Focus on Superconductivity for Cryo-Electrification of Aviation and Other Transportation Applications

High temperature superconducting (HTS) Maglev is a promising technology owing to its excellent electromagnetic properties of non-control stability and frictionless. As a critical component of HTS Maglev, the turnout poses challenges due to the use of the permanent magnet guideways (PMGs) that are difficult to switch mechanically. With simple structure and rapid responsiveness, electromagnetic turnout has become an interesting research field of HTS Maglev. The electromagnetic turnout is designed to control the electromagnetic force of the HTS bulks by adjusting the magnetic field using an electromagnetic-permanent magnetic structure. However, the magnetic field generated by the combination of the electromagnet and the PMs inevitably differs from the magnetic field above the PMGs, affecting the electromagnetic force behavior of the HTS bulks. To guarantee optimal performance of the electromagnetic turnout, the variation process of the magnetic field peak above the turnout is analyzed. Furthermore, a three-dimensional superconducting model based on the H-formulation and motion equation is built to obtain the electromagnetic force of the HTS bulks passing the turnout. It's concluded that higher cooling heights in the preparation and higher working heights in the turnout area are recommended to reduce the magnetic resistance and avoid the destabilizing effect. In addition, the smaller bulks experience greater resistance, and the longer bulks are prone to nodding, while the wider bulks are prone to shaking above the turnout. Properly matching HTS bulks with operating conditions improves electromagnetic force behavior and benefits HTS Maglev turnout passability.

Superconductors in practical use can be subjected to spatially non-uniform and time-varying external magnetic field as well as carrying a transport current, for example, in an electrical machine. This paper verifies that the integral method can be used in situations in which the external field is spatially non-uniform, by providing both theoretical reasoning and simulation results. Variations in the integral method are reviewed, such as how to impose transport current. Further, the integral method is applied to calculate ac loss in superconducting tapes in an air-cored electrical machine in a two-stage process: the external field is calculated in a COMSOL model without superconducting tapes, and exported into the integral method model that consists of the tapes only. The time taken by the integral method is a small fraction of the time taken by the full machine model in COMSOL, which uses the T-A formulation. There are good agreements between the full COMSOL model and two-stage method incorporating the integral method.

This research presents a comprehensive and innovative approach to investigating the magnetisation and cross-field demagnetisation behaviour of high-temperature superconducting (HTS) coated conductors (CCs) in practical superconducting machines. This study introduces several novel contributions, including the operation of the machine in propulsion energy conversion mode, the exploration of harmonics interaction in a real electric machine environment involving CCs, and the extraction of these harmonics as cross-field components. A 2D electromagnetic-thermal coupled numerical model employing the finite element method has been developed and validated against experimental data to simulate a partially superconducting machine. Upon magnetisation, the HTS stacks effectively operate as trapped field magnets, generating rotor fields for motor operation. With a peak magnetic flux density of 462 mT of the trapped field stacks (TFSs) in the air gap, the average values of the fundamental and fifth harmonics of the tangential magnetic flux density experienced by the TFSs were observed to be 25 mT and 1.75 mT, respectively. The research has thoroughly examined the impact of cross-field demagnetisation parameters including amplitude and frequency on the demagnetisation of TFSs. Furthermore, the study has also investigated the magnetisation losses occurring in various layers of HTS tapes, encompassing the HTS layer, magnetic substrate layer, and silver stabiliser at different amplitudes and frequencies. Two tape structures, namely a semi-homogenised model and a multi-layered model, have been analysed in terms of magnetisation loss. Additionally, insights into the shielding effect and skin effect at high frequencies were obtained, offering valuable information on the performance of HTS TFSs exposed to high frequency scenarios especially in high-speed machines for electric aircraft. The research outcomes are anticipated to provide valuable knowledge for the design and optimisation of HTS rotors employing TFSs in superconducting machines, contributing to the advancement of superconducting machine technology.

Regenerative braking technology has become increasingly attractive due to its ability to recover and reuse the energy that would otherwise be lost. In recent years, a new superconducting energy storage technology is proposed and it has been proved experimentally and analytically that the technology has promising application potential in urban rail transit for regenerative braking. However, a comprehensive assessment of the new technology has not been conducted up to date. In this paper, the currently available energy storage technologies for regenerative braking, such as batteries, supercapacitors, flywheels, and SMES are introduced along with the new superconducting energy storage technology. Comparative studies between the existing technologies and the new one are conducted in terms of energy density, energy conversion efficiency, energy storage duration, capital cost and environmental impact. It is concluded that a regenerative braking system with the new superconducting energy storage has very high cycle efficiency and is superior to the existing energy storage systems. It has the potential to revolutionize the regenerative braking technology and to develop more efficient and sustainable urban rail transportation systems.

Superconducting electric motors offer the potential for low weight and high power in applications such as electric aircraft and high speed marine transport. Combined with renewably-sourced cryogenic fuels and advanced fuel cells they offer a path to zero-carbon mass transport. The proposed architectures of these extreme machines, operating at temperatures around 20 K–50 K and employing very high alternating magnetic fields, require materials for the stator that are not electrically conducting and at the same time have good cryogenic structural performance. Additively manufactured (AM) materials can play a key role in these designs, and a collaboration between the Robinson Research Institute and Auckland University of Technology is studying the performance of a range of composite polymers in superconducting machine applications. There are significant challenges to be met, including understanding the effect of the build process on material properties at low temperatures, and also the effect of formulation changes on thermal properties. AM metals can be employed in the rotor components, where the magnetic field fluctuations are very small for our synchronous designs. In this usage case, we can achieve dramatic reductions in the weight of the rotor assembly by minimising the number of joints and facilitating the design of multi-functional components in our helium cooled, vacuum cryostat architecture. Novel design solutions have been developed for several key components in our prototype machines and these are discussed, along with cryogenic testing results for selected AM polymers and composites.

Climate change has spurred a shift to electric transportation, but aviation faces challenges with electric energy storage and propulsion. Cryogenically cooled superconducting motors, along with cryogenically cooled power electronics, offer a solution to increase the efficiency and power density of electric aircraft. This paper evaluates the feasibility of cryogenic power electronics by characterising new technologies (GaN, nanocrystalline) using new experimental techniques. It is found that the on resistance reductions of GaN E-high electron mobility transistors at cryogenic temperatures depend on the maximum blocking voltage of the device, and the size of the gate resistor for ohmic p-GaN devices. Different types of nanocrystalline cores are shown to vary greatly in their behaviour at cryogenic temperatures, which is measured using a modified core loss measurement circuit. Further analysis shows that the losses of a GaN based cryogenic inverter could potentially halve that of an equivalent Si based inverter.

High-temperature superconducting traction transformers (HTSTTs) have the merits of small size and lightweight in comparison with their conventional counterparts. This article reports the development progress of a 6.6 MVA HTSTT operating at 65 K, including the design, testing, and system cooling. The introduction of flux diverters and an optimized winding design realized a short-circuit impedance higher than 43% and AC loss less than 3 kW. The insulation structure was designed to pass insulation tests specified in standard in China GB/T 25120-2010. An open cooling system with reduced pressure was developed, which realized the efficiency of the 6.6 MVA HTSTT above 99%. Before assembling the prototype transformer, we conducted tests for critical current and dielectric performance of the HTS double pancake coils (DPCs) used in high-voltage (HV) and low-voltage (LV) windings to verify the current-carrying and insulation performances of each DPC. Finally, we measured the critical current and no-load loss of the HTSTT prototype at 77 K. Test results showed that the mass of the transformer is 33% less than conventional transformers. At 77 K, the critical current of the LV winding and HV winding is higher than 700 A and 50 A, respectively. Moreover, the HTSTT on a no-load test reached the test voltage of 25 000 V and loss of 6 kW. In the next step, we will continue to conduct experimental research, and verify the feasibility of the HTSTT on the train, and develop a circulating cooling system, all meeting the commercial requirements of the HTSTT.

The transition to electric propulsion for aircraft provides an effective way to reduce fuel consumption and achieves low-carbon aviation. Due to the advantages of high magnetic field and ultra-compactness of superconducting disk-up-down-assembly ('DUDA') magnets, they have a promising use in superconducting motors. Therefore, this paper presents a design of a fully superconducting motor using superconducting DUDA magnets with Halbach arrays. In order to study the feasibility of the superconducting DUDA magnets in electric motors, preliminary studies of two sets of 4-layer superconducting DUDA magnets were carried out. The manufacturing method with lap joints of the DUDA magnets was proposed and then the manufactured magnets were tested in liquid nitrogen. The contact resistance and critical current at each lap joint have been calculated and the magnetic field distribution of the magnets has been measured. The magnetic fields of the magnets were also verified by simulation and then the magnets were scaled up in size to meet the magnetic field magnitude for the motor. It has been proved that the DUDA magnets can generate a constant magnetic field above 1.11 T along the x-axis without iron materials, which meets the requirements of motors. Based on the analysis of electromagnetic performance, the structural parameters of the superconducting DUDA magnets were optimized with different pole-slot number combination in order to obtain higher efficiency and specific power density. To calculate the efficiency, finite element models in Comsol evaluated the AC losses of the superconducting DUDA magnets. By changing the slot type and winding configuration, the optimized motor is able to achieve a specific power density of 11.55 kW kg⁻¹ with an efficiency of 98% at 30 K.