Recycling permanent magnets from offshore wind turbines

Abstract

OWTs and WEEEs have both been found to contain NdFeB permanent magnets, which are both valuable sources for the recovery of REEs. At the same time, OWTs have been found to contain many electrical components that may not officially fall under the WEEE direct but share many similarities with WEEE. While there is little known about the recycling of OWTs and their electrical components due to their recent introduction, the recycling of WEEE has been studied more extensively and is a more mature industry.

To determine the preferred recycling route for permanent magnets from OWTs, a comparison was made with the characteristics and the recycling routes of permanent magnets from WEEE. The disassembly was found to be the most critical part to enable an efficient recycling process, leading to the objective of how can permanent magnets be disassembled from OWT generators. It was found that the disassembly of magnetised magnets at the scale of OWTs is challenging due to the forces required to move them and the brittleness of the materials. For efficient disassembly, the permanent magnets need to be locally and thermally demagnetised. Through a series of interviews, desk research, a lab visit and a site visit to Cuxhaven a concept for the disassembly of permanent magnets from OWTs was developed.

It was found that induction heating a copper coil wrapped around U shaped core to transfer heat to the permanent magnet via conduction is a potential scalable, automatable and time-efficient solution for demagnetisation. The power required to heat the coil depended predominantly on the time taken to heat the core as well as the contact surface area of the core with the permanent magnet, the mass of the conductor and the material chosen for the conductor. After thermal demagnetisation, the magnet can be removed while the magnetic is still hot or after cooling, dependent on the thermal expansion coefficient, effect of re-magnetisation and remanence of the magnet. Depending on the acceptable remanence to be able to handle the magnet, it may not be necessary to heat the magnet up all the way to the Curie temperature. The solution proposed in this study is just one of many options towards the recycling of permanent magnets, thus alternative solutions such as direct induction of complete thermal demagnetisation should also be considered.