A flywheel energy storage system can stabilize the fluctuating output of a solar photovoltaic power generation system, and it can also improve the efficiency of a railway system. To increase storage capacity, Railway Technical Research Institute (RTRI) has invented superconducting magnetic bearings with superconducting coils and bulk superconductors. That is, “total-superconducting magnetic bearing.” RTRI have started with basic research concerning “total-superconducting magnetic bearing,” and developed several prototypes of flywheel energy storage systems using “total-superconducting magnetic bearing.”At last, a demonstration machine for superconducting flywheel energy storage systems has been developed as a joint project of five enterprises subsidized by the New Energy and Industrial Technology Development Organization. This paper describes the background and outline of the research and development of this superconducting flywheel energy storage system.

The superconducting flywheel energy storage systems (FESS) can stabilize the fluctuation of the output from solar photovoltaic power generation systems. The FESS has been developed as a joint project of five enterprises subsidized by the New Energy and Industrial Technology Development Organization. Four key technology of the FESS are the magnetic fluid seal (MFS) technology for vacuum seal using a multi-material shaft, the active magnetic bearing (AMB) technology for non-contact levitated support, the dynamic break resistor (DBR) technology for allowing storage energy to be consumed as heat energy, and the touch-down bearing (TDB) technology for the safety device in case of the superconducting magnetic bearing’s failure. In this paper, the principles and features of those technologies are explained, and the validity of the design is also presented.

A superconducting magnetic bearing (SMB) has been developed with high-temperature superconducting (HTS) coils and bulks for a flywheel energy storage system (FESS). The FESS equipped with the SMB was tested at the megawatt photovoltaic power plant test site in Yamanashi Prefecture. Both the rotor and stator of the SMB are made of superconducting material, and are capable of supporting a flywheel weighing 4000 kg without any contact. So far, the system has remained in stable operation for 5000 hr. A further increase in storage capacity is required for the FESS to be applicable to railways as a system for preventing cancellation of regenerative braking. A SMB capable of supporting large 147 kN loads using a new type of REBCO tape has been developed and produced by SuperPower Inc. This paper describes the development of SMBs with loads 39.7 kN and 147 kN, and the development of REBCO wires.

This paper describes the development of RE-Ba-Cu-O (RE:Y or rare earth elements) bulk superconductors (“bulks”). HTS bulks have a great potential to pioneer a new application field of superconductors. The high-quality HTS bulks developed by Nippon Steel Corporation are called “QMG”, which is a single-grained material of REBa₂Cu₃O_x (RE123) with RE₂BaCuO₅ (RE211) inclusions finely dispersed in the superconducting matrix. The name QMG originally comes from the initial process, “Quench and Melt-Growth”. However, no quench step is required in the advanced QMG process, and thus “Q” now means “Quality” instead of “Quench”. The key point of QMG technologies is the simultaneous control of the crystal growth and the micro-structure. Although it is believed to be very difficult to produce large single-grained HTS bulks exceeding 80 mm in diameter, we have succeeded in enlarging QMG up to 150 mm using the RE compositional gradient method. These ultra-large HTS bulks have been used for the superconducting magnetic bearing (SMB) of the flywheel energy storage system in Yamanashi, Japan, contributing to non-contact support of a heavy 4,000 kg rotating flywheel. In addition to SMB, other promising application are HTS bulk magnets for ship propulsion motors, wind or tidal power generators, and desktop cryogen-free NMR/MRI. HTS bulks are also an ideal material for current leads. Different bulk properties are required for different applications. We have therefore customized QMG bulks depending on application requirements: Dy-QMG for current leads, Eu-QMG for NMR/MRI, and Gd-QMG for other HTS bulk applications.

A flywheel energy storage system (FESS) using a high-temperature superconducting magnetic bearing (SMB) with an electric power of 330 kW and a storage capacity of 10 kWh has been demonstrated at the Komekurayama photovoltaic power plant located in Yamanashi Prefecture. The storage capacity must be increased further before the FESS can be applied to railways as a system for preventing the cancellation of regenerative braking. In order to confirm the design validity of superconducting magnetic bearings, levitation tests up to 158 kN were conducted. In addition, a superconducting bulk flux creep test was performed for more than 200 hr, and it was estimated that the decrease in levitation force was slight, even after long-term operation. From these results, it was found that the current SMB design can be applied to FESS SMBs for railway applications.