A strategic approach to wind turbine blade recycling: Using life cycle assessment to enable data driven decision making

The persistent lack of circular, scalable, and low-impact solutions for decommissioned wind blades remains a challenge for the wind energy sector. With a saturation of proposed technologies, the sector needs to focus efforts on pulling through technologies both for the immediate and longer-term wind blade waste streams. In this work, models for high-profile EoL scenarios have been developed and, using lifecycle assessment, applied to representative onshore and offshore wind blade designs. In their current iterations, not all composite recycling approaches are environmentally beneficial when considering global warming potential (GWP). This is the case particularly for blades consisting of only glass fibre reinforcement, where only cement kiln and mechanical recycling scenarios resulted in GWP lower than landfilling. The introduction of carbon fibre reinforcement leads to reductions in GWP for almost all end-of-life scenarios, the most significant of which coming in the chemical and thermal scenarios. This indicates that to minimise carbon footprint a combination of approaches is required. These findings have been coupled with material circularity indicators and show that, while recycling can be beneficial, it is only one piece of the puzzle. Methods for integrating restorative materials into wind turbine blade designs, as well as finding circular solutions to reduce waste associated with current blade manufacturing practices, are also critical areas of research needed to mitigate the impact of future blade production.


Introduction
Most industrialised countries are relying heavily on the expansion of renewable energy generation to meet net zero targets, with wind being at the forefront of the global green energy transition.The amount of electricity generated by wind increased by almost 273 TWh in 2021, which represents the largest growth of all power generation technologies [1].While wind energy generation can provide electricity with just 1-3% of the Greenhouse Gas (GHG) emissions produced by traditional fossil fuels [2], there remains room for improvement within the industry to help accelerate toward global net zero targets.Although more than 80% of materials used in current wind powerplants are recyclable [3], the persistent lack of circular, scalable, and low-impact solutions for decommissioned wind turbine blades (WTB) remains a challenge for the wind energy sector.Several studies have demonstrated the potential environmental benefits that could be realised by WTB recycling [4][5] [6].These findings point to the development of low impact recycling solutions as a key area of strategic development across the WTB lifecycle with the potential to yield a reduction in the impact of wind energy generation.A strategic and data-driven approach to blade recycling should be followed to ensure a minimal environmental impact and a commercially viable proposition.The SusWIND programme has developed a life cycle impact approach that incorporates Life Cycle Assessment (LCA) and Material Circularity Indicators (MCI) to give a comprehensive picture of the environmental impact of WTBs from cradle-to-grave.Proprietary models have been developed to assess the environmental impact across a range of WTB end-of-life (EoL) scenarios, which cover all the predominant thermal, chemical, mechanical technologies, in addition to cement kiln co-processing, landfill, and incineration.LCA and MCI analysis has been applied to 1) a 5 MW glass fibre reinforced reference blade produced by the National Renewable Energy Laboratories (NREL) 5 MW [7] and, 2) a 15 MW glass/carbon fibre hybrid reference blade produced by the International Energy Agency (IEA) [8].This has enabled matching of present and future blade waste streams material compositions with preferred waste treatment options for lowest impact across the WTB lifecycle.

Reference blades
Two different WTB designs were analysed in this study (Appendix A gives the bill of materials of the two WTB designs): 1) "GF WTB": A WTB design reinforced with glass fibre (GF) (i.e.no carbon fibre (CF) reinforcement commonly seen in other WTB models).This design is representative (in terms of material composition) of legacy onshore WTBs comprising the majority of the first wave of WTBs to reach end-of-life (EoL) in the coming decade.The design for the "GF WTB" used in this work is based on the 5 MW reference blade reported by NREL in [7].
2) "GF/CF hybrid WTB": A WTB design with GF reinforcement in the shells and shear webs and CF reinforcement in the spar caps.This design is representative of future onshore/offshore EoL WTB waste streams following the widespread use of CF in spar cap regions.The design for the GF/CF hybrid WTB used in this work was developed from the IEA 15 MW reference wind turbine [8] developed as part of the IEA Wind Task 37 by NREL and Technical University of Denmark (DTU).Due to buckling and aeroelastic stability issues of this specific blade design (identified in a thorough analysis performed in [9]), a new round of calculations was conducted, and an optimised blade design was achieved satisfying all the problematic constraints.The design calculations as well as main characteristics of this blade can be found in [9].

Scenarios
Table 1 gives a summary of the WTB and EoL scenario combinations analysed.Appendix B gives a description of the EoL scenarios and assumptions made in generating the Lifecycle Inventory (LCI ) data.
Figure 1 presents a schematic showing an overview of process and material flows across the various EoL scenarios investigated.For disposal routes (landfill, incineration/Energy from Waste (EfW)), cement kiln, mechanical recycling, thermal recycling (fluidised bed, pyrolysis), and chemical (solvolysis) recycling it is assumed that all WTB substructures (shells, shear webs, spar caps) are processed using the respective EoL treatments.For recycling routes Cement kiln+ and Mechanical+ (shown in Figure 1), it is assumed that the spar cap is isolated from the rest of the blade structure during initial sectioning activities.The spar cap is then recycled using pyrolysis, and all other regions (shear webs, shells -which make up approximately 75% of the WTB mass) are then recycled via cement kiln co-processing or mechanical recycling.Cement kiln+ and Mechanical+ are investigated as they provide an opportunity to extract the valuable CF located in the spar cap, while diverting the other GF rich substructures to (potentially) lower impact recycling routes.Since the GF WTB scenario contains no CF, the Cement kiln+ and Mechanical+ EoL treatment scenarios have not been applied to that blade.Following discussion with stakeholders in the cement production industry, it is believed that CF cannot be processed in the cement kiln.As such, the GF/CF hybrid WTB scenario cannot be processed via cement kiln without prior spar cap removal (given that CF is only located in the spar caps for this design).

Lifecycle assessment
The LCA has been conducted with reference to the methods provided in ISO 14040 related to the preparation of LCA studies.The aim of the LCA was to evaluate the Global Warming Potential (GWP) of various EoL routes that have been proposed for WTBs both with and without CF reinforcement.
The LCA only considers the processes involved in the EoL treatment of the WTB (i.e., secondary material production in the case of recycling routes) and does not consider any processes associated with upstream phases of the WTB lifecycle (such as manufacture, installation, use, decommissioning).Processes considered at EoL are: 1) downsizing the blade using industrial shredding equipment; 2) transporting shredded waste to a waste management facility; 3) disposal / recycling of blade waste using the following processes: landfill, incineration (with energy recovery), cement kiln co-processing, mechanical recycling, thermal recycling (fluidised bed and pyrolysis), and chemical (solvolysis) recycling; 4) transporting recycling waste to a waste management facility; 5) disposing recycling waste.The impact of WTB cutting / sectioning processes performed prior to EoL treatment (including activities to isolate the spar cap in cement kiln+ and mechanical+ scenarios) is expected to be minimal and has not been considered within the LCA.
An avoided burden approach has been used for secondary material production (e.g., recycled GF and CF products from blade recycling), observing the following: 1) the burden for virgin materials used is allocated to the system, 2) the burden of secondary material production is included within the system boundary and is allocated to the system, 3) materials recycled at EoL offset the demand for a quantity of virgin counterpart materials, 4) knockdown in offset rates are included based on loss in performance and the impact this will have on products utilising recycled materials, 5) all blade waste is either recycled or disposed in the system boundary and an avoided burden credit is given to secondary materials produced.
The Lifecycle Inventory (LCI) secondary datasets used in this report have been sourced from the following databases: GaBi Professional Database 2022, Extension database XXII: Carbon Composites 2022, Extension database VII: Plastics 2022.No LCI primary data has been collected or measured.The materials and energy flows needed to manufacture the representative blade have been estimated/calculated based on data and assumptions reported in literature (see Appendix B).

Material circularity indicators
MCIs were calculated following the methodology outlined in [10].The "Product-level Methodology" was used as this was deemed the most suitable approach to assessing material circularity on a single WTB level.In this case, the MCI measures the extent to which linear flow has been minimised and restorative flow maximised for the WTB's materials, and how long and intensively it is used compared to a similar industry-average product.The MCI is essentially constructed from a combination of three product characteristics: 1) the mass of virgin raw material used in manufacture, 2) the mass of unrecoverable waste that is attributed to the product, and 3) a utility factor that accounts for the length and intensity of the product's use.The methodology used to calculate the MCI is detailed in [10] and will therefore not be reproduced in this report.A variation on the basic method was used to account for production losses and, when required, additional material inputs to the recycling process (e.g., solvents / chemical inputs in chemical-based recycling processes).The Multiple Production Steps approach was also used to account for the different material type inputs used in the blade manufacture [10].As is common practice within the wind industry, it is assumed that all raw materials input during the WTB production come from virgin sources.In this work it is assumed the blade life-expectancy and use intensity (e.g., annual energy production) is constant across WTB and EoL scenarios analysed for a given blade type.The utility factor was therefore the same across all scenarios and did not influence the MCI calculation.The MCI analysis gives a value between 0 and 1, where higher values indicate higher circularity.A fully circular system has an MCI of 1, which occurs when 1) all feedstock materials are from fully restorative sources, and 2) all waste / output materials are fully restored (e.g., reused, recycled).

Lifecycle assessment
Figure 2 gives the GWP results from the LCA for each of the WTB and EoL scenario combinations described in Table 1.The various sources of GWP have been categorised in Figure 2 to highlight dominant contributors for each treatment option and to give insight into areas of development that should be pursued to reduce the impact of the processes.A positive GWP indicates a contribution to GWP associated with GHG emissions (either directly or indirectly), whereas a negative GWP indicates a reduction in GWP (through the avoided burden of virgin material or energy production) enabled by the EoL treatment.The "Total recycling GWP" in Figure 2 is the sum of all GWP sources for a given EoL treatment, with a negative total recycling GWP indicating an overall reduction in GWP enabled by the treatment option.While an EoL treatment my increase circularity and reduce landfill burden, it is important to quantify the GHG emissions entailed by recycling processes and assess their contribution to Net Zero targets.Ultimately, optimal EoL solutions for WTB should both increase circularity and result in an overall reduction in GWP.For the GF WTB scenario (Figure 2a), it was found that cement kiln and mechanical recycling are the only EoL treatments able to provide an overall negative GWP.While cement kiln co-processing does result in direct emissions through combustion of the polymer fractions in the WTB, it is assumed that the WTB offsets energy input from petroleum coke which (relative to calorific value) generates more GHG emissions that WTB combustion.Development of green cement is critical for decarbonisation of the construction sector, and WTB scrap as an alternative lower carbon fuel source could play a role.In kilns where alternative fuels, such as solid recovered fuels, are already being used, it is yet to be established if substitution with WTB scrap will be an environmentally preferable alternative.To date, the LCA literature in this area has assumed only fossil fuel replacement (either coal or petroleum coke).It is therefore critical that future studies assess the impact when substituting alternative fuels in cement kilns, which may burn cleaner than fossil fuel counterparts and/or contain higher levels of biogenic carbon, the combustion of which is considered net zero.
Figure 2a shows that the secondary material produced during mechanical, fluidised bed, pyrolysis, and chemical recycling has low opportunity to avoid virgin material burden (as shown by "Recycling Material Output" in Figure 2a).This is for three key reasons, 1) GFs suffer significant mechanical performance loss during recycling which reduces the replacement rate of virgin GF counterparts, and 2) the GWP associated with virgin GF is already low (compared to other blade materials such as resins and CF) meaning that there is a limited amount of GHG emissions that can be displaced by replacing virgin with recycled counterparts, 3) GF is the only secondary material able to offset virgin material production.These factors mean that, for the total recycling GWP to be negative for a given EoL treatment, the impact associated with the recycling processes themselves must also be low to not supersede the limited avoided burden attributed through secondary GF production.
Figure 2a also shows that mechanical recycling is the only recycling technology which can produce secondary GF material and maintain a negative total GWP.This is primarily due to the low energy demand of the process in addition to the lack of direct GHG emissions from the process (which is found in thermal recycling technologies, as well as when incinerating organic residues from chemical recycling).A major barrier to the reuse of mechanical recyclate however is that, while fibre rich fractions can be extracted, contaminant persists on the fibre surface which may, 1) alter/limit the adhesion with polymers in secondary composite components, and 2) limit the number of technologies available to manufacturers due to material compatibility with existing processes / practices.It was demonstrated in [11] that the fibrous fractions can replace virgin GF in bulk moulding compound (BMC) production without sacrificing resulting composite strength.It is therefore assumed that the recovered fibrous fractions can offset the production of virgin GF, which would otherwise be used in the production of BMC products.
Figure 2b shows the LCA results for the GF/CF hybrid WTB.Recycling CF materials provides a much greater opportunity to offset GHG emissions by replacing high impact virgin CF production.This means that for all recycling scenarios in Figure 2b, the GF/CF hybrid WTB yields a negative total GWP and are therefore preferable to disposal routes (landfill and incinerations) when aiming to minimise GHG emissions.While mechanical recycling is found to be the lowest impact option for the GF WTB, it is the highest impact recycling scenario investigated for the GF/Cf hybrid WTB.This is because the CF is not being recycled into a format which can compete with virgin CF.It is assumed that fibrous mechanical recyclate from CF composite regions can only replace virgin GF, therefore the avoided burden associated with the process is low, as in the case for the GF WTB. Figure 2b shows that Cement kiln+ and Mechanical+ EoL scenarios provide the lowest GWP for the GF/CF hybrid WTB.This is because the WTB sub-structure material compositions are matched with preferred waste treatment options for lowest overall impact.This approach ensures that the additional GHG emissions associated with thermal or chemical processes (that is required to extract the CFs) are only applied to the CF rich spar caps, while deploying lower impact strategies to the shells and shear webs where there is less opportunity to reclaim valuable, high impact secondary materials.
It should be noted that the results presented in this LCA are a snapshot in time, the outcomes of which will likely change as developments to recycling technologies are realised.In the current iteration, pyrolysis suffers from having both high energy demand and direct GHG emissions from combustion of the polymer pyrolysis products.Future state of pyrolysis technology may enable 1) self-sustained processing through re-circulation and combustion of pyrolysis gases to power the system, and 2) condensation, collection and reprocessing of pyrolysis oils/waxes to produce feedstocks for the chemical industry.These solutions have the potential to reduce both the energy consumption and direct emissions, but also provide additional products from pyrolysis with the opportunity to offset virgin petrochemicals (or similar products).While this work has considered thermo-oxidative recycling using the fluidised bed, these systems can also operate under pyrolytic conditions and therefore has the same potential to reduce GHG emissions as the belt driven pyrolysis process modelled in this LCA.Future state of solvolysis IOP Publishing doi:10.1088/1757-899X/1293/1/0120077 technology may enable 1) a reduction in energy demand through catalysed polymer decomposition, and 2) the reprocessing of recovered organic residues to produce feedstocks for the chemical industry.

Material circularity indicators
Figure 3 presents a comparison of the WTB MCIs and total recycling GWP (as given in Figure 2) for the various EoL scenarios.The MCI analysis gives a value between 0 and 1, where higher values indicate higher circularity, and a fully circular system has an MCI of 1. EoL solutions existing within the upper left quadrant of the graphs in Figure 3 should be prioritised because they have lower GWP and higher circularity (measured as MCIs) and can better accelerate towards both Net Zero Emissions and Zero Waste to landfill targets.As would be expected, disposal routes (landfill and incineration) have the lowest MCI and are clearly the least favourable EoL strategy for GF/CF hybrid WTBs.

Figure 3 MCI and recycling GWP for the various EoL scenarios, a) "GF WTB", b) "GF/CF hybrid WTB"
Figure 3 shows that mechanical recycling has the highest MCI and lowest GWP for the GF WTB and is therefore identified as the priority EoL strategy to pursue for the first wave of EoL WTBs expected across Europe in the coming decade.Critical in realising economic viability will be the value of mechanical recycled granulates and the products there of.Developing use cases for the materials and strengthening routes to market is needed to ensure that fibrous recyclate can (in some applications) replace virgin GF, as is assumed in this LCA.On the balance of data presented in Figure 3, Mechanical+ is identified as the priority EoL strategy for the GF/CF hybrid WTB scenario.Mechanical+ has the lowest GWP of the EoL scenarios as described above and is approaching the highest MCI (with mechanical recycling alone having a marginally higher MCI).
In all scenarios in Figure 3, the determined MCI values are significantly lower than the theoretical maximum MCI of 1.This is because the MCI methodology also considers the sources of materials entering the product system during manufacture and use phases.As is common practice within the wind industry, it is assumed that all raw materials input during the WTB production come from virgin sources.These are by nature non-restorative and decreases the overall MCI.To enable a circular economy for WTBs, it is therefore important to extend research beyond recycling and focus on methods for integrating restorative material flows (e.g., bio-derived, reused, recycled materials) into blade designs and production.An example that is already done commercially by some turbine OEMs is the use of recycled PET core materials to replace traditional virgin PET material.Additionally, industry standard practices during WTB production result in significant levels of manufacturing waste which is currently not recycled.Identifying circular solutions to these materials will also be critical in transitioning to a circular economy for WTBs.
Recovery of usable matrix polymers from analogous composite structures has been demonstrated using chemical based recycling process for alternative "recyclable epoxy" systems (e.g., Recyclamine ) and standard epoxy systems using targeted catalsyed decomposition [13].The former requires the use of specific epoxy hardener systems during WTB production and there is currently a lack of published data to conduct independent LCA for the latter, therefore these recycling solutions have not been considered in this analysis.Regardless, both these approaches have the potential to increase WTB circularity by enabling the reuse of the polymer matrix which constitutes 21-23 %wt. of the WTB's bill of materials.

Conclusions
This investigation has quantified the GWP of various EoL treatment options for WTB designs with and without CF reinforcement.To compliment the LCA, the circularity across the WTB lifecycle has also been characterised using the MCI methodology for each of the EoL scenarios.Based on the current state of technologies, and the best available data the following conclusion can been made for GF WTBs: • It was found that cement kiln and mechanical recycling are the only EoL treatments able to provide an overall negative GWP.To date, LCAs investigating cement kiln coprocessing of WTBs have assumed fossil fuel energy replacement.With the growing use of alternative fuels in kilns, it is critical that future studies also assess the impact when substituting alternative fuels which may burn cleaner than fossil fuel counterparts.• Mechanical recycling provides the highest MCI and is the only recycling technology which can produce secondary GF material and maintain a negative GWP. Developing use cases for the materials and strengthening routes to market is needed to ensure that fibrous recyclate can indeed replace virgin GF as is assumed in this analysis.• Thermal and chemical recycling routes produce a positive GWP and in their current state of development are not priority routes for WTB recycling.Methods should be pursued which can reduce the energy consumption and direct emissions associated with these processes, in addition to reclaiming resin fractions capable of offsetting feedstock chemicals.As and when these developments are realised, LCA should be used to re-evaluate these technologies and alter recommendations were appropriate.
The following conclusions can be made for GF/CF hybrid WTBs: • Recycling CF materials provides a much greater opportunity to offset GHG emissions by replacing high impact virgin CF production therefore all recycling scenarios for GF/CF hybrid WTBs yield a negative GWP and are preferable to disposal routes.• Cement kiln+ and Mechanical+ EoL scenarios provide the lowest GWP, because the WTB substructure material compositions are matched with preferred waste treatment options for lowest overall impact.This approach ensures that the additional GHG emissions associated with thermal or chemical processes are only applied to the CF rich spar caps, while deploying lower impact strategies to regions where there is less opportunity to reclaim high impact secondary materials.• Mechanical+ was identified as the priority EoL strategy for the GF/CF hybrid WTB scenario when considering GWP and MCI environmental metrics.It is therefore critical that method(s) for spar cap extraction are developed in advance of future WTB waste streams where CF in these regions will be ubiquitous (e.g. in the case of offshore WTBs).
Enabling a circular economy for WTB means extending research beyond recycling alone.Methods for integrating restorative materials into WTB designs, as well as finding circular solutions to reduce waste associated with current WTB manufacturing practices, are also critical areas of research needed to mitigate the impact of future WTB production.
classifier.The four distinct fractions differ in composition and size, with the process producing two fibre rich fractions, a "coarse" resin rich fraction and a final "powder" fraction.Energy demand for both downsizing and classification stages required for mechanical recycling were modelled.Downsizing energy using a granulator was calculated using the method outlined in [16].All energy for mechanical recycling is assumed to be electrical and sourced from UK mains supply.It is assumed that the recovered fibre rich fractions can offset the production of virgin GF at a rate proportional to retained strength of GF in the recyclate.It is proposed that the powder fraction may be used as filler and offset the production of calcium carbonate filler material.The coarse, resin rich fraction, is assumed to be a waste product in the study and is assumed to be landfilled.

Fluidised bed recycling
Thermal recycling composite materials in a fluidised bed allows for oxidative decomposition of the polymer matrix, liberating the clean fibre (GF or CF) fractions which are transported by the gas stream out of the reactor and collected.The combustion gases are fully oxidised in an oxidising chamber to remove volatiles then passed through heat exchanger to recover heat to be fed back into the process.Heat is supplied into the system through oxidation of combustibles (e.g.polymers) present in the WTB waste stream, as well as through natural gas oxidation in the oxidiser.The fluidised bed requires fibre length <10 mm to operate efficiently; therefore, pre-treatment of waste WTB involves a secondary downsizing step in a granulator.The energy input requirements have been sourced from literature LCA [17].Fibre recovery rate is assumed to be 95% [17], with "waste fibres" assumed to be landfilled.It is assumed that all polymer materials in the WTB are fully oxidised during the fluidised bed process.It is assumed that recycled GF and CF can offset the production of virgin counterparts.Offset knockdown factors (proportional to retained strength of GF/CF in the recyclate) have been applied which dictates the replacement rate of virgin fibre counterparts.

Pyrolysis recycling
Thermal recycling of WTB waste using pyrolysis involves decomposition of polymers without (or in low) oxygen at elevated temperatures [18].This is often a multistage process, requiring different temperatures and/or atmospheric conditions to remove thermally stable char residues in order to reclaim contaminant free fibres.The polymer decomposition is endothermic and requires significant energy input.The sited energy input for pyrolysis recycling of composite materials varies throughout the literature.Energyrelated inventory data obtained from a commercial operation from ELG Carbon Fibre (reproduced [18]) was used in this study.It is assumed that all products from the polymer pyrolysis are eventually fully oxidised prior to releasing to the environment.Fibre recovery rate is assumed to be 95%, with "waste fibres" assumed to be landfilled.It is assumed that recycled GF and CF can offset the production of virgin counterparts.Offset knockdown factors (proportional to retained strength of GF/CF in the recyclate) have been applied which dictates the replacement rate of virgin fibre counterparts.

Chemical (solvolysis) recycling
Chemical recycling of WTB waste using solvolysis involves breaking down the polymeric fraction by utilising a solvent, often using elevated temperature and/or pressure.An acetone-based process has been used to model chemical recycling of WTB waste and gather the associated LCI data needed.Data has been extracted from a published LCA on this process using primary data of a lab scale reactor [19].It is assumed that all polymeric material fractions in the WTB can be broken down during solvolysis, which result in an "organic residue" fraction.It is assumed the organic residue is sent to energy from waste for disposal.Fibre recovery rate is assumed to be 95%, with "waste fibres" assumed to be landfilled.It is assumed that recycled GF and CF can offset the production of virgin counterparts.Offset knockdown factors (proportional to retained strength of GF/CF in the recyclate) have been applied which dictates the replacement rate of virgin fibre counterparts.

Table 1
Summary of WTB and EoL scenario combinations analysed, ✓ -scenario was analysed, X -scenario not analysed