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Micro-energy storage system using permanent magnet and high-temperature superconductor 2008


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Sensors and Actuators A 143 (2008) 106–112

Micro-energy storage system using permanent magnet and high-temperature superconductor
Kangwon Lee a,? , Ji-eun Yi b , Bongsu Kim c , Junseok Ko a , Sangkwon Jeong a , Myoungyu Noh d , Seung S. Lee a
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea b BK 21 Mechatronics Group at Chungnam National University, 220, Guseong-dong, Yuseong-gu, Daejeon 305-764, South Korea c Korea Electric Power Research Institute, 103-16, Munji-dong, Yuseong-gu, Daejeon 305-380, South Korea d Division of Mechatronics Engineering, Chungnam National University, 220, Guseong-dong, Yuseong-gu, Daejeon 305-764, South Korea Received 31 March 2007; received in revised form 10 August 2007; accepted 14 August 2007 Available online 12 September 2007
a

Abstract This paper presents the design and experiment with a micro-energy storage system using an axial-?ux permanent magnet and a high-temperature superconductor (HTS). This system consists of a three-phase planar stator, ?ywheel, and HTS. The three-phase planar stator is fabricated with ?ve lines, width of 100 m, and height of 50 m for low eddy current loss and high current density. The ?ywheel with a diameter of 50 mm is integrated by eight motor/generator magnets on the upper side and a single bearing magnet on the bottom side. The HTS is made of melt-textured yttrium-barium-copper-oxide (YBCO) and is used as the non-contact bearing by a levitation force. The maximum rotational speed of the ?ywheel is 51,000 rpm at 12 V and 0.68 A. At this speed, the ?ywheel stores a rotational kinetic energy of 337 J and the no-load three-phase electric output is 802 mVrms . By using a three-phase full-wave recti?er, an electric output power of 50 mW is measured. ? 2007 Elsevier B.V. All rights reserved.
Keywords: Power MEMS; High-temperature superconductor; Axial-?ux permanent-magnet motor; Planar stator; Energy conversion

1. Introduction The requirement for portable power generating devices is increasing worldwide and Power MEMS, which is a rapidly growing ?eld, includes micro-systems designed to produce electric power or mechanical work. Power MEMS comprises various systems such as micro-generators, gas turbines, combustion engines, turbo-pumps, fuel cells, and piezoelectric devices [1,2]. Among these systems, electric power generation is the main focus with a broad range of power output levels from milliwatts to watts. In particular, a micro-generator with an axial-?ux permanent-magnet motor is an interesting research area for high power density, which is operated by common power sources such as air-powered spindle and compressed air [3–5]. By using the air-driven spindle, the rotor can spin at high speed and deliver power output of the order of 1 watt. In order to use compressed

?

Corresponding author. Tel.: +82 42 869 3086; fax: +82 42 869 5046. E-mail address: kw5610@kaist.ac.kr (K. Lee).

air, the turbine blade at the bottom of a SmCo rotor is etched by electro discharge machining (EDM) and this rotor can rotate above a silicon stator using an air cushion. A high-temperature superconductor (HTS) is characterized by diamagnetism and ?ux pinning effects. The repulsion of the magnetic ?ux creates a levitation force and the lateral stiffness maintains the original position. Based on these principles, the permanent magnet (PM) can levitate above the HTS without an active control [6]. In this paper, we present the design and fabrication of the micro-energy storage system using the ?ywheel energy storage system (FESS) with a HTS bearing. The FESS with HTS bearing can store highly ef?cient energy due to the low friction loss of the non-contact bearing. Unlike MEMS levitation system using air bearing or electrostatic bearing, the micro-energy storage system with a HTS bearing is stable because of the large levitation force and lateral stiffness below a critical temperature 72 K. This system can also transform electric energy into rotational kinetic energy through a motor, and vice versa through a generator during the free rotation of the ?ywheel [7]. This generator

0924-4247/$ – see front matter ? 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.08.023

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Fig. 1. Schematic view of the micro-?ywheel energy storage system with the high-temperature superconductor bearing.

Fig. 2. (a) A conducting wire above motor magnets, (b) Lorentz force acting on the conducting wire, and (c) driving force and torque acting on the ?ywheel.

can be used in micro-satellites where highly durable batteries and compact systems are important. These two problems can be solved by using the FESS because of the integrated energy storage and attitude control. The FESS with HTS bearing is suitable for satellites due to its rapid charge and discharge cycles. The ?ywheel stores rotational kinetic energy generated by solar energy during the day and uses it to operate the system during the night. It can also use its momentum for attitude control of satellites [8]. In addition, compactness is realized without considerable power consumption, because of the low temperature and vacuum condition in space. 2. System design A schematic view of the micro-FESS with HTS bearing is shown in Fig. 1. The system consists of an Au electroplated planar stator, ?ywheel, and HTS. From the viewpoint of compactness of the system, the planar stator is designed as an axial-?ux type brushless DC motor/generator. The ?ywheel levitates above the HTS when the HTS is cooled below a critical temperature. During the levitation, the ?ywheel takes torque from the planar stator. When a conductor line is placed in a magnetic ?eld and a current is applied as shown in Fig. 2(a), a Lorentz force occurs in the radial components of the conductor line as shown in Fig. 2(b). In this system, the driving torque of the ?ywheel occurs in the opposite direction of the Lorentz force as shown in Fig. 2(c),

because the conductor line is ?xed. This torque and the induced voltage (back emf) can be obtained through a magnetostatic analysis. For the convenience of the analysis, we cut the motoring permanent magnets through an imaginary line at r = R, and unroll the axial surface as shown in Fig. 3. Assuming a uniform ?eld distribution in the radial direction, we can use 2D magnetostatic analysis to determine the ?eld distribution. If we consider only the ?eld due to the motoring permanent magnets, it is described by a potential equation ? 2ψ = ? · M (1)

where M is the magnetization vector representing the permanent magnets. In (1), the potential function ψ is de?ned as H = ??ψ (2)

where H is the ?eld intensity. Once H is determined from (1) and (2), the ?ux density B can be obtained from the material’s constitutive relations. The magnetization vector in (1) is a periodic function in our case, and it can be expressed as a Fourier series. If we consider only the ?rst term of this Fourier series (fundamental space), the magnetization vector can be approximated as M= p 4Br ? x z = M0 sin(qx)? sin z πμ0 2R (3)

Fig. 3. (a) Geometry of motor magnets and the imaginary cut line and (b) unrolled axial surface. Only one electrical cycle is shown.

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Fig. 4. Space distribution of ?ux density BZ and the planar stator coils. Only one phase of the coils is shown.

In (3), Br is the remanent ?ux density of the permanent magnets, μ0 the permeability of free space, p the number of poles, ? and z is the unit vector in the axial direction. Realizing that the magnetization vector has a component only in the axial direction, the right-hand side of (1) becomes zero. If we separate the solution domain into two regions (permanent magnets and air gap below the stator), the general solution of (1) can be written as ψ= (C1 eqz + D1 e?qz ) sin qx, (C2 eqz + D2 e?qz ) sin qx, 0 < z < tm tm < z < tm + g (4)

Fig. 5. Comparison between analytic calculation and numerical simulation of ?ux density BZ .

Hx is continuous at z = tm , ψ is ?nite at z = ∞. The resulting potential function in the air gap is ψ= M0 sinh(qtm ) e?qz sin qx q (5)

Here, we only consider the ?rst harmonic terms of the series solution. The four unknowns in (4) can be obtained by applying the boundary conditions which are as follows: Assuming in?nite permeability of the rotor yoke, Hx = 0 at z = 0, Bz is continuous at z = tm ,

and the ?ux density at the stator surface is Bz = μ0 M0 sinh(qtm ) e?q(tm +g) sin qx (6)

This ?ux density is plotted in Fig. 4 as a function of the electrical angle θ. When current-carrying coils are placed in the ?eld, a Lorentz force is developed. When one leg of the coils spans θ c electrically and the number of the coils turns nc , the

Fig. 6. Electromagnetic simulation of the ?ywheel: (a) the cutting plane above 2 mm above the motor magnet and (b) the cutting plane between the motor and bearing magnets.

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Fig. 7. Fabrication process of the planar stator.

total force on a single phase coil can be obtained from F =p
θ+(θc /2) θ?(θc /2)

and the result is (7) T = 3 nc kb F0 Ravg I0 2 (12)

nc I a Bz (R2 ? R1 ) dθ θc

In (7), R1 and R2 are the inner and outer radius of the motor magnets, respectively, and Ia is the current in the phase a coil. The result of the integration can be written as Fa = F0 nc kb sin θIa where F0 is F0 = p(μ0 M0 ) sinh(qtm ) e and kb is breadth factor. kb = sin(θc /2) θc /2 (10)
?q(tm +g)

(8)

In (12), I0 is the amplitude of the current and Ravg is the average of the inner and outer radius of the magnets. The back emf can be obtained similarly. The induced voltage in the phase a can be obtained from Ea = p
θ+(θc /2) θ?(θc /2)

Ravg ωBz (R2 ? R1 )

(R2 ? R1 )

(9)

nc θc



(13)

The result of this integration can be expressed as Ea = F0 Ravg ωnc kb sin θ (14)

Assuming that the three phases of the coils are spatially 120? (electrical) apart from each other, and the three phase currents are temporally 120? apart each other, the ?rst-order approximation of the torque can be obtained from T = Ravg (Fa + Fb + Fc ) (11)

Fig. 8. The 5-line electroplated planar stator with 100 m width and 50 m height.

Fig. 5 shows the axial component of the ?ux density computed by (6) (solid-line) and the ?ux density obtained from a ?nite-element analysis (FEA) (dotted-line). Since the results of FEA match well with the analytical results, we can con?rm the validity of the analysis. Throughout the analysis so far, we precluded the ?eld by the bearing magnets. From the perspective of the hysteresis losses, it is also important that the ?eld due to the motor magnets is completely decoupled from that by the bearing magnets. Fig. 6 shows the electromagnetic simulation that was used to con?rm the magnetic ?ux of the ?ywheel using MAXWELL 3D, because the non-axisymmetric magnetic ?ux creates hysteresis loss. Magnetic screening, which implies the uniformly distributed magnetic ?ux without interaction between magnets, is achieved by enclosing motor magnets in an iron disk with a highly magnetic permeability material. The magnetic ?ux of the bearing magnet shows an original ring shape without distortion and it is con?rmed that the motor magnets have no in?uence on the bearing magnet; further, the magnetic ?ux of the ?ywheel is axisymmetric.

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Fig. 9. Fabricated HTS and ?ywheel: (a) HTS on cold head, (b) eight motor magnets and magnetic screening disk on the front side, and (c) a ring type bearing magnet on the back side.

3. Fabrication The conductor line on the planar stator should have a narrow width and large number of turns per layer in order to minimize the eddy current loss. Further, the current density of the conductor increases when the conductor line is scaled down. This is because the heat dissipation is proportional to the surface area, and hence the ratio of heat generation to heat dissipation decreases [9]. Fig. 7 shows the fabrication process of the Au planar stator coil on the ?exible substrate using electroplating. A dry ?lm resist (DFR) is laminated on a 1 0 0 4-in. silicon wafer and this is treated on a 110 ? C hotplate for 5 min; a 125 m polyimide ?lm is then attached. The polyimide ?lm is used as a ?exible ? substrate to complete the Y-connection easily. Cr (100 A)/Au ? (400 A) is evaporated for the seed layer of Au electroplating using an E-beam evaporation. Omni coat is spin-coated and baked at 200 ? C on a hotplate for 1 min. SU-8 is spin-coated and baked at 65 ? C/95 ? C in a convection oven for 20 min/60 min. In order to build an electroplating mold, SU-8 is patterned and Omni coat is developed using O2 plasma. Au electroplating is performed and remained SU-8 mold is lift-off using a Remover PG in an ultrasonic cleaning bath for 2 h. During this process, the DFR is removed spontaneously and the polyimide ?lm is separated from the silicon wafer. After the seed layer is etched, a single layer of the planar stator is completed. Finally, the sin-

gle layer is electrically connected into the Y-connection. Fig. 8 shows the fabricated planar stator with ?ve lines, a width of 100 m, and a height of 50 m. The ?ywheel consists of four parts: an aluminum disk, a magnetic screening disk, eight motor magnets, and a single bearing magnet. The motor magnets are eight circumferential NdFeB magnets and the bearing magnet is a ring-type NdFeB magnet. The HTS is a melt-textured yttrium-barium-copper-oxide (YBCO) and 7 EA-disk-type HTS are placed on the cold head. Fig. 9 shows the fabricated HTS and ?ywheel. 4. Experiments and results There are two kinds of cooling states in a superconductor: ?eld cooling (FC) and zero ?eld cooling (ZFC). The FC state is a situation where the HTS is cooled below the critical temperature when the PM is closed to the superconductor. In this state, a more stable levitation is obtained because of the large stiffness in the lateral direction. The ZFC state is the opposite situation, that is, there is a slight magnetic ?eld near the HTS, so that the repulsive levitation force is larger in the ZFC state than in the FC state. For a stable levitation, the experiments are carried out at the 1 mm gap between the ?ywheel and the HTS. In order to operate under the vacuum condition, the experimental setup is composed of a cryocooler, vacuum chamber and pump, motor drive, and positioner. The operation sequence consists

Fig. 10. Operation sequence: (a) HTS is cooled, (b) ?ywheel is released, (c) planar stator is moved and electrical power is supplied, and (d) planar stator is separated.

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of the ?ywheel at the maximum speed is approximately 337 J. When the ?ywheel rotates up to a maximum of 51,000 rpm, the input voltage is removed. In contrast to the previous researches, the ?ywheel can hold the rotational kinetic energy during free rotation without any input voltage. FESS is also used as a generator to transform rotation kinetic energy into electric energy. Fig. 12 shows the no-load three-phase condition with an electric output voltage of 802 mVrms . From Eq. (14), the back emf is calculated as 820.5 mVrms , therefore these two results are in good agreement at 51,000 rpm. A three-phase full wave recti?er with a 1N5818 Schottky diode is designed to converter AC to DC and a low pass ?lter is used with a 100 F capacitor to remove the ac frequency components. As a results, the electric output power is 50 mW with a planar stator winding resistance of 1.8 .
Fig. 11. Rotation speed versus spin-down time during free rotation; the maximum energy storage is 337 J.

5. Conclusion A micro-generator using a ?ywheel energy storage system with a high-temperature superconductor (HTS) bearing has been designed and fabricated. In order to achieve a compact design, this system consists of a three-phase planar stator, ?ywheel, and HTS. For system design, a 2D magnetostatic analysis is achieved to calculate the back emf and this is calculated as 820.5 mVrms . An electromagnetic simulation for the magnetic ?ux of the ?ywheel is con?rmed, because the non-axisymmetric magnetic ?ux creates hysteresis loss. The ?ywheel includes an iron disk for magnetic screening which implied the uniformly distributed magnetic ?ux without interaction between magnets. From the simulation, it is con?rmed that the magnetic ?ux of the ?ywheel is axisymmetric and the motor magnets have no in?uence on the bearing magnet. A planar stator with ?ve lines, width of 100 m, and height of 50 m is designed as an axial-?ux type brushless DC motor/generator and fabricated using Au electroplating, which is connected in the Y-connection. In general photolithography process, it is dif?cult to remove the remained SU-8 mold from the substrate after the electroplating is performed. To solve this problem, the remained SU-8 mold is lift-off using a Remover PG in an ultrasonic cleaning bath. In order to rotate the ?ywheel, the voltage is supplied with PWM method using the motor driver. The ?ywheel rotates up to 51,000 rpm at 12 V, 0.68 A and the spin-down time is approximately 3 h 20 min under the vacuum condition at the 7 mm gap between the HTS and bearing magnet. At the maximum speed, the rotational kinetic energy of the ?ywheel is 337 J. Further, the no-load three-phase electric output of the micro-generator is 802 mVrms and the electric output power through a three-phase full wave recti?er is 50 mW. Acknowledgements This research is supported by KOSEF (Korea Science and Engineering Foundation) and we appreciate SFES laboratory in KEPRI (Korea Electric Power Research Institute) for providing HTS used in the experiments.

of four steps as shown in Fig. 10. First, the vacuum condition is decreased to less than 10?3 torr by using a vacuum pump and the HTS is cooled by the stirring cryocooler below the critical temperature 72 K. Second, the ?ywheel is released by the positioner and levitates above the HTS. Third, the planar stator is moved toward the ?ywheel using the positioner and the input voltage is supplied by a brushless DC motor driver; the ?ywheel rotates by means of the driving torque. Finally, the planar stator is separated from the ?ywheel by the positioner. In order to rotate the ?ywheel, the motor driver reads the rotor position by a Hall effect sensor at 60? (electrical angle) and the voltage is supplied with the pulse width modulation (PWM) method at the A, B, and C terminals. The ?ywheel rotates up to 51,000 rpm (3.4 (kHz) × 60 (s)/4 (pairs of poles)) at 12 V, 0.68 A. The spin-down time, which means the free rotation time from maximum speed to zero without any input electrical power, is 3 h 20 min for a 7 mm gap. Fig. 11 shows the rotation speed versus the spin-down time from a maximum of 51,000 rpm to zero rpm with a 10-min time interval. The rotational kinetic energy

Fig. 12. No-load three-phase 802 mVrms electric output at maximum 51,000 rpm.

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K. Lee et al. / Sensors and Actuators A 143 (2008) 106–112 [6] F. Hellman, E.M. Gyorgy, D.W. Johnson Jr., H.M. O’Bryan, R.C. Sherwood, Levitation of a magnet over a Flat Type II superconductor, J. Appl. Phys. 63 (2) (1987) 447–450. [7] K. Lee, Ji-eun Yi, B. Kim, J. Ko, S. Jeong, M. Noh, S.S. Lee, Micro generator using ?ywheel energy storage system with high-temperature superconductor bearing, in: IEEE MEMS 2007 Conference, Kobe, Japan, 2007, pp. 287– 290. [8] E. Lee, A micro HTS renewable energy/attitude control system for micro/nano satellites, IEEE Trans. Appl. Supercond. 13 (2) (2003) 2263–2266. [9] R. Rashedin, T. Meydan, F. Borza, Electromagnetic micro-actuator array for loudspeaker application, Sens. Actuators A (2006) 118–120.

References
[1] S.A. Jacobson, A.H. Epstein, An informal survey of Power MEMS, in: ISMME2003, 2003, pp. 513–520. [2] M. Marzencki, S. Basrour, B. Charlot, S. Spikovich, M. Colin, A MEMS piezoelectric vibration energy harvesting device, in: Power MEMS Conference, Tokyo, Japan, November 28–30, 2005. [3] S. Das, D.P. Arnold, I. Zana, J.W. Park, J.H. Lang, M.G. Allen, Multi-watt electric power from a microfabricated permanent magnet generator, in: IEEE MEMS 2005 Conference, Miami, USA, 2005, pp. 287–290. [4] H. Raisigel, O. Cugat, J. Delamare, Permanent magnet planar microgenerators, Sens. Actuators A (2006) 438–444. [5] A.S. Holmes, G. Hong, K.R. Pullen, Axial-?ux permanent magnet machines for micropower generation, J. MEMS 14 (1) (2005).


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