Stacks Of High-temperature Superconducting Tapes Research Articles

Alternating current losses in superconducting circular/stacked coils used in energy storage systems

Large transient currents and magnetic intensities are generally encountered by the superconducting magnetic energy storage systems thereby resulting in AC losses that occur during the charging/discharging durations thus, estimation of such heat loads must be known before the actual designing of cooling arrangements. Various analytical methods are accessible to approximate such losses however, not adequate to predict the same accurately. In this work, an extensive numerical model has been established to estimate AC losses among the stacked/circular coils used in superconducting magnetic energy storage applications under various transport currents (150 A to 225 A) and strong fields (1e5 A/m to 4e5 A/m). A homogenization method has been incorporated into the model stacks of high-temperature superconducting tapes (Ic=330 A @ 0 T self-field) instead of an actual multi-turned coil. The modeling has been done to achieve an anisotropic bulk equivalent for the stack keeping the overall electromagnetic identity of the model the same as that of the original multi-turned stacked geometry such that it eliminates the multiple boundaries among the geometrical layout consisting of metallic, insulating, superconducting, and substrate layers. In the present numerical model, E-J power law has been used which has considered the overcritical current densities. The results showed that transport currents affect AC losses significantly and as it approaches critical current, the AC losses increased abruptly. The novelty of the work is in the consideration of external fields which are present beforehand the actual current transportation through the coils using the homogenization approach. It has been concluded that perpendicular fields have a significant effect on AC losses compared to parallel fields. Considering the fact of high computational speed, this model can further be extended to solve more complex transient characteristics of HTS motors or generators. It has been concluded that the homogenized model can estimate average AC loss two orders faster than the original stacked model.

Experimental and numerical studies on current distribution in stacks of HTS tapes for cable-in-conduit-conductors

Cable-in-conduit conductors comprised of twisted stacks of high-temperature superconducting (HTS) tapes constitute a very promising technology by virtue of their easy manufacturing process, flexibility capabilities, and high current densities. In a cable, the current distribution among tapes is one of the key parameters affecting the cable performances. The distribution of current is affected mainly by the self-field configuration (ultimately related to the cable layout) and the termination resistances. In this paper we present a 2D finite element (FE) model, based on the T-A formulation, which computes the magnetic field and current distribution in stacked tapes. This model has been used to describe the experimental V–I results obtained in cables in which different current distributions among tapes are expected. The first case refers to V–I curves of stacks of HTS tapes inserted into ducts formed in the extruded aluminium cylindrical core for a straight cable. The excellent agreement between the experimental findings and the simulation results can be explained in terms of uniform current distribution within the tapes stack, up to the superconducting to normal transition. The second sample, an Al-slotted core Cable-In-Conduit-Conductor, has been bent down to a radius of 0.15 m, and from the measured V–I characteristic of each individual tape, a different tape degradation depending on the tape position within the stack was recorded. The model is able to reconstruct the V–I of the stacks from the characteristic curves of the individual tapes with a satisfactory agreement. The finite element analysis reveals non-uniform current distribution among the tapes, which could expose the cable to a potentially irreversible damage during operation. The proposed FE model constitutes a useful tool for the analysis and predictions of HTS CIC conductor performances and represents a suitable basis for the implementation of more complex models aimed at the design of specific and large applications of this conductor in the next future.

Semianalytical Modeling of AC Losses in HTS Stacks Near Ferromagnetic Parts

This article presents a semianalytical modeling approach for ac losses computation in high-temperature superconducting (HTS) stacks and coils near ferromagnetic parts. The Brandt's model is combined with the equivalent current model to take into account of the proximity of ferromagnetic materials with finite relative permeability. Large stacks of HTS tapes are considered. A homogenization is applied to reduce the computing time. The obtained results are compared with finite element analysis. Very good agreements are obtained on both local and global quantities.

Testing of Surface Mounted Superconducting Stacks as Trapped-Flux Magnets in a Synchronous Machine

Stacks of high-temperature superconducting tapes may offer a technically affordable solution for the application of superconducting materials as trapped-flux magnets in the rotor of synchronous electrical machines. Nevertheless, several concerns must be first addressed, among others, the optimal procedure to induce the current vortexes previously to operation (magnetization), the survivability of these vortexes in the electromagnetic environment present in an electrical machine and the accuracy of recently developed numerical models. With the aim of exploring such practicalities, this article presents a modified synchronous machine to test under liquid nitrogen conditions thin stacks of superconducting tapes. The machine is run under realistic conditions: currents in the stacks are induced from the stator, then the shaft is rotated, and finally, the machine is connected to a load, working as a generator. The results confirm previous numerical and experimental studies and establish a procedure for assessing the behavior of stacks in their actual operational environment.

Comparison of properties of a bulk HTS and a stack of HTS tapes after FC and ZFC

This paper presents the results of the analysis of properties of a high-temperature superconducting (HTS) material in two forms - HTS bulks and a stack made of a number of HTS tape layers. Stack of HTS tape is a new type of application of superconductors, which can be used instead of a convention bulk material. In this work, we study the levitation properties of HTS stack and compare it with the same characteristics of HTS bulks to find the effective way of modelling such type of HTS. Experimental measurements of the force interaction between a permanent magnet and HTS material, as well as a "trapped" magnetic field, are performed for both types of a superconductor. The HTS samples are analysed in the case of Field Cooling (FC) and Zero Field Cooling (ZFC). The aim of the research is to analyse experimental results to compose and justify mathematical models for calculation magnetic system with such types of superconductor.

A new model for the analysis of quench in HTS cable-in-conduit conductors based on the twisted-stacked-tape cable concept for fusion applications

Several new conductor designs for fusion applications, based on high temperature superconducting (HTS) materials, have been recently proposed worldwide. Most of them are based on the twisted-stacked-tape cable (TSTC) idea: a stack of HTS tapes is twisted, soldered and embedded in a copper tube, constituting what we identify here as a ‘strand’. Few strands are then twisted around a central core to make an HTS cable. The cable is finally inserted in a stainless-steel jacket, which provides mechanical support and confinement for the forced flow of the supercritical helium coolant through the interstitials among the strands.The thermal gradients that build up in a TSTC make the 1D thermal-hydraulic models developed for low temperature superconducting magnets unsuitable to study fast transients such as quench propagation.A new model is presented here, which allows describing each strand, and separately accounting for the respective superconducting (SC) stack and copper tube. Compressible Euler-like equations describe the He flow and thermodynamic state in each interstitial fluid region. The distribution of the transport current is computed with a distributed parameter approach, self-consistently with the temperature distribution in the different so-called thermal components of the TSTC.The model is applied to the study of quench propagation in a conductor made of CroCo strands—a recent design proposed by the Karlsruhe Institute of Technology and based on the TSTC idea. A comparison with the modelling approach adopted for quench simulations in LTS conductors is carried out, showing that, when the heating is localized, the new model predicts, for the average temperature on the cross section at the hottest location along the conductor and for the voltage along the conductor, an evolution much slower than that predicted by a standard LTS-conductor model.

A trapped field of 17.7 T in a stack of high temperature superconducting tape

High temperature superconducting (HTS) tape can be cut and stacked to generate large magnetic fields at cryogenic temperatures after inducing persistent currents in the superconducting layers. A field of 17.7 T was trapped between two stacks of HTS tape at 8 K with no external mechanical reinforcement. 17.6 T could be sustained when warming the stack up to 14 K. A new type of hybrid stack was used consisting of a 12 mm square insert stack embedded inside a larger 34.4 mm diameter stack made from different tape. The magnetic field generated is marginally higher than the previous trapped field record achieved by a bulk HTS magnet and 30% greater than previously achieved in a stack of HTS tapes. Such stacks are being considered for superconducting motors as rotor field poles where the cryogenic penalty is justified by the increased power to weight ratio. The sample reported can be considered the strongest permanent magnet ever created.

Magnetic Response of the Stacks of HTS Tapes

We have performed a numerical analysis of the magnetic response of a single piece of a high-temperature superconducting (HTS) tape and the stacks of rectangular pieces of HTS tapes. The number of tapes in the stacks was varied from $N=1$ to 100. We have carried out the simulation in the framework of the critical state model, taking into account the double-exponential dependence of the critical current density on the external applied field that was proposed by the authors earlier. The results of the calculation of the magnetization of rectangular packages of HTS tapes are presented and discussed. We found that the value of the magnetization per one tape is reduced as the number of tapes in the stack is increased. The decrease in the normalized magnetization was shown to be due to mutual screening among tapes in the stack. We have compared the calculated magnetization with the experimental data and found a good agreement between both results. The novel method of calculations of the magnetic properties of HTS tapes can be successfully used for the calculation of the magnetization of HTS tapes and stacks with arbitrary shape.

Relaxation of Levitation Force of a Stack of HTS Tapes

Nowadays, magnetic bearings are used to design motors, generators, separating platforms, and flywheel energy storage systems. High-temperature superconducting (HTS) magnetic bearing construction is based on YBa 2 Cu 3 O 7-x (YBCO) bulks through its ability to capture and hold significant magnetic field for a long time. However, that material requires external mechanical reinforcement and has low thermal conductivity, which provides additional restrictions on practical application. Modern HTS tapes do not have such disadvantages. Its structure provides better rigidity and thermal stability in comparison with HTS bulk. One more significant advantage of tapes is simplicity of cutting to required form. In order to construct magnetic bearings from stacks of HTS tapes, it is necessary to have data about stability of system stack magnet under different dynamic processes. In this work, relaxation of levitation force was researched in both static and vibration modes with vertical oscillation frequencies of 15, 20, and 30 Hz. The measurements were performed for stacks with different amounts of 12 mm × 12 mm REBCO tapes in a stack (n = 1, 5, 10, 15, 20) at temperature T = 77 K. Stacks were placed over permanent magnet with a minimal gap of 4 mm. FC cooling method was realized. All static and dynamic relaxation measurements were carried out at 11.6-mm gap.

Measurements and Numerical Simulations of Trapped Field in a Stack of HTS Tapes

Trapped field magnets are usually constructed on the base of bulk YBCO superconductors. However, recent achievements in the production of long-size coated conductors have opened the opportunity of using a tape stack for these applications. Better thermal conductivity, mechanical stability in high fields, and high critical current densities make such systems favorable. Moreover, the tape form of the conductor allows for organizing different spatial structures and optimizing device design. Trapped field was measured on the stack of high-temperature superconducting tapes in this work. An industrially stabilized (with copper shield) tape, which was produced by SuperOx company with GdBa 2 Cu 3 O 7-x superconductor inside, was used in experiments. The stack was made of 12-mm-diameter disk-shaped tapes, and the height was 20 layers. Magnetization was carried out by an 8-T superconducting magnet on the temperature of 77 K. To calculate the trapped field of a stack, field dependence and angular field dependence of critical current were measured on these tapes. The finite-element method was used for numerical simulations. It is shown that taking into account the magnetic field orientation in different areas of the stack leads to better agreement with the experimental measured trapped field values.

Design Optimization of Round Strands Made by Twisted Stacks of HTS Tapes

A twisted stack of high-temperature superconducting (HTS) tapes soldered into copper profiles was used as a strand in the fabrication of the 60-kA/12-T HTS cable prototype at the Center for Research in Plasma Physics (CRPP). Considering the strand in general as a modular element for the high-current cables, design parameters of the strand need to be optimized in order to best fulfill requirements of appropriate application field. During the development program of the HTS fusion cable at CRPP, influence of the design parameters on the strand's performance was obtained as input data for the optimization process. In this paper, we summarize the results of the twisting, bending, and transverse pressure tests on strands of various tapes and profile's geometries. Finite-element model for the transverse pressure test was developed, validated with the test results, and used for the design proposals. Effect of the stack aspect ratio, tape properties, and anisotropy, as well as selection of HTS material for the strand, will be discussed.

Computing AC losses in stacks of high-temperature superconducting tapes

Superconducting tape coils and Roebel cables are often modeled as stacks of parallelsuperconducting tapes carrying the same transport current. We solved, in the infinitelythin approximation, the transport current and magnetization problems for such stacksusing an efficient numerical scheme based on a variational formulation of the Kimcritical-state model. We also refined the anisotropic bulk approximation, introduced byClem et al in order to simplify AC loss estimates for densely packed stacks of many tapes;this was achieved by removing the simplifying a priori assumptions on the current sheetdensity in the subcritical zone and the shape of this zone boundary. Finally, we studied theconvergence of stack problem solutions to the solution of the modified bulk problem. It wasshown that, due to the fast convergence to the anisotropic bulk limit, accurateAC loss estimates for stacks of hundreds of tapes can usually be obtained alsousing a properly rescaled model of a stack containing only ten to twenty tapes.