Integrating energy storage into an electric power system has long been recognized as a way to maximize a utility's generation and/or transmission capacity. Electric power can be stored during off-peak periods and then recovered during high-peak conditions to offset the need for larger generation or expanded transmission capacity. In Japan, load-leveling or diurnal usage of stored energy is currently accomplished at pumped-storage installations, which are considered to be fairly inefficient, having only ~70% turn-around efficiency. Also, Japan's river system, highly dense settlement, and expensive real estate do not favor construction of additional pumped-storage facilities. The Japanese have thus been exploring alternative storage technologies, including SMES and flywheels. As discussed earlier, SMES research in Japan has been underway since the early 1970s, with many prototypes built and tested under actual "utility" conditions.
Japan's International Superconductivity Technology Center (ISTEC) conducted a three-year feasibility study starting in 1988 on electric power apparatus, including SMES, under a program sponsored by MITI's Agency of Natural Resources and Energy. As part of this study, the Subcommittee on Energy Storage recommended in 1989 the detailed design using available technology of a small-scale SMES, with either a solenoid or toroidal field configuration, that would be consistent with future R&D efforts (Katsuya 1990). This SMES program, much like Super-GM, did not mandate using HTS materials; thus, LTS conductors such as Nb-Ti and Nb3Sn were the primary choice. Demonstration of a small-scale SMES, whose size is closely related to that needed for power system stabilization, would also address many major technical issues facing the large-scale diurnal storage SMES, such as ac losses, power conditioning, and refrigeration. ISTEC's initial SMES effort was followed by a six-year program started in 1991 to implement the construction and test of a small-scale SMES pilot plant at a 100 kWh/20 MW rating. This ISTEC program, also sponsored by MITI's Agency of Natural Resources and Energy, involves Toshiba as the primary magnet manufacturer, the Electric Power Development Corporation providing power conditioning, and various utilities led by Chubu Electric with additional support from Tohoko and Kyushu Electric Power Companies (Kamiyama 1994).
The design approach for the 100 kWh/20 MW system, shown in Fig. 2.5, is a toroidal magnet with an outside diameter for the cryostat of ~12 m (Kamiyama 1994). A half-size prototype coil was constructed by Toshiba and had been recently tested at the time of the WTEC trip to Japan. The test coil used a forced-flow Nb-Ti cable-in-conduit conductor and demonstrated 20 kA at 2.8 T, which is the rated current for the basic design. The initial testing was conducted at the Japan Atomic Energy Research Institute (JAERI), with further tests planned at Lawerence Livermore National Labs (LLNL) in the United States. Additional information on this SMES program is presented in the ISTEC, Chubu, and Toshiba site reports (Appendix C).
Fig. 2.5. Conceptual design of the ISTEC superconducting coil for the 100 kWh small-scale SMES.
Another program looking at the use of SMES for power system stabilization has been led by Kansai Electric Power Company (KEPCO). A small three-coil torus (400 kJ per coil), shown in Fig. 2.6, was assembled and tested on the KEPCO power system through a transformer and chopper arrangement. Two of the coils were built separately by Sumitomo Electric and Mitsubishi Heavy Industries using Nb-Ti conductors (SuperCom 1996). The third coil, built by Mitsubishi Electric, used Nb3Sn. This program also involved Osaka University, which with KEPCO looked at how SMES could be used for power system control and stabilization (Tada, Mitani, and Tsuji 1995, 250).
Fig. 2.6. KEPCO 3-coil torus (400 kJ per coil).