High temperature superconductors for wind power
Contacts:
Asger B. Abrahamsen, Risø DTU, 46774741 (asger.abrahamsen@risoe.dk)
www.superwind.dk
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| 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. |
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. 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 and the DTU project Superwind is focussed on this goal. The suggested physics project is connected to the determination of the superconducting properties of commercial available wires as well as wires developed for the generator application within the Superwind project.
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 applying magnetic fields up to 16 Tesla and measuring the magnetization of the samples. The magnetization must be interpreted by applying a model describing the spatial density of flux lines, which are pushed in and out by ramping the applied magnetic field. Finally a measurement of the critical current must be done by sending a current through the superconducting wires and the results of the two methods must be compared and discussed.