Although the multiple benefits of renewable energies are well known: affordable energy for consumers, diversification of the energy matrix, and contribution against the climate crisis, some comments questioning the sustainability of the technologies that lead the current energy transition are also part of the conversation.
Wind and photovoltaic technologies make use of renewable resources to generate electricity without emitting greenhouse gases into the environment and without requiring water consumption. However, their sustainability is questioned when talking about the final destination of the turbine blades and solar panels at the end of the operational life of the power plant (or when the replacement of this equipment is considered to repower the plant and continue the operation due to the good renewable resource that is available on the site).
Current wind turbines will have recyclability at around 86%, which is mainly due to the high content of steel for which commercial recycling methods exist. Nevertheless, wind blades are the component that poses the highest uncertainty on how to dispose of them properly and safely .
The problem of handling end-of-life blades mainly lies in the composite parts. A standard wind turbine blade contains 90% polymer composite reinforced with glass fibers or carbon fibers, and the rest of the content is a mixture of PVC, balsa wood, metal, paint, and sealing .
The current blade recycling methods are divided into three categories: mechanical, thermal, and chemical. Mechanical recycling utilizes mechanical shredders to divide the composite material into smaller parts and pieces; the output of mechanical recycling can be used as fillers, reinforcements as raw material to produce new plastic products, or production of cement . Thermal recycling mainly consists of pyrolysis which is characterized by heating the material desired to be separated in the absence of oxygen under controlled conditions; the process itself aims to break down the organic materials (resin) into lower-weight molecules, the inorganic material (fibers and filler materials) is left intact for recovery . Chemical recycling, with the most common method being solvolysis, is a method that utilizes a solvent composed of catalysts/additives to depolymerize the chemical crosslinked bond present in thermoset polymer products. The reactive solvent will, in combination with temperature and pressure, diffuse into the composite material and break specific bonds in the resin, which abolishes the cross-binding between resin and fibers.
In light of this, a question arises, Why are landfill deposition and incineration still considered options despite the existing recycling technologies for end-of-life wind blades? The decision has an economical base, recycling is chosen only if it is the more cost-effective option for the decommissioned power plant.
The economic factors include the cost of recycling equipment, transportation to the recycling plant, and the selling price of the recycled output.
This context highlights the importance of establishing guidelines able to regulate the deposition of end-of-life wind blades, the creation of incentives for those power plants that opt for recycling, as well as support for technological processes to reach the maturity required to be considered a suitable option for decommissioned projects (Carbon Rivers is an example of a small business supported by the U.S. Department of Energy to develop and scale up a novel process, based in pyrolysis, to recycle turbine blades ).
In a similar way, methods for recycling solar modules are being developed to reduce the environmental impact of photovoltaic (PV) waste and to recover some of the value from end-of-life modules. As explained for wind blades, the methods can consist of mechanical, thermal, and chemical processes, and in some cases, a combination of them.
In a typical crystalline silicon PV module, 75% of the total weight corresponds to the module surface (glass), 10% polymer (encapsulant and back sheet foil), 8% aluminum (mostly the frame), 5% silicon (solar cells), 1% copper (interconnectors) and less than 0.1% silver (contact lines) and other metals (mostly tin and lead) .
The main recycling possibilities for silicon PV modules are: chemical etching (simple and efficient process able to recover high purity materials), thermal treatment (economically feasible process able to full removal of ethylene vinyl acetate (EVA) but presents harmful emissions and high energy requirements), dry and wet mechanical process (low energy requirements and equipment widely available), physical degradation (other separation processes required for full EVA removal), electro-thermal heating (easy removal of glass), mechanical separation by hotwire cutting (low cell damage and recovery of glass but other separation processes required for full EVA removal), organic solvents (easy access to the EVA but presents harmful emissions and wastes) .
It might be a concern that some recycling processes involve harmful emissions. In that regard it is important to highlight a comparison addressed by B. Huang et al. in , between PV modules recycling and landfill deposition, it was found that the environmental impacts from the recycling process are lower than for landfill, assuming that the recycled resources go back to the PV cells and modules manufacturing. These results considered that the recycling process involves dismantling, remelting, as well as thermal and chemical treatments.
The design of wind blades and PV modules plays an important role in the recycling processes and completes the circularity in the renewable energy industry. Design for recycling facilitates economic recycling methods and maximizes material recovery by designing products with a focus on increasing speed and ease of dismantling, improving the recycling rate and purity of recovered materials, and reducing waste . The application of this principle offers potential for environmental and economic benefits to manufacturers and end users as well.
 M. Hoefer. Wind turbine blade recycling: an economic decision framework.
 E. Paulsen and P. Enevoldsen. A multidisciplinary review of recycling methods for end-of-life wind turbines blades.
 V. Vladimirov and I. Bica. Mechanical recycling: Solutions for glass fibre reinforced composites.
 G. Oliveux, L. Dandy, and G Leeke. Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties.
 DOE, Carbon Rivers Makes Wind Turbine Blade Recycling and Upcycling a Reality with Support From DOE.
 M. M. Lunardi, J. Alvarez-Gaitan, J. Bilbao, and R. Corkish. A review of recycling processes for photovoltaic modules.
 B. Huang, J. Zhao, J. Chai, B. Xue, F. Zhao, and X. Wang. Environmental influence assessment of China’s multi-crystalline silicon (multi-Si) photovoltaic modules considering recycling process.
 IEA-PVPS. PV Module Design for Recycling Guidelines
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