Authors: Tim Schumacher, Lisa Bösch, Matthias Lindermann
The longer the rotor blade, the greater the amount of glue needed to bond the two blade shells together. Currently, rotor blade manufacturers need about 1,000 kilograms of adhesive for a blade around 80 meters in length, and this is cost intensive. Scientists at Fraunhofer IWES have therefore combined the experiences they have gained during a decade of research into rotor blade manufacture and developed a solution which simplifies and improves the adhesive application process: the Variable Glue Applicator (VGA). The VGA can apply the right amount of adhesive precisely where it is needed, thus enabling saving potentials of 10 to 20 percent per rotor blade. This corresponds to around 200 kilograms of adhesive in the case of an 80-meter rotor blade, which leads to immense cost savings for rotor blade manufacturers. Another advantage is that the VGA allows the glue to be applied in a continuous process, leading to a better bond line quality and hence longer rotor blade lifetime. The further automation of the glue application process shortens the production time, which is very important as blades continue to increase in length, but also offers a high and reproducible application quality.
Current state-of-the-art of rotor blade bonding
The process of applying the bonding pastes represents a bottleneck in the open mold cycle time during 24/7 serial production of rotor blades for wind turbines. The structural bonding paste for joining half shells and webs is currently applied by means of a set of glue shoes or a contour template which defines the cross-sectional bead profile geometry along the blade (Fig. 1). Therefore, these glue shoes need to be changed several times along the blade at so-called profile transition spots. These changes interrupt both the application process and the dosing system. Consequently, these profile transition spots are sites where void generation during the curing of adhesive joints or mixing problems as the dosing system is re-started are prone to occur. Voids in adhesive joints lead to stress concentrations which can cause tunneling cracks that may propagate into the blade structure during operation (Rosemeier et al., 2019). Interrupting the operation of the dosing machine can affect the mixing ratio between adhesive resin and curing agent, which can in turn affect the strength of the cured adhesive material. Moreover, the standard manual application procedure with glue shoes results in the amount of bonding paste being applied varying from blade to blade. This human factor can lead to the mass varying significantly from blade to blade.
Advantages of the VGA
The VGA was developed to overcome the above-mentioned drawbacks. It semi-automatically assists the application process for the bonding paste by continuously adjusting the requisite cross-sectional profile geometry of the bond line. This obviates the need to interrupt the process. Thus, using the VGA eliminates the presence of voids at profile transition spots and incorrect mixing ratios between adhesive resin and hardener. Benchmark studies with the VGA have challenged the traditional application process and revealed that the VGA shortens the open mold cycle time and reduces the mass of adhesive material applied and the degree of variability by removing the human factor from the equation.
The VGA can reduce capital expenditure per blade by reducing both open mold cycle times and material mass. The improved quality of the adhesive joints means the VGA can increase the number of cycles toward tunneling crack initiation in the field, which means fewer repairs and less downtime, thereby reducing operational expenditure. Furthermore, it incorporates special features like a highly elastic silicone inliner, the ability to adapt to different geometries, quick profile changes, easy handling, and minimal cleaning effort. With these characteristics, the VGA can apply the glue for the rotor blades in different geometries, e.g., triangular or trapezoidal, without the need for tool changes. This saves time and therefore money, because the conventional technology requires different tool parts to be attached for each tool change, which also increases the risk of air bubbles in the adhesive. Overall, the VGA can reduce the levelized cost (LCOE) of wind energy.
Approved workflow and further steps: Development of the next prototype
The IWES scientists tested the entire workflow within several research projects such as MultiMonitor, ReliaBlade, or RadkomQS in Lemwerder, in which five 30-meter research rotor blades were produced. More specifically, the shear web bonding was tested, as was the bonding of the leading and the trailing edges. Tests of 70-meter plus and 80-meter plus blades in serial production were also conducted successfully.
The researchers integrated the VGA into the automation testing environment at the BladeMaker DemoCenter in Bremerhaven. In this facility, the VGA can be connected to the 6-axis gantry portal, and the same program controls the motions and actions of the VGA and the dosing equipment. They also plan to add surface scanners to be able to react to deviations in production and to document the glue application. Further work will investigate the feasibility of the assisted application of the glue with the VGA and collaborating robots (Cobots).
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