Large Flywheel Ready to Store Power at 1MW Solar Plant

2014/03/12 13:07
Kenji Kaneko, Nikkei BP CleanTech Institute

The Railway Technical Research Institute (RTRI) and Furukawa Electric Co Ltd have developed a high-temperature superconducting magnet for large-size flywheels for the first time in the world.

They used the yttrium-based second-generation high-temperature superconducting wire manufactured by SuperPower Inc, which Furukawa acquired in 2012. The development was announced March 10, 2014. The newly-developed magnet is scheduled to be embedded in the large-capacity "superconducting flywheel power storage unit" and linked to a 1MW solar power plant to be built in Komekurayama, Kofu City, Yamanashi Prefecture, Japan, in 2015 on a trial basis.

The "flywheel power storage system" stores electricity by using surplus electricity of a mega-solar (large-scale solar) power plant, etc to rotate a large-size disk (flywheel) inside the system. When the amount of power generated decreases due to cloudiness, the system generates electricity to make up for the decrease. It can be used as a "battery" and has already been employed for effectively using electricity (regeneration invalidation prevention) for railway systems.

For the next-generation flywheel power storage system being developed, a superconducting magnetic bearing that RTRI developed by combining a superconducting bulk body and superconducting magnet was employed. Operating efficiency is improved by contactlessly floating the rotating disk to lower the friction loss of the bearing to zero.

Also, the product life of the bearing, which generally needs to be replaced on a regular basis, becomes semipermanent. The superconducting magnetic bearing being developed is aimed at floating an about 4t disk with a pair of bearings.

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To float such a heavy weight, it is necessary not only to generate a high magnetic field with a high-strength superconducting magnet but also to raise cooling temperature for efficient operation. Therefore, for the superconducting magnet, RTRI and Furukawa used a "double pancake coil" that (1) uses SuperPower's second-generation high-temperature superconducting wire and (2) has an "armor coil structure," which Chubu Electric Power Co Inc developed.

The internal and external diameters of the double pancake coil are 120mm and 260mm, respectively. It is a pair of flat coils made by rolling a tape-like superconducting wire to form a shape that looks like a sliced Baumkuchen.

RTRI and Furukawa (1) kept the temperature of the coil at 51K (-222°C) with a cooling method based on thermal conduction without using liquid nitrogen, (2) confirmed conduction and a magnetic field with a current of 110A (operation current) and (3) even succeeded in conduction with a current of 163A (the limit of the wire's performance). Also, they conducted a test using the superconducting bulk body and confirmed a levitation force higher than 2t and a high enough strength.

To generate a high magnetic field with the first-generation high-temperature superconducting wire, it is necessary to lower its temperature to 20K (-253°C). On the other hand, the second-generation wire can be operated at 50K (-223°C), enabling to lower cooling cost. RTRI and Furukawa plan to perform another test in which an actual-size flywheel is floated by adding coils.