Keywords

1 Introduction

Circular Economy (CE) is a system changing concept that could contribute to a more sustainable and resilient supply chain management (SCM) [1,2,3]. With an increasing frequency of disruptions with growing amplitudes a sustainable and resilient SCM becomes even more relevant [4]. A standard definition of CE does not yet exist, this paper follows the definition by Kirchherr et al. [5]. They define CE as a multi-level system changing concept that is based on business models with value obtaining strategies following a cascade – Reduce, Reuse, Recycle and Recover – and contributing to a sustainable development [5]. Embedding the circular thinking into SCM is stated as Circular Supply Chain Management (CSCM) [6].

Due to the expected benefits, CE is recently gaining attention in SCM practice (e.g. furniture industry, automotive industry, construction) and research [3, 6,7,8]. Nevertheless, investigations in regards to the wind energy SCM are still rare. Despite, research in this field is of importance as the industry mostly operates in a linear system. Ambitious climate laws by governments trigger an expansion of the industry that will also lead to increased waste volumes if not reflecting circular thinking [9]. The size of wind turbines and their components, as well as the decentralized geographical distribution of wind turbines, underscore the role of SCM in promoting a CE [10]. Hence, this paper investigates how the current literature on SCM in the wind energy industry is structured and if a link to CE exists.

2 Current State of Research

As highlighted in Sect. 1, applying CSCM in the wind energy industry has several potential benefits. Nevertheless, research on this topic is rare. This section with Table 1 provides an overview of relevant research and outlines the contribution of this review.

Table 1. Differentiation from existing research
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Jensen et al. [11] point out the need to embed CE in clean energy infrastructure by looking at offshore wind energy (offshore wind) in the United Kingdom (UK). They provide insights about strategies for the end-of-life (EoL) for rare earth elements, copper and composites. In another publication [12], Jensen et al. concentrate on remanufacturing of three case studies as of one is from the wind energy industry. Different economic, ecological and social factors through remanufacturing from the perspective of a wind turbine manufacturer are presented. Velenturf [13] develops a framework for integrating a sustainable CE in offshore wind. 18 CE strategies (e.g. dematerialise, repair, reuse) are stated for materials, components and infrastructure for offshore wind. She proposes to apply this framework to other energy infrastructure such as onshore wind energy (onshore wind) [13]. Nevertheless, these studies do not cover SCM aspects. Some researchers have investigated CSCM aspects in relation to wind turbine blades: For instance, Rentizelas et al. [14] examine the reverse supply network design for waste volumes from wind turbine blades in Europe. The analysis reflects on environmental and economic aspects and proposes a semi-decentralised network design with 3–4 facilities in Europe. Beauson et al. [15] provide a holistic view on the EoL-related processes of wind turbine blades and discuss potential waste volumes, the legislation and standards as well as technical processes in an European context. Nagle et al. [16] conduct a life cycle assessment of three repurposing scenarios for the second use of decommissioned wind blades from onshore wind in Ireland. Lapko et al. [17] focus on critical raw materials for photovoltaic panels and wind turbines and influencing factors to enable a closed-loop supply chain (SC). Koumoulos et al. [18] present a roadmap for challenges and practical applications for the composites industry. And finally, Franco et al. [19] conduct a systematic literature review (SLR) on the photovoltaic SC in the light of a CE.

Thus, none of the presented studies take a holistic view on the wind energy SC while embedding the concept of a CE. So far, no SLR on CSCM in relation to the wind SC exists. However, for example Farooque et al. [6] call for industry-specific research as a result of their review on CSCM literature. In addition, as CE is a system-wide approach it is of importance to gather a complete picture. This paper aims at closing the research gap by answering the research question: “What is the current state of research on wind energy SCM and which links to a CE exist?”. With conducting a SLR a basis to derive research trends and gaps is set.

3 Research Methodology

The research methodology of this paper consists of identifying a representative sample of literature on the wind energy SCM and of analyzing this sample in regards to its reference to a CE. The SLR process is based on Tranfield et al. [20] as the process is known to gather knowledge within scientific literature and to enable a holistic view. The procedure was already used in CSCM [19, 21]. Using the established databases Scopus and Web of Science [22], the SLR was conducted between 21/03/2022 and 12/05/2022. As also summarized in Fig. 1, the review process consists of six steps.

Fig. 1.
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Search process of the scientific literature review

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The research is conducted by applying the following search string to titles, abstracts and keywords: (“supply chain*” OR “value chain*”) AND (“wind”) AND NOT (“pv” OR “solar” OR “photovoltaic*” OR “biomass” OR “biofuel” OR “biogas” OR “hydro”). The research string follows the assumptions of Franco et al. [19] and Velenturf [13]. Franco et al. used “supply chain*” and “value chain*” and excluded other energy sources such as biomass to identify papers purely on photovoltaic in their case. This approach is applied for the case of wind energy in this publication. However, not “wind*” is used, but instead “wind” as done by Velenturf as “wind*” results in too many not related results. In contrast to Velenturf, the search does not include terms on CE, as CE has not yet fully migrated into the SCM and would eventually lead to overlooked publications [13, 21]. To underpin, a search on Scopus on 21/03/2022 with the search string TITLE-ABS-KEY ((“wind”) AND (“circular*”) AND (“value chain*” OR “supply chain*”)) led to 19 publications from 2013–2022.

A search with “wind” initially leads to 893,546 publications on Scopus and Web of Science. In a second step the literature is filtered to only scientific literature, journal and conference papers. Next, the search string is supplemented by “value chain*” and “supply chain*”, resulting to 1,261 scientific publications. In the fourth step, only German and English articles are selected and in the fifth step, the title and abstract of each article is checked for relevance for the objective of this paper. Thus, only articles are selected that have a linkage to the wind energy industry and to SCM. To limit the boarder of the investigation, papers that only thematize power trading, offsetting, balancing or energy storage and transport (e.g. grid) are excluded. The focus is on the wind turbine SC with its components and materials. Also, market studies and studies purely focusing on technical design are left out. The sixth step, removes duplicates between the chosen databases. Finally, a representative sample of 163 scientific publications on the wind energy SCM is considered as the body of analysis. For 122 paper a full-text is available and for 41 papers only the abstract. The content of the abstract is sufficient for conducting the analysis, otherwise they would have been excluded.

The analysis of the 163 articles foresees to first describe the sample regarding the publication year and secondly, a content analysis takes place. The object of research is characterized: material, component, wind turbine/ wind farm in onshore or offshore wind [13]. Materials for wind turbines are concrete, steel, electronical components (with its rare earth materials neodymium and dysprosium), copper, aluminum, polyvinyl chloride (PVC), operating fluids, composites (glass-fibre reinforced plastic (GFRP) and carbon-fiber reinforced plastic (CFRP)) [10, 23]. Out of those, the key components – foundation, tower, rotor blades, rotor hub, nacelle, generator, gearbox, grid connection technology – arise [10, 24]. Finally, the wind turbine and wind farm is assembled [13]. For a geographical classification, the region of investigation is stated. Finally, following Vegter et al. [25], the publications are classified according to an adapted SCOR model: Plan, Source, Make, Deliver, Use, Return, Recover, and Enable. The authors have extended the common and industry-neutral framework with the processes Use and Recover to promote CE. Hence, they extract recycling and remanufacturing from the process Make to Recover and repair and maintenance from the process Make to Use [25]. The model is already used by other researchers in CSCM [1, 26].

4 Results

This section presents the results of the applied research methodology that led to 163 scientific publications about the wind energy SCM since 2007. When comparing to SCM in general with its first mentioning of the term in 1982 and even earlier research on some aspects, it is still a young research field [4]. As shown in Fig. 2, it is astonishing that offshore wind has a longer track record as onshore wind was established to the market prior to offshore wind [10]. In total 8.6% of the articles focus on onshore wind, 36.2% on offshore wind and the remaining 55.2% did not specify or addressed both types. Authors who did not specify the field could possibly mean onshore, since the field is more mature. Otherwise, the research interest on offshore wind’s SCM could be due to the logistical and manufacturing complexity of maritime operations [10, 27].

Fig. 2.
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Number of publications per year and wind energy offshore or onshore

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Next, the investigated region and object of research are presented in Fig. 3. 31.9% of articles from the sample address Europe, 12.7% Asia-Pacific, 12.7% Americas, 3.0% Africa and Middle East and 39.8% were not classified or have a global scope. In Europe mostly Germany (12 articles) and the UK (8 articles), in Asia-Pacific predominantly China (18 articles) and in Americas mainly the United States of America (USA, 14 articles) were studied. China, USA and Germany are the biggest onshore markets that could eventually explain the research interest [10]. Europe being the largest offshore market, could justify why 49.2% of offshore wind research is located in this region [10]. Figure 3 also illustrates that 6.7% of the identified articles deal with materials, 27.6% with components and 65.6% with the wind turbine/ farm.

Fig. 3.
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Number of publications per region and object of investigation

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Only rare raw materials that are used for permanent magnets generators are represented in the sample. The research interest could be due to its high uncertainties on pricing and availability as supplier countries and mines are rare [10]. Research on key components mainly focus on blades and secondly on the generator and foundation. The publications on the foundation purely cover offshore wind. This could be due to the high technical requirements on the stability to cope with the extreme weather conditions [10, 24]. Most papers however address several components (e.g. spare parts). Finally, the sample is clustered according to examined processes as presented in Fig. 4. Most articles deal with the processes Plan (31.5%) and Enable (29.7%). Followed by Source (9.2%), Use (8.6%) and Make (8.3%). The results indicate that the wind SCM literature focuses on these processes. Studies on Return (2.7%), Recover (5.0%) and Deliver (5.0%) are less represented in research. In the sample, some papers show a link to CSCM and sketch aspects of reducing, reusing, recycling and recovering materials and components.

Fig. 4.
figure 4

Final sample of scientific articles clustered according covered processes [25]

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For instance, Menzel et al. [28] cover the Plan process and discuss the benefits of modularization for a German manufacturer. Mert et al. [29] reflect on product-service systems for onshore wind. Bonfante et al. [30] outline strategies for rare earth magnets production and analyze CE strategies. Cheramin et al. [31] design the reverse SC for rare earth magnets in the USA and Rentizelas et al. [14] for blades in Europe.

5 Conclusion and Future Research Agenda

This work has identified and characterized a representative sample of scientific literature on wind energy SCM. Insights into the year of publication and covered regions, products and processes were given. First CE links are outlined that could form the basis for further research: For example, a systematic CSCM design of the macro, meso and micro levels and its dependencies on different objectives could be of interest. To be able to handle the complexity while reflecting the entire lifecycle of a wind turbine, a regional focus (e.g. on Germany) could be beneficial. Then also specific market and policy conditions can be considered. Modelling the resource flows and characteristics would increase transparency and could enable the identification of viable circular business models and SC design. As CE relies on a system change, the enabling processes (e.g. technology, cooperation) also play an important role. The paper’s results indicate, that some CE strategies were addressed, but still rarely. Thus, researchers should continue to analyze the link of CE strategies and existing research on wind energy SCM. This counts for onshore and offshore, the material, component and infrastructure perspective and different regions. In this context, further empirical studies are needed that examine the effects on the three dimensions of sustainability as well as resilience. For instance, as turbines increasingly reach their EoL, research on reuse and recycle is required. Hence, a reverse SC and sufficient business models should be explored and lessons learned should be reflected in the design of new wind turbines. All in all, CSCM should enable to reduce, reuse, recycle and recover materials, components and turbines for keeping the highest possible value. To achieve this, the SC, company, products and processes levels have to be considered.