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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Innovative concepts for drive train components' taking place on Thursday, 13 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Asger Bech Abrahamsen Technical University of Denmark, Denmark
Co-authors:
Asger Bech Abrahamsen (1) F P Niklas Magnusson (2) Dong Liu (3) Ewoud Stehouwer (4) Ben Hendriks (4) Henk Polinder (3)
(1) Delft University of Technology, Roskilde, Denmark (2) SINTEF Energy Research, Trondheim, Norway (3) Delft University of Technology, Delft, The Netherlands (4) GL Garrad Hassan, Heerenveen, The Netherlands

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Asger B. Abrahamsen received the Ph.D. degree from the Technical University of Denmark (DTU) in 2003 for neutron scattering in superconductors. He conducted neutron scattering studies of superconductors and thermoelectric materials at the DANSCATT Centre from 2003-2005. He continued with x-ray synchrotron scattering on MgB2 superconductor wires during 2005-2006 in the Materials Research Division at Risø DTU. Since 2007 he became Senior Scientist with interest in the industrial applications of superconductors. In 2012 he joined the Department of Wind Energy of DTU with focus on advanced materials such as superconductors and permanent magnets for wind turbine

Abstract

Design study of 10 MW MGB2 superconductor direct drive wind turbine generator

Introduction

Offshore wind power is demanding large turbines in order to decrease the cost of energy. One challenge of up scaling the turbines above 10 MW is to provide a drive train with a sufficient torque rating larger than 10 MNm. A superconducting direct drive generator based on field windings of MgB2 superconducting tape is proposed as a solution by mounting the generator in front of the blades using a king-pin nacelle design. This configuration is further investigated in the INNWIND.EU project for power ratings up to 20 MW [1].

Approach

An experimental demonstration of a down-scaled MgB2 coil suitable as field winding for a direct drive wind turbine is planned in the INNWIND.EU project and the corresponding 10 MW direct drive generator obtained by increasing the length of the field coils and the number to poles is discussed. The demonstration coil is composed of 10 double pancake coils, which are stacked into a race track coil with a straight section of 0.5 m and an end winding diameter of 0.3 m. The cross section of the coil is about 8 cm x 8 cm [2].
The 10 MW direct drive generator is obtained by a 32 pole design with a straight section of 3.1 meters. The superconducting field coils are supported by a non-magnetic structure and inserted in a cryostat providing thermal insulation. An air-cored armature based on copper windings is placed at ambient temperature and enclosed in steel laminates to confine the magnetic flux in the generator. Finite element modeling is used to determine the magnetic flux density imposed on the inner part of the field coils in both the straight and end sections. This is compared to the MgB2 tape engineering critical current density JE defined as the of critical current of the tape divided by the cross section area of the tape including support metals as well as the insulation used in the coil winding. The load line of the generator field coils is thereby constructed by plotting the maximum magnetic flux density of the straight and end section as the current loading of the field coils are increased overlayed with the JE of the MgB2 tape as function of the applied magnetic flux density at different temperatures. This is used to determine the operation temperature of the generator and also the specifications of the cryostat. The cryostat will be developed in order to be integrated in a generator mounted in front of the blades in a King-pin nacelle design, which will be used in the INNWIND.EU project.


Main body of abstract

Superconducting direct drive (SCDD) wind turbine generators have been proposed as a lighter and more compact alternative to the permanent magnet direct drive (PMDD) generators for large offshore turbines, where direct drive is believed to reduce the cost of energy due to a higher reliability [3]. The advantage of superconductors is the vanishing resistance when cooled below the critical temperature TC. This can be utilized in wires, which can transport current densities in the order 100 – 1000 A/mm2 without the joule heating dictated by Ohms law, which is often limiting the current density in copper to below 5 A/mm2. Thus superconductors can produce high magnetic flux densities in electrical machines, which are usually prohibited by the saturation of the steel laminates around 1 Tesla [4].
Here we present a 10 MW MgB2 superconducting direct drive generator design aiming at an air gap flux density Bg ~ 1.5 Tesla, whereby the shear stress Fd = Bg*As can be increased while keeping the same electrical loading of the armature As. The MgB2 superconductor is chosen because TC = 39 K ( -234 oC), whereby the cooling can be obtained using cryocooler machines and there is no need for liquid helium.
The torque requirement of the generator is dictated by the 10 MW INNWIND reference turbine giving 10.6 MNm at a rotation speed of 9.65 rpm. The design in based on MgB2 race track coils made by stacking of 10 double pancake coils wound from a 3 mm x 0.5 mm tape holding 19 filaments of MgB2 embedded in Ni and stabilized by a copper strip. The tape is insulated with a Kapton tape and impregnated with epoxy. The length of the straight section of the race-track coil is 3.1 m, the end section diameter 0.3 m, and the cross section is about 8 cm x 8 cm [2]. 32 coils are placed on a cylinder with a diameter of 5.2 m and enclosed in a cryostat. An air-cored copper armature with steel laminates is placed around the superconducting field windings. The armature is at ambient temperature and cooled by conventional techniques. The current loading of the armature is assumed to be As = 100 kA/m. The outer diameter of the generator is 5.9 m as outlined in the table of figure 1.
Finite element calculations are used to investigate the magnetic flux density inside a pole of the generator as the current density of the coils is increase to the target of JE,coil = 70 A/mm2. Figure 2 is showing the geometry and the flux density distribution in different cross sections of a pole. It is seen that the maximum is about 2.9 Tesla on the edge of the superconducting coil and about 1.5 Tesla in the air gap of the generator. Figure 3 is showing the load line of the coil given by the maximum flux density in the straight and end section as function of the coil and tape current density [2]. The engineering critical current density of the MgB2 tape is also plotted as function of applied field. From this it can be seen that operation at T = 10-15 K should be possible.
Figure 4 is showing the integration of the 10 MW MgB2 generator into a nacelle, which holds the potential to be up-scaled towards 20 MW. The nacelle is based on a king-pin holding two main bearings mounted on each side of the hub with the blades mounted in the middle of the hub. This ensures a direct transfer of the load from the turbine blades to the tower. A direct drive generator can be mounted in front of the turbine blades by attaching the rotating outer ring of the generator to the hub. A central question is if to mount the superconducting field coils to the rotating frame or in the stationary frame of the nacelle.


Figure 1. Properties of 10 MW MgB2 direct drive generator.


Figure 2. Magnetic flux density in cross sections of the pole when the coil current density is at JE,coil = 70 A/mm2.


Figure 3. Load line of superconducting coils in 10 MW generator.


Figure 4. Integration of 10 MW generator into king-pin nacelle.


Conclusion

From table 1 it can be seen that the total amount of MgB2 tape needed for the proposed generator is in the order of 474 km. Using the present expected cost of the tape of 4 €/m and adding the cost of the active materials one obtains a cost per capacity of 226 €/kW. This should be compared to the 20% share of the offshore turbine price 1.5 M€/MW covered by the gearbox and generator giving a threshold of 300 €/kW. Taken into account that the cost of the MgB2 tape is expected to decrease to 1 €/m with an up-scaling of production to comply with a demand imposed by the wind sector then it seems plausible that the threshold can also be fulfilled when including the cost of the structural mass and the cooling system.
It should be noted that the MgB2 tape is still under development and that an increase of the critical current density in the order of 50 % can be expected with an introduction of a nano sized boron powder in the production of the tape [5]. Such an improvement can be utilized to increase the magnetic flux density in the air gap further, to decrease the amount of tape used and thereby decrease the cost of the generator or to increase the operation temperature to T = 20 K, whereby the cooling system can be simplified.
Finally the two possibilities of either having the superconducting coils rotating with the hub or fixed in the king-pin nacelle must be investigated to determine, which option that has the largest reliability. This is part of the future work of the INNWIND.EU project.



Learning objectives
This abstract will provide the delegates with an understanding of how the critical current density of a MgB2 superconducting tape is related to the magnetic flux density obtained in a 32 pole 10 MW direct drive wind turbine generator. Additionally they will learn how the INNWIND.EU project is considering to place the superconducting direct drive generator in front of the turbine blades using the King-Pin nacelle design.


References
[1] INNWIND.EU project web-site: www.innwind.eu
[2] A.B. Abrahamsen, N. Magnusson, B. B. Jensen, D. Liu and H. Polinder, “Design of an MgB2 race track coil for a wind generator pole demonstration”, EUCAS 2014, contribution 3P-LS2-07.
[3 ] H. Polinder, J. A. Ferreira, B. B. Jensen, A. B. Abrahamsen, K. Atallah and R. A. McMahon, “Trends in Wind Turbine Generator Systems”, IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS, VOL. 1, NO. 3, SEPTEMBER 2013, p. 174.
[4] B. B. Jensen, N. Mijatovic and A.B. Abrahamsen, “Development of superconducting wind turbine generators”, J. Renewable Sustainable Energy 5, 023137 (2013).
[5] M. Rindfleisch et. al.,2013, EUCAS conference 1M-WT-I1 , & S. Brisgotti et. al., 2013, EUCAS conference 1M-WT-O1