The introduction of the superconductor to magnetic levitation (maglev) trains greatly enhances the performances compared to those of normal conductor maglevs, e.g., from 430 km/h of the Transrapid (in Shanghai) to 603 km/h of the L0 Series in Japan. However, one of the important constraints on the development of superconducting maglevs is limited wireless feeding power for onboard superconducting magnets and cryogenic cooling. In this paper, a persistent-current superconducting magnets system with solid nitrogen (SN2) cooling preservation is proposed for the liberation of its demanding onboard power feeding requirement. The magnets are optimally designed with a no-insulation technique guaranteeing a safe operation with a magnetic field over 0.8 T. Lasting time of persistent current (at 96.5% magnetic field retained) and SN2 cooling preservation (up to 40 K) is all >9 h, covering a maglev traveling distance of >5400 km at an average designed speed of >600 km/h. The magnets have an anti-vibration ability of 15 g (147 m/s2) up to 350 Hz, which has covered the vibratory motion range in maglevs. This work is intended to provide a reference for superconducting maglev developments.
High-speed railways such as CRH, TGV, ICE, Shinkansen, and KTX, are running worldwide providing people with convenience. However, it is difficult to operate trains when speed is over 500 km/h due to the rail-wheel propulsion and catenary/pantograph power feeding system1. To acquire faster speed, relative research programs on magnetic levitation (maglev) train have started since the first publication2. A world record of ultrahigh speed at 603 km/h was made by the L0 Series superconducting maglev in Japan in 20153. The introduction of the superconducting technology to the maglevs is straightforward, to substantially enhance its performance because a superconducting magnet can easily provide magnetic field well above that possibility with a conventional permanent magnet, i.e., >0.5 T at centimeters even a decimeter away from magnet surface, while keeping a compact volume and light weight.
High-temperature superconducting (HTS) materials show great advantages on higher critical current density (Jc), critical magnetic field (Bc) and other performances in comparison to low-temperature superconducting (LTS) materials (e.g. Nb-Ti superconductor)4,5,6,7,8,9. Different from the LTS materials that are operated in 4.2 K liquid helium (LHe) bath, HTS materials have much higher critical temperature (Tc), which provide possibility of LHe-free and safe operation in cheaper liquid nitrogen (LN2) bath at 77.2 K with large thermal margin. Besides, the second generation (2G) HTS wires (e.g. YB2C3O7-δ) have advantages over 1G wires (e.g. Bi2Sr2Ca2Cu3Ox) including higher in-field Jc and enhanced mechanical property. Commercial 2G wires allow minimum bending diameter of 10 mm and maximum axial tensile stress of 427 MPa with >95% Jc retention10, which are satisfying in maglev applications. It is possible to optimally design HTS magnets for certain usages, and energize them conveniently by external current sources, persistent-current switches (PCSs), or flux pumps with high efficiency11,12,13,14. It is validated that on-board 2G HTS magnets are able to operate well for maglevs at rated speed of 620 km/h15. 2G HTS materials are promising in the superconducting maglev application.
However, obstacles are remained. Maglevs remove all the physical contacts to the ground to maximally reduce running frictions, thus, the kW-scale wireless power feeding is quite limited when considering the energy consumption during the operation of the on-board superconducting magnets including cooling and energizing with cryocoolers, compressors, large-current sources and sometimes water chillers besides the power for carriages. Attempts have been made by us for realization of persistent-current superconducting magnets without power source16. And solid cryogen auxiliary cooling for possible cryocooler-free operation is also investigated by groups17,18.
Inspired by the aforementioned advantages and limitations, the first demonstration of an on-board persistent-current superconducting magnets system with cooling-power-free operation especially for superconducting maglevs, is proposed in this work. Therefore, cooling and energizing facilities can be removed from train carriages. For the magnets system, designs combining electrical, mechanical, and thermal aspects are reported mainly including optimization of no-insulation magnets, analysis of field and energizing characters, performance of persistent-current mode, and strategies for enchantment of anti-vibration and cooling abilities. This paper is aimed at providing detailed designs and analysis of the on-board superconducting magnets system, and the methods can be transplanted to superconducting generators for wind turbines or NMR/MRI, etc.
A persistent-current superconducting magnets system with solid nitrogen cooling preservation is proposed for liberation of its demanding on-board power feeding requirement in ultrahigh speed maglevs. Firstly, the magnets are optimally designed guaranteeing a safe operation with magnetic field >0.8 T and total harmonic distortions of 5.98%, which is qualified for maglev operation. Then the magnets are wound by no-insulation 2G HTS wires for high in-field critical current, enhanced self-protective stability and volumetric compactness. Especially, persistent-current switches are carefully considered for realization of persistent-current operation of the magnets. Next, performances of the magnets system are analyzed. Electrically, lasting time of persistent-current operation of the magnets is >9.08 h (at 96.5% magnetic field retained), and corresponding traveling distance of a maglev is >5400 km without stoppings at average designed speed of >600 km/h. Mechanically, the magnets with pressing plates, which also acting as cooling plates for better thermal performance, have anti-vibration ability of 15 g (147 m/s2) up to 350 Hz (i.e., vibratory motion range in the maglevs) without performance degradations. Thermally, solid nitrogen provides a 9.2-hour cooling preservation period from 25 K to 40 K with satisfying temperature uniformity less than 1 K. And nitrogen remains solid even though unexpected quench of magnets happens. Conclusively, this work provides a demonstration of 2G HTS magnets system in maglev applications.
Cite this article
Dong, F., Huang, Z., Hao, L. et al. An on-board 2G HTS magnets system with cooling-power-free and persistent-current operation for ultrahigh-speed superconducting maglevs. Sci Rep 9, 11844 (2019). https://doi.org/10.1038/s41598-019-48136-x
- DOI : https://doi.org/10.1038/s41598-019-48136-x
This work was supported by National Natural Science Foundation of China (Project No. 51707120) and Science and Technology Commission of Shanghai Municipality (Project No. 17511102306).
The authors declare no competing interests.
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Professor Siavosh Kaviani was born in 1961 in Tehran. He had a professorship. He holds a Ph.D. in Software Engineering from the QL University of Software Development Methodology and an honorary Ph.D. from the University of Chelsea.