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Experiment and analysis for a small-sized flywheel energy storage system with a high-
temperature superconductor bearing
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INSTITUTE OF PHYSICS PUBLISHING SUPERCONDUCTOR SCIENCE AND TECHNOLOGY
Supercond. Sci. Technol. 19 (2006) 217222 doi:10.1088/0953-2048/19/2/011
Experiment and analysis for a small-sized
flywheel energy storage system with ahigh-temperature superconductor bearing
Bongsu Kim1, Junseok Ko, Sangkwon Jeong and Seung S Lee
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology,
373-1, Guseong-Dong, Yuseong-Gu, Daejon 305-701, Republic of Korea
E-mail: [email protected]
Received 16 August 2005, in final form 16 November 2005Published 10 January 2006Online at stacks.iop.org/SUST/19/217
AbstractThis paper presents a small-sized flywheel energy storage system that uses ahigh-temperature superconductor (HTS) bearing characterized by anon-contacting bearing with no active control. The small-sized flywheel ismade up several magnets for a motor/generator as well as an HTS bearing,and they are fitted into a 34 mm diameter, 3 mm thick aluminium disc. Forsimplicity and miniaturization of the whole system, the small-sized flywheeltakes torque directly from a planar stator, which consists of an axial flux-typebrushless DC motor/generator.
The small-sized flywheel successfully rotated up to 38 000 rpm in a
vacuum while levitated above the stator with a gap of about 1 mm. However,there are some eddy current losses in the stator and non-axisymmetry in themagnetic field causing large drag torque. In order to solve these problems, animproved magnet array in the flywheel, including magnetic screening, isproposed and 3D electromagnetic simulations have been conducted.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
There has been much effort to apply high-temperature
superconductors (HTS) to practical uses since their discovery.
One of the HTS applications is the HTS bearing, which is a
non-contacting bearing with extremely low frictional losses
requiring no active control [1]. A flywheel energy storage
system can be operated with very high efficiency when it
employs the HTS bearing, and many studies of HTS flywheel
energy storage systems have been conducted. Most of the
previous studies for this energy system have focused on large-
capacity energy storage [25], and some of them considered
special applications, such as an integrated energy storage
and attitude control system for a spacecraft [6, 7]. The set
of several flywheels stores excess energy generated by solar
panels during periods of exposure to the sun and is used to
provide power for the spacecraft subsystems during eclipse.
At the same time speed control of the flywheels, without
1 Author to whom any correspondence should be addressed.
influencing energy storage, is used to rotate the spacecraft forattitude control. Many studies of integrated energy storage
and attitude control have been conducted and they show that
simultaneous torquing of the flywheels for two functions is
possible [8]. The flywheel energy storage system also offersadvantages such as high energy density and a great number of
charge/dischargecycles [9].The present study concentrates on the development of a
small-sized flywheel energy storage system that uses an HTS
bearing. An attitude control function may be integrated into
the flywheel system by adding a speed controller after the
energy storage function; this leads to the development of aflywheel using an HTS bearing which rotates smoothly with a
low loss. Our small-sized HTS flywheel energy storage system
has different configurations from a large system in terms of the
motor/generator as well as the size of the flywheel. From the
viewpoint of compactness of the system, the motor/generator
should be designed such that the axial flux-type planar statoris located under the flywheel, as this configuration occupies
minimal volume. The operating frequency should also be
0953-2048/06/020217+06$30.00 2006 IOP Publishing Ltd Printed in the UK 217
http://dx.doi.org/10.1088/0953-2048/19/2/011mailto:[email protected]://stacks.iop.org/SUST/19/217http://stacks.iop.org/SUST/19/217mailto:[email protected]://dx.doi.org/10.1088/0953-2048/19/2/0118/8/2019 sust6_2_011
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B Kim et al
Motor magnet
Flywheel
S-pole
N-pole
Flywheel
Bearing magnet
Planar stator
Planar stator
HTS bearing
HTSCold head
Motor/generator
Figure 1. Schematic view of the micromachined flywheel energystorage system using an HTS bearing.
increased to allow greater energy storage, since the momentof inertia of the flywheel decreases according to the square of
its radius. Fabrication of a planar stator with a smaller volume
is not compatible with the conventional windingmethod. From
this point of view, the approach to a small-sized system should
be somewhat different from that for a large system.
This paper introduces the development of a small-sized
HTS flywheel energy storage system and the results of its
operation. The idling characteristics of the flywheel are very
poor for several reasons, including non-axisymmetry of the
magnetic field. Causes and solutions for this are considered
at the end of the paper.
2. Principle and system design
A schematic view of the small-sized flywheel energy storage
system with a HTS bearing is shown in figure 1. It consists of
a disc-shaped flywheel, an HTS bearing and a planar stator.
The cold head under the HTS is cooled by liquid nitrogen
or a cryocooler. Disc and ring type magnets for the HTS
bearing are embedded in the centre of the flywheel. The
flywheel also has eight magnets for a motor/generator around
its circumference. During levitation the flywheel receives
torque from the three-phase brushless DC (BLDC) type planar
stator. Three Hall effect sensors detecting the position of the
flywheel give signals to a motor driver in order to control the
currents of the stator coils. The planar motor structure has
some merits, including not only system compactness but also
the fact that the charge/discharge power rate can be regulated
by controlling the gap between the flywheel and the stator. For
example, the extraction power rate is lower during discharge
when the gap becomes wider. The reason for this is that the
torque constant is reduced by the lower magnetic flux density.
Figure 2(a) shows the construction of the single layer of
the axial flux BLDC stator (planar stator) which has three
pairs of layers for three phases. The layers are aligned with
each other by an electrical angle of 120. A single layer has
several turns of a zigzag winding, in which the parts in theradial direction generate a Lorentz force in the circumferential
direction (figure 2(b)). The torque on the single layer can be
O
Ri
Ro
O
r
Ri
Ro
T
B
FI
dr
O
(a) (b)
Figure 2. (a) A conductor path in the stator. (b) Scheme of torquegeneration.
expressed as follows [10]:
T = Nph Ro
Ri
r (I B) dr (1)
where B, I, and Nph are the magnetic flux density, the current
in the conductor and the number of the lines along radial
direction in the single layer (number of turns number of
poles), respectively. Ri and Ro are the inner and outer radii
of the stator. Assuming that the stator carries a continuous
current Irms, which is the RMS value of the actual currents,
and the rotor magnets have a mean magnetic flux density Bzm ,
which is the component in the axial direction, then the ideal
torque is simply expressed as follows:
T = 12NphBzm (R2o R
2i )Irms = KT Irms (2)
where KT is the torque constant. The back EMF is also derivedas
Vemf=12NphBzm (R
2o R
2i )F = KEF (3)
where KE and F are the back EMF constant and the rotational
speed of the flywheel, respectively. The starting torque, the no-
load speed and the torquespeed relationship of the axial flux
BLDC motor are almost same as those of the general radial
flux BLDC motor because torque and backEMF characteristics
are identical to the radial one as shown by (2) and (3). The
flywheel energy system needs torque only to accelerate the
flywheel and should have a rotational speed that is as fast
as possible. Nph, V and the number of layers could mainly
determine the highest speed of the flywheel. When the stator
has more turns or layers it has larger KT, so that the startingtorque increases and the no-load speed decreases. Both the
starting torque and the no-load speed are certain to increase
immediately once the electromotive force, V, increases.
In addition, a cryocooler, a vacuum chamber and a
positioner are needed. The HTS can be cooled below its critical
temperature by liquid nitrogen or a cryogenic refrigerator.
In this work, liquid nitrogen is used in the first operation
test, and then a vacuum chamber and a Stirling cryogenic
refrigerator are subsequently employed to examine the high-
speed operation and idling characteristics. The positioner
seizes the flywheel and releases it after the HTS is in
superconducting mode.
The stator and the HTS bearing mainly determine theoverall efficiency of the system. Inhomogeneity and non-
axisymmetry in the magnetic field cause hysteresis losses in
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Small-sized flywheel energy storage system with HTS bearing
(a) Attaching copper plate
on polyimide film
(d) Etching copper plate
(b) Spin coating with photoresist(e) Removing photoresist
(c) Patterning photoresist
photoresistcopper platepolyimide film
(f) Wiring and stacking multilayer
Figure 3. Fabrication process of a planar stator.
the HTS. Hence uniform magnetization and homogeneousmagnetic materials are important in realmagnets. In the case of
axisymmetry, as the flywheel designed in this system integrates
the motor magnet, an inappropriate array causes unexpected
distortion of the magnetic field [11, 12].
Eddy current losses are generated on the stator and other
conductive structures when variation of the magnetic field is
sensed. Variations of the magnetic field are inevitable because
the flywheel has alternative magnets and rotates. Therefore, to
reduce the area perceiving magnetic variations a perpendicular
orientation is the optimal way to minimize eddy current
losses [13].
3. Fabrication
The conductor path on the stator should have a narrow cross-
sectionand thenumberof turns per layerwith as dense an array
as possible in order to minimize eddy current losses. However,
this work puts priority on ease of the fabrication process and
we have made a relatively wide path of 600 m with three
turns per layer. The fabrication process is shown in figure 3.
A 50 m thick copper plate is attached to polyimide film and
then coated with photoresist. The photoresist is patterned with
the shape of the conductor path using photolithography. Ferric
chloride is then used to etch the copper plate. After removing
the photoresist, each layer is soldered and connected. Three
phases that have three layers in series each are each linked in a
Y-connection.The flywheel is made up of eight circumferential NdFeB
magnets of alternate polarity for the rotors poles and two
central concentric NdFeB magnets of opposite polarity for the
HTS bearing, which are fitted into a 34 mm-diameter, 3 mm-
thick aluminium disc. The magnetic flux density on the surface
of the magnets was measured by Gauss meter and its mean
value was 2.5 kG. The flywheels mass and moment of inertia
are 12.75 g and 1.84 106 kg m2, respectively. Figure 4
shows the fabricated flywheel and stator. Their specifications
are presented in table 1. The HTS is melt-textured YBCO.
4. Experimental details
Previously, we conducted preliminary operating tests with
a relatively rough stator and liquid nitrogen cooling under
Table 1. Specifications of the flywheel, the stator, and the HTS.
Flywheel Diameter 34 mmThickness 3 mmMass 12.75 g
Moment of inertia 1.84 106 kg m2
No. of poles 8Stator Thickness of the conductor 50 m
Width of the conductor 0.6 mmNo. of turns 3Phase type Three-phaseConnection type Y-connectionNo. of layers 9
HTS Diameter 10 mmThickness 1.8 mmMaterial Melt-textured YBCO
NdFeBMagnet
(a) (b) (c)
10mm Hall effectsensor
Figure 4. Fabricated parts: (a) a flywheel with NdFeB magnets;(b) a single-phase stator; (c) nine-layer three-phase stator with Halleffect sensors.
atmospheric conditions. This stator, which consisted of six
layers with two layers per phase, was a 1 mm wide, 50 m
thick copper conductor path with two turns per layer.From the results of this test we confirmed that the flywheel
levitating above the cooled HTS successfully rotated when
the motor driver circuit was turned on (figure 5). The spin-
down time was very short, within several tens of seconds,
and this short spin-down time appeared to be mainly due to
air drag. Figure 6 shows the rotational speed according to
the input current to the circuit at 12 V. The input power also
contains the power consumption of the driver circuit [14]. It
was difficult to measure the forcedisplacement relationship of
the HTS bearing in this system because the flywheel including
the HTS bearing was small and the levitation force was
feeble. So we directly watched the whirling resonantfrequency
(n), which was about 209 rad s1 (2000 rpm). From this
resonant frequency, we estimated the radial stiffness as about
560 N m1 (n =
kradial/mflywheel) and the axial stiffness as
about 1.12 kN m1 (kaxial = 2kradial [15]).
In order to confirm the energy storage functions of the
small-sized flywheel system, a new experimental arrangement,
which is illustrated in figure 7, was prepared. A cylinder
vacuum chamber made with acrylic plates and a cryogenic
refrigerator were added. The vacuum is less than 1104 Torr
when the cryocooler is cold. The temperature of the cold-
head of the refrigerator is 65.7 K and the temperature of the
HTS, which is estimated from the heat transfer model, is less
than 72 K. A positioner that holds the flywheel at its initiallocationreleases the flywheel when the temperatureof the HTS
stabilizes below the critical temperature.
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B Kim et al
(a) (b)
Figure 5. Rotation with a speed of about 6000 rpm: (a) top view;(b) side view; (c) at rest.
Rotation
alspeed[rpm]
8000
7000
6000
5000
4000
3000
0.2 0.3 0.4
Current [A] @ 12 Volts
0.5 0.6
Figure 6. Speed versus current for a 1 mm wide, two-turn, six-layerstator under atmospheric conditions.
Positionercontroller
Positioner
Flywheel
HTSStator
Cyrocooler
Power supply
Motor driver
Oscilloscope
Vaccumpump
Vaccumchamber
Figure 7. Experimental set-up under vacuum conditions.
5. Results and discussion
The rotational speed was recorded to be much faster than that
of the earlier test system. The speed according to the input
current is shown in figure 8. The maximum speed of the
flywheel was 38 000 rpm, which was an intersection point
between the torque-speed curve and the load-speed curve. The
loads mainly include friction load in the bearing and eddy
current load in the stator. The indicated part in the graph is
the region where the current no longer increases, regardless of
turning the knob of the motor driver. At maximum velocity
the flywheel carries only about 14.5 J, which is not sufficient
for practical application. If a motor driver supporting a higherfrequency and a composite flywheel were to be employed, the
storage capacity would be greatly increased.
Rotationalspeed[rpm]
40000
35000
30000
25000
20000
15000
10000
5000
0
0.0 0.1 0.2
Current [A] @ 12V
0.3 0.4 0.5 0.6 0.7 0.8 0.9
Figure 8. Speed versus current for a 0.6 mm wide, three-turn,nine-layer stator under vacuum conditions.
Contrary to our expectations, the spin-down characteris-
tics were very poor, within several tens of seconds, which was
similar to experiments in atmospheric conditions, although theflywheel startedidling at a speedof over 30 000 rpm. The mean
drag torque during spin-down was about 6.62 105 N m
and the friction coefficient was 0.152, which are excessively
large values. The assumption that the short spin-down time in
the previous experiment was mainly due to air drag appears
to be incorrect. A difference of only several per cent in the
spin-down time between atmospheric and vacuum conditions
was found. Clearly there is another larger loss in our systems.
Hence, we conducted several qualitative tests in atmospheric
conditions in order to identify the major losses.
There are some loss factors including eddy current loss,
inhomogeneity in the magnetic field and air drag loss, as
mentioned. Idling with and without the stator are compared
to investigate the effects of eddy current losses on the stator.
The eddy current loss mainly occurs in the stator because there
is no conductor under the flywheel except the stator. Although
the spin-down time is about 29 s when the stator is separated
from the idling flywheel, it is about 20 s with the stator at
10 000 rpm. If acceleration is constant during spin-down, the
contribution of the eddy current losses in the stator to the drag
torque increases by only 31%. The total drag torques include
the HTS bearing drag. While the portion of eddy current losses
is large, idling time without eddy current losses is extremely
short. Moreover, drag torque in the HTS bearing was larger
than that in a mechanical bearing thatwas engaged temporarily.
This means losses in the HTS bearing are the largest losses in
the system.In the experiments, we observed that the flywheel stopped
at one of the angles n/2 (where n is a natural number). This
indicates that the magnetic field on the central magnets of
the bearing is not symmetric but rather it is square-like. We
initially considered that the gap between the central bearing
magnets and the circumferentialmotor magnets should be wide
enough to avoid interaction of the magnet fields. However, the
spin-down tests reveal that the gap is not sufficient. Figure 9
shows the magnetic flux density on the flywheel simulated by
MAXWELL 3D. The flux density around the circular magnets
is distorted into a near square, which is on the cut plane above
1 mm from the surface of the flywheel. This non-axisymmetry
and azimuthal inhomogeneity of the magnetic field in thebearing parts caused by eight alternate poles creates hysteresis
losses in the HTS.
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Small-sized flywheel energy storage system with HTS bearing
Figure 9. Magnetic flux density on the flywheel.
Magnetsfor the bearing
Ironfor magnetic
screening
Magnetsfor the motor
Figure 10. Scheme of improved flywheel with magnetic screening.
The problem of distortion of the magnetic field in the
bearing parts can be mitigated by changing the arrangement
of the magnets on the flywheel. If the circularbearing magnets
were located on the opposite side of alternative motor magnets
and the stator, the influence of the magnetic field of the motor
magnets on that of the bearing magnets would be reduced. In
addition, the magnetic screening method could significantly
weaken the magnetic field penetrating towards the bearing
magnets. A new flywheel designedaccording to these concepts
is shown in figure 10. The half with the motor magnet
array is enclosed by a high-permeability material such as iron.
Figure 11 shows simulation results for this flywheel in the cut
plane. There is almost no trace of magnetic flux from themotor magnetsaround the bearing magnets in figures 11(b) and
(c). Consequently, the magnetic field of the bearing magnets
shows good azimuthal homogeneity and axisymmetry, as if
the bearing magnets were the only magnets. This improved
arrangement would reduce the drag torque significantly.
Eddy current losses essentially occur when a magnetic
fieldvarieson a conductor. However, they can be minimized by
reducing the area meeting the magnetic field perpendicularly.
It is difficult to fabricate a fine conductor path using the
wet etching method employed in this work because the non-
uniformity of the lateraletchingadversely affects the formation
of thin features. A desirable stator having a narrow width
with proper thickness could be fabricated by an electroplatingmethod instead, so that less eddy current is generated by the
magnetic variation.
(a)
(a)
(c)(b)
(b)
(c)
Figure 11. Magnetic flux density for a new conceptual flywheel withmagnetic screening: (a) the cut plane above 1 mm from the motormagnets; (b) the cut plane between the motor and bearing magnets;(c) the cut plane below 1 mm from the bearing magnets.
6. Conclusions
We have designedand fabricateda small-sizedflywheel energy
storage system with a HTS bearing. The motor that is merged
into the flywheel has a planar stator fabricated by a wet
etching process. The preliminary system was successfully
operated in atmospheric conditions, and then the small-sized
flywheel energy storage system was improved in terms of the
stator, the cooling system and vacuum circumstances. The
improved system stably rotated up to 38 000 rpm. Althoughthe system rotates at high speed, it has a large drag torque
due to some factors including eddy current loss in the planar
stator and non-axisymmetry of the magnetic field caused by
an inadequate combination of the motor magnets and flywheel.
An improved magnet arrangement of the flywheel including
magnetic screening is considered for axisymmetry of the
magnetic field affecting hysteresis losses on the HTS.
Acknowledgments
This work is supported by KOSEF (Korea Science and
Engineering Foundation) and the authors are indebted to the
SFES laboratory in KEPRI (Korea Electric Power Research
Institute) for providing the HTS used in the experiments.
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