Development of Flywheel Energy Storage Systemwith Multiple Parallel Drives
Jun-ichi Itoh, Tsuyoshi Nagano, Kenta Tanaka, Koji Orikawa, Noboru Yamada
Nagaoka University of Technology
Nagaoka, Niigata, Japan
itoh@vos.nagaokaut.ac.jp
Abstract—This paper introduces performance of a power
leveling system with a 3.0-MJ, 2900-r/min of flywheel energy
storage for multiple parallel operations. In terms of cost
reduction and improvement efficiency, this system uses low
cost ball bearings at the low speed. Moreover, the system iscomposed by not a large capacity flywheel but many flywheels
with small energy capacity but the cost reduction of the
flywheel system. As a result, the cost reduction can be achieved
by mass production. The proposed flywheel system is designed
by a low-loss design method that focuses on the mechanical loss
aspect. From the experimental results, it is confirmed that the
charge and discharge efficiency of the mechanical part of the
proposed flywheel system is 95.2% (charge 97.9%, discharge
97.5%). In addition, the suitable drive system with permanent
magnet synchronous motors (PMSMs) are developed for themultiple parallel flywheels system. The PMSMs which consistof a main winding and a damping winding are driven by asingle main inverter and small auxiliary inverters. From theexperimental results, the validity of the parallel drives control
method using two PMSMs is confirmed.

I.

INTRODUCTION

Renewable energy systems, especially a wind turbine or aphotovoltaic cell, generate power fluctuation and it depends
on the meteorological conditions. Therefore, these systems
require energy buffers such as the electric double layer
capacitors (EDLC), chemical batteries or flywheels tosuppress the power fluctuations. One of the problems in thechemical battery or the capacitor energy storage is a shortlife-time because the lifetime depends on the ambient
temperature and the numbers of charge and discharge time.
Especially, in high temperature environment, the lifetime ofthose storages will drastically become shorter. In addition,
the chemical battery cannot repeat rapid charge anddischarge by a large internal resistance. On the other hand,the EDLC provides high charge and discharge efficiencies.
Moreover, the rapid of charge and discharge is possible
because the internal resistance is very small. However,similar to the battery, the lifetime becomes short due to the
influence of the ambient temperature.
The flywheel has a long lifetime due to no chemical

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structure, thus the charge/discharge characteristics of highcycle has high performance in contrast to the chemical
battery. In addition, the flywheels provide environmental
friendly and low maintenance cost factors. Additionally, thekinetic energy which stored in the flywheel is proportional
to the square of the rotational speed. For this reason, ultrahigh speed rotation using magnetic bearings have beenstudied in recent years in order to achieve high energy
density [1-7]. However, the structure of the ultra-high speed
flywheel will results in high cost. Therefore, it is necessary
to reduce the costs and simplify the structure of the flywheel
system toward the realization of a practical system.
In this paper, the energy storage system which consists ofa small energy capacity flywheel is proposed. In addition,
the design method of the flywheel which aims to reduce themechanical loss is established. In the proposed system, theinertial of the flywheel that able to store enough energy at
low rotational speed is designed. Therefore, inexpensive ballbearings are adopted in the proposed system. Furthermore,
the adjustable speed method for the plurality flywheels is
proposed. By using several numbers of flywheels, theproposed system has some advantage and it as follows; (i)flexible power capacity, (ii) high reliability, (iii) cost
reduction due to mass production.
This paper is organized as follows; at first, the design
method of the flywheel and prototype flywheel system is
introduced. Second, the standby loss of the proposed
flywheel system is experimentally analyzed. Third, a design
approach of the permanent magnet synchronous motor(PMSM) for improving the efficiency and the energy density
is discussed. Finally, a multiple parallel drives system withPMSM is proposed. In addition, the basic operation of theparallel drives is verified.
II.

PROPOSED FLYWHEEL SYSTEM

Fig. 1 shows the configuration of the proposed flywheel
system that comprises a general purpose motor and ballbearings. This flywheel is designed by a low-loss design
method as described in the following paragraph. In thissystem, it is possible to store the kinetic energy of the 3.0

Fig. 1. Configuration of a proposed flywheel system thatcomprises a general purpose motor and ball bearings.

MJ at 2900 r/min. Such a low rotation speed region can beapplied to a typical ball bearing and general purpose motor.
In addition, the flywheel vacuum case and the motor are
separated by the magnetic coupling. As a result, windage
loss can be greatly reduced because it is possible to reduce apressure in the vacuum case by the vacuum pump.
Moreover, vacuum in the vacuum case does not affect theheat dissipation of the motor because the motor and thevacuum case are separated. Therefore, the general purpose
motor can be applied to drive the flywheel without adding
special cooling mechanism.
Fig. 2 shows the low-loss design algorithm. Thisapproach focuses on the design freedom of the shape of the
flywheel to store the same energy [8]. The characteristics ofthe proposed flywheel are as follows; (i) a low rotational
speed is considered for suppressing windage losses andtypical ball bearings is used, (ii) the use of a magnetic
coupling in order to separate between the flywheel vacuum
case and the motor. It is difficult to cooling the motor whenthe motor is built into the vacuum case. A magnetic
coupling or a magnetic fluid seal can be chose in order toseparate between the vacuum case and motor. However, themagnetic fluid seal has a problem that loss generated in thesealed portion is large [8]. On the other hand, the magnetic
coupling magnetically connects axis of the flywheel and the
motor. Therefore, the losses due to mechanical contact are
reduced as compared to the magnetic fluid seal.
In addition, windage loss can be greatly reduced because
it is possible to reduce pressure in the vacuum case by thevacuum pump. Moreover, vacuum in the vacuum case does
not affect the heat dissipation of the motor because between
the motor and the vacuum case are separated. By using a
structure as described as before, it is possible to easilychange the maximum rotational speed and output power ofthe flywheel by changing the specifications of the motor.
Fig. 3 shows the block diagram of the flywheel system
including the measurement system and auxiliary devices. In
the flywheel system, the induction machine is operated as a
generator during deceleration mode and then the kinetic

Fig. 2. Low-loss design algorithm of the flywheel with afocus on mechanical loss.

Fig. 3. Block diagram of the flywheel system including
the measurement system and the auxiliary devices.
Table 1. Specifications of the flywheel which is calculated from thelow loss design.
Material
SCM440Radius
500 mmThickness
65 mm
Weight of the
Flywheel
392 kgFlywheel
3.0 MJ at
Stored Energy
2900 r/min
Motor/Generator

MLC1115C (Fuji Electric)

Inverter

FRN 37G11S-2 (Fuji Electric)energy is converted into the electrical energy. On the other
hand, during acceleration mode, the electrical energy isstored as a kinetic energy, which is the machine working as
a motor. Therefore, this system uses a regenerative
converter. Furthermore, over temperature of the bearing andthe motor can be prevented by the oil cooler.
III.

ANALYSIS METHOD OF THE STEADY STATE LOSSIn this section, the losses that occur in a steady rotation
are separated into mechanical losses and electrical losses.
From this analysis, the factors of the steady state loss that
occur in the system will be clarified. Moreover, the further
efficiency of the flywheel system is considered.

A. Windage loss
The frictional losses that occur between the surface ofthe flywheel and the surrounding fluid are calculated by (1)from the shape of the flywheel and the friction moment.
P =C
ρω 3 ( r 5 − r 5 ) + C
ρω 3 r 2 r 2 t i ... (1)
w

ms

o

i

mc

o

i

where, Cms is the moment coefficient of the friction of the top
and bottom, Cmc is the moment coefficient of the friction of
the side, ω is the angular velocity of the body revolution[rad/s], ro is the radius of the vacuum case, ri is the radius of
the flywheel, ti is the thickness of the rotational axis of the
flywheel.
However, the friction moment fluctuates depending on
the state of the fluid in the vacuum case. In this case, it isdifficult to obtain a simple formula. Therefore, the windage
loss is analyzed by using the computational fluid dynamics(CFD). Unstructured grid-based thermal fluid analysis
system SCRYU / tetra (software cradle) is used for the
analysis. In the model which faithfully reproduces the realsystem, the computation time increases the complexity of theanalysis. Therefore, the simplified model eliminates
extraneous parts and mechanisms are used for analysis. Asan analysis condition, the fluid flow is turbulent and SST k-ωmodel is used as a model for turbulence. SST k-ω is a modelthat combines the k-ε model based on the k-ω model. In thevicinity of the wall, k-ω model's predictive accuracy is better
separated flow is used. On the other hand, the k-ε model isused in an area away from the wall. As the initial condition,
the pressure in the vacuum case is 500 Pa, the temperature is
10 degree Celsius. In addition, the fluid is rotated around theaxis of the flywheel.
Fig. 4 shows the analysis of the ambient pressure of the
flywheel. The analysis conditions are as follows: the steadyrotational speed is 2900 r/min and the vacuum in the vacuum
case is 500 Pa. From the analysis, the pressure increases
toward the radial direction from the center of the flywheel.
Therefore, losses caused by the friction of the fluid byincreasing the peripheral speed.

Fig. 4. Visualization of the ambient pressure of the flywheel at the
rated energy storage.
Table 2. Condition for bearing loss calculation.
f0

2

fl

0.0002

Viscosity ν

23 mm2/s

Pitch diameter dm

102.5 mm

Rotation speed n2900 r/min

Bearing load P

0N

f0

2

fl

0.00025

Viscosity ν

23 mm2/s

Pitch diameter dm

70 mm

Rotation speed n2900 r/min

PB = 1 .047 × 10 － 4 nM ............................................ (2)Bearing load P

3975.9 N

Additionally, the frictional moment can be separated
into frictional moment Mo that depends on the load and thefriction moment Ml that is independent from the load.

magnitude and the direction of the load, P is the load on the
bearing.

B. Bearing loss
The bearing loss PB is obtained by (2) from therotational speed n and the friction moment of the bearing

M = M o + M l ....................................................... (3)2M o = f o × 10－7 (un) 3 d m 3 ................................... (4)M l = f l Pd m .......................................................... (5)where, fo is the coefficient that depends on the format andthe method of the bearing lubrication, u is the viscosity of
the lubricant, dm is the inner diameter of the pitch of therolling elements, fl is the coefficient that depends on the

Bearing1

Bearing2

Table 2 shows the value used to calculate the bearing
loss. In this system, two bearings are arranged at the bottom
and one at the top of the flywheel. The bearing losses at
each rotational speed are calculated by (4) and (5).
C. Copper loss with respect to the fundamental wave
In this section, the losses generated by the motor are
separated into the copper loss and the iron loss.
Fig. 5 shows the equivalent circuit of the induction
machine with respect to the fundamental wave. Table 3

shows the specifications of the motor that is used in the
prototype system. From the equivalent circuit, therelationship between secondary input P2 and the mechanicaloutput Pmec is represented by (6).
P2 : Pmec = 1 : (1 − s ) ............................................... (6)From this equation, the secondary resistance is calculated by(7).
R2 ' =s 1
I 2 ' 2 ..................................................... (7)1− s 3Note that the slip s is calculated from the actual speed andthe synchronous speed of the induction motor. In addition,
the secondary current I2’ is calculated from the no-load testof the motor itself. In the no-load test, the output resistanceR2’/s can be regarded as almost infinity because the slip s isalmost zero. At this time, the current flowing through thecircuit will be all the excitation current Io. Therefore, thesecondary current I2’ is calculated by (8).

Fig. 5. Equivalent circuit of the induction machine with respect to thefundamental wave.
Table 3. Specifications of the motor that is used in the prototype
system.
Rated Voltage

200V

Rated Current

14.4A

Output Power

3.7kW

Primary resistance R1

1.01Ω

Excitation current Io

5.90A

I 2 = I1 − Io ............................................................. (8)From these results, the copper loss for the fundamental
wave is expressed by (9).

Pc = 3R1 I 1 2 + 3R 2 ' I 2 '2 ......................................... (9)D. Copper loss for harmonic
In the driving of the motor by an inverter with pulse
width modulation (PWM), the harmonic voltage is applied tothe induction machine. As a result, the losses due to theharmonics occurs [8]. In this section, the copper loss causedby the harmonic component of the motor current iscalculated.
Fig. 6 shows the equivalent circuit of one phase of theinduction machine for harmonic. The harmonics generated
by PWM is sufficiently high frequency region for thefundamental component. Therefore, the exciting componentof the equivalent circuit with respect to the fundamental
wave is ignored. In addition, the mechanical output can beignored because it can be regarded that the slip by the
harmonic component is 1. For these reasons, the loss for the
harmonic component is represented by (10).

Pn = 3( R1 + R2 ' ) I n '2 ............................................. (10)E. Iron loss
From the equivalent circuit of the induction machinewith respect to the fundamental wave, the iron loss isexpressed by (11) by using the excitation conductance.
I 2
Pi = 3 Mgo....................................................... (11)

In addition, the excitation conductance go is expressed by
(12).
go =

Po − W m................................................. (12)V12Fig. 6. Equivalent circuit of the induction machine for harmonic wave.

where, Po is input power at no load, Wm is the mechanicalloss at no load.
IV.

EVALUATION OF THE PROPOSED FLYWHEEL SYSTEM

Fig. 7 shows the measurement results of the charging anddischarging efficiency. In this experiment, the flywheel isaccelerated or decelerated to charge and discharge theenergy of 1MJ, respectively. Note that, the charging
efficiency is the ratio of the input energy to the stored
energy. On the other hand, the discharging efficiency is theratio of the decreasing of the kinetic energy to regenerative
energy. If only the mechanical loss is considered, it isconfirmed that the achieved charging efficiency is 97.9 %and the achieved discharging efficiency is 97.5 %.
Therefore, the total achieved efficiency is 95.2 %. From theanalysis, the usefulness of the proposed flywheel using lowloss design method is confirmed.
On the other hand, if the motor loss is considered, theachieved charging efficiency is 80.7 % and the achieved
discharging efficiency is 75.1 %. Therefore, the total
achieved efficiency is 60.7 %. The main cause this of low
efficiency is the additional loss is introduced during theinduction motor driving the flywheel. Thus, it is necessary
to apply the PMSM to achieve high efficiency of the entire
flywheel system.

(a) Charging efficiency
Fig. 8. Loss analysis results at the rated energy storage. In theexperiment, the steady rotation speed is 2900 r/min and thepressure of the vacuum case is 20 Pa.
Table 4. Comparison of the characteristics between inner rotor
type motor and outer rotor type motor.

(b) Discharging efficiency
Fig. 7. Measurement results of the charging and discharging
efficiency.

Evaluation criteria

Inner rotor type

Outer rotor type

Limitation of magnet size

LimitedNo Limited

Winding coils

Difficult

Easy

Heat dissipation

Easy

Difficult

Inertia moment of motor

Small

Large

Next, the loss reduction effect due to applying of PMSM
in the standby state of flywheel is evaluated by the loss
analysis technique.
Fig. 8 shows the analysis result of the steady state loss inthe experiment with prototype flywheel system. In thisexperiment, the rotational speed is 2900 r/min and thepressure in the vacuum case is kept at 20 Pa. The
mechanical loss and electrical loss are separated
respectively by a free-run test. From the analysis results
using the induction machine, it is confirmed that the motor
loss contributes 68.1% of the total loss. On the other hand,
the loss estimation results when applying the PMSM withhigh efficiency is shown on the right side of Fig. 8. From
the results, it is expected that electrical loss is reduced by
31% when the PMSM is applied [9].
V.

DESIGN OF THE SUITABLE PMSM FOR THE FLYWHEEL

Table 4 shows comparison of the characteristics between
inner rotor type motor and outer rotor type motor. In thispaper, the possibility of achieving a high efficiency of thesystem by considering the PMSM and the flywheel into thesystem is confirmed. The general purpose induction motor
can be used when the motor is located outside of thevacuum case. However, due to the limitation of the magnet
size, the inner rotor type motor is relatively large in therotation axis direction. Thus, the energy density of the
flywheel system becomes lower. In contrast, the outer rotor
type motor can be easily designed with large output torqueas compared to the inner rotor type because the magnet size

Fig. 9. Configuration of prototype motor.

is not limited. Therefore, the reduction in the axial directionof the motor is achieved. For these reasons, an outer rotor
type PMSM can be applied in order to improve the energy
density and efficiency of the flywheel system. This section
discusses the important point of design of the suitable
PMSM with outer rotor structure for the flywheel.
Fig. 9 shows the configuration of the proposed PMSM. In
this structure, it is possible to integrate the inertial body ofthe flywheel and the rotor of the motor. Thus, improvementof the energy density is achieved.
Fig. 10 shows the loss analysis results of the flywheel
during standby mode based on experiments with respect tothe rotor support material. In the experiment, A2017 of nonmagnetic material and SS400 of the magnetic material areused as the rotor support material. In addition, the loss iscalculated by the free-run test. Free-run test is a test that theinverter is electrically disconnecting and the motor with theenergy stored in the flywheel. In this state, the stored energy
is only consumed by the mechanical loss. Therefore, themechanical loss is calculated from the time until the stored
energy is consumed. From the experimental results, it isconfirmed that when the non-magnetic material is applied to
the rotor supporting material, the loss increases as comparedto the magnetic material. Next, the loss during the standby
state is analyzed by the electromagnetic field analysis inorder to verify the cause of the loss increasing.
Fig. 11 shows the distribution of joule loss density whenthe A2017 material is used and this analysis is based on
Finite Element Method (FEM) technique.
Fig. 12 shows the comparison of loss analysis results that
occur in the fixed stator base. Note that, the analysis isbased on 1500 r/min of speed condition. From this analysis,
it is confirmed that a large loss occurs on the fixed statorbase when a non-magnetic material is used in the rotor
support. The increasing of loss is because of the leakage
magnetic flux from the rotor when a non-magnetic material
is used. In addition, the loss is also because of the changes
in the magnetic flux linking in the stator base. In contrast, inthe case of using a magnetic material, the interlinked
magnetic flux with the fixed stator base is small and this isbecause the magnetic flux is leaking from the rotor througha support material. Therefore, the generated loss in the fixed
stator base is small when a magnetic material is used.
For the outer rotor type flywheel, the inertial body is
added to the periphery of the rotor in order to improve thestored energy. In addition, the flywheel is stored in a sealedcontainer in order to reduce windage loss. Therefore, formotor design to be applied to the flywheel, it is necessary toconsider the structure of the sealed container and thematerial of the flywheel.
VI.

Fig. 10. Standby loss of the prototype motor.

Fig. 11. Analysis result of the Joule loss density (A2017).

Fig. 12. Comparison of the stator base loss.

PROPOSAL OF THE PARALLEL DRIVES SYSTEM

From the loss analysis of the prototype system, it isconfirmed that the proportion of the motor loss is very largeof a total loss. A general-purpose induction motor is used todrive the flywheel in the prototype system and this willcauses of a large motor loss. Therefore, in order to reducethe motor loss, the PMSM is used to drive the flywheel.
However, in general it is not possible to drive the pluralityof the PMSM with single inverter. Thus, the parallel
operations of the proposed flywheel system cannot be
achieved by simply applying a PMSM.
In this study, the drive system that can drive multiple
flywheels using single inverter is proposed. In the proposed
drive system, parallel driving is achieved by using a PMSM
with an auxiliary winding.
Fig. 13 shows the configuration of the proposed parallel
driving system. In PMSMs, the auxiliary windings which

Fig. 13. Configuration of the proposed parallel driving
system.

1sFig. 15. Analysis and verification model of the PMSM in
addition the auxiliary windings for damping control.
Fig. 14. Control block diagram of the proposed parallel
driving system.

are used in the damping control (with the auxiliary inverter)are placed in the slots together with the main windings. The
proposed system uses two different power rating inverters.
The first inverter is the large power capacity inverter for themain windings to control the speed of the parallel connected
PMSMs. The second inverter is a small power capacity
inverter for the auxiliary windings to suppress the torquevibration. In term of the effectiveness of the proposedsystem, it is very important that the power capacity of theauxiliary inverter is small enough.
Fig. 14 shows the control block diagram of the proposedsystem. In the proposed parallel driving system, the V/fcontrol is applied to the main inverter. In addition, the fieldoriented control and the damping control are applied to theauxiliary inverter. Each of the auxiliary inverter controls thecurrent in auxiliary windings of the PMSM in order tosuppress the torque and speed vibrations. Since thesevibrations are caused by the phase difference between therotational coordinates of the inverter and of the PMSM, it
can be suppressed by the damping control in the auxiliary
inverter.
Fig. 15 shows the simplified system for confirming theoperation of the damping control. The proposed system uses
the PMSM that placed the auxiliary windings in the slots
together with the main (conventional) windings, and so thatthe mutual magnetic interference occurs between the main
and the auxiliary windings. Due to the mentioned reason,
the control for the auxiliary inverter becomes complicated.
Therefore, the proposed system is validated using a model
where two PMSMs are connected in series via single shaft.
Then, the rear end of the main PMSM is connected to theload machine. It means that the magnetic coupling was
ignored in the simulation and the experiment conditions.
Fig. 16 shows the experimental results that illustrate themotor speed vibration when the proposed system is applied(a) without the damping control and (b) with the damping
control ( ζ = 0.3 ) in an acceleration and deceleration test. In
this experiment, it is difficult to measure the torque response
directly. Therefore, the speed vibration is evaluated insteadof the torque response. In Fig. 16(a), the proposed system isimplemented without the damping control. The speed
vibration occurs during the acceleration. After theacceleration, a 400 r/min of speed vibration and the 10 Ap-pof current vibration in q-axis of the main inverter are
maintained. On the other hand, Fig. 16(b) demonstrates theexperimental results, where the proposed system isimplemented with the damping control ( ζ = 0.3 ). The
effectiveness of the auxiliary inverter from the results
confirms that the speed vibration is reduced from 400 r/min
to nearly 0 r/min in compared with the acceleration test of
Fig. 16(a). The 10 Ap-p of the current vibration in the q-axisof the main inverter is suppressed as well. Then, the output
power of the auxiliary inverter is suppressed to 13% of themain inverter when the damping factor is designed at 0.3. In
the same way as the acceleration test of Fig. 16(a), after the
deceleration, a 500 r/min of speed vibration and the 15 Ap-pof current vibration in q-axis of the main inverter are
maintained as shown in the deceleration test of Fig. 16(a). In
contrast, it is confirmed that, as shown the deceleration testof Fig. 16(b), the speed vibration is reduced from 500 r/min
to nearly 0 r/min and the current vibration is reduced from 15
Ap-p to 0 Ap-p in compared to Fig. 16(a) in the same way as
acceleration test. Nevertheless, it is confirmed that the q-axis
current of the auxiliary inverter flows only during
acceleration and deceleration. Moreover, the maximum qaxis current of the auxiliary inverter is 20% of the q-axis
current of the main inverter. Therefore, it is confirmed thatthe auxiliary inverter can suppress the speed vibration via
auxiliary windings with a small q-axis current of theauxiliary inverter even if in the acceleration and deceleration
test.
VII. CONCLUSION
In this paper, a low-cost flywheel system that suitable formultiple parallels driving method is proposed. In the
proposed system, it was confirmed that it is possible toreduce the windage loss by separating between the motorand flywheel with a magnetic coupling. As a result, the total
efficiency of the mechanical part for charge and discharge
operations is up to 95 %. In addition, the drive system that

Fig. 16. Acceleration and deceleration test without/with damping control in motor-generator set. (a) After theacceleration, the 400 r/min - speed vibration is maintained. (b) The speed vibration is reduced from 400 r/min to nearly 0
r/min in compared with (a)can drive multiple flywheels using a large main inverter andsmall auxiliary inverters whose power capacity are 1/10 of
the main inverter was proposed. From the experimental
results, the effectiveness of the parallel drive control using
two PMSMs was confirmed. In the future, the design
approach of the auxiliary winding with a PMSM thatsuitable for flywheel will be established.
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