Cryogenics 39 (1999) 53–58 Superconducting magnetic energy storage device operating at liquid nitrogen temperatures A. Friedman * , N. Shaked, E. Perel, M. Sinvani, Y. Wolfus, Y. Yeshurun Institute for Superconductivity, Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel Received 7 July 1998; received in revised form 7 November 1998 Abstract A laboratory-scale superconducting energy storage (SMES) device based on a high-temperature superconducting coil was developed. This SMES has three major distinctive features: (a) it operates between 64 and 77K, using liquid nitrogen (LN 2 ) for cooling; (b) it uses a ferromagnetic core with a variable gap to increase the stored energy while retaining the critical current value; (c) it has the option for simultaneous energy charge and discharge which increases the power available at the SMES output by a factor of  2 when operating as a converter. The present prototype of liquid nitrogen operating SMES stores 130 J at 64K and 60 J at 77K.  1999 Elsevier Science Ltd. All rights reserved. Keywords: High T c superconductors; Critical current density; SMES; Power applications 1. Introduction A device for storing electromagnetic energy is an attractive potential application for high-temperature superconductors (HTS). The feasibility of a high-tem- perature superconducting magnetic energy storage (HT- SMES) device has been extensively discussed [1–4] and a few experimental projects aiming at operating tempera- tures of  30K were reported [5–8]. The two main obstacles delaying the development of a LN 2 operated SMES are the relatively low critical cur- rent density J c of the BSCCO wires and the strong decrease of J c with magnetic field at LN 2 temperature. These two factors result in low values for the stored energy, making the liquid nitrogen operated SMES inef- ficient i.e. with poor ratio of the maximal energy stored in the coil to the power required for its cooling. Calcu- lations [9] show that for BSCCO-based SMES, maximum efficiency is achieved at about 30K. We report here on an HT-SMES, based on HTS coil made of Bi– Sr–Ca–Cu–O (Bi-2223) wires, operating at liquid nitro- gen (LN 2 ) temperatures. In order to improve the efficiency of this SMES we have introduced a ferromag- netic core and designed a special converter circuit. The * Corresponding author. 0011-2275/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII:S0011-2275(98)00126-X core, made of laminated iron surrounding the HTS coil, contributes a gain of  14 and  6 in the stored energy at 77 and 64K, respectively. Moreover, the core is designed to prevent the coil from being exposed to the magnetic self-fields, allowing for relatively high oper- ation currents in the presence of high self-fields. A speci- ally designed converter enables simultaneous charge and discharge of the HTS coil, adding another factor of  2 in the transmitted power. The details of this SMES and its operational states are described below. 2. SMES structure 2.1. The HTS coil The three double pancakes coil, purchased from American Superconductors Corporation (ASC) is made of 160 m multifilamentary silver-sheathed BSCCO wire. Its critical current (1 V/cm criterion) is 22.2 A at 77K. The energy storage capacity at this temperature is 4.2 J. Table 1 summarizes the characteristics of this coil. Typical I–V curves for the coil are shown in Fig. 1 for 64 and 77K. All measured I–V curves between 52 and 77K exhibit a power law dependence: V = V 0 (I/I c ) n , with a power n between 10 and 12. The critical current, I c , was found to increase, approximately lin-