- Email: [email protected]
Progress of superconducting bearing technologies for flywheel energy storage systems
Physica C 386 (2003) 444–450 www.elsevier.com/locate/physc
Progress of superconducting bearing technologies for flywheel energy storage systems N. Koshizuka a,*, F. Ishikawa b, H. Nasu b, M. Murakami c, K. Matsunaga c, S. Saito d, O. Saito d, Y. Nakamura e, H. Yamamoto e, R. Takahata f, Y. Itoh g, H. Ikezawa h, M. Tomita h b
d
a Morioka Laboratory, ISTEC-SRL, 3-35-2 Iioka-shinden, Morioka 020-0852, Japan Shikoku Research Institute Inc., 2109-8 Yashimanishi-machi, Takamatsu, Kagawa 761-0192, Japan c Tamachi Laboratory, ISTEC-SRL, 1-16-25 Shibaura, Minato-ku, Tokyo 105-0023, Japan Ishikawajima-Harima Heavy Industries Co., Ltd., 3-2-16 Toyosu, Koto-ku, Tokyo 135-8733, Japan e Sumitomo Special Metals Co., Ltd., 3-13-2 Takada, Toshima-ku, Tokyo 171-0033, Japan f Koyo Seiko Co., Ltd., 333 Toichi-tyo, Kashihara, Nara 634-0008, Japan g Imra Material R&D Co., Ltd., 5-50 Hachiken-cho, Kariya, Aichi 448-0021, Japan h ISTEC-SRL, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan
Abstract We report present status of NEDO project on ‘‘Superconducting bearing technologies for flywheel energy storage systems’’. We fabricated a superconducting magnetic bearing module consisting of a stator of resin impregnated YBaCuO bulks and a rotor of NdFeB permanent magnet circuits. We obtained levitation force density of 8 N/cm2 at 81 K and rotation loss per levitation force of 3 mW/N at 77 K. We confirmed that both pre-loading and excess cooling methods are effective for suppressing gradual fall of rotor due to flux creep. We designed a 10 kW h class flywheel energy storage test system and investigated feasibility of active magnetic bearings for controlling rotation axis vibration under high speed rotation of the flywheel. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk; 75.50.Ww; 85.25.Ly; 85.70.Rp Keywords: Superconducting magnetic bearing; Flywheel energy storage system; Levitation force; Rotation loss; Active magnetic bearing
1. Introduction Comparing with conventional energy storage systems such as chemical batteries, flywheel stor*
Corresponding author. Tel.: +81-19-635-9015; fax: +81-19635-9017. E-mail address: [email protected] (N. Koshizuka).
age systems have advantages on the following points: numbers of charge/discharge cycles, weight and size, replacement, reliability, safety, and pollution and toxic materials disposal problems. However, the flywheel storage systems using mechanical or magnetic bearings can deliver power for only a short period, a few seconds or minutes, by various parasitic losses such as
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02206-2
N. Koshizuka et al. / Physica C 386 (2003) 444–450
friction, hysteresis and eddy current losses of the bearings. Thus the use of lower loss superconducting magnetic bearings (SMBs) is expected for coming flywheel energy storage systems [1]. There are, nevertheless, following issues to be solved in realizing superconducting (SC) flywheel systems using SMB: (1) How to get the levitation force for supporting a heavy flywheel rotor. (2) How much we can reduce the rotation loss of SMB. (3) How to suppress the decrease of levitation force and the gradual fall of rotor during operation caused by the flux creep of SC material. (4) How we solve the rotation axis vibration problem caused by the low stiffness and damping of SMB. This is an inherent problem in SMB for flywheel applications. We use active magnetic bearings (AMBs) for controlling rotation axis vibration in our system. (5) The issue of mechanical strength of flywheel rotor becomes serious in its high speed rotation. Basically there are two types on the structure of SMBs: axial- and radial-type. The radial-type seems more preferable to the axial-type for future large-scale systems. Because it is not necessary to increase so greatly the radius of a ring shape permanent magnet circuit for scaling up the system. The mechanical strength of a large radius permanent magnet ring, which is necessary for a large axial-type SMB, could not be tolerable to the strong centrifugal force due to high circumferential velocity. We have eventually employed the radialtype structure based on a comparison of the two types in preceding NEDO project, Phase 1 (FY1995–1999) [2–4]. In present project Phase 2 (FY2000–2004), we aim to establish basic technologies on the SC bearings for 10 and 100 kW h class flywheel energy storage systems [5,6]. The target specifications are as follows; levitation force density of 10 N/cm2 , rotation loss of 2 mW/N, and proposal of measures for the gradual fall of rotors due to levitation force creep. Another aim is to fabricate a 10 kW h class prototype flywheel system using a SMB and evaluate various characteristics under real operation. These tests will reveal various issues taking place on the system during the operation.
445
2. Experimental results and discussion 2.1. Superconducting magnetic bearings [6] We fabricated SMB modules (180 mm diameter) for a 10 kW h test system consisting of a SC stator and a magnetic circuit, and evaluated the levitation force and rotation loss of these modules. The upper photo in Fig. 1 shows a SC stator module including YBCO bulk crystals. The stator is composed of eight pieces of YBCO bulk crystals with roof-tile shape [7]. The trapped field of the YBCO bulk crystals in the c-direction (perpendicular to the roof-tile plane) was 0.6–0.7 T at 77 K. In order to enhance the mechanical strength of these bulks we made a resin impregnation treatment [8]. The magnetic circuit is composed of three or four stages of ring shape NdFeB permanent magnets. We developed two types of ring magnets; split type consisting of 12 segments with joints,
Fig. 1. Photo of a prototype SC stator module of SMB for 10 kW h test system. Lower left: calculated levitation force vs. axial displacement between stator and permanent magnet circuit. Lower right: schematic view of SMB.
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and one-body type without joints. In order to enhance the levitation force of SMB we need to increase the output field strength of the magnetic circuit. We made several split type permanent magnets with differences in size, number of magnet segments and thickness of yokes. Then we selected several ring magnets with high homogeneity in the radial field direction and integrated them into a magnetic circuit. As for one-body type magnets, a higher homogeneity in field is expected because it has no joints. Actually abrupt dips of output field at joints disappeared. The top photo in Fig. 2 shows a onebody type ring magnet. We successfully made such a large one-body type ring magnet with a significantly high homogeneity in the field distribution. By using these ring magnets we fabricated a magnetic circuit as shown in the middle. The structure and sizes are given in the bottom. The levitation force is obtained by calculation using several parameters of the SC stator and
Fig. 2. One-body type magnetic circuit. From top to bottom: photo of a ring shape NdFeB permanent magnet, a magnetic circuit and structure and size of a magnetic circuit.
magnetic circuits. The lower left in Fig. 1 shows the calculated levitation force vs. axial displacement of the stator to the permanent magnet circuit. This curve shows that the maximum levitation force is 2000 N, which corresponds to the levitation force density 9 N/cm2 . This value was in good agreement with experiment. The real levitation force of a SMB was measured by using an apparatus developed in Phase 1. The levitation force density of a SMB at excess cooled 71 K reached our target value of 10 N/cm2 . As for the gradual fall (or drop) of rotor due to levitation force creep, ‘‘pre-loading method’’ and ‘‘excess cooling’’ (or supercooling) of the bearing are known to be effective for suppressing the fall. We confirmed effectiveness of the both methods on the problem. The effectiveness of excess cooling is clear in the upper of Fig. 3. The displacement (or the fall) of rotor during 3 h is much decreased at lower temperature 70 K than 81 K. On the other hand, the lower figure on pre-loading effect shows that the displacement is almost perfectly suppressed by 350 N excess loading, i.e., 1050 N preloading before starting the measurement at 700 N loading. We measured the rotation loss characteristics of SMB modules by using a test machine developed at Kenya Seiko Co., in Phase 1 [3,4]. Fig. 4 is a typical data on the rotation losses of a SC bearing. In the upper figure we can see the increase of rotation loss with rotation speed and initial load, Finit . In the lower figure we find each contribution to the total loss, i.e., SC bearing loss, cryostat loss and AMB loss. The details on this measurement are described in Refs. [3,4]. The SC loss is about 2.5 W at around 800 N loading, which corresponds to 3 mW/N. This value is slightly larger than our target of 2 mW/N. A significant amount of rotation loss of SMB is ascribed to magnetic hysteresis of SC bulk crystals which is caused by the rotation of magnetic circuit in the presence of inhomogeneous field distribution along the circular direction. Thus homogeneous field distribution is necessary for the magnetic circuit. A low rotation loss mentioned above for our SMB may be due to improvement of the field homogeneity in the magnetic circuit.
N. Koshizuka et al. / Physica C 386 (2003) 444–450
447
Fig. 3. Suppression of gradual fall of rotor by excess cooling (upper) and pre-loading method (lower). Displacement of rotor during 3 h is almost suppressed for both cases of loading 350 and 700 N by pre-loading of 1050 N in the lower figure.
In order to further reduce the rotation loss, it turned out that eddy current loss generated on the magnetic circuit should be reduced [6,9]. The eddy current is caused by AC field originated from sine wave like distribution of trapped field in the SC stator. In the SC stator composed of several pieces of bulk crystals the distribution of trapped field is of sine wave like along the circular direction. AC field is applied on the surface of magnetic circuit when it rotates. Therefore, together with the magnetic circuit we need to reduce the trapped field variation of the SC stator as much as possible. In that sense such technologies as joining two bulk crystals without weak links [10] or others
which suppress the induction of eddy current will play an important role for lowering the loss of SMB. In the present project we used Y123 bulk crystals as the material for SC stator. However, light rare earth 123 bulk crystals such as Sm123 and NdEuGd 123 are more promising, because they have higher trapped field under applied field. Recently we have successfully improved the trapped field of Sm and NdEuGd123 crystals. The trapped field maximum of Sm123 bulk crystal with diameter 38 mm reached 1.53 T, and that of NdEuGd123 crystal with the same size as Sm123 did 1.4 T both at 77 K. If we can fabricate a SMB
448
N. Koshizuka et al. / Physica C 386 (2003) 444–450
Fig. 5. SC stator module for 100 kW h class flywheel energy storage systems.
of 10 kW h class bearing. The magnetic field of nearly 1.1 T was obtained at the inside yoke region 1 mm apart from the surface. Fig. 4. Rotation loss vs. rotation speed (upper) and rotation loss vs. initial load Finit (lower) of a SMB. Total loss consists of SMB, AMB, and cryostat loss. Several contributions to the loss are given.
by using these materials, the levitation force will increase much more than that of YBCO crystals. We have also started to develop a SC bearing module for 100 kW h class flywheel systems. The size of the bearing including a magnetic circuit is about 300 mm in diameter. Fig. 5 shows a SC stator module for 100 kW h class flywheel systems. One segment of the SC stator module consists of a semicylindrical shape Y123 bulk crystal. The lower photo shows two segments of the crystals. Fig. 6 shows a 300 mm diameter magnetic circuit for 100 kW h class SC bearing [6]. The lower figure shows the structure and size of this magnetic circuit. Basically the structure is the same as that
2.2. 10 kW h flywheel energy storage test system [6] As for the total flywheel system, we have designed the basic concept of 10 kW h test system shown in Fig. 7. The flywheel, radial SMB, motor/ generator, radial and thrust AMBs are arranged as shown here. The major specifications of the system are given in Table 1. The rating rotation speed is over 15,000 rpm, stored energy 10 kW h, weight of the total flywheel rotor about 400 kg, and diameter 1 m. A 10 kW permanent magnet synchronous type motor/generator, a SMB consisting of five stories of a 60 mm height bearing module, and thrust and radial AMBs are installed. We use touchdown bearings called zero clearance auxiliary bearing (ZCAB) for system safety. The mechanical strength of the flywheel rotor is a critical issue for high speed rotation, because the centrifugal force on the outer rotor is quite strong.
N. Koshizuka et al. / Physica C 386 (2003) 444–450
Fig. 6. 300 mm diameter magnetic circuit for 100 kW h class SMB.
449
In our system, a mechanically strong carbon fiber reinforced plastics (CFRP) is used for the flywheel material. In order to fabricate a strong CFRP flywheel rotor, a homogeneous winding of carbon fibers in the radial direction is necessary. We are now investigating fabrication conditions such as temperatures of resin and mandrel, winding speed of fibers, and winding patterns. We have successfully produced flywheel rotors available for 10 kW h test operation system. In our SC flywheel systems, AMBs are used for stabilizing the high-speed rotation of the flywheel rotor. However, if the power and eddy current losses of AMBs are quite large compared with the rotation loss of SMB, the advantage of using SMB would be lost. Therefore, the reduction of such parasitic losses due to AMB may be crucial for practical systems. If we could demonstrate the high performance especially in loss characteristics in our SC flywheel energy storage system, a real development towards commercialization will start. When commercially available 10–100 kW h class SC flywheel energy storage systems are realized, they will be used for a variety of applications such as uninterruptable
Fig. 7. Basic structure of 10 kW h flywheel energy storage test system.
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N. Koshizuka et al. / Physica C 386 (2003) 444–450
Table 1 Specifications of 10 kW h class flywheel energy storage test system Rating rotation speed Storage energy FW Weight
15,860 rpma 10 kW h class
Diameter Rotation inertia
403 kg FRP ring: 125 kg Hub: 55 kg Main shaft: 223 kg 1000 mm 2.61 105 kg cm3
M/G Capacity Type Acceleration time
10 kW PM synchronous About 3 h
SMB Levitation force Density Type Gap
0.35 kg/cm3 (design data) SC: £ 123:2 £ 93:2 60 (5 stories) Radial 1.8 mm
AMB Thrust Radial Touch down
PID control Examining ZCAB (examining)
Vaccum degree
1 10
a
3
Torr
tion force creep is significantly suppressed by both pre-loading and excess cooling methods. As for the SC flywheel system, we established the basic concept and specifications of 10 kW h class test system, which uses a radial-type SMB and two AMBs for control of the lateral vibration of rotor axis. It was pointed out that the power loss and eddy current loss ascribed to the AMB should be eliminated. A further development of a mechanically strong flywheel rotor will be necessary for scaling up the SC flywheel systems. Acknowledgements We would like to thank K. Matsui (IHI), R. Yabuno, T. Oka (IMRA), H. Kameno (Koyo Seiko), K. Demachi (U. Tokyo), K. Miya (U. Keio), M. Tomita, N. Yamachi, T. Horigami, K. Nakazato, M. Anjyu and S. Tanaka (ISTEC) for discussions and contribution to the Project. This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Superconducting Bearing Technologies for Flywheel Energy Storage System.
17,200 rpm for 10 kW h storage by only FRP ring.
References power systems for data centers and stabilizing systems for power utilities.
3. Summary We described the present status of NEDO project ‘‘R&D of superconducting bearing technologies for flywheel energy storage system’’. We developed several SMB modules consisting of YBaCuO bulk stators and NdFeB permanent magnet rotors. The levitation force density was enhanced to 8 N/cm2 at 81 K. The rotation loss per levitation force 3 mW/N was obtained by improving the homogeneity in the field distribution of a permanent magnet rotor of the SMB. We confirmed that gradual fall of rotor due to levita-
[1] W.V. Hassenzahl, IEEE Trans. Appl. Supercond. 11 (2001) 1653. [2] Y. Nakagawa, Cryog. Eng. 34 (1999) 541. [3] H. Kameno, Y. Miyagawa, R. Takahata, H. Ueyama, Cryog. Eng. 34 (1999) 678. [4] H. Kameno, Y. Miyagawa, R. Takahata, H. Ueyama, IEEE Trans. Appl. Supercond. 9 (1999) 972. [5] N. Koshizuka et al., in: Proc. ISS2001, Physica C, to be published. [6] 2001 NEDO Report on R&D of superconducting bearing technologies for flywheel energy storage system. [7] K. Matsunaga, M. Tomita, N. Yamachi, K. Iiida, J. Yoshioka, M. Isono, M. Hirose, H. Nasu, H. Kameno, A. Kubo, R. Takahata, N. Kitai, H. Yamamoto, Y. Nakamura, N. Koshizuka, M. Murakami, in: Proc. ISS2001, Physica C, to be published. [8] M. Tomita, M. Murakami, Cryog. Eng. 34 (1999) 616. [9] K. Demachi, K. Miya, et al., in press. [10] J. Yoshioka, K. Iida, et al., Supercond. Sci. Technol. 15 (2002) 712.
Progress of superconducting bearing technologies for flywheel energy storage systems N. Koshizuka a,*, F. Ishikawa b, H. Nasu b, M. Murakami c, K. Matsunaga c, S. Saito d, O. Saito d, Y. Nakamura e, H. Yamamoto e, R. Takahata f, Y. Itoh g, H. Ikezawa h, M. Tomita h b
d
a Morioka Laboratory, ISTEC-SRL, 3-35-2 Iioka-shinden, Morioka 020-0852, Japan Shikoku Research Institute Inc., 2109-8 Yashimanishi-machi, Takamatsu, Kagawa 761-0192, Japan c Tamachi Laboratory, ISTEC-SRL, 1-16-25 Shibaura, Minato-ku, Tokyo 105-0023, Japan Ishikawajima-Harima Heavy Industries Co., Ltd., 3-2-16 Toyosu, Koto-ku, Tokyo 135-8733, Japan e Sumitomo Special Metals Co., Ltd., 3-13-2 Takada, Toshima-ku, Tokyo 171-0033, Japan f Koyo Seiko Co., Ltd., 333 Toichi-tyo, Kashihara, Nara 634-0008, Japan g Imra Material R&D Co., Ltd., 5-50 Hachiken-cho, Kariya, Aichi 448-0021, Japan h ISTEC-SRL, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan
Abstract We report present status of NEDO project on ‘‘Superconducting bearing technologies for flywheel energy storage systems’’. We fabricated a superconducting magnetic bearing module consisting of a stator of resin impregnated YBaCuO bulks and a rotor of NdFeB permanent magnet circuits. We obtained levitation force density of 8 N/cm2 at 81 K and rotation loss per levitation force of 3 mW/N at 77 K. We confirmed that both pre-loading and excess cooling methods are effective for suppressing gradual fall of rotor due to flux creep. We designed a 10 kW h class flywheel energy storage test system and investigated feasibility of active magnetic bearings for controlling rotation axis vibration under high speed rotation of the flywheel. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk; 75.50.Ww; 85.25.Ly; 85.70.Rp Keywords: Superconducting magnetic bearing; Flywheel energy storage system; Levitation force; Rotation loss; Active magnetic bearing
1. Introduction Comparing with conventional energy storage systems such as chemical batteries, flywheel stor*
Corresponding author. Tel.: +81-19-635-9015; fax: +81-19635-9017. E-mail address: [email protected] (N. Koshizuka).
age systems have advantages on the following points: numbers of charge/discharge cycles, weight and size, replacement, reliability, safety, and pollution and toxic materials disposal problems. However, the flywheel storage systems using mechanical or magnetic bearings can deliver power for only a short period, a few seconds or minutes, by various parasitic losses such as
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02206-2
N. Koshizuka et al. / Physica C 386 (2003) 444–450
friction, hysteresis and eddy current losses of the bearings. Thus the use of lower loss superconducting magnetic bearings (SMBs) is expected for coming flywheel energy storage systems [1]. There are, nevertheless, following issues to be solved in realizing superconducting (SC) flywheel systems using SMB: (1) How to get the levitation force for supporting a heavy flywheel rotor. (2) How much we can reduce the rotation loss of SMB. (3) How to suppress the decrease of levitation force and the gradual fall of rotor during operation caused by the flux creep of SC material. (4) How we solve the rotation axis vibration problem caused by the low stiffness and damping of SMB. This is an inherent problem in SMB for flywheel applications. We use active magnetic bearings (AMBs) for controlling rotation axis vibration in our system. (5) The issue of mechanical strength of flywheel rotor becomes serious in its high speed rotation. Basically there are two types on the structure of SMBs: axial- and radial-type. The radial-type seems more preferable to the axial-type for future large-scale systems. Because it is not necessary to increase so greatly the radius of a ring shape permanent magnet circuit for scaling up the system. The mechanical strength of a large radius permanent magnet ring, which is necessary for a large axial-type SMB, could not be tolerable to the strong centrifugal force due to high circumferential velocity. We have eventually employed the radialtype structure based on a comparison of the two types in preceding NEDO project, Phase 1 (FY1995–1999) [2–4]. In present project Phase 2 (FY2000–2004), we aim to establish basic technologies on the SC bearings for 10 and 100 kW h class flywheel energy storage systems [5,6]. The target specifications are as follows; levitation force density of 10 N/cm2 , rotation loss of 2 mW/N, and proposal of measures for the gradual fall of rotors due to levitation force creep. Another aim is to fabricate a 10 kW h class prototype flywheel system using a SMB and evaluate various characteristics under real operation. These tests will reveal various issues taking place on the system during the operation.
445
2. Experimental results and discussion 2.1. Superconducting magnetic bearings [6] We fabricated SMB modules (180 mm diameter) for a 10 kW h test system consisting of a SC stator and a magnetic circuit, and evaluated the levitation force and rotation loss of these modules. The upper photo in Fig. 1 shows a SC stator module including YBCO bulk crystals. The stator is composed of eight pieces of YBCO bulk crystals with roof-tile shape [7]. The trapped field of the YBCO bulk crystals in the c-direction (perpendicular to the roof-tile plane) was 0.6–0.7 T at 77 K. In order to enhance the mechanical strength of these bulks we made a resin impregnation treatment [8]. The magnetic circuit is composed of three or four stages of ring shape NdFeB permanent magnets. We developed two types of ring magnets; split type consisting of 12 segments with joints,
Fig. 1. Photo of a prototype SC stator module of SMB for 10 kW h test system. Lower left: calculated levitation force vs. axial displacement between stator and permanent magnet circuit. Lower right: schematic view of SMB.
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N. Koshizuka et al. / Physica C 386 (2003) 444–450
and one-body type without joints. In order to enhance the levitation force of SMB we need to increase the output field strength of the magnetic circuit. We made several split type permanent magnets with differences in size, number of magnet segments and thickness of yokes. Then we selected several ring magnets with high homogeneity in the radial field direction and integrated them into a magnetic circuit. As for one-body type magnets, a higher homogeneity in field is expected because it has no joints. Actually abrupt dips of output field at joints disappeared. The top photo in Fig. 2 shows a onebody type ring magnet. We successfully made such a large one-body type ring magnet with a significantly high homogeneity in the field distribution. By using these ring magnets we fabricated a magnetic circuit as shown in the middle. The structure and sizes are given in the bottom. The levitation force is obtained by calculation using several parameters of the SC stator and
Fig. 2. One-body type magnetic circuit. From top to bottom: photo of a ring shape NdFeB permanent magnet, a magnetic circuit and structure and size of a magnetic circuit.
magnetic circuits. The lower left in Fig. 1 shows the calculated levitation force vs. axial displacement of the stator to the permanent magnet circuit. This curve shows that the maximum levitation force is 2000 N, which corresponds to the levitation force density 9 N/cm2 . This value was in good agreement with experiment. The real levitation force of a SMB was measured by using an apparatus developed in Phase 1. The levitation force density of a SMB at excess cooled 71 K reached our target value of 10 N/cm2 . As for the gradual fall (or drop) of rotor due to levitation force creep, ‘‘pre-loading method’’ and ‘‘excess cooling’’ (or supercooling) of the bearing are known to be effective for suppressing the fall. We confirmed effectiveness of the both methods on the problem. The effectiveness of excess cooling is clear in the upper of Fig. 3. The displacement (or the fall) of rotor during 3 h is much decreased at lower temperature 70 K than 81 K. On the other hand, the lower figure on pre-loading effect shows that the displacement is almost perfectly suppressed by 350 N excess loading, i.e., 1050 N preloading before starting the measurement at 700 N loading. We measured the rotation loss characteristics of SMB modules by using a test machine developed at Kenya Seiko Co., in Phase 1 [3,4]. Fig. 4 is a typical data on the rotation losses of a SC bearing. In the upper figure we can see the increase of rotation loss with rotation speed and initial load, Finit . In the lower figure we find each contribution to the total loss, i.e., SC bearing loss, cryostat loss and AMB loss. The details on this measurement are described in Refs. [3,4]. The SC loss is about 2.5 W at around 800 N loading, which corresponds to 3 mW/N. This value is slightly larger than our target of 2 mW/N. A significant amount of rotation loss of SMB is ascribed to magnetic hysteresis of SC bulk crystals which is caused by the rotation of magnetic circuit in the presence of inhomogeneous field distribution along the circular direction. Thus homogeneous field distribution is necessary for the magnetic circuit. A low rotation loss mentioned above for our SMB may be due to improvement of the field homogeneity in the magnetic circuit.
N. Koshizuka et al. / Physica C 386 (2003) 444–450
447
Fig. 3. Suppression of gradual fall of rotor by excess cooling (upper) and pre-loading method (lower). Displacement of rotor during 3 h is almost suppressed for both cases of loading 350 and 700 N by pre-loading of 1050 N in the lower figure.
In order to further reduce the rotation loss, it turned out that eddy current loss generated on the magnetic circuit should be reduced [6,9]. The eddy current is caused by AC field originated from sine wave like distribution of trapped field in the SC stator. In the SC stator composed of several pieces of bulk crystals the distribution of trapped field is of sine wave like along the circular direction. AC field is applied on the surface of magnetic circuit when it rotates. Therefore, together with the magnetic circuit we need to reduce the trapped field variation of the SC stator as much as possible. In that sense such technologies as joining two bulk crystals without weak links [10] or others
which suppress the induction of eddy current will play an important role for lowering the loss of SMB. In the present project we used Y123 bulk crystals as the material for SC stator. However, light rare earth 123 bulk crystals such as Sm123 and NdEuGd 123 are more promising, because they have higher trapped field under applied field. Recently we have successfully improved the trapped field of Sm and NdEuGd123 crystals. The trapped field maximum of Sm123 bulk crystal with diameter 38 mm reached 1.53 T, and that of NdEuGd123 crystal with the same size as Sm123 did 1.4 T both at 77 K. If we can fabricate a SMB
448
N. Koshizuka et al. / Physica C 386 (2003) 444–450
Fig. 5. SC stator module for 100 kW h class flywheel energy storage systems.
of 10 kW h class bearing. The magnetic field of nearly 1.1 T was obtained at the inside yoke region 1 mm apart from the surface. Fig. 4. Rotation loss vs. rotation speed (upper) and rotation loss vs. initial load Finit (lower) of a SMB. Total loss consists of SMB, AMB, and cryostat loss. Several contributions to the loss are given.
by using these materials, the levitation force will increase much more than that of YBCO crystals. We have also started to develop a SC bearing module for 100 kW h class flywheel systems. The size of the bearing including a magnetic circuit is about 300 mm in diameter. Fig. 5 shows a SC stator module for 100 kW h class flywheel systems. One segment of the SC stator module consists of a semicylindrical shape Y123 bulk crystal. The lower photo shows two segments of the crystals. Fig. 6 shows a 300 mm diameter magnetic circuit for 100 kW h class SC bearing [6]. The lower figure shows the structure and size of this magnetic circuit. Basically the structure is the same as that
2.2. 10 kW h flywheel energy storage test system [6] As for the total flywheel system, we have designed the basic concept of 10 kW h test system shown in Fig. 7. The flywheel, radial SMB, motor/ generator, radial and thrust AMBs are arranged as shown here. The major specifications of the system are given in Table 1. The rating rotation speed is over 15,000 rpm, stored energy 10 kW h, weight of the total flywheel rotor about 400 kg, and diameter 1 m. A 10 kW permanent magnet synchronous type motor/generator, a SMB consisting of five stories of a 60 mm height bearing module, and thrust and radial AMBs are installed. We use touchdown bearings called zero clearance auxiliary bearing (ZCAB) for system safety. The mechanical strength of the flywheel rotor is a critical issue for high speed rotation, because the centrifugal force on the outer rotor is quite strong.
N. Koshizuka et al. / Physica C 386 (2003) 444–450
Fig. 6. 300 mm diameter magnetic circuit for 100 kW h class SMB.
449
In our system, a mechanically strong carbon fiber reinforced plastics (CFRP) is used for the flywheel material. In order to fabricate a strong CFRP flywheel rotor, a homogeneous winding of carbon fibers in the radial direction is necessary. We are now investigating fabrication conditions such as temperatures of resin and mandrel, winding speed of fibers, and winding patterns. We have successfully produced flywheel rotors available for 10 kW h test operation system. In our SC flywheel systems, AMBs are used for stabilizing the high-speed rotation of the flywheel rotor. However, if the power and eddy current losses of AMBs are quite large compared with the rotation loss of SMB, the advantage of using SMB would be lost. Therefore, the reduction of such parasitic losses due to AMB may be crucial for practical systems. If we could demonstrate the high performance especially in loss characteristics in our SC flywheel energy storage system, a real development towards commercialization will start. When commercially available 10–100 kW h class SC flywheel energy storage systems are realized, they will be used for a variety of applications such as uninterruptable
Fig. 7. Basic structure of 10 kW h flywheel energy storage test system.
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N. Koshizuka et al. / Physica C 386 (2003) 444–450
Table 1 Specifications of 10 kW h class flywheel energy storage test system Rating rotation speed Storage energy FW Weight
15,860 rpma 10 kW h class
Diameter Rotation inertia
403 kg FRP ring: 125 kg Hub: 55 kg Main shaft: 223 kg 1000 mm 2.61 105 kg cm3
M/G Capacity Type Acceleration time
10 kW PM synchronous About 3 h
SMB Levitation force Density Type Gap
0.35 kg/cm3 (design data) SC: £ 123:2 £ 93:2 60 (5 stories) Radial 1.8 mm
AMB Thrust Radial Touch down
PID control Examining ZCAB (examining)
Vaccum degree
1 10
a
3
Torr
tion force creep is significantly suppressed by both pre-loading and excess cooling methods. As for the SC flywheel system, we established the basic concept and specifications of 10 kW h class test system, which uses a radial-type SMB and two AMBs for control of the lateral vibration of rotor axis. It was pointed out that the power loss and eddy current loss ascribed to the AMB should be eliminated. A further development of a mechanically strong flywheel rotor will be necessary for scaling up the SC flywheel systems. Acknowledgements We would like to thank K. Matsui (IHI), R. Yabuno, T. Oka (IMRA), H. Kameno (Koyo Seiko), K. Demachi (U. Tokyo), K. Miya (U. Keio), M. Tomita, N. Yamachi, T. Horigami, K. Nakazato, M. Anjyu and S. Tanaka (ISTEC) for discussions and contribution to the Project. This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Superconducting Bearing Technologies for Flywheel Energy Storage System.
17,200 rpm for 10 kW h storage by only FRP ring.
References power systems for data centers and stabilizing systems for power utilities.
3. Summary We described the present status of NEDO project ‘‘R&D of superconducting bearing technologies for flywheel energy storage system’’. We developed several SMB modules consisting of YBaCuO bulk stators and NdFeB permanent magnet rotors. The levitation force density was enhanced to 8 N/cm2 at 81 K. The rotation loss per levitation force 3 mW/N was obtained by improving the homogeneity in the field distribution of a permanent magnet rotor of the SMB. We confirmed that gradual fall of rotor due to levita-
[1] W.V. Hassenzahl, IEEE Trans. Appl. Supercond. 11 (2001) 1653. [2] Y. Nakagawa, Cryog. Eng. 34 (1999) 541. [3] H. Kameno, Y. Miyagawa, R. Takahata, H. Ueyama, Cryog. Eng. 34 (1999) 678. [4] H. Kameno, Y. Miyagawa, R. Takahata, H. Ueyama, IEEE Trans. Appl. Supercond. 9 (1999) 972. [5] N. Koshizuka et al., in: Proc. ISS2001, Physica C, to be published. [6] 2001 NEDO Report on R&D of superconducting bearing technologies for flywheel energy storage system. [7] K. Matsunaga, M. Tomita, N. Yamachi, K. Iiida, J. Yoshioka, M. Isono, M. Hirose, H. Nasu, H. Kameno, A. Kubo, R. Takahata, N. Kitai, H. Yamamoto, Y. Nakamura, N. Koshizuka, M. Murakami, in: Proc. ISS2001, Physica C, to be published. [8] M. Tomita, M. Murakami, Cryog. Eng. 34 (1999) 616. [9] K. Demachi, K. Miya, et al., in press. [10] J. Yoshioka, K. Iida, et al., Supercond. Sci. Technol. 15 (2002) 712.