I. Ishii

Feasibility study of low-Voltage DC Superconducting distribution system

This paper describes the advantages of using superconducting cables in low-voltage DC distribution systems, focusing on total cost reduction. Demand for DC electric power has been rapidly increasing, requiring reductions in total costs and power supply system losses. The effects of using superconducting cables were investigated by changing parameters such as the current and length of cables, and it is shown that DC superconducting distribution systems are feasible even for small-capacity power systems and savings can be obtained beyond the extra cost of the superconductors and refrigeration systems. In this study, a test model of a DC superconducting distribution system, constructed by modification of an AC superconducting power transmission system, is presented and evaluated.

Hydraulic characteristics in superconducting power transmission cables

The hydraulic characteristics of sub-cooled nitrogen in superconducting power transmission line have been investigated to design the transmission line. To estimate the friction factor in corrugated pipes, the pressure drop was measured across a 10-m length corrugated pipe for various mass flow rates and compared with that in smooth pipes (Proc. 17th Cryo. Eng. Conf. (1998) 475). Another concern is the cooling method of the transmission line. A usual counter-flow cooling method has been found to be very difficult at liquid nitrogen temperature, because the thermal conductivity of the dielectric materials between the counter-flows at liquid nitrogen temperature are 10 times larger than that at liquid helium temperature. Instead, an externally cooling method was proposed (Proc. 17th Cryo. Eng. Conf. (1998) 475). As the results, the friction factor of outer corrugated jacket within three cables has become an important parameter to design the transmission lines. This friction factor is measured and compared with various formulas in this paper.

Characteristics of high-stiffness superconducting bearing

Magnetic bearings using a high-Tc superconductor have been studied. Generally the bearing makes use of the pinning effects to get the levitation force. The stiffness of the bearing, however, is extremely low as compared with industrial-scale conventional one. To improve the bearing stiffness we propose a disc-type repulsive superconducting thrust bearing with a slit for the restraint of the flux. Both theoretical and experimental evaluation on the load performance was carried out, and it is clarified that the proposed superconducting bearing has higher stiffness.

Numerical analysis of a superconducting bearing

High-Tc superconducting bearings are expected as a key technology for flywheel energy storage systems (FWES). However, low bearing stiffness behavior becomes a serious obstacle to realize FWES. We have so far suggested various kinds of bearing construction to improve the bearing stiffness, but the various improvements of superconducting bearings bring complicated magnetic circuit structure, which makes the analysis of the bearings and realization difficult. For this reason, we have been constructing the numerical analysis program using three-dimensional sealer potential method based on the critical state model to proceed with the stiffness improvement study. In this paper, we show the validity of the developed program for the disc-type superconducting bearing, which consists of a superconductor and a permanent magnet. This means that we can estimate the behavior of real size superconducting bearings for FWES.

Heat transfer characteristics of liquid nitrogen for cyclic heating

Bubble formation and the critical heat flux to transit to boiling state for cyclic heat inputs have been investigated using a platinum - 0.5at% cobalt (Pt-Co) wire and a silver sheathed Bi-2223 tape, in order to evaluate the cooling stability for superconducting AC power apparatus. Sinusoidal cyclic heat inputs ranging from 2 to 150 Hz were applied to a horizontal thin wire immersed in a pool of saturated liquid nitrogen under normal atmospheric pressure. A high-speed video camera with a frame speed of 1000 frames/s has been used to observe bubble formation around the wire. The bubble frequency and the critical heat flux to transit to boiling state have been measured for each frequency. As a result, it has been found that the bubble frequency and the critical heat flux have depended on the heating frequency. The same experiment has been performed on a silver sheathed Bi-2223 tape, however, the mechanism of bubble formation is different. The dependence of the critical heat flux on the heating frequency has been also observed.

Transient Heat Transfer Characteristics of Liquid Helium in Centrifugal Acceleration Field

Transient heat transfer of liquid state helium in centrifugal acceleration fields up to 1300G have been investigated for studying the stability characteristics of the rotor field windings of a superconducting generator. In the experiment, temperature resistive platinum-0.5% cobalt (Pt-Co) thin wire was used as the temperature sensor and heater to measure the heat transfer characteristics. The thermal mass of the Pt-Co wire was small enough to observe the temperature fluctuation due to bubbling in the boiling heat transfer. In this paper, influence of the centrifugal acceleration on the transient heat transfer and its mechanism are discussed based on the experimental results. It was observed that, for step heating with heat flux higher than maximumheat flux at steady state nucleate boiling, quasi-nucleate boiling appeared also in centrifugal acceleration field at the beginning of the heating, and that duration of the quasi-nucleate boiling was not much dependent on the acceleration field for relative high heat flux. This phenomenon was explained by bubble formation process in the boiling heat transfer. In this study, a high speed video camera was used to observe the relation of the bubble formation process and the temperature fluctuation of the Pt-Co wire in the normal gravity.

A study on stabilities of rotating magnets for superconducting generators

Abstract Stabilities of magnets in a high gravitational field up to 2000G at a rotating speed of 3, 600rpm, are studied to establish the fundamental design criterion of field windings of superconducting generators. It is clarified that magnet stability is greatly improved in a rotational field compared with stationary state, because of the enhancement of natural convection for the cooling of the coils caused by a high gravity. Heat transfer to liquid helium (LHe) are measured using a clean copper and a Formvar coated copper under rotational field. The coefficients are 0.7 × 104Wm−2K−1 and 0.4 × 104Wm−2K−1 at the gravitational field of about 1000G, respectively. They are almost identical to the calculated values based on the normal zone propagation velocities under the same condition.

Transient Heat Transfer of Supercritical Helium in High Centrifugal Acceleration Field

Transient heat transfer of supercritical helium in centrifugal acceleration field has been measured for studying the stability characteristics of superconducting rotor windings of a superconducting generator. A temperature resistive platinum-0.5% cobalt (Pt-Co) thin wire, having small heat mass, was used as a temperature sensor and heater to measure the heat transfer characteristics. It was observed that the steady state heat transfer of supercritical helium in 3600G (G; gravity acceleration) was established in 200μsec. In the paper, data of transient heat transfer of supercritical helium in centrifugal acceleration field are shown and their specific characteristics are studied comparing with those of liquid helium.

Discussion of the stable thermal equilibrium current of a superconductor during tests of high-speed rotor windings

Research and development of superconducting generation equipment has been conducted. As a part of the design research for the superconducting generator, the stability of superconducting field winding is being studied. The goal of this study is to formulate a design philosophy for a stable superconducting field winding in the presence of large disturbances. In the present paper, we describe the stable thermal equilibrium current of the superconductor, which is measured during the stability test. Modifying the equal area criterion presented by Maddock et al., a simple graphical scheme has been presented and the minimum stable thermal equilibrium current calculated.

Can’t matrix bindings to superconductor be evaluated by its ultrasonic spectrum?

Abstract In the practical use of superconducting magnet, regardless of low or high temperature’ superconductor, combination of the superconductor and high conductive matrix of normal metal is now indispensable. This binding property is one of the key points whether superconducting characteristics are fully attained or failed prematurely in the practical magnet. But this binding evaluations is not so easy, because even in the mechanical well-jointed conductor in the room temperature, it often appears poor in electric and magnetic property in cryogenic temperature by the contraction of the conductor. And sometimes, these often happen that critical current change time to time of an every trial of current loading. This must be understood that the binding condition between the metals changes by time to time of heating and re-cooling at/after every quench. So it must become very important to evaluate these bindings’ properties at cryogenic temperature and with real current charging. Here we report new technique for evaluating these bindings by observing ultrasonic spectra of acoustic emissions when a normal transition is propagated along the superconductor. We have discovered a simple but important fact that good contacting conductor has an emission of this ultra-sound with sharp resonating frequency peak or peaks at around highest spectrum in near 1.0 MHz area.