Contacts:
Asger B. Abrahamsen, Risø DTU, 46774741 (asger.abrahamsen@risoe.dk)
www.superwind.dk
The drive train of present wind turbines consists of a gearbox and a generator converting the kinetic energy of the rotor into electricity. A large mechanical load is acting on the drive train and the resulting wear of the gearbox is critical in the scaling up of future off-shore turbines to 10-20 MW (www.innwind.eu) A simplification of the drive train can be obtained by omitting the gearbox and mounting the rotor directly onto a slow rotating multi-pole generator, which tend to be heavy and bulky. This problem can however be solved by introducing superconducting coils to produce the magnetic field inside the generator. The suggested fagpakke project is connected to the determination of the superconducting properties of commercial available wires as well as innovative wires developed by a DTU-spinout company from DTU Energy Conversion.
Scanning electron microscope image of a superconductor, where Fe nano-particles evaporated onto the surface reveals the vortex lattice. A transport current density J result in a force f = J ´ F0 on the vortex lines and the critical current Jc is reached when the vortex lines start to move.
Superconductivity is caused by the pairing of conduction electrons into Cooper pairs, which condense into a common ground state and can conduct an electric current without resistance. An applied magnetic field will tend to create a rotational flow of the condensate either at the sample edge or inside the superconductor in the form of a vortex flow line. Each vortex line holds a quantized amount of magnetic flux F0= h/2e, which is confined by a supercurrent circulating around a normal core, where the Cooper pairs have been broken. The figure above shows a scanning electron microscope image of a superconductor, which was holding many flux lines when nano-particles of Iron were evaporated onto the surface of the superconductor. The Fe particles are concentrated at the position of the vortex lines and the image reveals an ordered vortex lattice. The presence of the vortex lines becomes a problem when a transport current J is passed through a superconductor, because a force f will be acting perpendicular to the current and magnetic field direction of the vortex line. Thus work is done if the vortex line moves, and the superconductor does not have a vanishing resistivity anymore. In all practical superconductors the vortex movement is prevented by incorporating nanometer sized defects and impurities acting as pinning sites.
The objective of this physics project is to determine the critical current density JC of high temperature superconducting wires by sending a transport current through the wire at different applied magnetic fields and cooled in liquid nitrogen boiling at T = 77 K. An experimental setup for IV measurements is already at the DTU Risø campus, but a new sample mount must be designed and constructed. Additional measurements of the magnetization curves of the sample must be performed and the critical current should be determined from suitable models of the vortex pinning. Finally the transport and magnetization measurement should be compared and discussed in relation to the wind power application.
NB ! It should be noted that DTU will cover public transport expenses for students commuting to the DTU Risø Campus as part of the project work.
Related projects: INNWIND.EU (www.innwind.eu): FP7 project targeted at the challenges of building a 10-20 MW offshore wind turbine. Coordinated by DTU Wind Energy.