ThermoBond: Development of a resistance welding process for the joining of thermoplastic rotor blades for wind turbines

At a glance

• The production of rotor blades currently involves the bonding together of two half shells – a method associated with a lot of manual labor, high costs, and susceptibility to errors. 
• Rotor blades made of thermoplastic resins offer the option of welding the half shells together, but the process is not yet sufficiently adapted to the requirements of the wind energy industry. 
• In the ThermoBond project, the project partners are therefore designing a novel welding process for thermoplastic rotor blades tailored for industrial use.
• Fraunhofer IWES is responsible among other tasks for concepts for the construction of the rotor blades and the production of a rotor blade segment.

The challenge

The current state of the art in rotor blade production sees two half shells bonded together. However, bonding methods are associated with a number of disadvantages. The manual application of around 1,000 kg of adhesive for a rotor blade measuring around 80 meters in length with 45–60 minutes can lead to numerous bonding defects. In addition, too much adhesive is often applied to reliably compensate for manufacturing tolerances. The consequences can be costly reworking in production, insufficient fatigue strength of the rotor blade or increased production costs. 

One approach for improving the recyclability of rotor blades involves the use of thermoplastic resin systems for new developments of rotor blades. Thermoplastic resin systems offer the option of melting and reprocessing individual components at the end of the rotor blade service life for reuse in new products. Using pyrolysis, the source materials can also be partially recovered and reused for new material.

Alternatively, thermoplastic resins can also be welded instead of joined with adhesive. Corresponding processes are already in use for the construction of aircraft and vehicle production but are not yet to be adapted to the requirements of the wind energy industry. 

The solution

In the ThermoBond project, the project partners are developing a novel welding process for thermoplastic rotor blades tested under near-industrial conditions. For this purpose, a rotor blade demonstrator measuring up to five meters in length is to be produced from a commercially available thermoplastic resin system and welded using the new process. In the process to be developed, resistance welding is adapted using carbon fiber or metal mesh. The fabrics are inserted between the joining surfaces and heated by applying a current. The heat generated in this way melts the neighboring joining surfaces, thus welding them together.

Fraunhofer IWES is responsible among other tasks for the development of concepts for the manufacturing of the joining surfaces of both rotor blade halves and the production of critical blade segments like the trailing edge, leading edge, and root of the blade. In addition, the demonstrator will be produced at Fraunhofer IWES using the powerful heating electronics available at the BladeMaker DemoCenter. 

The added value

Upon successful completion of the ThermoBond project, it will be possible to weld together rotor blades with joining surfaces in a scalable manner for the first time. The avoidance of expensive adhesives and acceleration of the process have the potential to significantly reduce production costs. At the same time, welded connections also offer advantages when the time comes to dismantle a wind turbine. For example, the components of the rotor blade can be separated again by applying current along the original welding seams, which can significantly reduce the number of cuts required when dismantling the rotor blade.