EARLY HIGH FIELD MAGNETS AND ENERGY STORAGE

In 1961, Kunzler et al. prepared the first practical powder-in-tube conductor by drawing a Nb tube filled with Nb3Sn powder into a wire. Following reaction, the wire showed very high critical current density of ~105 A/cm2 at magnetic fields up to 8.8 T (Kunzler et al. 1961). This result provided the first real demonstration of high field superconductivity and at that time produced as much excitement for superconductivity as seen in the subsequent HTS discovery. Kunzler and his coworkers eventually wound the Nb3Sn wire into a solenoid, which produced a field of 7 T (Kunzler 1987).

Kunzler's experiment thus opened the "Type II" superconductor era, offering enormous potential for superconducting magnets and electric power applications. Shortly after Kunzler's work was reported, Hulm and Blaugher (1961) published their studies on the Group IV, V, and VI transition metal alloys, four of which included the body-centered-cubic Nb-Ti and Nb-Zr systems. The Nb-Ti alloys were eventually shown to be the most technologically important because researchers were able to inexpensively fabricate long lengths of conductor with high current properties in useful magnetic fields of 3 to 5 T. Within a year, these hard superconducting Nb-Ti and Nb-Zr alloys were fabricated into wires and wound into solenoids, producing magnetic fields up to 7 T. The successful test of these solenoids quickly established that the transition alloy conductors could be easily applied to a wide range of electric power-related applications.

The idea of using large SC magnets for energy storage follows directly from the expression describing the energy stored in an inductor, which is simply, E(J) = ŻLI2, where L is in henrys (H) and I, the current, is in amps. Following the demonstration of high field magnets in the early 1960s, magnetic energy storage was immediately considered, but advances in fabricating a high-current cabled conductor were necessary before serious programs could be pursued. The major programs on superconducting magnetic energy storage (SMES) started in the early 1970s and continued through the mid-1980s. The primary interest for these magnets was directed at diurnal storage for load leveling and the need for a pulsed power source for current induction in the plasma of Tokamak fusion power devices. The SMES effort during this period, which was concentrated in the United States and Japan, is reviewed by John Rogers (1981).

The energy storage effort in Japan from the 1970s to the 1980s was fairly widespread, with most of the major electric power companies such as Hitachi, Mitsubishi, and Toshiba participating. In addition, storage programs were pursued by some of the utilities, in particular, by Chubu Electric Power and Kansai Electric Power companies. Hitachi built and tested a 5 MJ SMES system in 1986, which was connected to the 6.6 kV power line of the Hitachi Works to evaluate transmission line stability (Ishigaki, Shirahama, and Kuroda n.d.). In 1989, Chubu Electric and Hitachi jointly studied and developed a 1 MJ SMES system to evaluate how a SMES could provide power system stability (Fujita 1989). It is important to note that a "SMES system" includes the SC magnet, a fairly sophisticated solid state ac-dc power conditioning system (PCS), and usually a closed-cycle refrigerator. The SMES system is also interfaced with additional circuit breakers and a control system for protection and isolation. The response time for the SMES is thus dependent on the PCS and associated switch gear, which is fairly fast, allowing a SMES system to demonstrate high-speed response with capability of the order of one cycle or approximately 10 ms.

The major achievement in the United States during this period was the construction and successful test on the Bonneville Power System of a 30 MJ SMES to damp low frequency oscillations between two ac transmission lines running from the Pacific Northwest to Southern California (Rogers et al. 1985).


Published: September 1997; WTEC Hyper-Librarian