Abstract Body

Introduction - In transportation systems employing belts, wheels, or gear conveyors, issues such as friction and wear arise at connecting points. A contactless transporter system has been developed in response to these challenges, particularly suited for specialized environments like clean rooms. This study focuses on a magnetic levitation system incorporating permanent magnets and high-temperature superconductors (HTS). The load stage is equipped with powerful permanent magnets generating levitation force through repulsion, while the stability is maintained by the pinning force of HTS, eliminating the need for a continuous levitation control system [1]-[6]. The transporter smoothly glides above a magnetic rail, and when electric current excites the propulsion coils integrated into it, the transporter is propelled through the generation of a magnetic gradient [7]-[11]. This paper presents an experimental analysis investigating the propulsion force generated by two different sizes of HTSs when the propulsion coils from both the front and rear sides of the transporter are magnetized, and the coil pitch is varied.

The transporter system consists of a magnetic rail composed of neodymium magnets in a Halbach Array, enhancing the magnetic field in the upper of it. The main body of the transporter has containers for the HTSs and a free space in the middle to install the load stage. Here, the main body and the load stage are connected by a vertical linear guideway that allows an independent and free movement of the stage related to the main body. Also, the load stages are composed of neodymium magnets in a way that produces a repulsive force against the magnetic rail. Using four HTSs, the yawing and rolling motion of the transporter is suppressed, and it can move freely along the magnetic rail.

The propulsion method consists of magnetizing the rear propulsion coil and demagnetizing the front one of the transporter. Then, a magnetic gradient is generated, which tilts the HTSs and, consequently, the transporter, creating a forward momentum. This paper considers two types of coil pitch: the coil pitch and excited coil pitch. The coil pitch d is the distance between the central axes of neighboring propulsion coils and for this paper, and the excited coil pitch d_e is the distance between the central axes of the propulsion coils that will be excited, that is, the distance between the rear and front side propulsion coils. Also, two HTSs are used for the experiments, with diameter values of 55 mm and 60 mm.

The present experiment parameters are excitation currents I from 1 to 3 A per coil, coil pitches d = 25, 30 [mm], excited coil pitch d_e = 350, 360 [mm], and levitation gap g = 23 [mm]. Considering the rear side coil’s central axis as the origin position x = 0 mm, the experiment position range x is from 0 to 50 mm with a measuring interval of 5 mm. Load cells installed at the front and rear sides of the transporter measure the propulsion force at each position.

Figure 1 shows the propulsion force for each excited coil pitch and HTSs. For both excited coil pitch values, d_e = 350, 360[mm], the HTS with ⌀ =60mm has a maximum propulsion force that is 4.78% and 0.59% larger, respectively, than the HTS with a diameter of 55 mm. This is likely because, in the magnetic rail length direction, the larger HTS is under the effect of the magnetic field in a larger range than the other HTS. However, the small HTS presents local and global maximum propulsion force while the larger HTS shows only one global maximum force in both results.

Now, analyzing the average propulsion force, there is another discussion. Considering the average calculation range length L = 25[mm] for coil pitch d = 350[mm], the smaller HTS shows an average force of 0.169 N, which is 6.41% larger than the larger HTS. The L = 30[mm] (coil pitch d = 360[mm]), the smaller HTS shows an average force of 0.179 N, which is 8.38% larger than the larger HTS. These results are likely because, although the HTS surface area under the magnetic field influence is smaller for the small HTS, the total area under this influence across the entire experiment range (x = 0-60[mm]) is larger than the larger HTS.

In conclusion, the influence of the relation magnetic field – HTS surface area in the propulsion force was analyzed. From the results, the larger HTS showed a larger maximum propulsion force, but a small average propulsion force. In contrast, the smallest HTS showed a slightly small maximum propulsion force, but it generated a larger average propulsion force. From that, the larger average propulsion force will be considered the optimal result due to the smoothest characteristics, which will impact positively in future research involving velocity control systems.

References

[1] F. C. Moon, Superconducting levitation, John Wiley & Sons, New York, 1994.
[2] J. Hull, M. Murakami, “Applications of bulk high-temperature Superconductors”, Proc. of IEEE, vol.92, 2004, pp1705-1718.
[3] J. Jiang, Y. Zhao, Y. Li, and L. Zhao, "Trapped field and levitation performance of a YBCO superconductor magnetized in different external magnetic fields", J. Supercond. Novel Magnetism, vol. 32, no. 16, pp. 1-4, Jul. 2019.
[4] E. Maruo, M. Komori, K. Asami, N. Sakai, “Basic Study on Super-conducting Magnetic Bearing (SMB) With Superconducting Coil”, IEEE Trans. Appl. Supercond. vol.22, No.3, 2012, 201704.
[5] Yiming Zhang, Kaizhong Ding, Shuangsong Du, “Design Study of a High Loading Superconducting Magnetically Levitated Planar Motor”, IEEE Trans. Appl. Supercond. vol.31, no.5, 2021, 3601504
[6] S.B. Kim; S. Ozasa; M. Sawae, “Dynamic Characteristics of a 3-D Superconducting Actuator With Arranged Permanent Magnets and Electromagnets”, IEEE Trans. Appl. Supercond. vol.26, no.4, 2016, 3601604
[7] S. Ohashi, D. Dodo, “Influence of the Propulsion System on the Levitation Characteristics of the HTSC-Permanent Magnet Hybrid Magnetically Levitated System”, IEEE Trans. Appl. Supercond. vol.17, no.2, 2007, pp2083-2086.
[8] Y. Takaki, T. Sumida, S. Ohashi, “Improvement of Velocity Con-trol in the Permanent Magnet-HTS Hybrid Magnetically Levitated Conveyance System”, IEEE Region 10 Conference (TENCON), 2016, pp3105-3108.
[9] T. Sumida, Y. Kamitani, N. Yamada, S. Ohashi, “Propulsion Char-acteristics using Pinned flux of the HTS in the Permanent Magnet-HTS Hybrid Magnetically Levitated Conveyance System”, IEEE Transaction on Applied Superconductivity, 2016, Vol.26, No.4, 3600905.
[10] Y.Sakai, T. Sumida, Y. Takaki, S. Ohashi, “Improvement of aver-age velocity error in the HTS magnetically levitated conveyance system”, IEEE Xplore Digital Library 2017 11th International Symposium on Linear Drives for Industry Applications (LDIA), 2017, 17389323.
[11] A. H. Takinami, R. Saeki and S. Ohashi, "Improvement of the Propulsion Force by the Excitation Principle of the Propulsion Coil in the Permanent Magnet-HTS Hybrid Magnetically Levitated Transport System," in IEEE Transactions on Applied Superconduc-tivity, vol. 32, no. 6, pp. 1-4, Sept. 2022, Art no. 3602504, doi: 10.1109/TASC.2022.3174188.

Figure 1. Propulsion force for d_e = 350[mm] (left) and d_e = 360[mm] (right).

Keywords: High-Temperature Superconductor, Levitation, Permanent Magnet, Pinning, Coils

Abstract Body

Future carbon neutral society demands us major improvement of infrastructures with greenhouse gas emissions. Especially, focusing on some infrastructures such as factories, power plants and substations, rotating machines can be found in many places. Nowadays, 40-50% of electric energy is consumed by electric motors in all over the world. While almost 95% is usual value of motor efficiency, if we can improve 1% motor efficiency, it is possible to reduce 80 billion kWh and CO2 emissions by 32 million tons in all over the world [1]. Therefore, some research groups like electric machines and superconducting (SC) applications have been studying to realize higher efficiency rotating machines. In general, well known motor loss factors are copper loss, iron loss, mechanical loss at bearings, stray loss and so on. Then, SC wires contribute to reduce copper losses when we apply the wires to field windings because of the zero-dc resistance of the SC wires. And iron loss can be reduced by means of high-quality electromagnetic steel sheet. However, it is impossible for rotating machines to do without bearings. In other word, mechanical loss by bearings is quite difficult to remove.

Magnetic bearings can contribute to reduce mechanical frictions of usual bearings and therefore increase of efficiency of rotating machines is expected in the future. However, it is known that the magnetic bearings have complicated control systems as well as a merit of no mechanical contact.

SC magnetic bearings (SMBs) can simplify the components of the usual magnetic bearings via pinning effect between superconductors and permanent magnets (PMs). The physical phenomena means that it is possible for us to obtain stable magnetic levitation without complicated control systems.

We have been proposing a SMB using a PM and a ring shape layered REBCO SC tapes [2, 3]. The proposing SMBs have a potential to enlarge radial direction easily in comparison with bulk superconductors. And now we gave been investigating φ140 mm SMBs shown in Fig. 1.

In this presentation, we report some electromagnetic and mechanical characteristics of the φ140 mm SMBs using PMs with different magnetization direction such as radial or axial directions. And the effect of radial direction stabilization using other components using a PM and a cylindrical superconductor.

References

[1] N. Tachi, H. Koibuki and K. Takahashi, “Fuji Electric’s Top Runner Motor – Loss-Reduction Technology fof “Premium Efficiency Motor”,” Fuji Electric Review, Vol. 61, No. 1, pp. 31-35, 2015  

[2] Y. Terao, S. Fuchino and M. Ohya, "Stiffness and loss characteristics of superconducting magnetic bearings using layered HTS tapes and a permanent magnet." Physica C: Superconductivity and its Applications, Vol. 614, 1354401, 2023.

[3] Y. Terao, S. Fuchino and M. Ohya, “Electromagnetic Characteristics of Stacked Superconductors and Permanent Magnets Applying for Magnetic Bearings,” TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan), Vol. 58, No. 5, pp. 245-251, 2023, in Japanese.

Acknowledgment

This research was conducted as a commissioned project as JPNP14004 by New Energy and Industrial Technology Development Organization (NEDO).

(a)Layered Ring Shape Superconductor (b) Ring Permanent Magnet

(b)Fig. 1. Components of aφ140 mm superconducting magnetic bearing.

Keywords: Permanent magnets, Rotating machines, Superconducting magnetic bearings, Superconducting tapes

Abstract Body

The Magnetic bearings support rotors without contact by the magnetic force. Therefore, friction, noise, and energy loss can be eliminated. In this study, the HTS magnetic bearings are used, featuring a rotor consisting of permanent magnets and the HTS on the stator side. The rotor is levitated by the pinning force of the HTS which interacts with the magnetic flux from the rotor. This pinning force allows the maintenance of both levitation and guidance directions stability without the need for a control system [1]-[5].

On the other hand, the stability of the system under vibration, such as when mounted on a vehicle, has not been clearly understood. In this paper, the entire HTS magnetic bearing system is vibrated by an external force. It is assumed that the HTS magnetic bearing system will be mounted on a vehicle or other moving object and applied as a superconducting flywheel for power storage.

The rotational characteristics will be measured under three conditions: without vibration, with vibration, and with step vibration. The flux variation, calculated from the measured the rotational characteristics and the oscillation of the rotor, will be used to evaluate stability. The flux variation is one of the factors of rotational attenuation of the HTS magnetic bearing rotor [6][7].

The HTS magnetic bearing rotor is levitated by the pinning effect when HTS is cooled in a magnetic field. The HTS used in this study is a yttrium-based superconducting bulk material (YBaCuO). The rotor consists of a ring-shaped neodymium permanent magnet with an attached yoke. The yoke improves the utilization of the magnetic flux and improves the vibration suppression effect of the rotor.

In the experiment with vibration, the entire HTS magnetic bearing system is mounted on a bogie and subjected to vibration. The vibration is applied to the entire device for 100[s]. The amplitude is x = 300[mm], the period is T = 2.0, 4.0[s], and the height of the step is 20[mm]. The levitation gap of the rotor is g = 8 [mm].

Figure 1 shows the comparison of rotational attenuation. In the vibration experiment, the oscillation and rotational attenuation of the rotor increased within the vibration range for both periods T = 2.0 and 4.0 [s]. Comparing the results with and without steps, the rotational attenuation increased when the step was present. This is likely because not only lateral vibration, but also vertical vibration was added to the rotor when it passed through the step.

Comparing the period T = 2.0 and 4.0[s], the displacement of the rotor was larger, and the amount of the flux variation was larger for period T = 2.0[s]. The oscillation of the rotor causes a change in the flux linked to the HTS. This flux variation on the HTS surface is considered to have increased the rotational attenuation due to the electromagnetic force.

However, in the vibration experiment, the rotation remained stable after the vibration range, and the rotational attenuation was not significantly different from that in the experiment without vibration. Additionally, the levitation of the rotor was also maintained.

References

[1] E.Maruo,M.Komori,K.Asami,N.Sasaki,“Basic study on superconducting magnetic bearing (SMB) with superconducting coil ”, IEEE TransAppl Supercond, vol. 22, no. 3, 5201704, June 2012 ”J. Clerk Maxwell,A Treatise on Electricity and Magnetism,3rd ed., vol. 2. Oxford:Clarendon, 1892, pp. 68-73.

[2] J.Hull, M.Murakami,“Apprications of bulk high temperature superconductors ”, in Proc of IEEE, vol. 92, no. 10, October 2004, pp1705-1718

[3] M. Okano, N. Tamada, S. Fuchino, I. Ishii, T. Iwamoto, 2000 Numerical analysis of a superconducting bearing IEEE Trans Appl Supercond, vol.10, no. 1, pp909-912

[4] Z. Yu, G. Zhang, Q. Qiu, L. Hu, D. Zhang, M. Qiu, 2015 Analysis of levitation characteristics of a radial-type high-temperature superconducting bearing based on numerical simulation IEEE Trans Appl Supercond,vol. 25, no. 3, 3600605

[5] F. C. MOON, “SUPERCONDUCTING LEVITATION”, JOHN WILEY & SONS, NEW YORK, (1994)

[6] R.Taniguchi,S.Ishida,K. Yagi and S.Ohashi, “Analysis of the Amount of Flux Variation on the HTS Surface by the Oscillation of HTS Magnetic Bearing Rotor,” IEEE Transaction on Magnetics(Volumes: 59,Issue: 11, 8001004, 2023).

[7] K.Yagi,R.Taniguchi,S.Ishida and S.Ohashi, “Effects of the External Vibrations on Rotational Stability of the HTS MagneticBearings,”2023 IEEE International Magnetic Conference(INTERMAG), Sendai, Japan, 2023, doi:10.1109/INTERMAG50591.2023.10265098

Figure 1. The comparison of rotational attenuation

Keywords: Magnetic Levitation, High-Temperature Superconductor, Flux Pinning

Abstract Body

Introduction: In the drive to meet net-zero targets EU based wind turbine manufacturers such as Siemens Gamesa, Vestas and GE Energy are building turbines with ratings in excess of 15 MW. In China Mingyang has built a 22 MW demonstrator, pushing the EU manufacturers to follow suit. A low speed permanent magnet synchronous generator (PMSG), either direct drive or coupled to a 2-stage gearbox, is used. Due to low speed these generators are very heavy. According to the IEA 15 MW Standard Design, the PMSG used weighs in the region of 372 tons, with the active iron weighing 182 tons [1]. It is very challenging to transport and install such heavy generators as part of the nacelle structure at the top of a tower in an offshore wind turbine. For floating offshore wind turbines such a nacelle mass is challenging for maintaining stability during installation and operation, adding significantly to the cost. The active iron can be reduced by using air-cored copper windings, but this reduces the airgap magnetic field requiring more permanent magnet material, which is more expensive than iron. Edinburgh has demonstrated such a machine, the so-called CGEN modular generator [2], which has been successfully applied to small wind, wave and tidal systems. In [3] the authors have shown how the axial flux multi-stage CGEN air-cored concept has evolved into an HTS machine with all the active iron being eliminated. Each permanent magnet is replaced by a superconducting coil forming an array of air-cored coils as shown in Figure 1. Zhang et al showed that if such an array of coils replaced the permanent magnet structure in the CGEN machine the power per volume could increase by a factor of 4, whilst retaining the conventional air-cored stationary copper winding. However, it was a conceptual design that involved only preliminary numerical modelling analysis. As a follow up, a ten-year project SuperMachine funded by the Royal Academy of Engineering Chair in Emerging Technologies program was awarded to Mueller for development of this technology. In this paper the authors will describe the engineering design, build and test of a prototype Halbach HTS array module, in order to verify the concept.

Method – The main objective of the proof of concept is to show that the coil array is able to produce a magnetic field distribution similar to the permanent magnet module of a CGEN pole pair module, and to verify the design and modelling techniques used.  The sizing of the proof of concept module is based on the volume of the cryostat available within the Edinburgh HTS lab. A 2-stage module is therefore proposed so that it can be accommodated within the cryostat, with dimensions shown in Figure 2. AMSC Type 8502, 12 mm, YBCO tape will be used to wind the coils, 8 in total. Testing is undertaken in 2 stages: (i) Test in a LN2 bath; and (ii) Test in the Edinburgh cryostat. The first test allows rapid testing at lower current levels to verify the models, and to check that all coils are working. In the second test we will design a conduction cooling system for the cryostat to allow testing at lower temperatures, 30 K, so that the array can be tested at higher currents. The conduction cooling design will be used to design a custom modular cryostat for building a complete machine in a future project. The coils and cooling system have been designed, and manufacture is taking place. Results of the experimental work will be presented at the conference.

Results – In order to design the Halbach array module an understanding of the electromagnetic performance is required to calculate the coil forces acting so that the module structural support can be built. COMSOL multiphysics was used to evaluate the electromagnetic, mechanical, and thermal characteristics of the demonstrator The magnetic field distribution is shown in Figure 3(a) and the resulting forces acting on the stator coils is presented in Figure 3(b). With these forces a structural support has been designed as shown in Figure 4, in which the HTS coils can be moved to investigate the magnetic field distribution for different coil positions. The coils and cooling system have been designed, and manufacture is taking place. Results of the experimental work will be presented at the conference to prove the air cored HTS Halbach array concept, providing confidence to design complete machines using such modules as the basis.

References

[1] Definition of the IEA Wind 15-Megawatt Offshore Reference Wind Turbine, IEA Wind TCP Task 37, March 2020

[2] A. McDonald et al. “1MW Multi-stage air cored permanent magnet generator for wind turbines”,  6th  IET International Conference on Power Electronics, Machines & Drives, Bristol, April 2012.

[3] H. Zhang et al, "High temperature superconducting Halbach Array topology for air-cored electrical machines," J. Phys.: Conf. Ser., vol. 1559, 012140, 2020

Acknowledgment

This work has been funded by the Royal Academy of Engineering Chair in Emerging Technologies project awarded to Prof. Mueller.

Abstract Body

High temperature superconductors have the characteristics of high current carrying capacity, high mechanical properties and low anisotropy, and are used in applications requiring strong magnetic fields or high current carrying capacity, such as particle acceleration and fusion and other high-field large magnets. When a superconducting conductor is used in a magnetic field, the changing magnetic field will produce a certain amount of AC loss, which will put a burden on the refrigerator and cause a series of negative effects on the thermodynamic stability of the equipment. Here, we report the experimental methods and results for AC losses in three typical HTS conductors, i.e. REBCO CORC, REBCO TSTC, Bi2223 TSTC.

Abstract Body

High-Tc superconducting (HTS) synchronous machines have promising application prospects due to their high power-to-mass ratio and high efficiency. Most existing HTS synchronous machines are excited by traditional driven mode through current leads which incur considerable cryogenic heat load and maintenance difficulty of the rotating-stationary parts. Using HTS dynamo to wirelessly charge HTS rotor magnet has been a promising alternative solution, but is limited by the drawback of rotation-excitation coupling.

In this work, we demonstrate a synchronous machine with HTS rotator windings which are wirelessly powered by a transformer-self rectifier. The new approach not only dealt with the disadvantages of traditional driven mode, but also managed to decouple the excitation from rotation. This work may be of practical significance for the design and implementation of HTS synchronous machines in the future.

Abstract Body

To reduce carbon dioxide emissions from aircraft, an electric propulsion system is attracted. Regarding this problem, we are developing a superconducting propulsion system with superconducting synchronous motors and generators with a high output density compared to conventional ones. For aircraft propulsion systems, it is a crucial point to higher the output density of the superconducting motor. As the output is proportional to the revolution speed, we apply a saddle-shaped winding coil to the field coil of the motor using REBCO wires. The coil is directly replaced and fixed on the shaft, hence the mechanical strength of the rotor with the coils required in terms of a high revolution can be stronger than the rotor with the conventional racetrack coils. Thus, it is essential to investigate the effect that the end part of the saddle-shaped coil on the output of the motor. The coil end must be shorter than an accelerator application. The end part of the saddle-shaped coil is generally longer than the conventional racetrack coil since REBCO wire is tape-shaped. That results in the increase of the axial length and the weight of the motor. However, it is necessary to design the coil end so as not to degraded beyond the critical torsion of the REBCO tapes. First, we designed the saddle-shaped field coil so that the coil end contributes to the output of the motor. We calculated the output by the three-dimensional (3D) finite element analysis. In this paper, we proposed the design of the saddle-shaped field coil where the coil end contributes to the output.

Acknowledgment

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under Project JPNP15005.

Keywords: superconducting synchronous motor, REBCO, saddle-shaped coil

Abstract Body

High-temperature superconducting induction motors (HTS-IM) have attracted considerable attention due to their near 100% energy efficiency and simple structure. To further advance this technology, we are incorporating an active-type bearingless mechanism into the HTS-IM, which will prevent the rotor from making mechanical contact while maintaining a compact design. This approach will help avoid issues related to mechanical wear and friction. Given that HTS-IMs are fundamentally induction motors, the primary challenge, as highlighted in a research on active-type bearingless mechanism for normal-conducting induction motors[1], lies in the mutual interference between the torque magnetic field and the bearingless magnetic field. To address this challenge, a solution is presented in Fig.1. As shown in Fig.1(a), the rotor will first be redesigned based on a standard HTS-IM. More specifically, as illustrated in Fig.1(b), the HTS rotor bar will be reconfigured as a "pole-specific circuit[2]," which allows only the 4-pole magnetic field to induce current, while the 2-pole magnetic field does not induce any current. This design enables the decoupling of the torque magnetic field from the bearingless magnetic field, allowing them to be independently controlled. Regarding the stator, as shown in Fig.1(c), the dual-purpose no-voltage (DPNV)[3]excitation method will be employed, which only requires minor alterations to the winding coils without the need to modify the stator core. Finally, as shown in Fig.1(d), a control system for the bearingless mechanism will be developed, utilizing magnetic levitation control algorithms such as PID control, sliding mode control, and feedback linearization to stabilize the system.

References

[1] J. Chen, Y. Fujii, M. W. Johnson, A. Farhan and E. L. Severson, "Optimal Design of the Bearingless Induction Motor," IEEE Transactions on Industry Applications, vol. 57, no. 2, pp. 1375-1388, 2021.

[2] A. Chiba, et al. "Magnetic bearings and bearingless drives," Elsevier, 2005.

[3] E. L. Severson, et al. "Design of dual purpose no-voltage combined windings for bearingless motors," IEEE Transactions on Industry Applications, vol. 53, no. 5, pp. 4368-4379, 2017.

Figure 1. Design process of the bearingless system for high-temperature superconducting induction motor

Keywords: Active-type bearingless system, High-temperature superconducting induction motor

Abstract Body

The global transition to sustainable energy highlights the importance of efficient use of resources and innovative energy solutions. In particular, ports that rely on commercial electricity and fossil fuels are a major contributor to CO₂ emissions and operating costs. Meanwhile, high-temperature superconductors (HTS) in rotating machinery are one of the key components to enhance hydropower generation systems, including power generation and ship propulsion motors with hydrogen cooling system[1]. Previous studies have investigated the integration of HTS generators with hydrogen systems to obtain significant benefits in line with the carbon neutral port (CNP) concept. As a result, CO₂ emissions could be significantly reduced by utilizing both electricity and hydrogen [2]. This study examines the feasibility and potential effectiveness of deploying rotating machines equipped with HTS generators to meet the long-term energy demands of port environments. An optimized system configuration is proposed, based on MATLAB-based simulations. The optimization process is guided by three evaluation criteria: Efficiency of Power Utilization (EPU), which aims to minimize surplus power; Effective CO₂ Reduction (ECR), which focuses on reducing external power dependency; and Rate of Stored Hydrogen (RSH), which assesses the adequacy of hydrogen storage in meeting operational requirements. These criteria are quantified, and the comprehensive evaluation value is calculated as the sum of these metrics. **To achieve an optimal configuration, this study uses the Simulink Design Optimization Toolbox's Response Optimizer to fine-tune generator specifications and hydrogen tank capacity. By iteratively adjusting these parameters within the simulation framework, the optimization process aims to improve the evaluation metrics. The results suggest that the proposed system configuration appropriately sizes the HTS generators and optimizes hydrogen tank capacity to meet the energy demands of specific port operations. Additionally, the study notes the operational advantages of using surplus electricity for hydrogen production, which may contribute to a more sustainable and efficient energy solution for port environments. The findings indicate that HTS and hydrogen integration could support efforts to develop more sustainable port infrastructures.

References

[1] S. Hara et al., "Development of Liquid Hydrogen Cooling System for a Rotor of Superconducting Generator," in IEEE Transactions on Applied Superconductivity, vol. 31, no. 5, pp. 1-5, Aug. 2021, Art no. 5202505, doi: 10.1109/TASC.2021.3065814.
[2]K. Tsuzuki et al., "Study of Superconducting System Under Hydrogen Demand for Carbon Neutral Port," in IEEE Transactions on Applied Superconductivity, vol. 34, no. 3, pp. 1-4, May 2024, Art no. 3601804, doi: 10.1109/TASC.2024.3360940.

Acknowledgment

This work was supported by JKA and its promotion funds from KEIRIN RACE

Abstract Body

The authors conducted a design study on a 1-MJ class mobile superconducting magnetic energy storage (SMES) system for on-site eigenvalue measurement of electrical power systems. The SMES coil consists of a force-balanced helical coil (FBC) for mobility and weight saving because the FBC can minimize the mass of the structures to support the induced electromagnetic forces.

The authors consider the use of MgB₂ superconducting wires from the viewpoint of reducing the refrigeration power requirement and the manufacturability of high-current conductors such as Rutherford cables. However, the bending strains imposed on the MgB₂ strands during the fabrication of Rutherford cables degrade the wire's critical current [2]. In addition, because the FBC has three-dimensional complex windings, the critical current of the MgB₂ strand degrades due to the bending strain caused by the complexity of the helical coil trajectories. Therefore, it is necessary to investigate the allowable strain of MgB₂ strands to fabricate Rutherford cables and helical coils.

In this research, the authors use the 0.67-mm diameter MgB₂ strands, fabricated by Hitachi, Ltd. [3]. The allowable strain after heat treatment of 0.67-mm diameter MgB₂ strands was confirmed to be 0.19% [4]. However, the Rutherford cables are generally fabricated before the heat treatment. Therefore, as a first step in this study, the authors evaluated the critical current characteristics of MgB₂ strands under the bending strains expected during the fabrication of Rutherford cables.

In this measurement experiment, the MgB₂ strands samples are cooled using a two-stage GM refrigerator. Figure 1 illustrates the cooling system and the experimental setup for evaluating the critical current properties of the MgB₂ strands. The cooling temperature ranges from 10 K to 20 K, controlled by a film heater placed on the second stage of the GM refrigerator. Four slots with different radii of curvature are cut into the surface of the sample plate. Four MgB₂ wire samples are set in the slots and heat-treated while applying the bending strain. These wire samples are connected in series. After heat treatment, the sample plate and the wire samples are set on the second stage of the GM refrigerator. The critical current is measured by the four-terminal method.

Based on the experimental results, this work plans to evaluate the allowable bending strain of the MgB₂ wires and to discuss the design considerations for the MgB₂ Rutherford cables and helical coil.

References

[1] S. Nomura, et. al., "Operation scenario of mobile SMES for on-site eigenvalue measurement of electric power system," IEEE Trans. Appl. Supercon., vol. 28, no. 3, Jun. 2018, Art. No. 441807.
[2] T. Yagai, et. al., "Development of design for large scale conductors and coils using MgB₂ for superconducting magnetic energy storage device," Cryogenics, vol. 96, pp. 75-82, 2018
[3] H. Tanaka, et. al., "Performance of MgB₂ Superconductor Developed for High-Efficiency Klystron Applications," IEEE Trans. Appl. Supercon., vol. 30, issue 4, Jun. 2020, Art. No. 6200105.
[4] H. Tanaka, et. al., "Influence of Sintering Conditions on Bending Tolerance at RT and Ic of In Situ MgB₂ wire," IEEE Trans. Appl. Supercon., vol. 29, issue 5, Jun. 2019, Art. No. 8401104.

Acknowledgment

This work was supported by the JSPS KAKENHI Grant-in-Aid for Scientific Research(B) (Grant Number 22H01477).

Figure 1. Schematic illustration of the cooling equipment (a) and the critical current measurement of curved MgB₂ samples.

Keywords: Superconducting magnetic energy storage (SMES), MgB₂, Bending strain, critical current test, GM refrigerator