Review existing strategies to improve circularity, sustainability and resilience of wind turbine blades – A comparison of research and industrial initiatives in Europe

To become the first climate-neutral continent by 2050, the EU is committing to ambitious targets such as reducing greenhouse gas emissions by at least 55% by 2030 (compared to 1990) and increasing the share of renewable energies in the EU energy mix to 45% by 2030. To meet these targets, the wind industry has to grow significantly while ensuring enough resources across the supply chains to scale the wind energy market and by striving to reduce its already shallow environmental impact. As material extraction and the production of wind turbines cause the most emissions, costs and risks on the scarcity of resources, the implementation of a circular economy could support the wind industry in meeting the EU target and improving its sustainability and resilience. However, the circular economy is a complex concept, and it can be challenging to translate it into precise action points, objectives and measurements. To clarify how a circular economy can support the wind industry, this paper takes the example of wind turbine blades, and it establishes a structured overview of research results and industrial initiatives aiming at implementing circularity for improving sustainability and resilience. The overview is used to investigate if objectives and clear actions are stated and how those differ between research and industry. By identifying gaps, future research and industry initiatives can be directed towards closing the bigger picture. The results show that many initiatives are ongoing, but only some circular strategies are comprehensively investigated, and clear objectives and measurements often remain to be included. The industry and research progressed the furthest on recycling. Future research and industry activities should further follow the path of closing the loop but need to also concentrate on reducing material use, extending the lifetime of blades and enabling a second lifecycle of blades.


Introduction
The wind industry is crucial for achieving climate neutrality globally [1].As such, an immense expansion of wind capacity and, with this, an increase of resource consumption (e.g.material, human, capital) across the wind supply chain is required [3,2].Simultaneously, the industry has to strive to reduce its environmental impact further, even though being already one of the energy sources with the lowest greenhouse gas emissions today [4].A circular economy could positively influence the sustainability and resilience of the industry's supply chains by decoupling from virgin material consumption [5][6][7].It should be noted that several definitions of a circular economy exist that differ in their scope and objectives [8].Within this paper, a circular economy is defined as a system-wide concept

Research methodology
The scope of this research is the installed onshore and offshore wind turbines in Europe.The research will focus on wind turbine blades to enable precise identification of objectives and actions for implementing a circular economy.Concentrating on a key component reduces the complexity, overcoming an often-discussed issue that a circular economy is too complex and concrete actions are difficult to grasp [12].Wind turbine blades were chosen as the object of research as they are of interest from a circular economy perspective: Blades are manufactured using the scarce material balsa wood [13] and emissions can be reduced by limiting the extraction of virgin materials [14].Moreover, suitable recycling processes [15] and further strategies to narrow, slow and close the resource flow still need to be developed or to become more mature.
The research methodology, shown in figure 1, aims to identify gaps within and across research and industry initiatives for improving the circularity, sustainability and resilience of wind turbine blades.It focuses on identifying and analysing objectives and actions for a circular economy.The first part of the research methodology consists of the data collection on wind energy research and industry initiatives on objectives and actions for a circular economy (Section 2.1).The second part is the data analysis of the identified data (Section 2.2).

Data collection
Data related to objectives and actions for a circular economy and circular wind strategies are collected in industry and research.Two themes are prioritized, circular economy strategies dedicated to a 2nd lifecycle of wind turbines and circular economy strategies dedicated to recycling.Strategies related to a 2 nd lifecycle have received little attention so far, but the need for further investigations is mentioned by literature review papers [15][16][17].As recycling gained attention by research and industry, it is interesting to investigate the link between industry and academia.For industry, the focus of investigation is on the communicated intentions and targets towards a circular economy and for academia, scientific publications and research results are reviewed.
To investigate the industry initiatives, suppliers for blade manufacturers, blade manufacturers and operators are considered as they represent the main actors in the wind turbine blade value chain.The suppliers Owens Corning, 3B Fibreglass, and Olin are selected.In addition, the manufacturers Vestas, Siemens Gamesa Renewable Energy (SGRE) and LM Wind Power and the operators Ørsted, RWE and Vattenfall are chosen for further investigation as those companies have major market shares in the European market [19,18].Data collected are publicly available information from the companies' websites, sustainability reports, circular economy reports or similar.The data were gathered during 23.02.2023-23.05.2023.Using secondary data collection instead of primary data collection (e.g. through workshops or interviews) is preferable at this stage as the officially communicated strategy with objectives and actions can be identified.
For all nine companies selected, the latest sustainability report was downloaded from the companies' website.When available, additional objectives and actions were collected from the websites and circularity roadmaps of the companies.The yearly sustainability reports are non-technical reports, they present the companies' strategies, objectives and achievements related to sustainable development goals, including circular economy objectives.These reports are intended for communication purposes and the title of the report is generally customized with a statement promoting the vision of the company, such as "Leading the energy transition" [20].There are no standard table of content for such report and each report is structured differently.Data from the reports are collected when circular economy or one of the circular wind strategies according to Velenturf [21] are mentioned.To make the collection of data on objectives as systematic as possible, each objective was identified as quantitative or qualitative one.An example of a quantitative objective is "We aim to send no wind turbine blades to landfill (committed since 2021)" [22].An example of a qualitative objective is "we aim to limit to the minimum the use of balsa wood and have a substantial volume of the PET from a recycled source" [23].While the quantitative objective mentions a quantity and a timeline and may act as a target, the qualitative objective expresses an intention.In additions, the actions planned to support the objective were also collected.An example of an action is "to clarify contracts on responsibility of blade waste and training the companies site personnel and contractors on how to comply with our landfill ban" [22].

Data analysis
The second part of the research methodology is the data analysis of the identified data, i.e identified scientific articles and companies' sustainability reports.In section 3.1, the content of the data is analysed and visualized on two different perspective, which are the circular wind strategies as defined by Velenturf [21] and the level of investigation according to Kramer and Schmidt [26].
Velenturf [21] formulates 18 circular wind strategies as a result of reviewing literature, workshops and including feedback from different stakeholders of the offshore wind industry.The strategies are assigned to either narrowing, slowing, closing resource flows or the integration into the environment.Narrowing resource flows includes the strategies design for circularity, dematerialise, prevent waste, and modularisation.Maintain & repair, reuse & repurpose, refurbish & remanufacture, disassemble, and lifetime extension is allocated to slowing resource flows.Repowering is a strategy that belongs to slowing and closing of resource flows.The closing of resource flows is represented by the strategies recertify, decommission, recycle.Re-mine and recover energy are at the interface of closing and integrating.Finally, the strategy restore site and landfill are considered to be strategies belonging to the integration into the environment.Her framework aims for a comprehensive overview of the process steps along the lifetime.Some strategies are equal to the circular economy strategies used in research on conceptualizing a circular economy [6,27].For example, repair, reuse, repurpose, refurbish, remanufacturing, recycle and recovery are common stated circular economy strategies, also known as R-principles [27].Design for circularity is highlighted as the overall aim to design out waste and pollution by enabling the R-principles across different levels of the system [6].Next to those strategies, Velenturf states strategies that could be more seen as facilitating strategies.For example, modularisation is seen as a strategy to ease assembly, disassembly and transport and thus could promote the establishment of the R-principles that slow and close the resource flows [28].Definitions of each strategy are given in [21] and by the stated references.Strategies that are predominately used in this paper are introduced in the following [21]:  Design for circularity: Considers the entire lifecycle and foresees more radical, not just incremental, changes in design to enable the comprehensive range of circular strategies, e.g.design for repair, design for reuse, design for upgradeability or design for recycling. Lifetime extension: Keep the wind turbine "in use beyond designed service life" at the site. Reuse & repurpose: Reuse is the structural reuse of components, e.g.blades, for the same function and repurposing is a structural reuse with a different function. Refurbish & remanufacture: Refurbishment is the replacement or repairment of some components that leads to an overall "upgrade" of the multi-component product.
Remanufacturing aims at bringing the product's function at least to the originally manufactured quality by following a fully documented industrial process. Recycle: Recycling considers the material level and aims at "turning waste into a new substance or product" [29].Recycled materials are not directly sourced from the natural environment as it is the case for primary resources, but instead from waste.The levels of investigation according to Kramer and Schmidt [26] differ between organisational, product and processes level.The organisational level reflects tasks related to organisational management, information systems, and (digital) technologies.For instance, stakeholder involvement, choice of technologies and data management or choice of a business model.The product level, in the case of this paper, considers the design of the blade structure and its materials.The processes level includes actions related to the supply chain processes.Hence, tasks on the design, planning, managing, and execution of material, information, and financial flows within a supply chain [30].This includes processes on plan, source, make, deliver, use, return, recover, and enable as outlined by Vegter et al. [31].In this paper, processes related to blades are considered.For instance, the sourcing of materials, manufacturing, delivering, installation, decommissioning, refurbishing, repurposing, recycling and reverse logistics of blades.
For clustering the collected data from industry and academia the following procedure was taken: For industry, the authors of this paper have analysed the data according to the covered circular wind strategies and level of investigation independently and discussed when findings deviated.For research, the circular wind strategies used in the title, abstract or key words formed the basis.For the literature on a 2 nd lifecycle, the authors have checked the papers according to the above set definitions of the circular wind strategies.For classifying the identified papers according to the covered level(s) of investigation, the authors followed the above-mentioned definitions.The collected data from industry and academia can cover multiple strategies and levels of investigation.
For the research output, to visualize the results, a graphical overview is obtained by counting and categorizing all research papers identified.This way, the most researched or most targeted circular economy strategies can be easily seen.The development over time is also investigated.To evaluate evolving trends, an overview of the number of research papers per year of publication considering all circular economy strategies and the level of investigation is prepared.Furthermore, a more detailed analysis of the addressed objectives and actions for a 2 nd lifecycle of blades (Section 3.2), and recycling of blades (Section 3.3) is provided.Hence, the actions in industry that are linked to objectives can be compared to the existing research.Finally, a broader discussion takes place in Section 3.4.

Circular wind strategies
The analysis of the nine companies selected is visualized in figure 2. The figure displays the circular economy strategies and indicates the number of companies contributing to them based on the content of their respective public available information, in particular from the latest sustainability report and website.The content, objectives and actions used for figure 2 is gathered and presented in a table in Appendix 1.Some companies mention a wide scope of a circular economy [32,23], but objectives and actions are predominantly set on recycling.From figure 2, it appears that all companies are mentioning and setting qualitative and/or quantitative objectives on recycling for wind turbine blades.The second most important topic addressed falls in the category of reduced or no landfill.In third and fourth position, two strategies come forward: dematerialise & prevent waste as well as design for circularity.
Analysing the level of investigation, it appears that most of the content belongs to the blade-related processes, material, and organisational level.This seems reasonable as recycling gains most attention and this strategy tackles the material design and the development of new recycling technologies.The blade structure level was not considered as often.Figure 4 shows the results on scientific publications on wind turbine blades using the term of at least one circular wind strategy in their title, abstract or keywords.It shows a focus on the strategies "maintain and repair" (1,483 papers), "lifetime extension" (457 papers) and "decommissioning" (601 papers).Research on maintenance and repair as well as lifetime extension eventually slows down the material use within the 1 st use phase.The 2 nd lifecycle and the strategies reuse, refurbish, remanufacturing, and repurposing have not gained the same attention.Furthermore, the narrowing strategies have only being rarely investigated.For the strategies "repower" and "restore site" it is reasonable that those haven't been researched for blades as those strategies are more likely to be researched from a turbine perspective.Comparing between industry and research indicates that the industry communicates mostly on recycling and research focuses on maintain and repair.However, the 20 research publications that used the term "circular economy" were mostly on recycling.Narrowing and 2 nd lifecycle strategies have not gained a lot attention by industry and academia.The 2 nd lifecycle is closer analysed in the next section (3.2).In addition, as recycling has gained a lot attention by industry and research, a more detailed analysis of the link between research and industry is conducted (section 3.3).Hence, the next sections zoom in at the objectives and actions for enabling a 2 nd lifecycle of blades and recycling of blades' materials.

2 nd lifecycle
After reaching the end of a wind turbine's first lifecycle -hence after the original designed lifetime or lifetime extension -a reuse of the turbine or its components (e.g.blades) should be prioritized according the EU Waste Hierarchy [33].Therefore, enabling a 2 nd (3 rd , and so on) lifecycle of the turbine.If the entire turbine cannot be reused, then some components could be reused at the same site in the course of a repowering project or at a different site.The 2 nd lifecycle comprises of strategies that foresee a reuse with the same function (direct reuse, refurbishment, remanufacture) or a different function (repurpose).
Objectives by the industry collected on the topic of a 2 nd lifecycle of wind turbine blades are presented in table 1.The only quantitative objective formulated among the industries is the one by Vestas.SGRE formulates a qualitative objective on lifetime extension, which is not the same as 2nd lifecycle, however it was still collected as it is part of the higher ranked circular economy strategies.The structural reuse of components could be part of achieving the objective of lifetime extension.However, no information is provided on this.The actions by Vestas to support its objective on reuse of component and a 2nd lifecycle is to create new repair loops for minor components to regionalize their repair and refurbishment infrastructure where possible.Looking at the research publications, in total, 130 papers use key words linked to a 2 nd lifecycle of turbines by applying reuse, refurbish, remanufacturing, and/or repurposing.After analysing the content, only 27 papers were identified that address the 2 nd lifecycle of wind turbine blades.Figure 5 shows the identified papers according to the addressed circular wind strategy and their level of investigation.Most papers are linked by the authors to reuse (20 papers), followed by repurpose (10 papers), and remanufacturing (4 papers).No paper was found on refurbishment.When checking the content of the papers, none of those deal with remanufacturing or refurbishment and 23 papers focus on repurposing.Two papers research the modularisation of blades.Two papers provide waste volume predictions that are aimed for planning recycling capacities.However, this information could also be beneficial for the planning of capacities for a 2 nd lifecycle.When looking at the level of investigations, 5 papers have addressed the organisational, 11 papers the blade structure, 6 papers the material, and 7 publications the blade-related processes.The development over time shows a slight increase of publications since 2018, howsoever, the total number of publications is still very limited.As such no detailed trends in regards to the circular wind strategies or the level of investigation can be derived from the sample.The inconsistent use of the terms reuse, refurbish, remanufacturing, and repurposing shows a lack in common definitions of the used terms.This is underpinned by looking at the example of the term "reuse": Mishnaevsky [16] uses the terms reuse and refurbishment and assigns it as primary recycling to recycling of blades.He foresees reuse for the same use, either through lifetime extension, refurbishment or reusing in different structures (e.g.bus shelters).Woo and Whale [17] see reuse to take place after decommissioning of a turbine and providing a 2 nd lifecycle for the entire turbine or parts.The authors mention remanufacturing processes in relation to recycling and refurbishment in relation to reuse and lifetime extension.Beauson et al. [15] frame reuse as continued use of a blade as blade after decommissioning.Also, Martinez-Marquez et al. [25] mention remanufacturing, refurbishment, and repurposing, but do not distinguish consistently between remanufacturing and refurbishment throughout their conducted review.The term "reuse" is not defined by them.Similar to the industry activities, concrete actions in research are rare.On reusing with the same function (i.e.direct reuse, refurbishment or remanufacturing) only a paper by Kaczmarek et al. [34] present structural modularization for providing the possibility to reuse and recombine modules.Further actions could be derived when looking at repair, as indicated by Mishnaevsky [16].On repurposing, industry hasn't set any quantitative objectives or actions.Owens Corning mention that they aim to "repurpose or recycle the remaining waste, including recycling waste back into our own processes whenever possible" [35].However, as a fibre manufacturer they eventually relate this objective to the fibre and not the repurposing of wind turbine blades.Research has investigated different actions recently.For example, research on repurposing blades as playground, bridge or bus shelter was carried out [36].
It becomes clear, while analysing the collected data, that uncertainties concerning the future development and technical feasibility for a 2 nd lifecycle exist.Mishnaevsky [16] refers to a technical lifetime of blades of up to 50 years.Beauson et al. [15] expect that manufacturers are designing blades for suiting the designed lifetime and as such reducing residual life.Researchers highlight the importance of reuse for achieving a circular economy [15,16].However, for instance, Kwon et al. [36] skip the pathway of reuse as blade entirely by stating safety concerns, without providing explanations.Joustra et al. [37] write that blades of a turbine cannot easily be installed at a different turbine and as such they conclude that material recycling remains the only suitable option.Research by Martinez-Marquez et al. [25] state that structural repair procedures are not well developed, but Mishnaevsky outlines a technical readiness level (TRL) of 9 for repair and reuse.Furthermore, it is noted by Martinez-Marquez et al. that with an increase in size of blades a structural reuse becomes more difficult [25].Ortegon [38] mentions that remanufacturing would be possible.This is supported by Martinez-Marquez et al., who refer to a TRL of 10 for refurbishment/remanufacturing [25].Due to the different use of terms across the studies as well as few provided details, an assessment of the potential for a 2 nd lifecycle is difficult to conduct.
Investigations on the technical feasibility and the historical and expected development of the residual lifetime across turbine and blade types could provide clarity.Also, an assessment of the secondary market and predictions of supply and demand for a 2 nd lifecycle of blades are currently missing in research.The gathered knowledge can be used to assess if more research in this field is required and if so, objectives and actions can be formulated.

Recycling
When a blade cannot be reused anymore, recycling of the materials is considered as the next step of the EU Waste Hierarchy [33].Table 2 shows, the quantitative objectives by the industry collected on the topic of wind turbine blade recycling.The wording of those objectives (e.g."100% recyclable turbine" or "zero waste") as well as the targeted achievement date (e.g.2030 or 2040) differs slightly.Some additional qualitative objectives were mentioned by the remaining companies, these are not listed in table 1, but shown in the Appendix 1.
Figure 7 shows a schematic representation of the lifecycles of a wind turbine.The stakeholders analysed in this paper: the wind turbine blade material supplier, the wind turbine blade manufacturer and the operators are represented in the figure together with the objectives stated in their respective sustainability reports.From this figure, it is clear that all stakeholders involved in the wind turbine's lifetime are committed to recycle wind turbine blades.It seems that the industries from the wind energy sector are joining forces to solve the challenges of recycling wind turbine blades.As mentioned in the introduction, this aspect is key in realizing and implementing circular economy strategies.
The actions from industries are to support research and be part of research projects to develop recycling solutions.An additional action from Ørsted is to clarify contracts on responsibility of blade waste and training Ørsted site personnel and contractors on how to comply with a landfill ban for blades.Results from the identified research are presented in figure 6.In total, 183 papers used keywords related to recycling of composites, balsa wood, resin, fibre and/or PET.Out of those, 112 papers are appropriate for analysing as they address wind turbine blade recycling.The right part of figure 6 outlines that the overall research interest has increased since 2017.A peak is shown in 2022, representing 27.6% of papers from the sample.A reason for this peak could be the announcement of the industry to call for a ban on old turbine blades in landfills that was announced in June 2021 [39].
Looking at the terms used by authors in the title, abstract and keywords, recycling of composites (86 papers) is the most researched topic within recycling of wind turbine blades.For 35 papers, researchers have used the term "recycle resin" and in 33 papers the term "recycle fibre".Research on recycling PET (2 papers) and balsa wood (3 papers) is rarely shown.Detailed reviews on the state of the art of recycling are given for example in [15], [16], [40] and [41].Researchers are investigating the replacement of currently used materials for the manufacturing of new blades.For the fibres and resin of composites, research paths are biocomposites, thermoplastic matrices, and modified epoxies [15].For the core, mostly balsa wood is used.Referring to above, research on alternative materials for the core is rare.Further, the manufacturing, recycling and reuse of materials from existing and new blades is being researched.In particular for materials of existing blades, the development of suitable recycling solutions is required as decommissioning volumes are expected to increase.Recycling methods can be clustered into mechanical (e.g.shredding), thermal (e.g.pyrolysis), or chemical recycling (e.g.solvolysis) [15,16].However, recovering energy of composites to use in the cement kiln production is the only existing solution which runs at an industrial scale and indicates a positive environmental impact over landfill [42].The use of recycled materials from blades in the manufacturing of new blades has not been achieved in scale [15].An interesting comparison can be made on the different recycling solutions needed for existing blade reaching end-of-life today and the ones built with a recyclable resin and that will be recycled in a few decades from now.Recycling processes for existing blades, such as mechanical recycling for the construction of sound insulation panels close the loop as long as the sound insulation are not sent to landfill at the end-of-life.These solutions rely on the proper recycling of the following application made of the materials.Recycling processes for future blades pushes for the separation and the recovery of the individual material constituents with the objective, in some cases, of reusing these materials in the production of new blades.In this case, the loop is closed and a new loop starts.
When distinguishing between the covered level of investigation, it shows that 12 papers address the organisational, 15   A comparison of the industry and research activities is, due to the different nature of data, difficult to conduct.It seems that research is more focusing on material development and the related manufacturing or recycling processes for recycling composites and with this the fibre and resin.In the collected data, companies do not communicate a lot on these engineering activities, but rather on organisational actions.For example, choosing to collaborate in research projects or training personnel.The first mentioned task implies to work with research on the above-mentioned topics.The recycling of the core material (e.g.balsa wood) has not been addressed prominently by research or industry.Eventually the interest will raise as, for example, the Netherlands have proposed the recycling of balsa wood as one criterion within the circular economy-related auction criteria in their upcoming auctions IOP Publishing doi:10.1088/1757-899X/1293/1/01203911 for offshore wind [44,43].PET, a fossil-based material, has also not received a lot attention so far.With the required phase-out of fossil fuels, alternative materials and the reuse of existing PET from different industries could be a field for further research.

Broader discussion
For a broader discussion of the findings, figure 7 summarises some of the key objectives for a circular economy from industry and puts those into context of the different strategies of a circular economy.Qualitative objectives are marked with a yellow box and quantitative ones with a blue box.The figure also sketches how the different lifecycle processes and circular strategies on narrowing, slowing and closing resource flows are interlinked.It should be noted that a circular economy does not foresee a complete closed system, but rather cooperation with other companies within and across industrial sectors [9].Moreover, in practice there is not one ideal circular economy and instead it depends on the local environment, e.g.available reverse supply chain, suitable refurbishment and recycling facilities, country's energy mix or potential synergies with other industries.First of all, waste and pollution are to be designed out of the material extraction and processing, manufacturing, remanufacturing, refurbishment, installation, operation, decommissioning, reusing, repurposing, and recycling processes.
The material extraction and processing of the fibre, resin and core material should be reduced to an absolute minimum and be replaced by recyclable or regenerative materials.Within the production of the materials and the blade itself, manufacturing waste should be eliminated.A blade can be produced either through manufacturing, refurbishment or remanufacturing.When installing a wind turbine, manufactured, refurbished, remanufactured or directly reused blades can be used.The three last mentioned strategies are preferred from a circular economy perspective as they eventually have a lower environmental impact than manufacturing a new blade.However, currently it is not possible to disassembly a blade after a use phase for inspecting and returning it to an as-new condition as it is for example being done for the engines from Rolls-Royce [45].Hence, remanufacturing for blades is, as of today, technically not possible.Furthermore, due to the technical progress and launch of larger and larger turbines and with this, larger blades, a reuse of older and smaller blades might be less attractive.For instance, an onshore wind turbine is designed with an operational lifetime of 20 years according to the IEC standard [46].An aim can be to extend the lifetime, as such operating the turbine at the same site for longer than originally designed.During the operational phase, maintenance and repair work are carried out.After the end of the first lifecycle, the turbine is decommissioned.The decommissioned blades preferable enter, in a cascading order, the direct reuse, refurbishment and remanufacturing loops.If the blades cannot be reused as a blade, they could be repurposed, i.e. reused with a different function.When finally reaching the end of the structure's life, recycling of the material is foreseen.The materials of the blades are aimed to be recycled with the same quality of material properties so they could be reused for the production of blades.The presented objectives, that have their origin from different companies, address a wide range of processes, e.g.reduction of virgin materials, design fully recycle turbines or enable a lifetime extension.However, not all of these objectives have a quantitative nature and the level of ambition differs across the companies.Finding a solution for the recycling of composites is an aligned objective the entire industry has and research is investigating different solutions for the already installed blades and designs for the new to be produced blades.This is one of the few topics addressed in the reports of the companies where a lot of data is available and a general consensus on the target exists.Interesting to note is that objectives are set by companies that lead to benefits for their suppliers and customers.For instance, wind turbine blades manufacturers can develop recyclable blades design and may participate in developing recycling solutions (however they cannot develop the technology).Glass fibre manufacturer may develop the technology to remelt recycled fibres and may also support the development of recycling technology by providing their needs.The establishment of feedback loops on lessons learned, e.g. from the operation, disassemble or recycling processes can in return support the material and blade manufacturer on new designs.For the lifetime extension objectives by the manufacturers, who also offer services along the operational phase, and by operators are set.Most research publications were found on repair and maintenance as well as lifetime extension.A few objectives on further slowing (e.g.establish refurbishment of components) and narrowing (e.g.decrease waste intensity) were identified.The research coverage on those objectives was limited.

Conclusion and outlook
The paper provides a comprehensive review of objectives and actions from industry and research for transforming to a circular economy.The example of wind turbine blades is used.The review shows that different circular wind strategies and levels of a system are addressed by different stakeholders.The findings were put into perspective by sketching out the lifecycle processes in a circular economy for wind turbine blades and outlining the interdependencies between the stakeholders (refer to figure 7).The main results and observations from this review are the following: IOP Publishing doi:10.1088/1757-899X/1293/1/01203913 1) Despite the different nature of the content, we see good agreement between different supply chain industry players and research on recycling.2) Second lifecycle of blades lacks attention from research and suffers from lack of definition and characterization.
3) The interdependencies across stakeholders, product lifecycle phases, and system levels are to be considered for transforming to a circular economy for wind turbine blades.
This review highlights three major challenges standing on the way: 1) Lack of standard and definitions.
2) Lack of traceability and transparency.
3) Second lifecycle shows potential but lacks attention and policies.
The review's findings, in particular on recycling, indicate two potential enablers for a circular economy: 1) Promoting a transition of industry through policy change.
2) Progress on circular economy strategies through alignment of objectives across stakeholders.
In the following the major challenges and enablers identified herein are described and with this a research agenda is derived: The provided review illustrates that there is no standard in used terms for articulating circular economy objectives and actions, neither in industry, nor in research.Industry and research often do not define their understanding of the used terms.For instance, the term "refurbishment" is sometimes used interchangeably with "remanufacturing", even though remanufacturing for blades, when referring to the above set definition, does not make sense as of today.
The review of scientific publications on wind turbine blades has shown that only a few researchers use the term "circular economy" that could indicate that the concept is not yet embedded into the research field.The lack of definition will also have major impact on the calculation of recyclability rate.Recyclability rate are not presented in this paper, however, they are being increasingly used in regulations to promote circularity and the recycling of product before the products are produced and used.If definitions are unclear, recyclability calculation will be highly unreliable.
There is also a lack of traceability and transparency on the objectives and actions.As mentioned the non-standardised companies' sustainability reports were used as main source for analysing objectives and actions for a circular economy.Due to the lack of standardisation of the reports, but also of the used terms, aspects could be understood differently if analysed by someone else.Also, it cannot be guaranteed that all relevant information on the website were identified.To provide transparency, the underlying data is provided in Appendix 1.The reviewed companies do not provide a comprehensive report on the progress of the objectives' fulfilment.There is also no standard yet on how to measure progress.The data quality might improve in the future through the Corporate Sustainability Reporting Directive that aims to standardise reporting on sustainability for companies being active in the EU [47].Further standardisation could be achieved through agreeing on circular design criteria as targeted by the Global Alliance for Sustainable Energy [48].In future research methods to collect primary data (e.g.expert interviews) could be used.Additionally, further companies, along the entire supply chain should be investigated.In particular, to include companies working on the refurbishment, resell, decommissioning and recycling of blades could add further insights.For the identified data on academic research, further findings could appear if other databanks (e.g.Web of Science) are used.In addition, only a detailed look into recycling and a 2 nd lifecycle took place, but the other circular wind strategies could also be reviewed in more depth.Further insights on current activities and intentions of research could be assessed through reviewing ongoing research projects.In those, more broader objectives on sustainability, resilience and circularity might be outlined.
The 2 nd lifecycle shows potential but lacks attention from researchers and policies.As stated above terms are not well defined and the potential of reuse has not been assed.There is also a lack in traceability and transparency.For instance, North Europe, in particular Denmark and North Germany have an aging fleet of onshore turbines.Predictions of waste volumes have not materialised so far that mostly assumed a decommissioning after 20 years [49].Thus, annual expected quantities are highly uncertain, which complicates the planning of strategies at the end of a lifecycle.When and where which turbines will be decommissioned is of importance for the capacity planning of reverse logistics, decommissioning, direct reuse, refurbishment, repurposing, and recycling in this region.An assessment if blades were reused could provide insights on the expected quantities of subsequent strategies such as recycling.Additionally, an assessment if the reuse of blades is a strategy for this region that should be promoted from a circular economy perspective was not conducted yet.Further insights could help to set or redefine objectives by the industry, define research needs, and inform policy for eventually required changes.
It looks as if a transition of industry could be promoted through policy change.It seems that the call for a landfill ban in June 2021 has triggered circular economy strategies, in particular recycling, across the industry.Further investigations on the role of policy and public perception for transforming to a circular economy, could bring valuable insights for further progress.The achieved progress and ongoing activities on recycling of blades indicate that progress on circular economy strategies is achieved through an alignment of objectives across stakeholders.Research on the transition process towards more sustainability, resilience and circularity could help to understand the mechanism and thus provide insights on how to further accelerate the transition in the wind industry.For the herein conducted analysis, a new dimension of analysis could be added by investigating the development of the companies' strategies over time.For instance, when was circular economy first communicated by the companies and how did it evolve over the years.
It remains the question if all necessary objectives have already been defined and set objectives are progressive enough to stick within planetary boundaries and reduce dependencies on materials to actually achieve the build-out of the wind industry.There is no consensus on what the circularity objectives should be across the wind industry, except of promoting recycling.As there is not one ideal circular economy, empirical research on the narrowing, slowing, and closing of resource flows for different regions and with different stakeholders across the onshore and offshore wind industry as well as other industries could provide more insights.Hence, taking into consideration different interdependencies along the lifecycle and stakeholders with a system-wide perspective.The formulation of aligned objectives with stakeholders across supply chains, research, and policy would be the first step towards more progress on transforming to a circular economy and thus decoupling from virgin material consumption.

Figure 1 .
Figure 1.The research methodology of this paperCovered circular economy wind strategyLevel of investigation2.DATA ANALYSIS

Figure 2 .
Figure 2. Industries' documentation on circular economy and circular wind strategies for wind turbine blades

Figure 4 .
Figure 4. Research on circular wind strategies for wind turbine blades

Figure 5 .
Figure 5. Papers on a 2 nd lifecycle of blades by addressed circular wind strategies and level of investigation papers the blade structure, 65 papers the blade material and 47 papers the bladerelated processes.When looking at the development over time, research on blades materials and processes starts in 2005.The blade structure and organisational level was first mentioned in 2010 for the structure and 2012 on the organisational level.Especially, since 2018 the number of publications on materials has increased.In all levels of investigation, a peak appears in 2022.The topic recycling of composites has a steady research output since 2010 with first publications dated back to 2005.First publications on recycling resins also date back to 2005, but more regular annual output appears since 2016.The term "recycle fibre" is first used by a publication from 2010, however, 57.6% of the publications using this term were published in 2021 and 2022.The three publications on recycling of balsa wood are from 2020, 2022, and 2023.The two publications on recycling of PET are from 2005 and 2023.

Figure 7 .
Figure 7.A schematic circular economy for wind turbine blades with narrowing, slowing and closing resource flows

Table 1 .
Industries' objectives on a 2 nd lifecycle for blades

Table 2 .
Objectives on recycling for blades