The SMES is characterized by several superior features such as allowing highly efficient, high-speed, input and output of electric energy. The application of a SMES to an electric power system contributes to stabilizing the power system and maintaining power quality, and it is anticipated that there will be extensive beneficial effects, such as upgraded controllability of power systems. Previous national development projects established the basic technology necessary to make the SMES feasible; however, challenges related to reducing system cost have remained. In the project phase from FY1999 to FY2003, the focus of development was to produce an SMES dedicated to power system stabilization, while placing a particular emphasis on finding ways to reduce the costs of the superconducting coil and peripheral devices. As a result, we now expect to be able to develop a low-cost SMES that is competitive with other power-control devices including SVC.
High-Tc superconducting technology is thought to provide many merits for SMES systems. For example, a cryo-cooled system can be used as a cooling system for high-Tc superconducting coils, indicating that a wide range of operation temperatures can be selected. As a result, the heat capacity of the coil system becomes much larger than that at 4.2 K. If we could absorb transient heat generation with the heat capacity of the coil, SMES systems can be designed under the over-current state of critical current for a short duration. As the cooling capacity for an average heat load will be enough to cool the High-Tc superconducting coil system for an SMES, it is expected that refrigeration system cost can be reduced. Moreover, we are developing a high critical-current superconducting wire for the SMES system. The Bi2212 Rutherford conductors can carry 4 kA at 26 K under cryo-cooling. We also estimate the superconducting wire cost of the whole coil system, which is designed to minimize the superconductor volume. The conclusion is that the cost of a high-Tc SMES system can be reduced remarkably by using low-cost YBCO superconducting wires in the future.