A typical wind turbine is designed for around 20 years of service meaning that many of the first-generation wind farms are at or approaching their end-of-life (EOL) stage. Approximately 85–90% of the wind turbine can be recycled including the tower, foundation and nacelle, which are made up of metals (steel, copper or aluminium) and concrete [1]. The blades, however, are composed of fibre-reinforced polymer (FRP) composite materials (including glass and carbon fibres, thermosetting polymers, epoxies and structural adhesives), core materials (including balsa wood and/or polymeric foams) and some metals such as steel, aluminium or copper ^[2][3][4][2,3,4]. This mix of materials presents a challenge for recycling and is energy-intensive to separate [5]. Until recently, the EOL stage was not considered a problem or priority, which resulted in a lack of industry guidelines and standard procedures for the removal and disposal of these blades when decommissioned ^[6][7][6,7]. Recent media have highlighted the growing concern as many of the blades are being sent to landfill sites across the US [8]. Current EOL technologies that may be considered are reuse, repurposing, recycling, recovery, co-processing, incineration or landfilling [9]. Some of these EOL technologies also require a continuous supply of material and therefore may be hindered by fluctuations and insufficient supplies [10]. Accurately predicting EOL blade material becomes a crucial aspect for the planning and development of sustainable circular strategies as well as to motivate governments and policy makers to take action to prevent a large build-up of blade materials [11]. Some EOL technologies are capital-intensive, and a lack of certainty in EOL blade material flow represents an investment risk for commercial processors. A number of journal and magazine articles have addressed the issue of blade waste material; however, there are still many uncertainties associated with the EOL material forecasts [12].

2. Recycling Technologies and Potential Material That Can Be ‘Reused’

A circular economy strategy for blade management involves the reuse of blade material in a new product. Before a waste management strategy is needed, the blades should be used and reused for as long as possible [1]. The reuse potential of wind blades depends on what reuse strategy is chosen as methods yield different amounts of reusable material ^[13][18]. The different EOL options for wind turbines blades can be assessed in terms of sustainability using the waste hierarchy. At the top of the hierarchy is the lifetime extension or reuse of the turbines at another site. Next on the waste hierarchy is blade repurposing, which involves the reuse of full blades or large sections of the blade in new industrial or architectural applications. Some examples of repurposing applications include pedestrian bridges (e.g., Blade Bridge along the Midleton-Youghal Greenway in County Cork, Ireland) ^[14][15][16][22,54,55], power transmission lines ^[17][56], children’s playgrounds (e.g., Wikado playground, Rotterdam) ^[18][57], bicycle shelters (e.g., Aalborg Harbour) ^[19][58], affordable housing ^[20][41], among others ^[21][59]. Full blade repurposing implies that the entire blade will be reused in one or more applications and will therefore yield a 100% reuse potential. In this scenario, different sections of the blade (root, tip or mid-span) could be reused in different applications ^[22][60]. Several start-up companies have been launched to commercialize blade repurposing (e.g., Anmet (Szprotawa, Poland), BladeMade (Rotterdam, Netherlands), and BladeBridge (Cork, Ireland)). In some cases, if the second life application requires little processing, testing and fabrication, the blades may be cut and prepared directly on site before being transported to their new location ^[22][60]. After reuse and repurposing come material-scale recycling methods. There are three key types of recycling methods based on mechanical, chemical or thermal processes. Mechanical recycling refers to the shredding, cutting or grinding of the blade material, reducing it in size. This material may then be used as a replacement filler for concrete or as a reinforcement in plastics and other products ^[23][24][25][61,62,63]. Only 70–75% of the FRP composite material (15% is already discounted since it is not FRP material) can be reclaimed in this grinding process ^[26][27][28][64,65,66]. In some cases, the blades may be cut and shredded on site to reduce transportation costs ^[29][40]. This may be completed using a mobile waste grinding unit. The fibres may be recovered through thermal or chemical recycling processes. In the case of thermal recycling such as pyrolysis or Fluidised Bed Combustion (FBC), high temperatures and pressures and vacuum may be used to recover the fibre materials. In the case of chemical recycling, the fibre materials are recovered from the resins using chemical solvents leaving behind the fibres. Co-processing, a form of material recycling, involves the substitution of blade material to replace virgin-mined materials such as clay, sand and limestone used to manufacture cement in a cement kiln. The polymeric materials in the blades provide energy recovery. This process yields a 50% recycling potential since only the fibre portion is recycled ^[12][30][12,67]. The energy recovered from burning the polymer is not considered material recycling however, it may contribute to life cycle assessment (LCA) benefits since many cement kilns are coal- or lignite-fired. While the European Composites Industry Association (EuCIA) encourages the co-processing of blade waste, not all countries have the ability to recycle using this method. There are very little data on which cement kilns are involved, except for a single Holicim plant in Lagerdorf, Germany and a single unidentified plant in Missouri, USA working in conjunction with Veolia ^[31][32][68,69]. This means that for companies in places such as the United Kingdom (UK) blades have to be transported, which can be expensive and energy-intensive ^[33][70]. Qureshi ^[34][71] provides an up-to-date review of the EOL options for FRP composite materials, including the strengths and limitations of each process in terms of energy demand and costs. The study shows that landfill and incineration are the most common and cheapest strategies for dealing with composite waste material; however, it is recognised that the composites industry needs to find more sustainable and circular practices such as reuse, repurposing and recycling. Beauson et al. [11] provide a review of the legislative and technical challenges of EOL blade management and composite material recycling. Coughlin et al. [5] and Fitzgerald et al. ^[35][72] provide estimates of costs and market potential of EOL processes. These factors further contribute to the uncertainties of EOL blade management. More recently, blade material passports ^[36][73] have been developed to document the material composition of specific blades with the aim to develop a standardised approach for blade disposal and aid potential recycling processes (e.g., Vestas (Arrhus, Denmark), Siemens Gamesa (Zamudio, Spain) and LM Wind Power (Kolding, Denmark)).