As the world gets more and more wind turbines - which at some point have served their lifetime - the challenges of recycling the turbine blades increase. The blades are designed to last for a long time without breaking or weathering - not for reuse.
As part of the sustainable transformation, it is now a demand from the authorities and from the industry itself. The turbine blades are not to be disposed of, but to be transformed or recycled. Yet, there are some major challenges.
Recycling in three categories
As a start to put things in perspective, composite waste are coming from many sources, e.g. building and construction, transportation, marine, in addition to wind. The composite materials from wind turbines is representing only a smaller part of the generated total composite waste. Also, the type of composite material (typically glass fibres in a cured thermoset polymeric/plastic matrix) used is not toxic or harmful; it presents however a challenge for recycling it, i.e. to avoid landfilling.
Recycling techniques for thermoset based polymer composites are commonly presented in three categories, depending on the type of processes involved: mechanical, thermal or chemical based.
Mechanical recycling
Mechanical recycling involves cutting, crushing, shredding and grinding techniques, which reduces the material to granulate like mixture. Depending on the intended application, the material can be processed down to different particles dimensions. The applications for shredded composite material has been investigated as filler or reinforcement in new polymer composite materials or concrete. There are a few applications of this in US and Europe (e.g. for railroad ties, decking boards, and panels). By mechanical recycling, the glass fibres become shortened and misaligned, and this leads to reduction in reinforcement efficacy when the fibres are used in new materials. The challenge is to find applications having sufficient low requirement for mechanical properties, and at the same time, the cost of using recycled materials should be lower than other filler materials.
Thermal recycling
Thermal recycling is a group of recycling processes that involves thermal treatments in order to separate the fibres from the matrix and recover to some extent energy from the combustion of the matrix material. Thermal recycling processes includes pyrolysis, fluidised-bed pyrolysis and microwave pyrolysis. The processing temperature varies from 450°C to 700°C. It is well-known that exposure of the glass fibres to these high temperatures are known to significantly reduce their strength; the fibres become very brittle. This makes it a challenge to re-use the glass fibre part, e.g. as reinforcement in new composites. Recent work (e.g. at University of Strathclyde) has demonstrated that the strength of the glass fibres can be recovered to a large extent by etching and re-sizing techniques. This adds of course cost to the recycled fibres, which must be compared with the low cost of virgin glass fibres.
Chemical recycling
Chemical recycling typically covers solvolysis-based processes. These processes use chemicals, heat and pressure to dissolve the matrix material. Compare to thermal recycling processes, the processing temperatures for solvolysis are reported to be lower, and potentially this means that glass fibres can be recovered with their full strength; however, still glass fibre recovered by solvolysis are shown to have reduced strength. The matrix part is recovered in a liquid form, and with the potential to re-use it for matrix in new composites. Main challenge for the chemical recycling is the upscaling, i.e. to be able to process composite waste in large quantities.
All three recycling methods have their pros and cons. They are currently extensively explored in joint projects with universities and the key industrial players of wind energy.
To find the best solutions
Recycling is truly a cross-disciplinary area; this article is addressing aspects related to materials (and not all the other aspects, such as legislation, environmental impact, standards, societal acceptance etc). All aspects are however closely linked, and a holistic approach must be taken to solve the challenges, and to find the best solutions. This is maybe the overall greatest challenge for recycling of wind turbine blades; to bring knowledge and experts together, and match this with the practical requirements and constraints of industry and society.
Justine Beauson, Malcolm McGugan, and Bo Madsen, DTU Wind Energy.