When wind energy blades reach the end of their lifetime they cannot simply be decommissioned and taken to the nearest landfill. Most are as large as the wings of an airplane and need to be cut into several pieces small enough to be loaded onto a truck. Once deposited at the landfill, they remain there because the blades, as they are currently constructed, cannot be recycled. This is why the number of spent turbine blades is expected to grow exponentially over the next few years. Each megawatt of installed capacity equates to about 9 metric tons of composite waste. This may create massive environmental and economical problems in the future.
Now (2020), researchers at NREL have designed wind turbine blades using a thermoplastic resin system developed by Arkema, Inc. Unlike thermoset resins, which cannot be reheated, thermoplastic materials can potentially be recycled at the end of the wind turbine's lifespan. Because they can be thermally welded, thermoplastic resins eliminate the need for additional adhesive materials, expediting blade manufacturing and eliminating highly stressed bond lines.
A commercial blade is generally constructed from epoxy or other thermoset resins. These types of materials are both difficult and expensive to recycle. Because of this, spent thermoset blades usually end up in a landfill after decommissioning. Thermoplastics represent an interesting alternative to the thermoset matrices. Thermoplastic resins soften when heated and harden again when cooled. In general, they are more ductile and tougher than thermoset resins and are used for a wide variety of non-structural applications without fillers and reinforcements. Thermoplastics can be melted by heating and solidified by cooling which makes it possible to repeatedly reshape them. Thermoplastic molecules do not crosslink and therefore they are flexible and re-formable.
Using thermoplastic resins in wind turbine manufacturing has been at the centre of scientific interest for almost a decade. In 2016, scientists developed a process for producing a thermoplastic-fibre composite during which a thermoplastic resin was first heated to a liquid state; then the fibres were directed unidirectionally and impregnated with the thermoplastic resin in a liquid state to create composite laminae. Finally, through an automated machine lay-up process, a composite laminate was produced comprising a plurality of the composite laminae.
In 2018, scientists invented a new manufacturing method which enabled turbine blades to be produced in smaller segments that could be joined after transportation to the field. The process employed vacuum-assisted thermoforming of thermoplastic composites which could be recycled after use. Six turbine blades were created from two separate segments composed of two elements each. They were combined using fusion welding and adhesives. A set of three blades was tested at a small-scale wind farm, producing power outputs of 20 W at low wind speeds, comparable to an existing commercial turbine.
In 2019, scientists investigated fusion joining wind turbine blades, produced using thermoplastic resin. Their research showed that, compared to typical adhesives used in wind turbine blades, fusion welding led to an increase in both the static and fatigue lap-shear strength as compared to bonded thermoplastic composite coupons. This initial coupon-scale research suggested that there was potential for developing fusion welding techniques for full-scale wind turbine blades.
The advantages of the new blade material are numerous: thermoplastic resins enable wind turbine blades to be recycled at their end of life and also reduce the time, cost, and energy involved in manufacturing. Thermoplastic blade production reduces labour costs due to its faster curing cycle compared to the thermoset resin/epoxy. Also, because thermoplastics do not require heat to cure, equipment and labour costs are lower than they would be for the business-as-usual epoxy blade in which heated tooling or a post-curing oven would be required. Moreover, thermoplastic blades can be thermally joined, instead of relying upon adhesive bonds. Blades can potentially be made stronger and lighter, and they can also be more easily repaired compared to the business-as-usual blade. The thermoplastic resin system enables on-site manufacture of blades so that larger blades can be built with much lower transportation costs. These blades also perform well when saturated with seawater, opening the door for countless marine energy applications.
Thermoplastic blades have yet to be validated at a full scale and in realistic operating conditions. NREL researchers are working hard to bring these innovative, recyclable blades and processes to market.