In the field of applying superconducting technology to electric power, superconducting magnetic energy storage (SMES) features highly efficient electric power storage as well as high-speed energy storage and output in comparison with conventional energy storage equipment. With superior qualities of this kind, SMES is expected to bring about a wide range of benefits, such as a stabilization of the power system, maintenance of power quality, and power load leveling. However, extensive R & D for SMES is required to achieve further cost reductions because many issues are yet to be resolved with respect to cost-performance and practicality. Given this situation, a 5-year national government project was started in 1999, focused on the development of cost reductions for small-scale SMES associated with power system control to meet market needs and soon become feasible. This paper outlines the ongoing R & D under the national governmental project, the need for cost reductions, and the study results of marginal costs, which can accommodate SMES costperformance.
Optimal SMES system concepts were developed for power system stabilization (available capacity of 54MJ, available output power of 100MW) and load fluctuation compensation or frequency regulation (available capacity of 1, 800MJ, available output power of 100MW) applications. The system details designs mainly composed of the superconducting coil, which aimed to achieve a cost decrease for each usage, based on four different schemes. The details of the designs and cost-reduction efforts were reviewed. Furthermore, to verify the validity of the development concept and the performance based on the detailed design result of the superconductor, five kinds of short superconductors (about 10m) selected respectively from among four kinds of superconductors were made for trial purposes. The performance of these prototypes was analyzed, including the measurement of current-carrying characteristics, stability and AC loss. Especially, the AC loss characteristic was measured with two devices: “superconductor coupling loss measurement device” of Kagoshima University; and “actual AC loss measurement device”, equipment was that built to enable the measurement and assessment of AC loss under conditions closely approximating the actual magnetic fields.
Superconducting magnetic energy storage (SMES) is a promising technology for electric utility stabilization of transmission, load compensation, and frequency regulation. This paper presents the results of a study to reduce the capital cost of an SMES system. The study focused on a toroidal-shaped superconducting coil system and concerned both 100MW/15kWh SMES for stabilization on transmission and 100MW/500kWh SMES for load compensation and frequency regulation. The charge/discharge ratio (k) was optimized in consideration of the economical efficiency of the whole SMES system, and thereby k=0.52 for small-scale SMES and k=0.86 for medium-scale SMES were obtained. To reduce the cost of a superconducting coil system, the system's design should be based on the concepts of a compact, a simple structure and small material amount. The aspect ratio of a toroidal coil system and that of a solenoid unit coil are determined from the viewpoints of minimum material amount. Aluminum-stabilized forced-flow superconductors have been newly proposed to achieve high voltage and high magnetic-field design in correspondence with small coils and reduced coil-turn numbers. New oxidization techniques for the aluminum surface of the stabilizer have been developed and the structure of the superconductor is designed simply and symmetrically to reduce manufacturing cost and ac loss. The supercondutor's favorable performances have been confirmed through short sample tests. These results provide a strong incentive for utilities to encourage and support the development of SMES cost-reduction technology.
A cost reduction study of 100MW/15kWh SMES for power system stabilization and 100MW/500kWh SMES for fluctuating load compensation and frequency control is performed. The optimization of SMES capacity factors and rated magnetic fields for each system shows that the optimum capacity factor of 100MW/15kWh SMES is lower than that of 100MW/500kWh SMES. And the optimum magnetic field at the point of minimum cost is about 3T, lower than that of usual NbTi magnets. A multipole solenoid system with NbTi CIC conductor is proposed as a cost-reduced SMES system. To reduce the number of superconducting strands, the copper stabilizer segregated from the strand, and a strand cross-section structure is simplified from the first-phase SMES project design for cost reduction. To reduce long-term AC loss, the strand has a CuNi sheath, and cables are twisted up to the second degree, which is much lower than that of the cable designed for the first-phase SMES coil.
The investigation of the possibility of low-cost SMES is an indispensable theme if that practical use is to be realized. We optimized the system composition by selecting the most suitable charge/discharge rate. We conducted the optimum design on the structure of superconductor and the coil system under special consideration of the material unit price, the total material amount, the manufacturing efficiency, and so on. The pool-cooling double-row solenoid system using NbTi base conductor is the most suitable one as 15kWh-100MW SMES for the system-stabilizing use and 500kWh-100MW SMES for the load-fluctuation-compensating and frequency-controlling use.
A cost-reduction design of the SMES system is proposed, using Nb3Sn conductors that in recent years have shown progress in cost and performance. The main feature of this design should be a combination of Nb3Sn single-strand conductors and a small bore solenoid (=CELL) coil. The cost reduction was intended to use mass production methods and to operate small or medium-sized factories. However, this simple and compact design brings up new issues. Quench protection is especially a key issue. It needs high-speed quench detection and forced-quench that will reduce the unbalance magnetic force at quench and prevent the CELL coil from burning up. This cost-reduction design of the SMES system using Nb3Sn conductors is sufficiently competitive with the other designs that use NbTi conductors.