Physica C 408–410 (2004) 930–931
www.elsevier.com/locate/physc
A superconducting high-speed flywheel energy storage system
R. de Andrade Jr. a,*, A.C. Ferreira a, G.G. Sotelo a, W.I. Suemitsu a,
L.G.B. Rolim a, J.L. Silva Neto a, M.A. Neves a, V.A. dos Santos a,
G.C. da Costa a, M. Rosario a, R. Stephan a, R. Nicolsky a
a
LASUP/UFRJ, Cx.P. 68553, 21945-970 Rio de Janeiro, RJ, Brazil
Abstract
High-speed flywheel systems have been studied as compensators of voltage sags and momentary interruptions of
energy. Besides the complexity of these systems, the main concerns are bearing losses. This work is part of the
development of a superconducting high-speed flywheel energy storage prototype. In order to minimize the bearing
losses, this system uses a superconducting axial thrust magnetic bearing in a vacuum chamber, which guarantees low
friction losses, and a switched reluctance motor-generator to drive the flywheel system. Dynamic simulations made for
this prototype, connected to the electric power network, show the viability of use it as a compensator.
2004 Elsevier B.V. All rights reserved.
PACS: 85.25.Ly; 85.25.Am
Keywords: Flywheel; Superconducting magnetic bearing; YBCO
1. Introduction
High-speed flywheel systems, in comparison with
conventional batteries, present some interesting characteristics as energy source for compensators of voltage
sags and momentary power interruptions [1,2]. Flywheels have been used from ancient times to store energy used to equalize the mechanical energy demand.
Flywheel energy storage systems (FESS), coupled to an
electrical motor-generator, also have been used to
equalize the electrical power demand. These systems
draw energy, smoothly, from the electrical system, store
and return it at the demand peak. At the moment, most
systems use heavy flywheels that operate at low speeds
with a low energy density. The energy stored in a flywheel is proportional to the moment of inertia and to
the square of angular velocity. Then increasing the flywheel angular velocity may increase the energy density.
*
Corresponding author. Address: DEE/EE UFRJ, Ilha do
Fund~
ao, Cx.P. 68515, 21945-970 Rio de Janeiro, RJ, Brazil.
Tel.: +55-21-2562-8031; fax: +55-21-2562-8017.
E-mail address: [email protected] (R. de Andrade Jr.).
The main concerns about increasing the angular velocity
are the viscous air drag, flywheel strength, due the high
rim speed, and the increasing bearing loss. The viscous
air drag can be avoided placing the flywheel and the
motor-generator inside a vacuum chamber. The recent
development of carbon fiber composite flywheel allows
very high rim speed (so high as 1 km/s) [1]. There are
several types of bearings that can minimize the friction
losses. The most used are: active magnetic bearings
(AMB); passive magnetic bearings (PMB) stabilized
with special mechanical bearings and superconducting
magnetic bearings (SMB). The SMB needs to be cooled
and the energy used to do this has to be computed as an
energy loss. However, there is very low energy dissipation in superconductors and with a good cryogenic
project they can be cooled with small power cryocoolers.
This work is part of the development of a superconducting high-speed flywheel energy storage prototype,
Fig. 1. In order to minimize the bearing losses, this system uses a SMB as the axial thrust one; a PMB is used as
the radial bearing and an AMB will be used mainly to
overcome the instabilities at the critical flywheel speeds.
The system was placed in a vacuum chamber, preventing
viscous air drag. A switched reluctance machine (SRM)
0921-4534/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2004.03.168
R. de Andrade Jr. et al. / Physica C 408–410 (2004) 930–931
Fig. 1. Scheme of the proposed flywheel energy storage system.
was chosen to make the energy conversion due its
robustness and null idle losses [3].
2. Superconducting magnetic bearing
The superconducting magnetic bearing was prepared
with Nd–Fe–B magnets and YBa2 Cu3 O7 d superconducting blocks. The magnets are assembled with magnetic flux shapers, in order to increase the levitation
force and the stiffness [4]. In spite of the azimuthal
symmetry of the bearing, a set of brick shaped magnets,
20.0 · 14.0 · 2.8 mm3 , arranged in ring shaped magnetic
flux concentrators was used in order to minimize costs.
The field mapping, made using a homemade device,
showed that the resulting magnetic field does exhibit
azimuthal continuity.
The levitation force measurements show that it has a
levitation force of 150 N with an air gap of 6 mm. In
order to analyze the stability of the SMB, dynamic
measurements were made following the same procedure
given in Ref. [4]. These measurements show a low
damping (0.01) for radial and axial directions in all
conditions analyzed. Like another SMB previously
analyzed [4] the stiffness, axial and radial, increases
strongly when the superconductors are cooled in the
magnetic field of the bearing, reaching 74.5 N/mm for a
cooling distance of 3 mm, that results in a gap of 1.9
mm. These dynamic measurements will be used in the
mechanical simulations that will specify the AMB.
931
Fig. 2. Result of the power electronics simulation showing the
system response to a voltage sag.
flywheel. Fig. 2 shows a simulation example of compensation considering a voltage sag in the input voltage;
the output voltage remains constant in spite of the
voltage sag in the input voltage. It is shown also in Fig. 2
the variation of flywheel speed due to the energy drained
for the compensation system.
The SRM parameters were obtained from static finite
element method (FEM) simulations. FEM simulations
were validated by comparison with previous experimental results taken from a similar machine.
4. Summary
The development of FESS with SMB and SRM was
described. The SMB levitation force and stiffness were
measured and will be used in the mechanical simulations
that will evaluate the need of an AMB. The simulation
of the preliminary power electronics projected to control
the FESS shows that this system will be able to compensate voltage sags.
Acknowledgements
The authors acknowledge the financial support of the
Brazilian agencies CNPq and FINEP and to WEG
Motors.
References
3. Dynamic simulations
The complete system for voltage compensation was
simulated using a commercial program, EMTDC/
PSCAD. The system consists of series-connected power
network coupling converter, SRM driver, SRM and
[1] R. Hebner et al., IEEE Spectrum (April) (2002) 46–50.
[2] S. Nagaya et al., IEEE Trans. Appl. Supercond. 11 (2001)
1649–1652.
[3] L.G.B. Rolim et al., in: Proc. of ICEM 2002, Brugge,
Belgium, 2002.
[4] R.M. Stephan et al., Physica C 386 (2003) 490–494.