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Glass fiber and glass fiber-reinforced polymers are of interest to engineers for a wide variety of applications, owing to their low weight, high relative strength, and relative low cost. However, management of glass fiber waste products is not straightforward, particularly when it is part of a composite material that cannot be easily recycled. This is especially the case for physically large structures such as wind turbine blades. This chapter deals with the challenges of managing this growing waste stream and reviews the structure and chemistry of glass fiber and glass fiber-reinforced polymers used in wind turbine blades, the separations processes for extracting the glass fiber from the thermoset resin, and end-of-life options for the materials. Thermodynamic evidence is reported and evaluated for a novel end-of-life solution for wind turbine waste: using it as a supplementary cementitious material.
Institute for Energy Studies, Western Washington University, Bellingham WA, USA
Engineering and Design, Western Washington University, Bellingham, WA, USA
Advanced Materials Science and Engineering Center, Bellingham, WA, USA
Layla Russell
Institute for Energy Studies, Western Washington University, Bellingham WA, USA
Paul Striby
Institute for Energy Studies, Western Washington University, Bellingham WA, USA
*Address all correspondence to: glossed@wwu.edu
1. Introduction
Glass fiber reinforced polymer (GFRP) is manufactured from the high temperature conversion of raw materials into a homogenous melt which is fabricated into fiber and bonded to a polymeric resin [1]. Raw materials used for the glass fiber (GF) portion are generally derived from geologic sources and may include silica sand [2]; clay [3]; calcite [4]; and borate minerals [5]. GFRP is used in multiple applications including textiles, electronics, aerospace, boats, and wind turbines, to name a few [6]. Despite the advantages of GFRP composites – such as low weight, high compressive strength, and relative low cost – the thermoset nature of the polymeric matrix presents a challenge for recycling of these materials, as the resin is not easily depolymerized from the original constituents and tends to undergo a loss of mechanical strength following separation from the resin [7]. This chapter describes the physical and chemical properties and use of GFRP with a specific focus on wind turbine blades, and evaluates current and prospective end-of-life strategies for dealing with this complex waste stream. Also discussed in this chapter are currently available recycling and end-of-life solutions for GF and GFRP, as well as a review of the separations processes for depolymerization of the resin. This chapter also recapitulates and discusses the results of a study on its prospective revalorization as a supplementary cementitious material (SCM). The strategic re-use of wind turbine GF in cement has the potential to not only to deal with this challenging waste stream, but also to offset the carbon emissions from cement clinkering [8].
1.1 Chapter objectives
This chapter is motivated by the need to understand limitations of current end of life options for GFRP used in wind turbine blades, and to systematically characterize the materials (chemical composition, structure) and technologies used in producing and processing wind turbine GFRP so that an assessment of its end-of-life uses as an SCM and otherwise may be presented End of life options for GFRP based materials are complicated by the thermoset nature of the composite material [9]. For this reason, repurposing of as-received GFRP-based parts is often the most economical solution. This makes the waste management from wind turbines – particularly their blades challenging - as the parts are physically quite large: they have an average rotor diameter of 127.5 meters and weigh on average 22,000 kg for the blades and hub alone [10]. Furthermore, the average lifespan of a wind turbine is 20 years, and by the year 2050, there is predicted to be over 40 million tons of GFRP based wind turbine materials entering the waste stream [9]. Although wind power is a lower-carbon option for energy generation than hydrocarbons, strategies are needed to deal with the growing waste stream. One such strategy is recycling the GF portion of wind turbine waste for use as a supplementary cementitious material (SCM) [8]. While GFRP has previously been used by the cement industry for several applications it has not been commercially used as an SCM. This chapter will discuss the viability of using GF as an SCM based on a previously reported study, and will also describe the chemistry and structure of GFRP in relation to the current end-of-life options for the materials.
1.2 Review of wind turbine and GFRP anatomy
A wind turbine is a device that extracts energy from wind and converts it to mechanical energy, which is further converted into electricity by the wind turbine generator. Structurally, wind turbines are comprised of rotor blades attached to a base. The nacelle is the housing attached to the base, which stokes the turbine’s generator. The anatomy of a wind turbine is shown in Figure 1, along with a key showing the primary materials used in each component.
Figure 1.
Schematic diagram of wind turbine showing its parts and composition.
As seen in Figure 1, the primary material of the rotor blades is GFRP, whereas concrete, rare earth elements, and e-metals and steel are the main materials of the remainder of the turbine. Of the GFRP in the blades, roughly 60% of the mass is comprised of GF, and the remaining 40% is comprised of resin. This chapter is primarily concerned with the turbine blade GFRP.
Figure 2 shows a colorized backscattered scanning electron microscope (SEM) image of GFRP, magnified x267. The yellow filaments apparent in the image are the glass fiber components, and the smooth green plate-like structures are the resin. Visual inspection of the orientation of the filaments in the resin material shows that the glass fibers are reinforcing the resin and are primarily oriented along the sagittal plane. This dispersion and orientation contribute to the high tensile strength of GFRP materials. The high surface area to volume ratio of the GF is also apparent upon visual inspection. Compositionally, the glass fibers are derived from geologic materials that have been melted and processed into glass filaments, and the resin is a thermoset epoxy-based material. The SEM image reveals the nature of the composite material: that is to say, the degree to which the GF filaments are integrated into the resin. This is a helpful frame of reference when considering the technologies and methods available to separate the composite.
Figure 2.
Colorized backscattered scanning electron microscope image of GFRP.
2. Review of current wind turbine GF and GFRP end-of-life options
End-of-life options for wind turbines generally fall under two categories: repurposing of old blades and wind turbine parts, and separation and recycling of the materials. Recycling processes require either the mechanical crushing of whole GFRP, or the extraction of specific materials from the GFRP and resin components in the composite, whereas repurposing uses the composite material with minor modifications in the as-received state. A waste hierarchy is conceptually shown in Figure 3, which describes the order of preference for the currently available methods to reduce the environmental impact of GFRP waste from wind turbine blades. While the design of materials to extend the service life of the parts is shown to be the least environmentally impactful option, repurposing and recycling represent intermediary options between life-extending design and landfilling/disposal. Repurposing and recycling of wind turbines are described below, along with a critique of the limitations and benefits of each.
Figure 3.
Waste hierarchy showing order of preference for available methods to reduce the environmental impact of GFRP waste. Adapted from Nagle et al.
2.1 End of life: repurposing
Repurposing of wind turbine parts by making minor modifications to their parts in their as-received state is the most common end of life option, due in large part to the higher cost of recycling [11]. Examples of repurposing include transforming the blades into load-bearing structural systems; cutting the blade for use in roof frames for small houses and/or affordable housing communities [12]; using the cut root section as a foundation for homes along flood plains [13]; and using the shear webs that connect the top and bottom halves of the blade as doors, window covers and more [14]. Other reuse options include use in community structures such as playgrounds, benches, bridges, and transmission poles. The defining feature of repurposing is that turbine parts are reused in their whole state, with the only modifications being cutting of the sections into smaller parts.
2.2 End of life: recycling
Although repurposing of wind turbine parts is the most economical and therefore common end-of-life option [11], recycling of the blades offers additional opportunities. Recycling refers to the mechanical crushing and reuse of the GFRP or the extraction of GF fibers from the composite for use as feedstock for other processes. Mechanical recycling, whereby scrap composites are grinded into smaller pieces and sieved based on particle size, is currently one of the few processes with current commercial operations, given the low cost and energy expense associated with it [15]. In a mechanical recycling process, the composite material is reduced by successive cutting, grinding, and then shredding. The resulting materials are reclaimed as either aggregate, powder, or clusters of GF. Some uses for mechanically recycled GFRP include use as a 3D printing feedstock; production of laminates; and reuse in new composites [15]. Current uses of recycled materials from these methods are limited by the presence of contaminants such as metals, mineral oils, and paints in the scrap, therefore it is imperative that the use cases for scrap recycling be agnostic to such impurities.
Although GF and GFRP have not been used as an SCM to date, the use of recycled GFRP in the cement industry is not without precedent: Mechanically recycled GFRP has been used as an aggregate replacement in concrete [16]. When used as an aggregate in concrete, tensile, flexural strength and shrinkage of the concrete has been found to improve, However, a review of several studies reveals that the influence is inconsistent, and varies depending on the chemical composition of the feedstock GFRP; the physical properties of the GFRP components; the presence of impurities in the materials; and imperfect dispersion of the materials in the concrete [17]. It should be noted that concrete aggregates are fundamentally distinct from SCMs: In the former case, granular materials ranging in sizes from roughly 10-40 mm are added to cement paste to form concrete. While the aggregate may chemically react with the cement paste, its main purpose is to improve the mechanical strength of the mixture. SCMs, on the other hand, are very fine ground particles that are added to the cement clinker (powder) after it is removed from the kiln, and participate in a series of chemical reactions to form reaction products which favorably impact the hardened cement paste. This is further described in Section 3 and demonstrated conceptually in Figure 4, which shows the differences between cement powder, SCM, aggregates, and concrete.
Figure 4.
Conceptual diagram showing difference between cement powder, SCM, paste, aggregate, and concrete.
Recycling of GFRP may also refer to the extraction of GF fibers from the composite: due to the high energy cost and infrastructure requirement for this process, it has not been widely adopted [11]. However, another precedent for use of recycled wind turbine GFRP is cement co-processing. A small number of plants in Europe have been built specifically for the co-processing of wind turbine waste as a cement clinker replacement [18]. In a co-processing operation, the GFRP from wind turbines is shredded and then mixed with a solid recoverable fuel (SRF), about 50% of which is SFR and 50% GFRP by mass [19]. The resin portion of the GFRP in the wind turbine blade is used as fuel to heat the cement kiln, which offsets a portion of fossil fuel that would otherwise be used for this purpose. The clinker (powder) material in the cement feedstock is then replaced by the GF portion of the which reduces raw material consumption [8, 20].
2.3 Review of separations processes of glass fiber from polymer/composite material
As discussed in Section 2.2, recycling of the GF component of GFRP requires that the materials be separated from the matrix. Since the matrix in these composites often consists of thermoset resin which is not easily depolymerized (Figure 2), separation of the fibers from the resin can be an energetically expensive process [20]. The reader is referred to Figure 2 for a visual examination of the physical integration of the glass fibers in the resin of a typical GFRP. Separations processes to extract the GF from the resin fall into two categories: chemical and thermal. The following sections describe thermal and chemical separations processes to elutriate GF materials from the resin. The limitations described below (energetic expense; presence of impurities; low yields) are indicative of why recycling of GFRP falls below repurposing in the waste management hierarchy (Figure 3), although it should be noted that technological advances may tip these balances in coming years.
2.3.1 Thermal separation
Currently, fluidized bed separation and pyrolysis are the only available thermal processes to accomplish separation of the GF from the composite, and the high operation temperatures required can damage the GF and negatively impact its mechanical properties [15]. This is critical to consider when evaluating how the extracted materials may be used in subsequent applications. The intended use of the resin must also be considered as it is a thermoset material cannot be usefully reshaped upon separation. Hence, any reuse of the resin must not require such a process.
In a fluidized bed process to separate the components, ground scrap composites are fed into a fluidized bed and heated to approximately 450 C with a velocity air jet, which causes the resin to volatilize and decompose. The GF can then be elutriated and sieved [21]. Organic contaminants that did not volatilize with the polymer must be removed in a bed grading process. Oxygen is required for this process in order to minimize formation of char. Fiber yields from fluidized bed processes are roughly 40% of the total fiber product. This is to say that approximately 60% of the feedstock fibers are not successfully rescued from current separations processes, which is a considerable limitation when considering the environmental and economic scalability. Technological improvements to increase the recovery of GF materials from fluidized bed separation are needed to make this process more efficient.
Pyrolysis is the second thermal separation process for extracting fibers from resin. In a pyrolysis process to thermally separate the components, GFRP composites are heated in an anoxic environment, which causes the matrix to decompose. This process involves the destructive distillation of the matrix, and produces hydrocarbons which can be harvested for their chemical value. Currently pyrolysis has only been proven at the laboratory scale and does not have any known commercial operation [21]. This is predicted to change in coming years as research and demand grow.
2.3.2 Chemical separation
In a chemical separation process for recycling GF components, the GFRP composite’s polymetric matrix is dissolved into a chemical solution. Methods such as hydrolysis and glycosis are used to recover the raw materials. In a chemical process, glass fibers are extracted after the polymer is broken down into basic monomers. Nitric acid [7], sulfuric acid [22], and alternate acids can be used as a solvent. Glycol, methanol, ethanol, 1-propanol, and acetone may also be used in solvolysis to depolymerize the matrix and extract the GF [11]. The chemical yields from these processes are quite low, and the solid residue is often contaminated with organic material which requires further treatment. These are similar to the limitations of the pyrolysis process described in the previous section, and therefore, further research and investment is needed for these operations to become commercially valuable.
The previous sections detailed current end of life options for wind turbines, as well as the processes that can be used to separate the GF from the resin so that it can be recycled. The strategic revalorization of GF as a SCM requires that 1) the GF components be reactive with cement clinker, and 2) that they contain the chemical constituents need to form the primary reaction products (hydrates) of cement [23]. To that end section provides an overview of the chemistry of cement clinker, and the major reaction mechanisms and hydrates in cement. The chemistry of wind turbine GF specifically is then described and compared to the chemical constituents used in existing SCM in Section 3.2.
Ordinary Portland cement (OPC) clinker is derived from geologic materials (primarily clay and limestone). The clinker is produced by heating raw materials in a kiln at temperatures of more than 1450°C, where aluminum and iron-based oxides are added as fluxing agents [24]. The primary constituents of OPC clinker are calcium silicates, ferrite, and calcium aluminates, with lesser amounts of potassium and sodium sulfates and magnesium oxide [24, 25, 26]. Cement chemistry notation is given in Table 1.
The reaction of these constituents in the cement pore solution yield solid hydration products such as calcium-silicate-hydrate (C-S-H) and calcium hydroxide (CH), the former of which gives cement its strength, and the latter of which helps to maintain the alkalinity of the pore solution [27, 28]. The alkalinity of cement pore solution is crucial to controlling deleterious processes such as corrosion and alkali-silica reaction [29]. To that end, the concentration of these elements in pore solution – and its pH – are of great importance when designing cement mixtures [30].
3.1 Supplementary cementitious materials
SCMs, when used in conjunction with OPC, contribute to the beneficial properties of the hardened cement by reacting hydraulically, and/or through the pozzolanic reaction (Eq. 1) in geochemical notation and Eq. (2) in cement chemistry notation to form desirable reaction products and pore solution chemistry.
Ca(OH)2+H4SiO4→CaH2SiO4·2H2OE1
CH+S→C−S−HE2
SCMs are generally added to the clinker after the kilning process, and before the addition of water to the mixture (Figure 4). Materials generally used as SCMs include fly ash, silica fume, ground blast furnace slag, and metakaolin, although any material with appropriate chemical composition and reactivity may be used for this purpose [31]. Compositionally, SCMs must be rich in silica, alumina, and calcium, and the proportion of glassy to crystalline materials should be high to ensure that the material will be reactive in an alkaline environment [32]. The proportions of the glass oxides in SCMs will influence the reaction kinetics in a blended OPC/SCM system, as well as the thermodynamic stability of the reaction products [33].
3.2 Wind turbine GF composition and chemistry
The GF used in wind turbine composites is compositionally similar to that of traditional SCM (Figure 5); however, its amorphous nature may make it more reactive than highly crystalline materials, suggesting that it may be more pozzolanic than many fly ashes. The GF is initially derived from geologic sources, which are melted in a furnace at high temperatures (approx. 1400°C) and passed through a perforated bushing before mechanical attenuation and fiberization [34, 35]. The resulting fibers exhibit varying compositions that depend on the initial proportions of raw materials, but are generally high in silica, calcium, and aluminum oxides (Table 2 and Figure 6) [8].
Figure 5.
Ternary diagram showing GF (closed circles) and class C (open triangle) and class F (closed triangle) fly ash from [8].
Chemical name
Formula
Notation
Name
Tricalcium silicate
Ca3SiO5
C3S
Alite
Dicalcium silicate
Ca2SiO4
C2S
Belite
Tricalcium aluminate
Ca3AlFeO6
C3A
Aluminate
Tetracalcium aluminoferrite
Ca2AlFeO5
C4AF
Ferrite
Calcium hydroxide
Ca(OH)2
CH
Portlandite or CH
*Calcium silicate hydrate
C3S2H3
C-S-H
C-S-H
Table 1.
Cement chemistry notation (*note that C-S-H is non- stoichiometric).
HC (g/100 g)
MC/MS (g/100 g)
HS (g/100 g)
CaO
38.00 (34.63)
17.79 (28.95)
22.82 (26.06)
SiO2
58.00 (52.86)
29.94 (48.73)
51.01 (58.27)
Al2O3
8.58 (10.64)
8.58 (13.96)
8.58 (9.81)
Fe2O3
3.50 (4.32)
3.50 (5.69)
3.50 (3.99)
Na2O
0.54 (0.49)
0.54 (0.88)
0.54 (0.62)
MgO
0.56 (0.41)
0.56 (0.91)
0.56 (0.64)
SO3
0.06 (0.05)
0.06 (0.10)
0.06 (0.07)
K2O
0.20 (0.18)
0.20 (0.33)
0.20 (0.23)
Sr
0.18 (0.16)
0.18 (0.29)
0.18 (0.21)
Ba
0.03 (0.02)
0.03 (0.05)
0.03 (0.02)
Table 2.
Chemical composition of notional GF end members raw and (normalized to 100) from [8].
Figure 6.
GF composition as derived from literature. Figure from Glosser et al. [8].
Chemical compositions for GF used in general applications [16, 36, 37, 38] as well as wind-turbine-specific applications [19, 39] are characterized in literature (Table 2 and Figure 6). Glosser et al. [8] condensed the compositional variation of GF into three notional end member compositions that are statistically representative of GF used in wind turbines and normalized to 100% to account for differences in reporting of LOI and resin between the derivative studies (Table 2). As seen in both Figure 6Table 2, despite the variation in specific GF compositions, the primary chemical constituents of GF used in wind turbines – SiO2, CaO, and Al2O3 – are congruent with those of OPC and SCMs like fly ash and slag. Section 4 discusses further assessment by Glosser et al. [8] regarding the potential of GF from wind turbines as an SCM, using thermodynamic modeling to predict important characteristics of cement + GF mixtures.
3.3 Review of thermodynamic modeling in cementitious systems
Thermodynamic modeling of cementitious systems is a proven technique to predict the type and amount of solid reaction products and pore solution chemistry, based only on a-priori knowledge of the chemistry of the reactants [24, 40, 41, 42, 43]. Equilibrium calculations can be performed using a Gibbs Energy Minimization algorithm – which assumes that the reaction proceeds to infinite time [44]. This makes thermodynamic modeling an attractive alternative to experimental designs of OPC/SCM mixtures, particularly when the goal is to predict and optimize for a particular proportion of reaction products.
To date, only one known study has evaluated wind turbine GF for use as an SCM. In this study by Glosser et al. [8], thermodynamic simulations were performed to model the pore solution chemistry and major reaction products of cement + GF mixtures of varying GF end-member compositions derived from literature reports [16, 19, 38, 39]. Replacement levels of GF up to 60% by mass were evaluated and compared to results for OPC. Three notional GF end members were evaluated to capture a probable range of compositions: 1) high CaO (HC); 2) median CaO, median SiO2 (MC/MS); and 3) high SiO2 (HS). These end members (Table 2) are the CaO and SiO2 values (g/100 g) of the minimum, median, and maximum CaO:SiO2 values in grams/100 grams as reported in literature.. In all cases, the mean of Fe2O3 and Al2O3 (grams/100 grams) of all literature values were used. For the trace metals, the maximum of the reported mass concentrations were selected. The compositions of each notional GF composition were then normalized to 100%. Details of the methods can be found in the original study [8].
As discussed in Section 3 of this chapter, two main solid reaction products of interest in a cementitious system are CH and C-S-H. The study modeled the CH and C-S-H content of fully hydrated GF + OPC systems for all GF end members with replacement levels ranging from 0–60% by mass [8]. Results of this study found that for all replacement levels, mixtures with any GF end member contained less CH by mass than OPC only mixtures. This was found to be a result of the consumption of CH in the pozzolanic reaction, separate from dilutionary effects. The authors compared the GF + OPC CH results to that of a fly ash of identical bulk composition to the MC/MS GF, but 40% glass content, and found that the CH would not be fully consumed under these experimental conditions. The implications of this result suggest that GF is a strongly pozzolanic SCM. Examination of the C-S-H content of the GF mixtures supports this interpretation [8].
In keeping with the observations of CH, C-S-H amounts were found to be greater for all GF end members than in OPC, and showed varying trends based on the overall silica content of the GF (Figure 7). Recall from Eqs. (1) and (2) that C-S-H forms from the pozzolanic reaction between silica and CH, so it would be expected that greater concentrations of C-S-H would be present in mixtures where more CH is consumed by this reaction. As seen in Figure 7, this trend was found in the study, with C-S-H concentrations peaking for each end-member mixture at the same GF replacement level as the CH depletion. In all cases and at all replacement levels, the GF + OPC mixtures were found to contain more C-S-H than an OPC only mixture. The study further compared the GF + OPC mixtures to a fly ash with an identical bulk composition as the MC/MS end member, but 40% glass content, and found that the fly ash mixture produced less C-S-H than the GF + OPC mixture. [8]. Since C-S-H is generally considered to be a favorable hydrate, the implications of these simulations are clear: GF may be a powerful SCM for optimizing the amount of C-S-H in cementitious mixtures. The study further found that certain trace metals such as strontium reacted with the cement clinker to form insoluble strontium hydrates, which has the benefit of immobilizing potentially ecotoxic minerals from the GF.
Figure 7.
C-S-H content of GF + OPC end members at replacement levels from 0–60%, with CH depletion point demarcated by arrow. Adapted from Glosser et al. [8].
The pore solution composition makes up the other important component of cement. The study also simulated pore solution pH and composition for each GF end member, up to replacement levels of 60%, and found that pore solution pH for each GF end member were similar up until roughly the 30% replacement level. After eclipsing the 30% replacement level the pH for all end members decreased, and eventually declined to values lower than that of OPC (pH = 13.6). These trends were found to correlate with the depletion of CH and the drop in alkali hydroxides in the pore solution. The results of these simulations show that cements made with GF may be used to achieve a range of optimal pH, depending on the mixture design specifications and the intended use of the mixture. The authors noted that the pore solution pH values of these mixtures, particularly at high replacement levels, is equivalent to those achieved with high reactivity fly ashes. This was found to be true despite the overall lower alkali content in the GF relative to fly ash. The authors stated that the similarity in results between GF and fly ash is likely due to the higher crystallinity and therefore lower reactivity of most fly ashes, which results in a lower available reactive alkali content than that the bulk concentrations of the ashes [8].
4.2 Discussion of GF as an SCM
In this section, the potential for revalorization of wind turbine GF as an SCM is critically discussed in the context of the waste hierarchy shown in Figure 3, as well with an appreciation for the economic viability and practical benefits and drawbacks based on chemistry and intended use.
As shown in Figure 3, recycling of GFRP falls below repurposing and prevention as a preferred waste strategy, but above disposal. The revalorization of wind turbine GF as an SCM would be categorized as a recycling process, insofar as the GF is separated from the polymer (Figure 2) and turned into a new substance: in this case, a powder comprised of ground glass fibers. A major limitation of this use of GF therefore, is the energetic cost and processing required to separate the GF from the resin, and then grind it into a suitably fine powder for it to react with the cement clinker. It is worth noting that the carbon emissions from such a process will be greatly reduced in future years, as the transition away from fossil fuels to renewable energy progresses. This may reframe the position of recycling in the waste reuse hierarchy. This will be particularly true if the separated resin material can be beneficiated in an energy capture process, such as is currently done in the case of cement co-processing as described in Section 3. Technological improvements which increase the yields of GF from chemical and thermal separations will also be needed for recycling of the fibers to become viable at the scale needed for wind turbine blade waste.
Another current limitation of the revalorization of wind turbine GF as an SCM is economic: As described in Section 2 of this chapter, the most common end-of-life option for wind turbines is repurposing, due to the expense associated with separating GF from the resin, as well as the low yields and loss of mechanical strength of the fibers following separations. The issue of expense is likely the limiting factor at present time for revalorizing the GF from wind turbines as an SCM. However, the European co-processing projects described in Section 3 are indicative that the costs need not be prohibitive when the processes are scaled. This may be particularly true in coming decades, when the amount of wind turbine waste entering the waste stream renders repurposing of such a large volume of waste impractical and uneconomical. Ongoing research into commercializing and upscaling thermal and chemical separations processes described in Section 2 of this chapter will also alter the economic balance of recycling wind turbine waste for use as an SCM. As these separations processes become more efficient and more common, the associated cost will decrease, making them more attractive options.
While the limitations and drawbacks to recycling the GF extracted from wind turbine GFRP as an SCM are well articulated, it is worth noting that there are some discrete benefits for this use, both at present as well as in future scenarios. The first present benefit of revalorizing the GF following separation from the resin is that the mechanical properties of the fibers are irrelevant to its use as an SCM. As discussed in Section 2 of this chapter, following chemical and thermal separation of the fibers from the resin, the resulting glass fibers suffer a loss of mechanical strength. This presents a physical and engineering limitation on how the fibers can be reused, as the intended application must not rely on the mechanical strength of the fibers being preserved. However, when considering the use of recycled GF as an SCM, it is the chemical composition and amorphous nature of the fibers that makes them valuable, rather than their mechanical properties. Thus, the loss of mechanical strength is immaterial to their performance for this use case.
Another advantage to the beneficiation of glass fibers for use as an SCM is that the chemical composition of the feedstock need not meet any particular specification, so long calcium, silicon, and aluminum oxides make up the bulk of the glass. The need for a precise fiber chemistry has hampered the use of these fibers in the cement co-processing use case. However, as shown in the study by Glosser et al. [8], a wide range of GF chemistries are acceptable and perform well in cement mixtures as an SCM. Therefore, variations in the proportions of chemical oxides in the GF feedstock will not adversely impact performance. In fact, a variety of compositions may be beneficial, as this would enable GF + OPC mixture designs that could meet a range of desired specifications.
An additional advantage of using the GF from wind turbine GFRP as an SCM is the ability of the trace metals present in the fibers to react with the cement powder to form insoluble hydrates. As shown in the study by Glosser et al. [8], the presence of contaminants – which renders recycled GF unusable for certain applications where impurities are detrimental – not only does not negatively impact its performance as an SCM, but may be an effective method to manage ecotoxicities present in the fibers. Furthermore, the glassy nature of the fibers has been shown to render them more chemically reactive than current SCM feedstocks such as fly ash, which is of benefit for the design of cement mixtures.
Finally, the use of GF from wind turbine blade GFRP as an SCM has the benefit of offsetting CO2 emissions from the cement clinkering process, which is a major environmental issue, as the cement industry accounts for 8% of emissions annually. When put in this context, the inefficiencies and energy expense inherent in current GF separations processes are partially offset.
This book chapter provides a review of the sources, processing, structure and chemistry of glass fiber materials used in wind turbines, its end-of-life options through both repurposing of whole glass fiber reinforced polymer parts and separations and recycling of glass fibers, and recapitulates the results of a study evaluating a novel use of the fibers as a supplementary cementitious material. The current and future benefits and limitations of such revalorization is presented within a critical structure that examines the scientific and engineering processes in Ref. to probable environmental impacts as well as economic constraints.
The physical structure of GFRP is examined in this chapter through both scanning electron microscopy as well as a review of literature. It is shown that GFRP composites are comprised of fibrous filaments made up of glass fibers, which are integrated into a thermoset polymeric resin. The fibers are derived from geologic sources that have been melted and processed, and are primarily comprised of silicon, calcium, and aluminum oxides.
This chapter has shown that GFRP blade waste from wind turbines presents a complex waste management challenge. This is due to in part to the large size of turbine blades, as well as the substantial amount of waste materials that will be entering the stream in coming decades as more wind turbines are retired. The thermoset nature of the GFRP, as well as the economic and energetic expenses associated with separating and refining the GF from the matrix also complicate its end-of-life uses.
A review of scientific literature described in this chapter demonstrates that at present, repurposing of wind turbine blades not only has the lowest environmental impact in terms of a waste management hierarchy, but also is the most common present solution in large part due to the low cost and substantial current commercial operations. Recycling of wind turbine blade GFRP is less common. Currently, the major limitations involved with separating and recycling GF components are low yield of glass fibers; high energetic expense of the processes; and the presence of impurities in the resulting products. However, the energetic and economic costs associated with recycling technologies are expected to be reduced in coming decades, as separations processes become more efficient and economical with scale.
The use of recycled GF from wind turbine blades is evaluated in the context of the cement industry, with a specific focus on its use as a supplementary cementitious material. Compositionally, the chemical constituents of glass fibers are shown to be nearly identical to that of cement clinker, and therefore, show promise for its use in cement mixtures. A thermodynamic study which models the reaction products of simulated glass fiber and cement mixtures is recapitulated and critically evaluated. Study results suggest that the chemistry and reactivity of glass fibers may make them suitable for this use, and that they are expected to react with cement clinker to produce pore solution compositions and solid reaction products that are beneficial for cement strength and durability. The major benefits of this use include the high chemical reactivities of the fibers; the reaction of impurities in the recycled fibers with cement constituents to immobilize contaminants; and the range of acceptable GF feedstock compositions that can form favorable cement reaction products. However, it is shown that the current cost and lack of commercial operations for separating GF from resin makes such revalorization uneconomical at present. In coming decades however, the increase in wind turbine waste may render this option both more practical as well as more economical. Furthermore, for a large and complex material like wind turbines, recycling may become the most attractive option, as repurposing the parts will become impractical given the large volume of waste expected in coming decades.
The authors wish to acknowledge Prannoy Suraneni, Eli Santkyul, and Eric Fagan who co-authored the original study reported in this chapter. We also wish to thank Dr. Michael Kraft of WWU for his expertise with the scanning electron microscope and Dr. Mark Peyron for the GFRP sample.
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Written By
Deborah Glosser, Layla Russell and Paul Striby
Submitted: 24 October 2022Reviewed: 02 November 2022Published: 21 November 2022
Open access peer-reviewed chapter
