Analysis of a small recyclable wind turbine blade

The wind energy industry is constantly seeking lower levelized cost of energy (LCOE) and also to improve its environmental and social sustainability. Thermoplastic resins are considered to be key to recycling wind turbine blades in the best possible way to reuse the main raw materials at the end of their life. In this work, a new thermoplastic liquid resin, AKELITE, patented by CSIC, is used to produce a new generation of 100% circular, sustainable wind turbine blades. The aim is to recover the two main components, the fibres and the resin, in optimal conditions to produce a new wind blade. First, the recycling protocol of composite laminates was optimised and the properties of the recycled composite were analysed. Interestingly, the first recycling process resulted in an improvement in the mechanical properties compared to the original laminate, which is ascribed to the presence of a thin layer of resin on the fibre surface that improves the adhesion between the layers. Thus, it is expected that the new blade will have a good performance, using almost 100% of recyclable materials. A 1.9 m recycled wind turbine blade is currently being manufactured to validate the results of this study. The successful development of such blades could have a significant impact on the sustainability of wind energy, providing a path towards circularity and reducing the environmental impact of wind turbine blades at the end of their life cycle.


Introduction
Currently, between 85% and 90% of the total mass of a wind turbine can be recycled.Wind turbine blades are the most difficult to recycle due to the composite materials used in their manufacture.Recycling solutions are not yet widely available and cost-competitive [1].
Some solutions are focused on achieving blade circularity.Siemens Gamesa installed in 2022 the world's first turbine equipped with recyclable blades (81 m) in RWE's Kaskasi offshore wind farm [2].LM Wind Power has manufactured a 62 m blade using Arkema's thermoplastic resin and highperformance glass fabrics from Owens Corning in the ZEBRA project [3].In 2023, MingYang Smart Energy has launched a 75.7 m turbine blade with over 95% recyclable materials, claiming to be the first Chinese OEM to offer recyclable blades [4].
In this work, a new infusible thermoplastic resin (AKELITE [5]) is processed by in mould polymerisation.A previous study of a 1 m blade made with Akelite showed that the static and centrifugal 1293 (2023) 012008 IOP Publishing doi:10.1088/1757-899X/1293/1/012008 2 performance were similar to those of a commercial epoxy [6].The study also showed the recycling capabilities of the resin through immersion in a suitable solvent.
This paper presents the optimization of the recycling protocol, analysing some parameters such as solvent/composite ratio, dissolution time and dissolution temperature, and the properties of the recycled material.Thus, samples of 75 x 20 cm composite materials have been made, tested, recycled and later re-manufactured in order to test the properties of the second generation composites.These steps are being repeated on a 1.9 m blade, in order to study the scalability for small wind turbine blades.

Materials and Methods
Two resins have been used to produce the composites: a thermoplastic resin, Akelite, patented by the CSIC, and a commercial epoxy resin from Sicomin [6].Akelite is a reactive acrylic resin that undergoes polymerisation through the addition of benzoyl peroxide as an initiator at a temperature of 60 ºC for 2 hours.Its density is 1.18 g/cm 3 .The epoxy resin is a two-component system based in an epoxy SR1280 and a fast hardener, SD4772 mixed in a specified ratio of 100:27 parts by weight.It is a very low viscosity system, especially suited for infusion processes.According to the supplier's specifications, the epoxy resin is cured at room temperature for 24 horas, and at 60 °C for 16 hours.Its density is 0.97 g/cm 3 .The used fibres were biaxial ±45º carbon fibre fabric 50 K at several areal weights supplied by Mel Composites and an unidirectional glass fibre with an areal weight of 625 g/m 2 supplied by Gavazzi Tessuti Tecnici.

Manufacturing of composites
Laminates of 75x20 cm were produced using three layers of carbon fibre fabric, one of 200 and two of 600 g/m 2 , by vacuum assisted resin infusion (VARI).The fibre content in the laminate is approximately 70 wt.%.The laminate is cut into three pieces of 25x20 cm.The first piece is used to analyse the mechanical properties of the material under bending and inter laminar shear strength (ILSS).The second piece is subjected to a recycling process and a new laminate is made from the recovered fibres and resin.The third piece is subjected to two recycling processes and its mechanical properties are analysed.

Testing of composites
The mechanical properties of all three composites, original, once and twice recycled, were measured in the longitudinal direction of the fibres using a three-point bending test according to ASTM D790-03 and ASTM D2344, respectively.Specimen dimensions were 50.8 mm long and 12.5 mm wide for flexural tests and 15 mm long and 7.5 mm wide for ILSS tests.An Instron 2204 machine was used for both tests, with a load cell of 1 kN for flexural tests and 50 kN for ILSS tests, at a rate of 1 mm/min.A minimum of seven specimens were tested for each composite.

Recycling of composites
The laminates were recycled by immersion in a solvent for 18 hours at room temperature without mechanical agitation.Once the fibre was recovered, the resin was extracted from the solvent using a rotary evaporator at 50 ºC.

Manufacturing of blades
Two 1.9 m blades were manufactured by VARI with a lay-up fibre configuration of satin/45C/0G/sandwich/0G/45C/0G (Figures 1-3).After the vacuum infusion process of the two valves, they were filled with Sicomin PB170+DM03 low density epoxy foam and glued with Sicomin Isobond 735 two-component epoxy adhesive. 3

Testing of blades
The wind turbine blades were analysed through static flap-bending test (Figure 4).Static loads were estimated according to the simplified load model of IEC 61400-2 Standard (Small wind turbines).The blade root was secured to a rigid steel test rig using a simple clamp bolted connection.The blade was mounted horizontally (flap-wise), with two winches used to apply the load.

Recycling of blade
A heating bath circulator has been designed that allows for direct temperature control of a bath tank and indirect heating of the blade immersion solvent, in this case acetone.To achieve this, the blade has been cut into two pieces to immerse them in the bath and reduce the required volume of acetone (as shown in Figure 5).The circulator heats the water in the bath tank, and the heat is transferred to the acetone, allowing it to reach the desired temperature.

Results and Discussion
A straightforward and scalable method was used to recycle the laminates.The recycling process involved immersing the laminates in four different solvents -acetone, chloroform, ethyl acetate, and tetrahydrofurane -for a duration of 18 hours at room temperature.A first trial was done with small laminate specimens to select the appropriate solvent.All solvents dissolved the polymer and the laminates were easily separated into the carbon fibres plies by gently pulling by hand.Among the solvents tested, acetone was selected considering the price, toxicity, and environmental impact to continue the study.
A resin:solvent ratio of 1:20 was found to be optimal.After separation, the plies were left to dry at room temperature.The resin is then extracted from the solvent using a rotary evaporator.This allows a complete recovery of the resin and solvent, resulting in a 100% circular process.Approximately, a 95% of the resin is recovered and 2-3% remains on the fibre surface.The recycled fibre and resin are reused to produce a new laminate for testing (Figure 6).This recycling process was repeated twice to analyse the mechanical properties of the recycled composites (Table 1), which were compared to those obtained with the epoxy resin.
Interestingly, the properties of the laminates with the thermoplastic resin are superior to those with epoxy resin, demonstrating the feasibility of this resin for the manufacture of structural materials.The laminate properties after the first recycling (Akelite 1R) tend to improve compared to the original laminate, probably due to the presence of a thin layer of resin on the fibre surface, which acts as a sizing agent and improves the adhesion between the layers.The mechanical behavior slightly deteriorated with the second recycling (Akelite 2R), although the properties were similar or even better than those of the virgin material.Once the recycling conditions for the laminates were successfully optimised in the laboratory, the current objective is to recycle the 1.9-meter blades and recover the fibres.Thus, 1.9 m blades were manufactured with both the epoxy and Akelite resins and characterised them.Table 2 shows the summary of property tests done to both blades, such as mass, centre of gravity and natural frequencies.For static tests, loads were applied at 0.6 m and 1.4 m from root blade and measured with two load cells.Deformation was measured with a strain gauge glued at 1.0 m from root blade, and tip deflection with a string potentiometer located at blade tip.Tables 3 and 4 show the results of the static test carried out to the epoxy and Akelite blades, respectively.The epoxy resin blade has a tip deflection of 166 mm while the Akelite resin blade has a 10% higher tip deflection of 183 mm.To avoid tower strike, IEC 61400-2 Standard requires that the tip displacement should be less than the clearance provided between the blade tip and the tower, even at worst case loading.In our case, this distance is 303mm, higher than 183 mm tip deflection.It is currently carrying out the recycling of Akelite wind blade through a similar immersion protocol.The next step involves separating the resin and acetone using a rotary evaporator.This would enable easy circularity of raw materials, fibers, and resin, making it possible to remanufacture a new blade.

Conclusions
It is presented the potential recycling protocol for a wind blade made from a thermoplastic fibre reinforced polymer.Recycling of composite samples revealed materials with improved properties compared to the original thermoplastic composite, which was attributed to the presence of a small percentage of polymer residue.Such residue is likely to act as a sizing agent, improving the interface between the fibre and the matrix.The static test showed small differences in the blade characteristics.In general, variations could be expected from manual blade manufacture.
Again, the Akelite static test was similar to the epoxy static test.This means that thermoplastic resin could be a substitute for epoxy resin for use in the wind energy industry, contributing to the recycling of raw materials at the end of their life.More tests need to be done to scale up the goals and results.

Figure 1 .
Figure 1.Laying glass and carbon fibres on the moulds.

Figure 6 .
Figure 6.100% circular process and recovery of fibre reinforced thermoplastics

Figure 7 .
Figure 7. Extraction of resin from acetone using a rotary evaporator

Table 1 .
Mechanical properties of the laminates after recycling process

Table 2 .
Results of property tests

Table 3 .
Static test of the epoxy resin blade

Table 4 .
Static test of Akelite resin blade