This paper describes the present status of high temperature superconductors (HTS) and of bulk superconducting magnet devices, their use in bearings, in flywheel energy storage systems (FESS) and linear transport magnetic levitation (Maglev) systems. We report and review the concepts of multi-seeded REBCO bulk superconductor fabrication. The multi-grain bulks increase the averaged trapped magnetic flux density up to 40% compared to single-grain assembly in large-scale applications. HTS magnetic bearings with permanent magnet (PM) excitation were studied and scaled up to maximum forces of 10 kN axially and 4.5 kN radially. We examine the technology of the high-gradient magnetic bearing concept and verify it experimentally. A large HTS bearing is tested for stabilizing a 600 kg rotor of a 5 kWh/250 kW flywheel system. The flywheel rotor tests show the requirement for additional damping. Our compact flywheel system is compared with similar HTS–FESS projects. A small-scale compact YBCO bearing with in situ Stirling cryocooler is constructed and investigated for mobile applications. Next we show a successfully developed modular linear Maglev system for magnetic train operation. Each module levitates 0.25t at 10 mm distance during one-day operation without refilling LN2. More than 30 vacuum cryostats containing multi-seeded YBCO blocks are fabricated and are tested now in Germany, China and Brazil.

http://iopscience.iop.org/article/10.1088/0953-2048/25/1/014007/pdf

Superconductor bearings, flywheels and transportation

An efficient three dimensional (3D) finite element method numerical model is proposed for superconducting coated conductors. The model is based on the T–A formulation and can be used to tackle 3D computational challenges for superconductors with high aspect ratios. By assuming a sheet approximation for the conductors, the model can speed up the computational process. The model has been validated by established analytical solutions. Two examples with complex geometries, which can hardly be simulated by the 2D model, are given. The model could be used to characterise and design large-scale applications using superconducting coated conductors, such as high field magnets and other electrical devices.

https://iopscience.iop.org/article/10.1088/1361-6668/30/2/024005/pdf

An efficient 3D finite element method model based on the T–A formulation for superconducting coated conductors

A 45-m-long high-temperature superconducting (HTS) Maglev ring test line, named “Super-Maglev,” has been successfully developed in Chengdu, China, in February 2013, 12 years after the birth of the first man-loading HTS Maglev test vehicle. The Maglev vehicle (2.2 m in length, 1.1 m in width) is designed for one passenger with a levitation height of 10–20 mm; the permanent-magnet guideway (PMG) (45 m in length, 0.77 m of track gauge) is a racetrack shape with a curve radius of 6 m; the driving is accomplished by a linear induction motor with a maximum running speed of 50 km/h. The linear motor is composed of four submotors installed at one straight section in the middle of the double PMGs, and the total length is 3 m. This second-generation HTS Maglev vehicle system is highlighted by the cost-performance and the wireless multiparameter onboard monitoring function. The current same-level load capability has been achieved over a small-section low-cost PMG whose cross-sectional area is only 3000 mm
 2
. On the vehicle, parameters of levitation weight, levitation height, running speed, acceleration, lateral offset, online position, and total running distance of the vehicle are real-time monitored and displayed on the onboard tablet computer. The system component and test data are reported in detail in this paper.

A High-Temperature Superconducting Maglev Ring Test Line Developed in Chengdu, China

This paper presents a topical review of the current state of the art in modelling the magnetization of bulk superconductors, including both (RE)BCO (where RE?=?rare earth or Y) and MgB2 materials. Such modelling is a powerful tool to understand the physical mechanisms of their magnetization, to assist in interpretation of experimental results, and to predict the performance of practical bulk superconductor-based devices, which is particularly important as many superconducting applications head towards the commercialization stage of their development in the coming years. In addition to the analytical and numerical techniques currently used by researchers for modelling such materials, the commonly used practical techniques to magnetize bulk superconductors are summarized with a particular focus on pulsed field magnetization (PFM), which is promising as a compact, mobile and relatively inexpensive magnetizing technique. A number of numerical models developed to analyse the issues related to PFM and optimise the technique are described in detail, including understanding the dynamics of the magnetic flux penetration and the influence of material inhomogeneities, thermal properties, pulse duration, magnitude and shape, and the shape of the magnetization coil(s). The effect of externally applied magnetic fields in different configurations on the attenuation of the trapped field is also discussed. A number of novel and hybrid bulk superconductor structures are described, including improved thermal conductivity structures and ferromagnet?superconductor structures, which have been designed to overcome some of the issues related to bulk superconductors and their magnetization and enhance the intrinsic properties of bulk superconductors acting as trapped field magnets. Finally, the use of hollow bulk cylinders/tubes for shielding is analysed.

/pdf/modelling-of-bulk-superconductor-magnetization-iet0fuf1ew.pdf

Modelling of bulk superconductor magnetization

As a bellwether transportation mode, albeit with hot controversy, evacuated tube transport (ETT) can dramatically reduce air friction, and incorporating Maglev technology could also thoroughly eliminate wheel-rail friction. It is not difficult to imagine that incorporation of these two technologies could establish an innovative transportation system, with particular advantages in terms of high speed, safety, energy saving, and environmental protection. The integration of these two technologies has been a great challenge, however, due to the composite technology. To realize this revolutionary idea, we have successfully developed the first proof-of-principle prototype of a 45-m-long high-temperature superconducting Maglev evacuated tube transport (HTS Maglev-ETT) test system, called the “Super-Maglev,” based on the passive self-stable HTS Maglev conceived in our group in 2000. The system mainly consists of three parts: an HTS Maglev-vehicle-guideway coupling system with 1-t load capability at a levitation gap of 10 mm, a 45-m-long racetrack-type evacuated tube with a 2-m-diameter circular cross section pumped by a hybrid air extraction system, and a 3-m linear induction motor to provide sectional propulsion. The system can achieve a pressure as low as 2.9 kPa in the tube. Experiments show that air drag on the vehicle is greatly reduced at that low air pressure, and a maximum speed of 50 km/h was recorded on the 6-m-diameter test guideway. Theoretically, the reduction of the aerodynamic consumed power could reach as high as to 90% under 10 kPa. This “Super-Maglev” strongly demonstrates the feasibility and potential merits of the HTS Maglev-ETT transportation concept.

A High-Temperature Superconducting Maglev-Evacuated Tube Transport (HTS Maglev-ETT) Test System

This paper presents a transverse flux linear motor (TFLM) with the superconductor-aluminum hybrid as the secondary and E-shaped ferromagnetic yokes wrapped the conductor windings as the primary to generate the transverse flux, the hybrid secondary is made of aluminum plate and close-ended coated superconductor coils, with the latter placed over the former to realize the stable levitation, guidance and self-propulsion. In this paper, a multi-degree of freedom 3-D finite-element dynamic model of such superconducting TFLM was established, in which the actual geometry of the motor and the nonlinear behavior of superconductor were represented. We investigated the transient characteristics and dynamic responses against disturbance and also studied the influence of aluminum plate on the overall performance. The obtained characteristics of the dynamic superconducting TFLM model may offer a reference for dynamic experiments and control strategy.

3-D dynamic simulation of an integrated levitation, guidance and propulsion system by a superconducting transverse flux linear motor

The paper proposed double-sided superconducting linear generator with YBCO tape windings. An analytical model of linear generator was developed based on electromagnetic field theory. A parameter-scanning method is used to search for optimal dimension parameters with the purpose of minimizing the non-sinusoidal components of the induced voltage curve. The results obtained show a significant improvement in the quality of air-gap magnetic field, and reduction of the higher harmonic in voltage wave obviously. The induction voltage in the generator coils at the speed of 130 m/s are studied. Finally, the feasibility of the superconducting linear generator was analyzed and confirmed.

Structure Optimization of a Superconducting Linear Generator With YBCO Tape Windings

A double-layer metal-insulation method using brass sheets as the double-layer insulators is proposed in this paper. It can enhance the contact resistivity while preserving greater thermal conductivity merit. The underlying mechanism of the contact resistivity enhancement is to increase the number of contact surfaces and to degrade the contact quality between the insulators. Then, we wound a single-layer brass-insulation coil and a double-layer brass-insulation coil to compare their contact resistivities, and confirmed the effectiveness of the double-layer metal-insulation method. Furthermore, since the capacity to withstand the overcurrent is weakened with the increasing contact resistance of the metal-insulation coil, we further investigated the influence of the contact surface resistivity distribution on the coil performance under different scenarios to optimize the double-layer metal-insulation coil for receiving superior thermal stability. The simulation results indicate that dominant second contact surface resistivity and minimal first and third contact resistivity is the optimal design for the double-layer metal-insulation coil to receive the best thermal stability, irrespective of the cooling environment, contact resistivity magnitude, operating current and coil dimension. In addition, with regard to the thermal performance differences caused by the contact surface resistivity distribution, we found that the increment of contact surface resistivity and the overcurrent enlarged the distinctions at different levels.

Performance enhancement of coated conductor magnet with double-layer metal insulation

A high-temperature superconducting (HTS) magnetic transmission device is introduced. The key part of the transmission device is an HTS magnetic helix transmission mechanism, which consists of a magnetic bolt and an HTS nut, and its strong points are being frictionless and having high efficiency. The transport platform is driven along the direction of the magnetic bolt. All moving parts of the device do not come into contact with the load that is transported by the transport platform. The basic mechanical properties of this device are studied. The position precision and angle precision of the transport platform are discussed.

A High-Temperature Superconducting Magnetic Transmission Device

The onboard superconducting magnets and null-flux coils (NFCs) on the track side are the crucial components for realizing the null-flux superconducting electrodynamic suspension (EDS). In this article, the multiobjective optimization of a null-flux EDS system was investigated to improve the performance using the nondominated sorting genetic algorithm-II (NSGA-II) method. First, a precise mathematical model of the superconducting EDS was established by adopting mutual inductance calculation method we proposed before. Then, the comprehensive sensitive analysis was carried out to analyze the influence of variables on different objectives. The ratio of levitation force to drag force (drag ratio), the ratio of levitation force to mass of NFCs (mass ratio) as well as the fluctuation of levitation force, were defined to be the optimization objectives. Third, an optimization scheme employing NSGA-II method was built, and the Pareto front was obtained. Afterward, the effectiveness of optimized results was verified by the finite element analysis. Finally, an equivalent experimental method was used to characterize the performance of a down-scale EDS system to confirm the proposed optimization methodology. To conclude, the optimal method can greatly improve the performance of EDS, and provide diverse design scheme choices. 

Guangtong Ma

Papers

3-D dynamic simulation of an integrated levitation, guidance and propulsion system by a superconducting transverse flux linear motor

Structure Optimization of a Superconducting Linear Generator With YBCO Tape Windings

Performance enhancement of coated conductor magnet with double-layer metal insulation

A High-Temperature Superconducting Magnetic Transmission Device

Design Optimization of Null-Flux Superconducting Electrodynamic Suspension for Cost-Saving and Performance Improvement