A Review of the Potential for the Recovery of Wind Turbine Blade Waste Materials

A successful circular economy can only exist when it relies solely on renewable energy sources. The adoption of resilient business models and the consequent redesign of legislation on all sectors are essential to ensure sustainable economic growth. Wind energy can offer clean and renewable energy with a low environmental impact. Nevertheless, waste in end of life composite materials resulting from wind turbines is a problem that needs to be addressed. Composite materials are commonly used in wind turbines due to their excellent mechanical properties, matched by low weight. Notably, the recycling technologies of such materials is limited. Material flows and estimations of end of life materials are of great importance and will convince stakeholders that markets for recycling composites are viable investments.


Introduction
The European Commission has set its course towards a circular economy via the adoption of resilient business models and consequent redesign of legislation on all sectors.The goal is to ensure sustainable economic growth.Finite stocks (already available within the economy) should be controlled, and renewable resource flows must be balanced in order to maximize the use of resources.Furthermore, redesign of the business models should rule out all negative factors.A successful circular economy can only exist when it relies solely on renewable energy sources.Wind energy can offer clean and renewable energy with a low environmental impact.Nevertheless, waste in end of life composite materials resulting from wind turbines is a problem that needs to be addressed.Two decades ago the wind energy industry was considered as niche.Currently, it has claimed a large percentage of the overall electricity consumption (10%-15% in Europe).According to the International Energy Agency (IEA), projections show that wind energy will be the largest energy source in less than two decades.Currently, the specified industry contributes 0.25% of the total EU Gross Domestic Product (GDP), while employing more than 250,000 people [1].It is well understood that with growth comes a greater footprint in the global supply chains, and thus a greater responsibility.Furthermore it is expected that a great number of currently operating composite structures are expected to reach their end-of-life (EOL) in the next decade.This will introduce a large amount of composite waste [2,3].
In actual fact, GRP's are the largest material group in the composites industry, with a Global demand about 10 times more compared to the CRP's [4].In Figure 1, the sectors of industry with the corresponding demand are presented.Wind energy has been developed rapidly throughout the world.The European Commission aims to reach the target of a high percentage of the overall primary energy consumption in the EU and almost one quarter of electricity provided by renewable energy sources [2].However, as more wind parks are built worldwide, the energy consumed, pollutants released and volume of composite materials increase as well, magnifying the environmental impacts associated to their manufacture and service [5].
These facts undeniably generate the need for adequate disposal life cycle stages or recycling regimes, which will need to be managed with minimum environmental impact.The main goal of the EU is to generate resource efficiency.This can be achieved via restructuring of waste into a resource.The nature of composite materials poses further difficulties in recycling, considering the limited means appropriate [6].More specifically, according to WindEurope (formerly the European Wind Energy Association) [7], there are three scenarios regarding the future of the wind energy industry.The temperate projection 323 GW of cumulative wind energy capacity would be installed in the EU by 2030 (of which 253 GW onshore and 70 GW offshore).With this capacity, wind energy would produce 888 TWh of electricity, equivalent to 30% of the EU's power demand.The wind energy industry would invest €239 bn by 2030 and provide employment to 569,000 people.With such growth rates, social and economic impacts must be taken into account so as to improvise and implement the optimal policy and accompanying measures required.
The current work focuses on emphasizing the necessity of creating markets for recycling composites via determination of material flows and projections of end-of-life material quantities, as the expected recovered materials from End-of-Life Wind Turbines are going to have significant volumes.

Wind Turbine Industry: Circular Economy and LCA
The concept of circularity must be introduced during the material's selection process and prior to the product design specification determination.Most of the metal components are currently being recovered after the end life in service for such structures, which typically reach a 20-year span.The RoHS (Restriction of Hazardous Substances) Directive is fully in effect in this industry, in practice by definition and in application of pro-active materials restriction lists.Wind turbine manufacturers are fully complying with the RoHS Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, and many manufacturers apply best to go beyond legal requirements in their ambitions; wind turbines produce renewable energy replacing fossil energy.Wind energy has been developed rapidly throughout the world.The European Commission aims to reach the target of a high percentage of the overall primary energy consumption in the EU and almost one quarter of electricity provided by renewable energy sources [2].However, as more wind parks are built worldwide, the energy consumed, pollutants released and volume of composite materials increase as well, magnifying the environmental impacts associated to their manufacture and service [5].
These facts undeniably generate the need for adequate disposal life cycle stages or recycling regimes, which will need to be managed with minimum environmental impact.The main goal of the EU is to generate resource efficiency.This can be achieved via restructuring of waste into a resource.The nature of composite materials poses further difficulties in recycling, considering the limited means appropriate [6].More specifically, according to WindEurope (formerly the European Wind Energy Association) [7], there are three scenarios regarding the future of the wind energy industry.The temperate projection 323 GW of cumulative wind energy capacity would be installed in the EU by 2030 (of which 253 GW onshore and 70 GW offshore).With this capacity, wind energy would produce 888 TWh of electricity, equivalent to 30% of the EU's power demand.The wind energy industry would invest €239 bn by 2030 and provide employment to 569,000 people.With such growth rates, social and economic impacts must be taken into account so as to improvise and implement the optimal policy and accompanying measures required.
The current work focuses on emphasizing the necessity of creating markets for recycling composites via determination of material flows and projections of end-of-life material quantities, as the expected recovered materials from End-of-Life Wind Turbines are going to have significant volumes.

Wind Turbine Industry: Circular Economy and LCA
The concept of circularity must be introduced during the material's selection process and prior to the product design specification determination.Most of the metal components are currently being recovered after the end life in service for such structures, which typically reach a 20-year span.The RoHS (Restriction of Hazardous Substances) Directive is fully in effect in this industry, in practice by definition and in application of pro-active materials restriction lists.Wind turbine manufacturers are fully complying with the RoHS Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, and many manufacturers apply best to go beyond legal requirements in their ambitions; wind turbines produce renewable energy replacing fossil energy.
An assessment of the environmental suitability of a recycling process must quantify all potential environmental impacts of the process.The majority of environmental impacts in a wind turbine come from materials and the supply chain.Optimization will be succeeded when transport for installation is limited to a minimum so as to keep fuel consumed for the process low.It is imperative that production-suppliers are chosen, amongst other criteria, by location [8].
Due to their nature, use of certain materials requires special attention given their potential environmental impact.The use of Life cycle assessment (LCA) is already well established in many industries and is growing in the composites field where it has been used to study the environmental effects of substituting more commonly used material types with composites in transport applications [9][10][11].A series of interesting overviews on the difficulties and novel technologies have been published [12][13][14][15][16], addressing mostly the recycling of carbon fibre composites for structural applications.
The framework of LCA relies on the series of environmental management standards (EMS) introduced by the International Standards Organization (ISO 14000) [17].In general, two basic methods for LCA are used to assess the life cycle of products, materials or processes: process-level analysis and economic input-output analysis.Process analysis is preferred due to the detailed assessment of resources, while in economic input-output analysis, (EIO) [18] takes into account various economic transactions, resource requirements and environmental emissions and uses input-output tables which model the economic system in its entirety.

Recycling Technologies for Composite Materials
Traditionally, composite structures/materials mostly from the aerospace and automotive industry, when reaching their in service lifetime, were disposed of in landfills or incinerated.In the last decade, the vast majority of the European Union have voted in favour of legislation forbidding landfill disposal of such materials.As far as incineration is concerned, the main problematic point is a potential release of toxic byproducts due to the incineration of the matrix constituent [19].
Recycling technologies (Figure 2) being the greener solution, is practical only if the value of the reclaimed raw materials exceeds the cost of the recycling process.
An assessment of the environmental suitability of a recycling process must quantify all potential environmental impacts of the process.The majority of environmental impacts in a wind turbine come from materials and the supply chain.Optimization will be succeeded when transport for installation is limited to a minimum so as to keep fuel consumed for the process low.It is imperative that production-suppliers are chosen, amongst other criteria, by location [8].
Due to their nature, use of certain materials requires special attention given their potential environmental impact.The use of Life cycle assessment (LCA) is already well established in many industries and is growing in the composites field where it has been used to study the environmental effects of substituting more commonly used material types with composites in transport applications [9][10][11].A series of interesting overviews on the difficulties and novel technologies have been published [12][13][14][15][16], addressing mostly the recycling of carbon fibre composites for structural applications.
The framework of LCA relies on the series of environmental management standards (EMS) introduced by the International Standards Organization (ISO 14000) [17].In general, two basic methods for LCA are used to assess the life cycle of products, materials or processes: process-level analysis and economic input-output analysis.Process analysis is preferred due to the detailed assessment of resources, while in economic input-output analysis, (EIO) [18] takes into account various economic transactions, resource requirements and environmental emissions and uses inputoutput tables which model the economic system in its entirety.

Recycling Technologies for Composite Materials
Traditionally, composite structures/materials mostly from the aerospace and automotive industry, when reaching their in service lifetime, were disposed of in landfills or incinerated.In the last decade, the vast majority of the European Union have voted in favour of legislation forbidding landfill disposal of such materials.As far as incineration is concerned, the main problematic point is a potential release of toxic byproducts due to the incineration of the matrix constituent [19].
Recycling technologies (Figure 2) being the greener solution, is practical only if the value of the reclaimed raw materials exceeds the cost of the recycling process.

Current Market Status
There are several commercially active companies that recycle composite waste and sell recyclate on to new markets.
Carbon conversions (formerly MIT-RCF) in the USA recycle carbon fiber-reinforced parts.They reuse reclaimed carbon fibers to produce new, high-performance components.CFK Valley Recycling in Germany, utilizes pyrolysis and takes in waste streams from transportation and end-of-life

Current Market Status
There are several commercially active companies that recycle composite waste and sell recyclate on to new markets.
Carbon conversions (formerly MIT-RCF) in the USA recycle carbon fiber-reinforced parts.They reuse reclaimed carbon fibers to produce new, high-performance components.CFK Valley Recycling in Germany, utilizes pyrolysis and takes in waste streams from transportation and end-of-life sporting goods to distribute the recovered fiber as chopped and milled products.Damacq Recycling International in Netherlands processes collected hardened thermoset composite parts and production waste (mostly wind turbine components) through their recycling method.Eco-Wolf, based in the USA, supplies equipment for incorporating recycled materials (e.g., GRP regrind) into a resin mix for spray up or casting.
ELG Carbon Fibre Ltd. of UK processes waste material in its patented pyrolysis process, including manufacturing waste and cured parts.Karborek IT of Italy offers a patented recycling process for the recovery of carbon and glass fibre from composite materials.Neocomp from Germany have developed a reprocessing and utilization option for glass fiber-reinforced plastic.
TRC from Spain is dedicated to the development of a highly energy-efficient processes rendering high-quality glass and carbon fibres, together with sub-products to be re-used as fuel to produce heat and energy.Procotex of Belgium recycles only unprocessed fibre waste primarily from weaving and cutting scraps, waste from carbon fiber producers and dry fibres.V Carbon in England reclaims carbon fibre primarily from aircraft and automotive structures and then use the fibers to manufacture new parts.

Decommissioning
Engineers consider technoeconomic facts so as to decide upon the decommissioning of a unit.Wind turbines contain 80-85% metal, which is well suitable for recycling without quality losses.A remaining challenge is the improvement of the footprint from blades which today principally contain virgin material with very limited opportunities for reusing materials [16].
Large-scale wind turbines (up to 3 MW), commonly are of a 3-blade rotor, whose weight is almost 4% of the total turbine and of which 40% can be considered to be of composite nature [20].Furthermore, at the manufacturing stage, approximately 10% of the composite results in waste, a rather significant amount.A major factor dictating the bill of materials is the overall size of the structure.Furthermore, the trends in design and manufacturing differ between small and large turbines.This is due to the differences in strength and fatigue loading requirements.Certain wind turbine components do actually receive high cycle fatigue through constant dynamic loading, in some cases more than expected in other high-performance engineering structures, e.g.aircrafts.Thus, material fatigue properties are an important consideration in wind turbine design and materials selection [21].
There are new component developments that will significantly change the materials usage patterns.Generally, there are trends toward lightweight materials especially on the moving parts in order to maximize energy yields [16,19].
The wind turbine blades are mixed structures in terms of materials, generally composed of various materials with different properties such as glass or carbon reinforcement fibres, thermoset or thermoplastic Polymer matrices and Sandwich panels.The fibre-reinforced polymer (FRP) composites represent the majority of the blades' material composition.Glass fibre represents the primary material in wind turbine blades.Furthermore, coatings such as polyurethane or polyethylene are applied for outer surfaces.Besides bonded joints, one should expect steel bolts in structural interfaces and copper wiring for lightning protection [22][23][24].Composite materials recycling flows are difficult to define and quantify.Waste and concurrent projections of resource demand are predicted; nevertheless, deviations can be of non-negligible magnitude due to falsely declared (due to comparability issues mainly) input quantities [25].
The evaluation of a wind turbine's performance should come after defining the equilibrium between environmental impacts during the lifecycle and the energy saved.The energy consumed by the entirety of processes is associated with the manufacture of a wind turbine, ranging from mining and processing of natural resources to transport and installation, while the emission payback time is the time during which avoided emissions, via use of the wind turbine, are equal to those released for manufacturing and all related processes [21].

Material Flows
The following materials inventory Table 1 has been drafted taking into account published data of wind turbines .The materials, grouped by wind turbine components, are accompanied by embodied energies offering a starting ground for analysis.As the rotor size increases, the trend will be toward high-strength, fatigue-resistant materials.As the turbine designs continually evolve, various composites will likely come into use in this industry, which means that a solution to the industrial recycling capability will be mandatory.The basic nature of the problem to be solved is that inherently, composite materials are hard to recycle.There are two facets to be addressed in order to reach a solution.The first is the development of optimal recycling technologies capable of maximum reclamation in terms of materials and energy.The second is the synthesis of novel composites that while maintaining excellent mechanical properties, are also receptive to recycling [22].
The above should comply with economy of scale linked to the availability of EOL material, the environmental regulations and the overall cost and probable new environmentally hazardous derivative substances which will eventually dictate the sustainability of the process [22].
The following estimation is based on the Global Wind Energy Council (GWEC) reports regarding globally installed (and cumulative) wind power capacity.The assumptions made were that the EOL is typically 20 years and that the most commonly used wind turbine is of a 1.5 MW capacity (Table 2), with an overall weight of 160 tons.End of life due to the failure of material quantities was omitted.The following projection presented in Figure 3 does not take into account the rather high percentage (Typically 10%) waste in composite manufacture, mainly because this will decrease drastically as the processes will be optimized [17].The total mass of composite waste from EOL wind turbines at the end of service life for such composite structures will escalate in the next decade as seen in Figure 2.With these projections regarding the wind industry, composites to be recycled are expected to increase fourfold in a decade.Until now, the supply of composite waste from blades has been limited.Recycling composite materials on an industrial scale is a very difficult task, underpinned by the few techniques currently available.The urge for optimized composite material recycling is driven by the expected volumes of obsolete material and the accompanying laws.Figure 4 presents an interesting set of data which helps identify the composite waste origins, i.e., the locations of large wind farms correlated with capacity [43,44].The availability of obsolete components-products and manufacturing scrap is very important for proper operation, as is the distribution per region.Nevertheless, taking into account that the specified materials will be available for recycling in greater volumes, limited availability will not constitute a threat of increasing the cost for running the process.[43,44]).With these projections regarding the wind industry, composites to be recycled are expected to increase fourfold in a decade.Until now, the supply of composite waste from blades has been limited.Recycling composite materials on an industrial scale is a very difficult task, underpinned by the few techniques currently available.The urge for optimized composite material recycling is driven by the expected volumes of obsolete material and the accompanying laws.Figure 4 presents an interesting set of data which helps identify the composite waste origins, i.e., the locations of large wind farms correlated with capacity [43,44].The availability of obsolete components-products and manufacturing scrap is very important for proper operation, as is the distribution per region.Nevertheless, taking into account that the specified materials will be available for recycling in greater volumes, limited availability will not constitute a threat of increasing the cost for running the process.With these projections regarding the wind industry, composites to be recycled are expected to increase fourfold in a decade.Until now, the supply of composite waste from blades has been limited.Recycling composite materials on an industrial scale is a very difficult task, underpinned by the few techniques currently available.The urge for optimized composite material recycling is driven by the expected volumes of obsolete material and the accompanying laws.Figure 4 presents an interesting set of data which helps identify the composite waste origins, i.e., the locations of large wind farms correlated with capacity [43,44].The availability of obsolete components-products and manufacturing scrap is very important for proper operation, as is the distribution per region.Nevertheless, taking into account that the specified materials will be available for recycling in greater volumes, limited availability will not constitute a threat of increasing the cost for running the process.[43,44]).

Conclusions
The wind power industry is one of the fastest growing consumers of FRP composites in the world, which correlates with the industry's rapid growth in recent years.A major factor dictating the bill of materials is the overall size of the structure.Furthermore, the trends in design and manufacturing differ between small and large turbines.This is due to the differences in strength and fatigue loading requirements.Certain wind turbine components do actually receive high cycle fatigue through constant dynamic loading.Thus, material fatigue properties are an important consideration in wind turbine design and materials selection.There are new component developments that will significantly change the materials usage patterns.Generally, there are trends toward lightweight materials especially on the moving parts.
As the rotor size increases, the trend will be toward high-strength, fatigue-resistant materials.As the turbine designs continually evolve, various composites will likely come into use in this industry, which means that a solution to the industrial recycling capability will be mandatory.
Existing practices and options for waste management are applicable to the post-decommissioning of wind turbines.Knowledge of data regarding the amounts of waste estimated and produced and the forecast of volumes to be expected is critical for the sustainability of recycling composites at an industrial level.Available data prove the need for addressing the composite recycling as in the next decades EOL volumes will vastly increase.
Furthermore, it is necessary to research the means of improving the wind turbine blade manufacturing processes, as the result will produce structures that have lower levels of embodied energy and reach equilibrium of emissions faster.Another point is the advancement of technologies that allow reuse or recycle of, as the case is, high-value materials.The driving force to achieve the above will be by leading manufacturers to realize the advantages and opportunities that can be reached by application of LCA.The challenges of reprocessing to be met include enhanced product quality, environmental regulations and processing costs.Optimization with regard to recycling processes will enable satisfying levels of quality and price compared to pristine composites so as to virtually generate a market for recycled composites.

Figure 1 .
Figure 1.Percent demand of composites for industry sectors.

Figure 1 .
Figure 1.Percent demand of composites for industry sectors.

Table 1 .
Materials used for wind turbines with corresponding embodied energies.