion of thick
sections (e.g., blade root areas), reduced void content and higher fibre volume fractions of 58 to 60 percent. (It also is
recommended as a solution for complete wetout during infusion of traditionally problematic carbon fibre laminates.)
Infusion rates are up to 10 times faster than standard infusion resins, and cured parts weigh less but have greater fracture
toughness and fatigue resistance [14-15].
2.3 Thermoplastic composites
Despite of the growing maturity and acceptance of thermoplastic composites for several industrial product applications,
it is important not to over-hype their promise for large structural developments. For instance, it is easy to overlook the
arising issues for processing blade structures of 60-80 m long as explained by the authors [2-4]. Several researchers
trying hard to improve design concepts for future thermoplastic blade [16], polymer performance and also to decrease
process temperatures by adding special particles like nano, fillers, etc in order to replace the current thermosets by
utilizing the benefits associated with the thermoplastics like emission free (styrene), process-ability, recyclability, and
weld-ability. Bersee and Rijswijk [17] working for the last ten years with thermoplastic reactive polymers, selected
Anionic Polyamide 6 (APA6) resin for developing future thermoplastic blade. The authors developed processing
technology as the resin is low viscous (10 times lower than that of epoxy), tooling system to account drying and a
process cycle which accounts both crystallization and polymerization simultaneously, with roughly half of its
crystallization formed at this point. When cooling, crystallization continues. But, due to the toughness of the material, no
matrix cracking occurs and finally leads to very high quality laminates [2]. Teuwen [18] demonstrated further, APA-6
retains the toughness of PA-6 and it achieves a higher interfacial bond strength, which improves fatigue strength. This is
expected to improve with the optimized sizing, and fatigue testing to demonstrate this is in process.
Thermoplastic materials offer higher strength to weight properties than thermosets, therefore leading to lighter aerofoils
which can lead to cheaper transport costs and reduced turret weights. Therefore few European research projects focused
in developing the future blades using thermoplastics [19-20]:
1.
“Green Blade” project (finished December 2004) – Eire Composites and their project partners developed a 12.6m
long turbine blade from CBT/Glass thermoplastic composite material. They even patented an innovative tooling
system designed for thermoplastic composite processing [19].
2.
“Blade King” 5 Yrs project [20] (started Oct 2008) – This project screened all thermoplastic composites (see Table
1) and selected few systems for further evaluation. Thermoplastics specialist (Dr. Hans Knudsen) from Comfill
industry, Denmark on developing new materials and technology jointly with project partners (Department of wind
energy, DK and Aalborg University, DK) which will significantly reduce the production time of large rotor blades.
Table 1. Mechanical properties – Unidirectional thermoplastic composites (prepreg, commingled and reactive polymers)
Sl.
No.
Material
System
Fibre
Volume
Fraction
Vf
(%)
1
2
3
4
5
GF/PP
GF/HDPE
GF/mPET
GF/PBT
GF/PA6
50.9
45.2
49.5
46.0
47.0
1
2
3
4
GF/PP
GF/LPET
GF/PA6
GF/PBT
50.3
49.4
47.8
50.0
1
2
GF/CBT160
GF/APA6 [21]
50
46
1
2
GF/Polyester
GF/Epoxy [22]
50
48
Compression
Modulus
Compression
Strength
EC
(GPa)
σC
(MPa)
Prepreg Laminates
36.2
335
34.8
205
32.9
372
31.3
517
34.6
534
Commingled Laminates
41.4
516
40.6
624
39.1
577
37.5
601
Reactive polymer based Laminates
36
487
39
634
Thermoset Laminates
38
910
36
650
Compression
Strain to
Failure
ϵC
(%)
Inter-Laminar Shear
Strength
1.0
0.6
1.2
1.6
1.6
25.9
11.1
22.6
48.6
59.3
1.3
1.5
1.5
1.7
37.4
59.1
69.8
35.5
1.4
1.7
48
---
1.5
1.7
76
---
ILSS
MPa
2.4 Natural fibre composites
Natural fibres have become popular reinforcement material for fibre reinforced polymer composite developments. The
reinforcement can replace the conventional fibre, such as glass, aramid and carbon as an alternative material [23-24]. The
main advantageous of natural fibres (flax, hemp, and jute fibres) include low cost, fairly good mechanical properties,
high specific strength, non-abrasive, eco-friendly and bio-degradability characteristics. In spite of impressive specific
mechanical properties, the main challenges associated with these reinforcements include severe moisture absorption, fire
resistance, mechanical properties and durability, variability, and manufacturing/processing of natural FRP composites
[23]. Bamboo is another interesting material considered as plant fibre and has a great potential to be used in polymer
composite industry. Bamboo has 60% cellulose with high content of lignin and its microfibrillar angle is 2–10 deg,
which is relatively small. This characteristic property has made bamboo fibre as fibre for reinforcement in variety of
polymeric matrices [25-26]. As the blade design requires very high strengths and stiffness with very less moisture
absorption criterias, natural fibres are not yet fully considered as future materials for blade developments. But in the
recent days, some research projects focusing on small wind turbine blades using natural fibres are listed below:
1.
“NATEX” is a collaborative project funded by the European Commission under 7th framework programme
[27]. The project partners solved several research issues associated with the natural fibre composites mainly – to
promote the use of natural fibres (hemp and flax) in structural applications, to identify and develop innovative
applications and processes, to shift from resource intensive to knowledge based industry, to significant the
potential impacts for various sectors and actors in the supply chain (see Figure 3).
2.
Nottingham Innovative Manufacturing Research Centre, UK is developing a sustainable small wind turbine
blade with fully natural fibre (flax, jute, hemp and sisal) and resin (polyester/epoxy) composite [6]. The small
wind turbine industries has expanded considerably following initiatives to reduce CO2 emissions but has faced
challenges in manufacturing efficient small-size wind turbine blades.
3.
A research project at “Roskilde University, Denmark” studied the properties of bamboo material and made a
LCA to compare the performance between bamboo turbine blades and glass fibre turbine blades with
sustainable perspective [25]. The result concluded that bamboo material fulfilled the requirement of
constructing a turbine blade and has a high performance associated with sustainable development.
Currently several researchers are working in the development of natural fibre composites for future blades, but to satisfy
the blade design requirements the possible options could be foreseen as the hybrid fibre reinforced polymer composites
with natural fibres.
Figure 3. a) Wind turbine car and b) blade geometry and blade with carbon fibre, flax fibre, and carbon/flax fibres [8]
2.5 Hybrid fibre composites
Reinforcement industry introduces advanced stitching technology bringing out a new hybrid fabric (Non-Crimp Fabric
(NCF)) as shown in Figure 4a (which is Devold’s proprietary technology [12]). The fabric consists of both glass and
carbon fibres with a predefined optimum mix ratios of fibre contents. Roman [7] from “Devold AMT” claims the hybrid
fabric permeability was increased four times higher than with the glass fabric allowing for a much more rapid, quality
infusion. The other way of developing a hybrid composites were by placing few layers of carbon and more layers of
glass in a predefined optimal stacking layups as shown in Figure 4b. The two methods of hybridization helps to tailor the
composite mechanical properties bringing up the properties in between glass and carbon fibre reinforced polymer
composites (close to carbon fibre reinforced composite properties). The biggest challenges with hybrid fabrics include
translate of the higher carbon fibre properties such as stiffness, strength, and fatigue properties in to accountable
composite properties. This is mainly possible, if the reinforcement layers of carbon and glass fibres have suitable and
compatible fibre surface treatments (i.e. sizings) with the polymeric resin used for infusion process. The author [7]
demonstrated the increase in tensile and compression properties for hybrid fabrics compared to glass fibre reinforced
epoxy resin systems.
Figure 4. Hybrid materials a) hybrid fabric (glass/carbon fibres)
b) hybrid layup with alternate layers of glass and carbon fibre reinforcements
Prabhakaran et. al. [28] demonstrated the benefits associated with hybridization regarding flexural performance of hybrid
fibre composites. The experimental assessments show outer skin layers with high strength fibres such as glass fabrics
combined with flax layers give better flexural properties (with the optimal stacking layups shown in Figure 4b). Whereas
Mikkelsen et al. [8] demonstrated the optimized hybrid blade for wind turbine car (figure 3a) with a dominating amount
of flax fibres perform almost as good as the pure carbon blade while the pure flax did not perform as well.
3. CONCLUSIONS
The present article presents a critical review of various material systems for future wind turbine blade developments. The
materials considered in this study include various fibre reinforcements, thermoset composites, thermoplastic composites,
natural fibre composites, and hybrid composites. Advantages and disadvantages of various materials and its limitations
are explained, which can give good insight for material selection for both large and small turbine blades. The processing
challenges for various materials systems needs to resolve first, in order to consider the materials for blade design.
At the end-of-life of the wind turbine blades, thermoplastics blades can be recycled, which is a major advantage
compared to its thermoset counterpart but it needs few more advances before implementing as blade materials. Whereas
the natural fibre composites cannot be an immediate solution for large blades, but for smaller blades these materials can
become a good choice. For future blades, another interesting material could be hybrid composites.
ACKNOWLEDGMENTS
The work reported was conducted at Department of Wind Energy, a research study sponsored by the Danish National
Advanced Technology Foundation. Special thanks to Hans Knudsen (Comfil, Denmark), Prof. Ole Thybo Thomsen
(Aalborg University (AAU)), Dr Saju Pillai (AAU), & Dr. Samuel M. Charca (AAU), Bent F. Sørensen (Technical
University of Denmark (DTU)), Povl Brøndsted (DTU), Helmuth Langmaack Toftegaard (DTU), Christen Malte
Markussen (DTU), Bo Madsen (DTU), Hans Lilholt (DTU), Lars P. Mikkelsen (DTU), Tom Løgstrup Andersen (DTU),
Aage Lystrup (Risø DTU), for supporting with the experiments and technical discussions.
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