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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: naucica@kaist.ac.kr

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:naucica@kaist.ac.krhttp://stacks.iop.org/SUST/19/217http://stacks.iop.org/SUST/19/217mailto:naucica@kaist.ac.krhttp://dx.doi.org/10.1088/0953-2048/19/2/011

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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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