Contacts:
Asger B. Abrahamsen, Senior Research Scientist, DTU Wind Energy, DTU Risø Campus, asab@dtu.dk, +45 24921613
Anders Christian Wulff, Researcher, DTU Energy Conversion, DTU Risø Campus,anwu@dtu.dk, +45 46775783
Nenad Mijatovic, Associate Professor, DTU Electrical Engineering, DTU Lyngby Campus, nm@elektro.dtu.dk, +45 45253525
The drive train of most 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 based on Nd2Fe14B permanent magnets. The magnetic field strength of these magnets is however limited by saturation and sets a limit of how compact the direct drive generator can be in future wind turbines. This limit can be removed by introducing superconducting coils to generate the magnetic field, but the challenge of cooling and mechanically stabilizing the superconducting coils are still being investigated (S. K. Moore, IEEE Spectrum, 26 July 2018).
The suggested fagpakke project is connected to the determination of the superconducting properties of commercial available wires as they are operated inside a vacuum cryostat resembling the environment of state-of-the-art superconducting direct drive generators.
Fig. 1: 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 x Phi_0 on the vortex lines and the critical current J_C 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 Phi_0 = h/2e, which is confined by a supercurrent circulating around a normal core, where the Cooper pairs have been broken. Figure 1 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 do 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 project is to determine the critical current density J_C of high temperature superconducting RBa2Cu3O7+x wires (R = Rare earth elements) by sending a transport current through the wire when it is installed inside a vacuum cryostat and cooled to T = 75 K using a cryocooler (see fig. 2). The design, construction and test of a wire mount suitable for testing the superconducting wire must be done and the obtained measurements of the superconducting wire must be compared to measurements using liquid nitrogen as coolant. A local suppression of the critical current using a permanent magnet might be incorporated to reproduce a local weak section of the wire. Finally, the transport measurement should be discussed in relation to the fabrication of coils for future wind turbine application.
The test cryostat used for the project is related to the MagLab of DTU Wind Energy and is operated in a collaboration between DTU Electrical engineering and DTU Energy conversion.
Fig. 2: The vacuum cryostat for testing of superconducting wires and coils is shown in the middle: The power supply(left) is used to pass up to I = 200 A into the cryostat, where a cold head at the bottom of the cryostat can cool down the test device to T = 75 K. The cold head is connected to a helium compressor shown on the right hand picture.
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.