Carbon Nanotube Wind Turbine Blades: How Far Are We Today from

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Carbon Nanotube Wind Turbine Blades - How Far Are We Today from Laboratory Tests to Industrial Implementation? Slawomir Boncel, Anna Kolanowska, Anna W. Kuziel, and Iwona Krzy#ewska ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01824 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 22, 2018

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ACS Applied Nano Materials

Carbon Nanotube Wind Turbine Blades – How Far Are We Today from Laboratory Tests to Industrial Implementation? Sławomir Boncel*, Anna Kolanowska, Anna W. Kuziel, Iwona Krzyżewska Silesian University of Technology, Department of Organic Chemistry, Bioorganic Chemistry and Biotechnology, Krzywoustego 4, 44-100 Gliwice, Poland *Corresponding author: tel.: +48 32 237 12 72, fax: +48 32 237 20 94, e-mail: [email protected] Abstract With a few works in 2010 on the application of carbon nanotubes (CNTs) in wind turbine blades (WTBs), new hopes have arrived promising to conquer the so-far used materials. From that moment, several dozens of publications have been published confirming that CNTs – typically considered as ‘universal soldiers’ in the composite, hybrid and hierarchical materials – could be a reliable component of WTBs. However, all of the experimental results, in a few cases supported by theoretical models, were obtained only in the laboratory scale. This review intends to answer the title question by summarizing the up-to-now efforts and comparing the levels of outperformance of CNT-based WTBs. The general conclusion from the review is that physicochemical and mechanical properties of CNT-based WTBs, although promising, must be balanced by economy and technological aspects with reliability of large-scale synthesis and standardization of CNT batches as the key ones. Keywords carbon nanotubes; wind turbine blade; composite materials; hybrid materials; hierarchical composites

1 ACS Paragon Plus Environment

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1 Introduction Carbon nanotubes (CNTs) have recently generated the most powerful ‘wind of change’ in materials science influencing modern technology and engineering [1]. In the same time, among the most abundant and conveniently transformable renewable energy resources, apart from sunlight and movements of water masses, it is the wind which has emerged as the increasingly important source of electric power [2]. Wind power is ecologically attractive – it has zero emission of greenhouse gases, zero water consumption and low requirements for the land [3]. Due to technological efficiency and economy, wind turbines are the most frequently arranged into farms (multiple turbines). In 2015 wind turbines provided 841 TW of electric power around the world, while – as exponentially growing – the number of wind powers installed across the globe in 2016 was more than 54 GW [4]. Wind energy production is constantly on the rise and has reached >4% of worldwide electric power usage [5]. This tendency – yielding inter alia ca. 20% growth of offshore wind energy capacity in EU annually – has arisen from accessibility, ecopolitics and the expansion of novel efficient technologies including nanotechnology as the most prominent one. Those technologies were generously supported by the rapid development of new construction materials for wind turbine blades (WTBs), including CNTs. The latter requirement for the new materials and nanomaterials derives from the continuous increase of rotor diameters as the ‘extracted’ wind power (transferred to the generator) is directly proportional to the surface area of the rotor (Fig. 1).


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If? How? When?

LCOE (2013$/MWh)

$600 $480

CNTs in WTBs?

$360 $240 $120 $0

40 m 0.5 MW

50 m 0.6 MW

112 m

126 m 5 MW

150 m 7.5 MW

178 m 10 MW

250 ? 252 m 20 MW

200 150 100

Hub height (meters)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

Discovery of CNTs

0

5 00 05 90 10 99 20 20 19 20 1 5 0 9 0 05 80 90 19 20 20 19 19

t en es r P

re tu Fu

Fig. 1 Time-dependent decreasing levelized cost of electricity (LCOE) in USD per MWh in 2013 (left Yaxis); development of rotor diameters in the wind turbines (right Y-axis); the first enthusiastic results in the application of CNTs in the lab-scale wind turbine blades emerged in 2010 but despite numerous efforts no industrial scale CNT containing blades have been applied up-to-date – the questions on added values, economy and specification (geometry, processing, incorporation/admixing, etc.) still remain open. On the basis of figure in ‘Wind energy – the facts: a guide to the technology, economics and future of wind power, European Wind Energy Association, Earthscan, 2009.

Hence, undeniably, the blade is the central point of the wind turbine design and manufacture as it harvests and converts mechanical energy into electricity. In parallel, the blade is the most damage prone and, apart from the tower, one of the most costabsorbing element of the turbine. Among new materials for the WTB components, due to superb mechanical properties, carbon (nano)fibres (CnFs) and other quasi-1dimensional (1D) carbon nanoallotropes with CNTs (and their macro-assemblies, also as a combination in the hybrid materials) have recently arisen as the most perspective ones [6]. Moreover, since morphology, geometry and physicochemistry of CNTs are increasingly ‘programmable’, they can be theoretically produced in a ‘properties-by-



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design’ approach. Indeed, recent years have seen a significant growth of the interest in the potential application of CNTs in WTBs (Fig. 2).

Fig. 2 CNTs in WTBs – the number of scientific publications until October 2018 per year; search words = carbon AND nanotube(s) AND wind AND turbine AND blade(s)

The highest increase in the number of scientific publications was recorded in 2012-2014 (Web of Science®, Scopus®) and 2017 (Google Scholar®). This momentum reflects the international policies. For example, since 2004, a number of countries promoting renewable energy with a direct policy support has nearly tripled (from 48 to over 140), and an ever-increasing number of developing and emerging countries are setting renewable energy targets and enacting support policies. In this review, considering the above ambiguity and non-monotonic trends in time, we intend to characterize CNT-reinforced composites – including hierarchical ones – with key morphological, physicochemical and mechanical properties (including stiffness, strength and fatigue life) in the background of unmodified and typical matrices, composites and hybrid materials. We also hope to shed more light on the problems and


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pending challenges in the publicized area of CNT-based WTBs. 1.1 Wind turbine blades – general characteristics and perspectives of development A typical large-scale WTB is arranged into a three-blade rotor with a horizontal axis of rotation. The blade itself is a two-face suction and pressure construction and it actually constitutes a developing model pioneered in 1889 by Danish engineer Poul La Cour. The faces are joined and reinforced by shear webs linking the upper part with the lower. The flapwise load is caused by the wind pressure while the edgewise load is caused by gravity and torque. As the consequence, laminate flapwise must work in a compressioncompression/tension-tension mode while laminate edgewise in the compression-tension one. Hence, the sandwich structures of aeroshells are designed and calculated against elastic buckling. The nature of loads-carrying specific sections of WTB and its components are presented in Fig. 3.

Fig. 3 A simplified cross-section of WTB with an indication of working loads with their directivities (left); the key elements of WTB which manufacture is based on assembling and bonding two aeroshells and shear webs (grey colour indicates the primary load-carrying composites) (right) [6]; Copyright MDPI, Basel, Switzerland.

Multi-layer composite materials (which have actually replaced heavy, expensing and



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corroding metals) dedicated for WTBs should be characterized by low density, high stiffness, high toughness, high interlaminar, interfacial and torsional shear strengths and long fatigue life. Moreover, those characteristics must be maintained under harsh atmospheric conditions such as high insolation, (acid) rain, strong icing or thunderstruck. Wood, canvas and their laminates as well as aluminium have been used for years in the manufacture of WTBs due to relatively low cost, moderate weight and availability, but reinforced polymer composites must have emerged as the natural rivals due to a combination of the superb physicomechanical properties. Among those properties, fatigue life, which is the number of stress cycles of a specified character that a specimen sustains before defined failure, is of a high economic importance [7]. Modern WTBs require lighter, of higher mechanical performance and of longer fatigue life than wood or glass fibre (GF) laminates. The latter ones – which are still in use [8,9] – offer a life cycle of a blade under real-life conditions from 20 to 25 years [6]. 1.2 Fundamentals of manufacture of wind turbine blades The blades are the most important and the highest cost component of a wind turbine which structure is defined in terms of the outer geometry and the inner structural layout. It can be manufactured from different and complex materials via a few technologies as it is subjected to varying loads and directions (wind, gravity, centrifugal force). A typical WTB design (one-piece construction) is based on load-carrying laminates in a rectangular hollow beam (spar). A spar is fixed into the first shell and the second shell is lifted over onto the spar. In another common design, there is no spar; instead, there is a combination of load-carrying laminates incorporated in the aeroshell together with two shearwebs. The beam spar and the sandwich face sheets of the aeroshell are made from


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fibre-reinforced polymer composites; the sandwich cores are made from polymeric foam or balsa wood and the blade is assembled with adhesives joining: (a) the aeroshells at the leading edge, (b) the spar and the aeroshell, and (c) the aeroshells at the trailing edge [10]. For both constructions, as the wind blades of increasing size have been produced, adhesive bondings must have been emerged as equally important areas of load transfer. Typical blade joints use paste adhesives several millimetres thick, of varying geometry [11]. Adhesive joints have fewer sources of stress concentration, higher toughness and more uniform stress distribution through the joined area as compared with mechanically fastened joints, such as rivets or screws [12]. The concept of chemical thixotropy to produce adhesives with a controlled flow exploiting nanoparticles – in order to enhance the final toughness – have been successfully applied in WTB bonding [13]. This aspect in the light of use of CNTs will be discussed later in this review in a separate subsection. Technologies of WTB manufacturing (Fig. 4) can be divided into four types: (A) wet hand lay-up, (B) filament winding, (C) prepreg, and (D) resin infusion.



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Fig. 4 Technologies of WTB manufacturing: A) wet hand lay-up; B) filament winding; C) prepreg; D) resin infusion technology.

Wet hand lay-up is a laminating technique in which fibres are impregnated by resins and cured at room or higher temperature. The upper and lower shells are adhesively bound together to form an airfoil structure. For larger blades, the webs are inserted into the foils to withstand bending and shear loads. In order to control stiffness of the blades, the fibres could have any specific orientation in the matrix [14]. In the filament winding, the fibre and resin are wound together around the mandrel. The process is primarily used for cylindrical components but can be adapted for any shape [15]. In the prepreg technology, the fibre material is pre-impregnated with resin at room temperature and then laid up onto the mould surface, vacuum bagged and then heated [14]. In the resin infusion, the dry fibres are sited in a mould which seals and encapsulates the dry fibres. Then the liquid resin is injected into the moulds and the components are cured [6]. 1.3 Damages of wind turbine blades The most common types of structural damages found in the wind turbines are blade and tower damages. The WTB had the highest number of reported damage occurrences 8 ACS Paragon Plus Environment

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among all other components (Fig. 5). Over the years of operation, WTBs undergo severe fatigue-induced deterioration due to tremendous cyclic loads. Thus, cracks may initiate in the blades and propagate to the failure point upon certain unusual environmental factors.

Fig. 5 Most frequently reported damages of wind turbine components [16]; data from GCube Insurance Services, Inc., New York, USA

Among seven types of WTB damages (Fig. 6), delamination and adhesive joint failures are the most frequent ones. Delaminations are the most critical and commonly studied failure modes in the laminated composite materials. There are many reasons for blade damage starting from the manufacturing floor to the field operation. Usually, manufacturing-induced defects, precipitation and debris, water ingress, variable wind loadings, operational errors, lightning strikes and fire or even collisions with birds are accountable for the cracking and damage of WTBs. Wind is the driving force of the turbine but, obviously, wind gusts or heavy storms may damage or completely destroy it [16].



Type 2 Adhesive joint defailure

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Type 7 Cracksin gelcoat (chanal cracks) Type 4 Delamination (+/-45°)

Type 1 Skin/adhesive debonding

Type 5 Split cracks Type 5 Splitting along fibres

Type 2 Adhesive joint failure

Type 3 Sandwich debonding

Fig. 6 Main damages of WTBs at various interphases; modified from a figure in [17]; Copyright MDPI, Basel, Switzerland.

Blades that encounter the impact of sand particles and/or water droplets will first show an increase in the surface roughness negatively affecting the aerodynamic performance. In the next step, erosion of the blade (a loss of material from the solid surface) continues, while other solid particles (stones, hail balls/lumps, etc.) or fluids (rain) intensify damaging of the blade [18]. This process usually starts with the formation of small pits near the leading edge which increase in density with time and combine to form gouges. If left to the forces of nature, the gouges then grow in size and density, and cause delamination near the leading edge [19]. Ice build-up on the surface of WTB is another major source of safety and operational concerns. The large centrifugal forces can be sufficient to dislodge and launch off the accumulated ice from the blades at high speeds. Additionally, the ice may build up unequally among three blades and cause unbalanced rotation. This in turn can overload the hub and one blade would vibrate with a higher amplitude than the others. This phenomenon can result in a catastrophic failure [20].


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The blade is also the highest point of the wind turbine and is thereby the component most exposed to the lightning strikes. The strikes may form a destructive internal shock waves within the blades. Moreover, lightning temperatures can force the interior moisture to transform into an expansive state with the overpressure causing significant failures [20]. Nevertheless, weather conditions are not the only cause of WTB damages. Insect impacts are also a source of contamination and hence problems for WTB operation. And although, at low wind speeds the insects do not affect the blade operation, in strong winds, the angle between the flow and the blades increases and the aerodynamic suction peak (the area of minimum pressure and maximum velocity) shifts to the leading edge. If this happens to the blades covered with dead insects, the output power of the turbine can drop even by 50% [21]. 1.4 Carbon nanotubes as a response to the urgent (nano)technological needs With their promising properties, CNT-based composites have attracted much interest as key component of the WTB materials. When strength and density factors form the foundation of the future applications, CNT-reinforced composites arrive with their superb performance. Additionally, CNTs and their composites could be seen as key additives enhancing the overall mechanical performance of eco-friendly and lightweight WTB materials. This aim could be achieved via hybridization of CNT and CNT composites with natural fibres [22]. CNTs can be tailored to multi-functional materials simultaneously augmenting Young’s modulus under tensile and bending, tensile strength and toughness as compared to the neat polymer and inorganic matrices. Among other critical properties, CNTs have a realistic potential to improve the lightning protection due to high electrical conductivity, excellent chemical and thermal stability and high thermal conductivity. CNTs could also act as flame-retardant additives [23].



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The earliest reports from 2012 by Loos et al. on the manufacture of WTBs based on CNT-polyurethane composites prompted the technologists to develop this approach [24]. The pioneers have prepared homogenic CNT dispersion by using simultaneous sonication and magnetic stirring after addition of a dispersing agent (B60H). This world’s first CNT-based wind blades were operating with a power of 400 W in a 12 Vwind turbine generator. 2 CNT composites for wind turbine blades – premises, manufacture aspects and enhancement of mechanical properties 2.1 Characteristics of individual CNTs Individual CNTs as sp2+-hybridized -conjugated macromolecules exhibit a unique combination of superb physicochemical properties (inter alia ballistic electroconductivity, extraordinary tensile strength, chirality-dependent reactivity, tailorable optical properties, etc.) hence promise numerous applications from electronics to medicine to high-performance composites. However, due to the interphase phenomena, i.e. ohmic contact resistance, van der Waals bundling of spaghetti-like morphologies or weak inter-tube bonding, and typically non-selective synthesis, the every-day life and hence scaled-up applications has yet to be accomplished in many areas [25]. Generally, on the one hand, individualization of CNTs plays a vital role in manufacturing CNT-based composites of enhanced mechanical, thermal and/or electrical properties at low percolation thresholds. Similarly, ‘debundling’ of CNT agglomerates is crucial in antibacterial or antifouling applications [26]. On the other hand, macroscopic assembling of CNTs into desired forms and geometries of superior electrical and mechanical properties requires ultra-long CNTs and hence problematic processing. An attempt to find a compromise between the two above aspects is one of


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the main research activities in the global science of CNTs. The potential of multi-tasking, not only in terms of mechanical performance, is indeed represented by CNTs as one of the most promising carbon allotropes. Individual CNTs exhibit a unique combination of excellent mechanical performance, although expressed in the nano-scale and in the absence of crystallographic defects [27]. Table 1 recalls the most important mechanical properties of CNTs upon transition from nano- to macroscale fibers. It should be here emphasized that spinning a fibre from MWCNTs – the less expensive CNT type – has not been successful up to date. Table 1 Mechanical properties for individual CNTs versus CNT macro-assemblies Crystal/bulk Tensile strength

Compressive strength

[GPa]

[GPa]

density

Nanotube

References

[g cm-3] SWCNT

30-100

153

2.1

[28,29,30,31,32]

MWCNT

20-90

28.5

2.1

[33,34,35,36,37]

SWCNT fibre

27-52

0.416

0.25-0.65

[38,39]

It can be immediately concluded therefrom that ‘translation’ of properties from individual CNTs their macro-assemblies still remains a challenge due to a sine qua non condition of homogeneity and uniformity of CNT batches (Fig. 7).



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Fig. 7 Typical variations of CNT batches (morphology, contaminations, etc.) and their macro-assemblies: CNTs may be arranged into isotropic (powder, films, etc.) and anisotropic objects (sheets, fibres, yarns, etc.)

This problem of scaling-up derives from the ‘replacement’ of strong covalent bonding in the single nanotubes by multiple though weak inter-nanotube van der Waals forces in the 3D-systems (networks, yarns, films etc.). Several works addressed the above challenge by growing ultra-long nanotubes [40,41] or covalent cross-linking of nanotubes [42]. Nonetheless, from the industrial point-of-view, the simplest use of CNTs would be their one-step incorporation into polymer matrices as the fibrous reinforcement. On the other hand, surface functionalization and/or high-energy mixing could furnish (intrinsically hydrophobic) CNTs compatible with a variety of matrices from polymers to metals to ceramics. This process actually resembles surface sizing broadly applied for GFs and CFs [43]. Hydrophobicity of CNTs causes also difficulties in their post-processing and assemblage to fibres, although semi-technical scale production of CNT fibres has been known since few years. There are three main routes of leading to spinnable CNT fibres: wet spinning [44], high temperature c-CVD [45] and room temperature spinning from


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vertically aligned CNT forests [46]. The first two methods emerge as the most promising towards high aspect ratio CNTs of excellent mechanical properties. Nevertheless, the initial enthusiasm must have been cooled since ecology (use of chlorosulphonic acid as nanotube solvent) and economy (high temperature, low yield per carbon) of production of high-performance CNT fibres still limit their industrial use. Consequently, CNTs – not only as free-standing systems but also as a key component of environment- and performance-stable composites – must still await their translation into the macro-scale functional assemblies and then fully applicable objects. 2.2 CNT composites as a function of characteristics of CNTs and polymer matrix Coming to CNT powder – as the isotropic – and fibrous (high aspect ratio) CNTs – as anisotropic – nanofillers of composites, a natural solution could be combining polymer matrix with CNTs. CNTs – as exemplary 1D-nanoparticles toward a WTB laminate – should lead to the enhanced operational stability. Fig. 8 shows three phases of timedegradation at full load of unmodified and 1D-nanoparticle modified epoxy resin composite laminate at various strength of the filler-matrix interactions.



III unmodified

II

I 1

Degradation

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Shift B

nanoparticlemodified

Shift A

0

Fatigue life Fig. 8 Changes in degradation of unmodified (solid line) and nanoparticle [weaker (dashed line) and stronger (dotted line) filler-matrix interactions] modified composite laminate [47].

For the unmodified laminate, phase I covers the initiation of growth of cracks in the material while phase II corresponds to the formation of defined cracks, delamination and longitudinal inter-fibre fractures. In phase III, fractures of the fibres appear and delamination progresses to a complete decay of performance [47]. Under otherwise perfect molecular architecture of the laminate components and conditions of its manufacture, addition of 1D-nanoparticles to the matrix could lead to the increased fatigue life at lower degradation rate. Both characteristics can be further enhanced by surface modification, e.g. by stronger adhesion which is indicated in Fig. 8 by shifts ‘a’ and ‘b’ which correspond to prolongation of phase I and phase II – both running at lower degradation level of the composite. The key variables deciding about the mechanical performance of CNT-based WTBs –


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up to now tested only in the laboratory scale – are type of the nanotube filler (morphology and directionality) and matrix characterizing the filler-matrix interactions. As for CNTs, morphology (open or closed tubes, smooth outer surface, twisted, entangled, bundled, etc.), geometry (number of walls, aspect ratio, i.e. length-todiameter ratio), surface functionalization (e.g. intentionally or non-intentionally introduced, e.g. terminal hydroxyl groups), degree of individualization, number/density/localization of structural defects (vacancies, interstitials, Stone-Wales 7-5-5-7 defects) and contaminations (e.g. metallic caps or nodules, sp3-carbon type in amorphous carbons, ceramic particles, e.g. from alumina supports of catalysts of CNT growth) are the most critical ones as they correspond to load transfer, homogeneity of forces and compatibilization with the matrix (Fig. 9).

Fig. 9 Typical variations in morphology and surface chemistry of CNTs

The type of matrix (co-responsible for the highest possible cohesion) and/or other carbon/non-carbon composites constituted the typical reference materials selected for the comparative studies (Fig. 10) – discussed further in details in this review.



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Fig. 10 Chemical formulae of matrices used in the manufacture of CNT-based WTBs; R = alkylene and/or arylene units.

CNT-reinforced composite prepared with the intention to be tested in the WTBs were: GF/epoxy/MWCNT buckypaper [48], SWCNT-polyether ether ketone (SWCNTPEEK) [49], SWCNT-methyl methacrylate (MMA)-styrene (Sty) [50], DWCNT-epoxy resin [51], MWCNT-PUR (synthesized from methylene diphenyl diisocyanate and a mixture of polyether polyols) [52], and CF reinforced polymers (CFRP)/epoxy resin with MWCNTs and few-layered graphene (FLG) [47]. Mechanical properties were determined for each of the reinforced composites and enhancements of stiffness, strength and fatigue life were observed as compared to the neat matrices for all cases.


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Notably, introduction of even small amounts of CNTs (single-, double- or multi-walled nanotubes) to a level guaranteeing uniform dispersion, has led to the increased mechanical (but also other, e.g. electrical or thermal) properties of the composites. As mentioned earlier, one of the most frequently CNT-based materials used for preparation of WTBs were nanocomposites based on epoxy resin or PUR matrices (PUR composites have been prepared via dispersing of CNTs and surfactant in polyol, degassing and mixing with isocyanate). Due to increased viscosity and formation of extensive nanotube 3D-networks, combining CNTs with resin components (at loadings leading to higher performance than matrix itself) requires more rigorous mechanical energy regimes (e.g. high-shear mixing, ultrasonication, etc.). Apart from air bubbles as a regular problem in the preparation of viscous blends, high nanotube surface area causes adsorption of air and moisture, and hence, in order to maximize filler-matrix interphase, pre-formulations of composites must be subjected to degassing supported by elevated temperature, diminished pressure, addition of degassing agents/surfactants [e.g. polyvinyl butyral, Triton X-100 (polyoxyethylene phenyl ether), BYK-9077™], centrifugation and/or thinning the samples. The preparation of CNT composites can be also achieved by multiple milling of CNTs, high-energy and/or and thermal pretreatment of the CNT-added resin [53]. Additionally, enhanced filler-matrix interactions could be achieved by CNT surface functionalization leading to further covalent crosslinking [51,53,54]. Another possible approach to modification of CNTs is based on the plasma treatment. This method has many advantages over the ‘wet’ methods. This is a relatively swift, controllable and single-step process yielding various functionalities on the CNT surface. For example, Williams et al. [55] showed that oxygen plasma treatment yielded carboxyl-CNTs yielding stable dispersions and it was an effective



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method for coating the prepreg surface with a nanomaterial phase. The process eliminates or significantly reduces problems related to off-filtration of CNT agglomerates, necessity of operating with larger quantities of liquid phase and viscosity problems associated with the resin transfer methods. 2.3 Potential of the scaling-up Trying to dissociate from the purely lab manufacturing and testing, one must immediately admit that multi-scale composites of CF and/or GF reinforced with CNTs are the ones to be the more likely used in the WTB manufacturing than isotropic CNT epoxy composites. CF composites already proved their excellent and industrial-scale properties, limited, however, by interlaminar and compressive performance. One of the possible solutions could be introduction thereonto CNTs (under various geometries, morphologies and localizations). Their role would be the enhancement of throughmatrix-thickness dominated properties in the intra- and interlaminar regions providing, consequently, higher strength and toughness of the final composites. CNTs were also recorded to improve electro- and thermoconductivity as well as adhesion to the polymer matrices [56,57]. There are two different ways for preparation CNT/CF composites (Fig. 11): attaching CNTs directly onto fibres (A) or dispersing CNTs in polymer matrix (B) [58].


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Fig. 11 Two different routes for the preparation CNT/CF polymer composites [23].

A simple addition of CNTs into CF composite preformulations, as one of the alternative routes toward CF/CNT composites, is limited by matrix viscosity and the filtration effect – in consequence – it is possible only with low CNT loadings. Furthermore, the flow of the resin during impregnation tends to align the CNTs parallel to the primary CFs which is the least desirable orientation for enhancing the matrix-dominated composite performance [59]. CNTs can be also attached to CFs through chemical grafting, electrophoretic deposition or by using a CNT-loaded fibre size. However, these methods do not provide strong attachment of the CNTs to CF. A superior approach to the above ones employs hierarchical composites with CNTs grafted onto the CF (CNTg-CF). Grafting the CNTs onto the CF surface provides higher CNT loadings with a radial orientation, and consequently improves the fibre-matrix interfacial strength and resistance of the composite to delamination [60]. Nevertheless, growth of CNTs on CF is a challenge. c-CVD is the most frequently and conveniently used process here. This route, i.e. when CF is used as a substrate, has, nonetheless, few disadvantages like



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degradation of intrinsic strength and stiffness (high temperature, dissolution of carbon from CF surface by catalyst, damage from water and oxygen) [61]. One must recall here the most important examples. And so, Lachman et al. [62] performed c-CVD for synthesis of CNTs on the CF surface and observed improvement in the filler-matrix adhesion. But simultaneously, heterogeneous coating/grafting and a decreasing in the fibre strength occurred. Antony et al. [63] used a continuous open-ended CVD for CNT-g-CF production. The use of a potential difference to the fibres provided in situ homogenous growth of a thin layer of CNTs. This approach allowed for the production of short, aligned and densely CNT-grafted CFs. Additionally, the composite maintained a high primary CF volume fraction in the material exhibiting enhanced electrical, mechanical and thermal properties. Importantly, no damages in the CF structures were observed as initially assumed which is a very important premise and pours new hope in the light of the future CNT-based WTB manufacture. 2.4 Mechanical high-performance CNT composites There is an increasing number of papers discussing the improvement of fibre composites by deposition of CNTs on fibre surface. Interfacial shear strength of hierarchical composites could be improved from 11 to 150% for MWCNT/CF in epoxy matrix [64,65,66] and 26% in PMMA matrix [67]. Fracture toughness of epoxy composites could also be improved from 30 to 200% [68,69,70]. CNTs was found to generate improvements in properties of the composites not only for CF but also for GFs [71,72,73] or alumina fibres [74,75]. Coming to details, Wan Dalina et al [48] incorporated two types of MWCNT (OD=10 nm; l=1-10 m) buckypaper [free-standing isotropic MWCNT buckypaper (CNTBP) and MWCNT buckypaper/epoxy (CNTBPE)] into three-ply glass fiber/epoxy laminated composites (3GF) (Fig. 12).


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Fig. 12 Schematic diagram of the manufacture of CNTBPE and CNTBP samples [48].

The amount of MWCNTs in both hybrid samples was 2.8 vol.%. It was observed that flexural moduli of both samples were increased up to 13% when compared to 3GF. Also, flexural strength was higher as compared to the reference material, i.e. by 20% for CNTBP and 50% for CNTBPE, respectively. As authors claimed, MWCNTs provided better load transfer from epoxy to the GF reinforcement, thus promoting an even distribution of forces throughout the composites. The strong adhesion forces might explain the increase of flexural strength of enriched MWCNT materials as compared with the original sample. Lower flexural strength for CNTBP composite was caused by inefficient epoxy impregnation through CNTBP and the presence of voids and hence weak filler-matrix bonding. Specific strength of the reference sample was 35% lower than for CNTBP and 70% lower than for CNTBPE composites. The effect of incorporation of CNTs into matrix on the cyclic fatigue behavior has been studied by Loos et al [76]. In this study, concentration of MWCNTs (OD=13 nm, l>1 m) in the composite was 0.19 wt.%. The authors reported that the fatigue life of MWCNT/epoxy



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composite has increased by 1550% over the neat epoxy (for 25 MPa peak stress). It has been demonstrated that MWCNTs could suppress failure in polymers via crack-bridging and a frictional pullout mechanism. On the other hand, the MWCNT composite exhibited more elastic behaviour than its neat epoxy counterpart but similar tensile strengths were recorded for both materials. Yu et al. [77] have studied the effects of MWCNT (OD=40-60 nm; l=5-15 µm) concentration on fracture toughness of the composite. Additionally, the composites were prepared in the presence and in the absence of degassing agent. The results indicated that fracture toughness of 1 wt.% and 3 wt.% MWCNT composites increased by 30% and 35%, respectively as compared to the neat epoxy (without degassing agent) or 30% and 60% respectively (with degassing agent). Furthermore, fatigue life of 0.5 wt.% MWCNTs/epoxy composite, as compared with neat epoxy, was 10.5 and 9.3 times higher with stress amplitudes of 8.67 MPa and 11.56 MPa, respectively. Godara et al [78] have studied composites containing 0.5 wt.% of amine-functionalized MWCNTs (OD=9.5 nm, l=1.5 µm) (Nanocyl NC3152™, NH2 content less than 0.6 wt.%) and also have observed increases in the interfacial shear strength as compared to the neat composites – from (i) 32% for MWCNT-sized GF reinforced composites with MWCNT/epoxy to (ii) 48% for virgin GF-reinforced composites with MWCNT/epoxy to (iii) 92% for MWCNT-sized GF-reinforced composites. Gojny et al [51] investigated fracture strain and fracture toughness in the epoxy composite reinforced with 0.1 wt.% of pristine and amino-functionalized doublewall carbon nanotubes (DWCNTs) (OD=2.8 nm, l>1µm). Fracture strain has increased from 7.25% (0.1 wt.% DWCNTs) to 7.65% for NH2-DWCNTs while fracture toughness from 16% (0.1% wt. DWCNTs) to 18% (0.1% wt. NH2-DWCNTs) to 26% (1 wt.% NH2-DWCNTs) as compared to the neat epoxy. As the authors claimed, covalent


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bonding between functionalized nanotubes and the epoxy resin prevented from the earlier failure at the agglomerate-matrix interphases. Indeed, not only CNTs have shown potential to augment a variety of physical and mechanical properties resulting in multifunctional composites. Also, other carbon nanomaterials like carbon nanofibers (CNFs) could offer remarkable properties of the future WTBs. For instance, Palmeri et al. [79] prepared CNF composites in the epoxy systems. The systems contained blends of two high-performance resins – high-viscosity tetraglycidyl 4,4’-diaminodiphenyl methane (resin A) and low-viscosity triglycidyl paminophenol (resin B). Both resins were cross-linked with a stoichiometric amount of 3,3’-diaminodiphenyl sulfone. The studies shown an improvement from 45% (for pure resin A) to 80% (for pure resin B) in the fracture toughness for composites with 0.68 wt.% of CNFs. Similar results were received by Bortz et al. [80]. The authors admixed 0-1 wt.% of helical-ribbon CNFs to epoxy matrix which caused an increase in the fracture toughness to 66% for 0.5 wt.% and 78% for 1 wt.% CNFs, respectively. In the same time, the observed increase in the fatigue life of composites was 180% and 365% for 0.5 and 1 wt.%. The cohesive forces between CNFs and the matrix are weak because typically lipophilic CNFs have small specific surface area and low surface energy. Increase of hydrophilic character of CNFs could be achieved by surface treatments and sizing, both of which are conducted after manufacturing of the fibres. All of the steps included: polymerization from a precursor, stabilization and carbonization followed by surface treatment and sizing. CNFs for composites are subjected to surface sizing which does more than just protects the fibres. It also facilitates strand formation, reduces fuzz, improves processability and increases bonding between fibre and matrix [6,81]. As



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demonstrated in modelling, an improvement of adhesion at the CNF/matrix interface could be attained by introduction of CNTs on the CNF surface emerged – similarly to CFs. Liu et al. [54] studied the influence of incorporating MWCNT-COOH or MWCNTCOOH covalently functionalized with phthalazinone-containing diamine (MWCNTDHPZDA) on the interfacial shear strength (IFSS) in the unsized CF/epoxy composites (Fig. 13).

Fig. 13 Schematic of manufacturing CNT/CF hybrid fibre; PPEK=poly(phthalazinone ether ketone) [54].

MWCNTs before oxidation had diameter 10-30 nm and length 5-15 µm, although their morphology after oxidation was not studied. IFSS for MWCNT-COOH/CF reinforced composite was found 10% higher than for the non-modified CF composite. Such a relatively weak enhancement could be attributed to the agglomeration of MWCNTCOOH and a partial oxidative damage of their sp2+ε-architecture. On the other hand, IFSS for MWCNT-DHPZDA composite was 70% higher than for the reference sample. The improvement could be assigned to ‘mechanical interlocking’ of modified MWCNTs within the matrix and the improved filler-matrix adhesion as the amine


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groups born on the phthalazinone moiety could be co-cured with the epoxy component. It might be hypothesized that less destructive covalent functionalization as proposed e.g. by Kolanowska et al [82] could yield even higher augmentations. Siddiqui et al [83] investigated CF-reinforced epoxy composites with 0.5 wt.% and 1 wt.% MWCNTs (OD=40-60 nm, l=20 µm). Here, interlaminar shear strength (ILSS) was found to increase by 12% for 0.5 wt.% MWCNTs as compared with the neat composite. For the same composite, torsional shear stress was improved by 19.5% while torsional shear modulus by 17%. It should be though noted that addition of 1 wt.% MWCNTs into the matrix was unsuccessful due to poor dispersion of nanotubes in the resin. Other CNT composites were prepared from poly(ether ether ketone) (PEEK) as the matrix. Diez-Pascular et al. [49] reported manufacturing of PEEK/SWCNT/GF laminates with 0.5 wt.% and 1 wt.% SWCNT (arc-purified OD=10-20 nm, l=1-10 µm; laser-grown OD=1.3 nm, l=1-10 µm) loading. With regard to tensile strength, experimental data revealed its increase with the increasing SWCNT content, i.e. ca. 17% for composites containing 1.0 wt.% SWCNTs. Addition of 0.1 wt.% and 0.3 wt.% MWCNTs (OD=13 nm, l> 1 µm) to PUR composite (synthesized from methylene diphenyl diisocyanate and a mixture of polyether polyols) was investigated by Loos et al. [52]. Here, fatigue life increased by 248% at 50 MPa peak stress for 0.3 wt.% MWCNT-PUR systems as compared with the neat PUR matrix. Composites containing 0.3 wt.% MWCNTs showed an increase up to 38% in the tensile strength in relation to the reference PUR sample. Marrs et al [50] have observed an increase in the fatigue life of MWCNT-modified (d=18-180 nm, l=50 µm) methyl methacrylate-styrene copolymer (MMA-co-Sty). And so, addition of 2 wt.% and 5 wt.% of MWCNTs have improved fatigue life for 20 MPa peak stress by 565 and 592%, respectively. CF/epoxy resin was



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also reinforced with MWCNTs (Nanocyl™ NC7000, OD=9.5 nm, l=1.5 µm) and few layered graphene (FLG) in a study by Knoll et al [47]. The fatigue life increased 5 times for MWCNTs and 15 times for FLG, respectively. Table 2 summarizes how type of non-modified matrix or composite, morphology and surface physicochemistry of CNTs affected mechanical properties of the composites prepared to be tested in WTBs. Table 2 Enhancement of mechanical properties of CNT-based polymers toward their application in WTBs Type / amount of CNTs; Mechanical Neat polymer/composite

nanotube dispersion /

Enhancement [%]

Reference

40-60

[48]

7.7-18

[84]

7.25-7.65

[51]

property orientation MWCNTs (OD=10 nm, l=1-10 nm); 2.8 vol.%; GF/epoxy homogeneous dispersion / random orientation Flexural strength SWCNTs; 0.5, 1.0 wt.%; non-homogeneous PEEK/GF dispersion / random orientation DWCNTs, DWCNT-NH2 (OD=2.8 nm, l>1µm); 0.1 Fracture strength

Epoxy resin

wt.%; homogeneous dispersion / random orientation DWCNTs; 0.1 wt.%; homogeneous dispersion /

16.9

random orientation DWCNT-NH2; 0.1 wt.%; Toughness

Epoxy resin

homogeneous dispersion /

18.5

[51]

random orientation DWCNT-NH2; 1.0 wt.%; homogeneous dispersion /

26.2

random orientation


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MWCNTs (OD=40-60 nm, l=5-15 µm); 1.0, 3.0 wt.%; Epoxy resin

30-33

[77]

1550

[76]

93-105

[77]

248

[52]

565-592

[50]

32-92

[78]

69.9

[54]

12

[78]

19.5

[78]

homogeneous dispersion; individualized nanotubes MWCNTs (OD=13 nm, l>1µm); 0.19 wt.%; Epoxy resin homogeneous dispersion / random orientation MWCNTs (OD=40-60 nm, l=5-15 µm); 0.5 wt.%; Epoxy resin homogeneous dispersion / individualized nanotubes Fatigue life cycles, MWCNTs (OD=13 nm; 30 MPa l>1µm); 0.3 wt.%; PU homogeneous dispersion / random orientation MWCNTs (OD=20 nm; l>1µm); 2.0, 5.0 wt.%; MMA-co-Sty

nearly homogeneous dispersion / random orientation MWCNTs (OD=9.5 nm; l=1.5 µm); 0.5 wt.%; from-

GF/epoxy resin GF radial alignment / high entanglement Interfacial shear MWCNT-COOH, strength MWCNT-DHPZDA; 9.8 Epoxy resin with CNF

wt.%; uniform distribution on the CNF surface / high entanglement MWCNTs (OD=9.5 nm, l=1.5 µm); 0.5 wt.%; from-

Interlaminar shear GF / epoxy resin

GF radial alignment / high

strength

entanglement Torsional shear

MWCNTs (OD=9.5 nm; GF / epoxy resin

strength

l=1.5 µm); 0.5 wt.%; from-



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GF radial alignment / high entanglement MWCNTs (d=9.5 nm; Torsional shear

l=1.5 µm); 0.5 wt.%; fromGF / epoxy resin

19

modulus

[78]

GF radial alignment / high entanglement

Flexural and tensile strengths of various CNT composites were presented in Fig. 14.

Fig. 14 Flexural (left) and tensile (right) strength of WTB composites as a function of CNT content; LG = laser-grown, AG = arc-discharge grown

Clearly, an improvement of both parameters was found for CNT-modified composites. The highest value of flexural strength was achieved for CNTBPE composite with 1 wt.% MWCNTs (OD=10 nm, l=1-10 nm) while the highest value of tensile strength was achieved for PEEK/LG-SWCNT composite with 1 wt.% SWCNTs (laser-grown CNTs; d=1.3 nm, l>1 µm). It can be noted that preselection of thermodynamically compatible matrix is a noticeable premise toward high-performance and long-living composites [85]. Nevertheless, the weight and costs of the matrices (epoxy ca. 10 €/kg, PEEK >100€/kg) limit applicability of the expensive ones since turbines with WTBs based thereon would be economically non-competitive [86]. Fig. 15 shows a relationship between toughness and fatigue life and type/amounts of CNTs in the composites.


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Fig. 15 Toughness (left) and fatigue life (right) of CNT composites as a function of CNT content (wt.%).

The highest values of toughness were achieved for epoxy composites containing 0.1 wt.% and 1 wt.% of DWCNT-NH2. It is evident that covalent filler-matrix bonding accompanied by uniform dispersion of nanofiller is the most prospective applicationoriented modification which again though must be balanced by economy. In turn, the highest value of fatigue life of composites was recorded for epoxy composites with 0.3 wt.% MWCNTs (OD=40-60 nm, l=5-15 µm) for a stress peak at 30 MPa. The lowest fatigue life cycles were observed for MMA-co-Sty composite with 10 wt.% MWCNTs which was associated with a poor dispersion of CNTs in the composite. Another work by Zhang et al. [87] proved that the fatigue crack growth rates were significantly reduced by using CNTs of smaller diameter and higher length which led to the improvement of CNT dispersion in the matrix. 2.5 CNT in WTB adhesive joints Polymer adhesives are commonly used for joining multi-part composites. This solution provides many advantages including high strength-to-weight ratio, superior fatigue and environmental resistance, low stress concentrations, less requiring process regimes and, last but not least, relatively low costs. Since adhesives are much weaker than the adherents they join, nanofiller-toughened adhesives, including CNTs, were considered



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as a potential route to the progress in this domain [88]. For example, Hsiao et al. [89] bonded two graphite fibre/epoxy composite laminates using MWCNTs. As low as 1-5 wt.% content of MWCNTs yielded the improved shear strength of adhesion. It must be emphasized that durability of the bonded components and CNTs not only improved the adhesive properties but also the overall durability of the composites. Yu et al. [90] prepared epoxy composite with a CNT fraction between 0-5 wt.%. The incorporation of CNTs increased the bonding strength and durability of the epoxy joints. Srivastava [91] studied carbon/carbon and carbon/carbon-silicon carbide composites bonded with pure epoxy resin containing 3 wt.% of MWCNTs. Here similarly, the CNT resin had a higher adhesive joint strength. CNT addition led also to: an increase in toughness and strength of the epoxy resin as well as resistance to: (a) growth of cracks, and (b) shear deformation. Konstantakopoulou et al. [92] investigated the effect of CNT incorporation (MWCNTs, OD=5-50 nm, l>0,5 µm) and the optimal MWCNT weight fraction toward improvement of mechanical properties. Here, MWCNT incorporation had a moderate improvement in the joint strength. The results revealed that 0.3 wt.% was the highest weight fraction which could be used without decreasing the strength of the composite. A significant influence on the improvement of joint properties was found for a type of dispersing method (sonication/mechanical stirring, time) and the surface preparation. Korayem et al. [93] investigated the effects of CNT-modified epoxy resin on the CFreinforced polymer-to-steel interfaces. The incorporation of CNTs into the epoxy resin led to an increase in Young’s modulus and tensile strength of the adhesive. Furthermore, Sanchez-Romate et al. [94] used spraying technology for the preparation of adhesive films. CNTs (0.1 wt.%) (NC7000, OD=10 nm, l=2 µm) were sonicated in water with sodium dodecyl sulphate as a surfactant. CNTs were homogeneously


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distributed along the adhesive film providing 3D-network. The authors observed that the electrical resistivity of the film increased with the increasing mechanical strain due to tunnelling effect on the CNT percolating networks. Further, this phenomenon caused breakage of the conductive paths due to the crack propagation. An increase of the electrical resistance was correlatable with the crack propagation along the bonding line. As the authors proved, CNT-doped adhesive films could be used for detection of crack propagation along the bonded joints. Kang et al. [95] incorporated CNTs (OD=10-15 nm l=10-20 µm) into the epoxy resin using a three-roll mill. This adhesive was used for an aluminium composite. The fatigue lives of adhesive joints with 2 wt.% of CNTs were higher than those of joints without CNTs. The prolonged fatigue life was probably connected with a longer crack initiation and propagation time. Consequently, crack initiation and propagation could be detected then by measuring the variation of equivalent resistance when CNTs dispersed into the adhesive. 2.6 Anti-icing and de-icing WTBs using CNT-based systems The presence of ice on the WTB surface has adverse effects on its safety and efficiency. It increases weight of the blade which causes the unbalance of the rotor and changes the silhouette of the blade. Larger ice accumulation on the blade may inevitably cause degradation of the aerodynamic turbine performance. Hence, anti-icing and/or de-icing systems become a necessary tool to avoid the irreversible damages of WTBs [96]. Antiicing and de-icing are two major pathways for suppressing adhesion of ice on surfaces. The anti-icing property prevents from freezing of the incoming water without any power input, while the de-icing provides an alternative function of removal of ice when the environmental conditions exceed anti-icing limits [97]. Here, electrothermally heating systems are a subject of a great interest for several reasons. They are, among others,



characterized

by:

(i)

high

energy

efficiency

and

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(ii)

lower

maintenance

requirements/costs, (iii) without increasing the noise during operation. CNTs with their excellent thermal and electrical properties have been frequently considered as the electrothermally heating elements [98,99,100]. Going to the latest reports, Xu et al. [101] indicated that surfaces with higher hydrophobicity demonstrated better anti-icing performance and benefitted from the active de-icing – surfaces of increased hydrophobicity led to lower weight of cumulated ice and negligible ice adhesion. Nevertheless, full exploitation of typically entangled and defective CNTs is difficult as non-uniform and viscous dispersions are formed therefrom. On the other hand, it must be highlighted that some of the trials emerged as successful. For instance, Yao et al. [102] produced CNT web by drawing a continuous sheet or film of horizontally oriented CNTs of a specific aspect ratio. The highly aligned CNT web was characterized by a negligible weight, uniform heating, efficient energy consumption and rapid antiicing/de-icing. Fischer et al. [103] prepared a CNT layer on a non-woven substrate and covered it by a GF layer. For this anti-icing system, the ice layer of a thickness 3-4 mm could be completely removed within less than half an hour at -5 C. Furthermore, Jaing et al. [104] applied the photothermal effect of CNTs. They sprayed superhydrophobic SiC/CNTs (MWCNT-COOH, OD=100 nm, l=10-20 µm) coatings on poly(ethylenevinyl acetate) (PEVA) surface. Under NIR irradiation, the surface temperature increased and the so-generated heat was rapidly transferred melting the ice. There are in fact several patented solutions with CNTs as the electrothermally active nanofillers suitable for WTBs [105,106,107]. CNTs, e.g. as paints, can be used here due to their low density, convenient processing, well-defined conductive capacity (leading to low power consumption) and simple manipulation.


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3 Conclusions and future perspectives It has been shown that CNTs could offer significant improvements when applied to WTBs. The level of mechanical outperformance of CNT-based components of complex composites and hybrid materials, although studied so far non-systematically, is typically a function of matrix physicochemistry, matrix–filler and filler–co-filler interactions (critically – arrangement of CNTs against and adhesion to the co-filler in the hierarchical composites), CNT morphology (aspect ratio, number/type of defects) and the filler density, distribution and homogeneity. Industrial scale manufacture of the CNT-based final material for WTBs emerges then as a trade-off between properties and economy. The critical argument of economy for new CNT-based nanomaterials in WTBs cannot be omitted: due to the highest production costs and limited uniformity of production batches as compared to GFs or CFs, CNTs are still en route to the global wind energy industry. On the other hand, market prices of CNTs are continuously declining. Exemplary prices of industrial grade CNTs are: 95 €/kg (Nanocyl NC7000™ MWCNTs), 100 $/kg (MWCNTs, Jiangsu XFNANO Materials Tech Co. Ltd), 600 $/kg (Cheaptubes™ MWCNTs) – just to mention few producers from different continents. But still, simplifying, a nanotube is ‘incomparable’ to other nanotubes not only between producers – the products may differ in numerous critical parameters: from contamination levels to within-batch differences in number of crystallographic defects to dispersion of geometrical parameters (length, diameters, etc.). The continuous absence of standardization of carbon nanomaterials is here manifested – paraphrasing Richard Feynman’s, ‘there is (still) a plenty of room at the bottom’… for improvement. The most up-to-date research on the application of CNT-based WTB materials suggest that the hierarchical composites manufactured from CNT-grafted CFs provide the most



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encouraging properties and hence possible to be scaled-up for wind energy industry in the nearest future. On the other hand, there is at least one most evident and promising aspect of the added value for CNT-based WTBs, i.e. de-icing which concerns either permanently or transitorily cold climate. As shown, ice accretion on the WTBs is detrimental to their performance, durability and safety of personnel in the vicinity of operating [108]. Maintenance of higher surface temperature at low wetting of superhydrophobic WTBs could be a promising way of retarding the icing [109] as conductive CNT-based composites could provide synergetic electrothermal heating of WTBs repelling water droplets. Such CNT-based systems are probably the closest ones to be implemented soon on the industrial scale. 4 Acknowledgements Slawomir Boncel greatly acknowledges the support from the Rector’s Professorial Grant: RGP 04/020/RGP18/0072, Silesian University of Technology. Anna Kolanowska is greatly indebted to Silesian University of Technology for the financial support in the framework of the Statutory Activity Grant for Young Scientists (BKM/525/RCH2/2018). 5 References 1 Zhang, Q.; Huang, J.-Q.; Qian, W.-Z.; Zhang, Y.-Y.; Wei F. The Road for Nanomaterials Industry: A Review of Carbon Nanotube Production, Post-Treatment, and Bulk Applications for Composites and Energy Storage. Small 2013, 9, 1237–1265. 2 Lu, X.; McElroy, M.B.; Kiviluoma, J. Global Potential for Wind-Generated Electricity. PNAS 2009, 106, 10933-10938. 3 Jaber, S. Environmental Impacts of Wind Energy. J. of Clean Energy Technol. 2013, 1, 251-254. 4 Sawyer, S.; Fried, L.; Shukla, S.; Liming, Q.; Global Wind Report 2016 – Annual Market Update,


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Time-dependent decreasing levelized cost of electricity (LCOE) in USD per MWh in 2013 (left Y-axis); development of rotor diameters in the wind turbines (right Y-axis); the first enthusiastic results in the application of CNTs in the lab-scale wind turbine blades emerged in 2010 but despite numerous efforts no industrial scale CNT containing blades have been applied up-to-date – the questions on added values, economy and specification (geometry, processing, incorporation/admixing, etc.) still remain open. On the basis of figure in ‘Wind energy – the facts: a guide to the technology, economics and future of wind power, European Wind Energy Association, Earthscan, 2009. 334x175mm (144 x 144 DPI)

ACS Paragon Plus Environment

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CNTs in WTBs – the number of scientific publications until October 2018 per year; search words = carbon AND nanotube(s) AND wind AND turbine AND blade(s) 212x177mm (144 x 144 DPI)



A simplified cross-section of WTB with an indication of working loads with their directivities (left); the key elements of WTB which manufacture is based on assembling and bonding two aeroshells and shear webs (grey colour indicates the primary load-carrying composites) (right) [6]; Copyright MDPI, Basel, Switzerland. 305x125mm (144 x 144 DPI)


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Technologies of WTB manufacturing: A) wet hand lay-up; B) filament winding; C) prepreg; D) resin infusion technology. 419x215mm (144 x 144 DPI)



Most frequently reported damages of wind turbine components [16]; data from GCube Insurance Services, Inc., New York, USA. 219x110mm (144 x 144 DPI)


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Main damages of WTBs at various interphases; modified from a figure in [ ]; Copyright MDPI, Basel, Switzerland. 333x151mm (144 x 144 DPI)



Typical variations of CNT batches (morphology, contaminations, etc.) and their macro-assemblies: CNTs may be arranged into isotropic (powder, films, etc.) and anisotropic objects (sheets, fibres, yarns, etc.) 169x140mm (144 x 144 DPI)


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Changes in degradation of unmodified (solid line) and nanoparticle [weaker (dashed line) and stronger (dotted line) filler-matrix interactions] modified composite laminate [47]. 334x200mm (144 x 144 DPI)



Typical variations in morphology and surface chemistry of CNTs 217x162mm (144 x 144 DPI)


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Chemical formulae of matrices used in the manufacture of CNT-based WTBs; R = alkylene and/or arylene units. 264x229mm (144 x 144 DPI)



Two different routes for the preparation CNT/CF polymer composites [23]. 334x187mm (144 x 144 DPI)


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Schematic diagram of the manufacture of CNTBPE and CNTBP samples. 341x174mm (144 x 144 DPI)



Schematic of manufacturing CNT/CF hybrid fibre; PPEK=poly(phthalazinone ether ketone) [54]. 244x115mm (144 x 144 DPI)


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Flexural (left) and tensile (right) strength of WTB composites as a function of CNT content; LG = lasergrown, AG = arc-discharge grown. 337x95mm (144 x 144 DPI)



Toughness (left) and fatigue life (right) of CNT composites as a function of CNT content (wt.%). 338x119mm (144 x 144 DPI)


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