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Successful development of a high-temperature superconducting magnet for next-generation flywheels

Fri, 03/21/2014 - 3:54pm

The Railway Technical Research Institute (hereinafter called "RTRI") and Furukawa Electric Co., Ltd. (hereinafter called "Furukawa Electric") have been the first in the world to succeed at the development of a high-temperature superconducting magnet for large flywheels, using the second generation high-temperature superconducting wires manufactured by SuperPower Inc., a subsidiary of Furukawa Electric. (note 1)

This development is being carried out in cooperation with Kubotek Corporation, Mirapro Co., Ltd. and the Public Enterprise Bureau of Yamanashi Prefecture, as part of the project for the "Development of Technology for Safe, Low-cost, Large-size Battery Systems" being promoted by the New Energy and Industrial Technology Development Organization.

Summary of development results

  • We succeeded in the development of a high-strength high-temperature superconducting magnet by using second generation high-temperature superconducting wires (note 2) that use yttrium.
  • It was demonstrated that a load exceeding 2 tons can be supported without contact when this magnet is cooled to 50K (minus 223°C) and a high magnetic field is generated.
  • This makes it possible to operate at 50K (minus 223°C), which is a temperature considerably higher than with the conventional high-temperature superconducting coils that are cooled to 20K (minus 253°C). We therefore have the prospect of being able to reduce cooling costs.

Details of the development

  

A flywheel power storage system stores energy by using surplus power from solar voltaic power generation or other sources to rotate a large disk (the flywheel) contained inside the machinery. When the amount of power generated decreases due to cloudy weather, the system then generates power to compensate for the amount of that decrease. It can be used as a "battery" that does not deteriorate and has a wide range of applications, including being useful for the effective use of electric power in railway systems (measure for regeneration cancellation).

The next-generation flywheel power storage system currently being developed applies the superconducting magnetic bearing proposed by RTRI, which is made by combining a superconducting bulk material with a superconducting magnet. The rotating disk is levitated without contact to eliminate friction losses on the bearing, which is done to improve the operating efficiency. It also makes it possible to use the bearings semipermanently, whereas on previous systems it was necessary to replace the bearings periodically.

The superconducting magnetic bearing currently being developed consists of a superconducting bulk material and a superconducting magnet. The target for levitation is for a disk of approximately 4 tons to be lifted by one bearing set, by using the diamagnetic effect that occurs in the superconducting bulk in response to the magnetic field generated by the superconducting magnet.

In order to lift such a large weight as this, it is necessary to generate a high magnetic field on a high-strength superconducting magnet. Furthermore, for efficient operation, it is necessary to raise the cooling temperature. For these reasons, we chose the "armor" coil (note 3) structure developed by Chubu Electric Power Co., Inc. for the coil to be used in the superconducting magnet in this development. This is a double-pancake coil (a flat coil made from a pair of windings of superconducting wire shaped like tape) with an internal diameter of 120mm and an external diameter of 260mm. We also chose the second generation high-temperature superconducting wires from SuperPower Inc., which Furukawa Electric purchased in 2012.

The coil that was produced was maintained at 51K (minus 222°C) by cooling using a compact refrigerator and heat conduction, without using liquid nitrogen. The electrification and magnetic field were confirmed at the operating current of 110A and we also succeeded in electrification at 163A, which is the performance limit for the wiring. In addition, we implemented testing of the combination with the superconducting bulk material and confirmed that the expected levitation force exceeding 2 tons was being created and that there were no problems related to the strength. The first generation high-temperature superconducting wires used previously needed to be cooled to below 20K (minus 253°C) in order to generate a high magnetic field. However, the second generation high-temperature superconducting wires can be operated at 50K (minus 223°C), so it is possible to expect reductions in cooling costs. From now onwards, we will add further coils and implement levitation testing on real scale flywheels.

Development structure
The development work in this project is being promoted by the RTRI bringing together Furukawa Electric, Kubotek, Mirapro and the Public Enterprise Bureau of Yamanashi Prefecture. The development of the superconducting magnetic bearings was carried out jointly by the RTRI and Furukawa Electric. The RTRI implemented the basic design for the superconducting magnetic bearing overall and Furukawa Electric implemented the design and manufacturing of the high-temperature superconducting magnet. This superconducting magnet bearing was developed by RTRI up to fiscal 2012 with government subsidies from the Ministry of Land, Infrastructure, Transport and Tourism, with the aim of applying it as power storage technology for railways. In the project this time, the aim of the development was to achieve increased size and practical application.

Future plans
It is planned that this magnet will be integrated into the large capacity superconducting flywheel power storage system being developed from fiscal 2012 to fiscal 2014 and that interconnection testing will start from 2015 at the megawatt-class solar power system to be newly constructed at Komekurayama in Yamanashi Prefecture. (note 4)  

Explanation of terms  

note 1 - Superconducting flywheel power storage system
A flywheel is a machine that converts electrical energy into kinetic energy on a high speed rotating body to store that energy. It is a machine that can input and output the electric power at high speed and also repetitively. In conventional systems, the mechanical bearing losses on the rotating body were large and also the wear on the bearings meant that long-term operation was difficult. However, by using superconducting magnets and superconducting bulk, it became possible to produce bearings that levitate the rotating body without contact and an electrical power storage system was realized that has low losses and does not require maintenance. 

note 2 - Second generation high-temperature superconducting wires (2G HTS)
These are superconducting wiring materials that are produced by forming an intermediate layer on a tape-shaped metal substrate of chromium-nickel base alloy, etc., and then forming layers by forming crystals of oxide superconductor made up of materials such as rare earth elements (yttrium, etc.), barium and copper.

These are in the superconducting state at the temperature of liquid nitrogen (minus 196 °C), they have high current density and even when in a magnetic field, they show little property degradation or AC loss. They are the highest performing materials among the high-temperature superconducting wires that have been put to practical application.

note 3 - Armor coil technology
This is technology developed by Chubu Electric Power Co., Inc. to dramatically improve the magnetic field of high-temperature superconducting magnets. The method used is to bear the electromagnetic force acting on the superconducting wire with the side plate that contacts with the superconducting wire. By combining this with insulation coating technology using liquid resin, a superconducting coil is realized that withstands the highest strength of magnetic force in the world, a force double that for conventional superconducting coils using yttrium and 6 times that for metal based superconducting coils.

note 4 - Yamanashi Prefecture Komekurayama mega solar power plant
Yamanashi Prefecture has one of the greatest amounts of solar radiation in Japan and the prefecture jointly constructed the "Komekurayama Solar Power Station" with Tokyo Electric Power Company in Kofu City in Yamanashi Prefecture, as the core part of its global warming measures execution plan. Around 80,000 solar panels have been installed on high ground in a vast area covering 44.7ha. It generates 1,200kWh per year, which is equivalent to the power used by 3,400 ordinary households. Yamanashi Prefecture is currently constructing a 1,000kW photovoltaic power plant for use in a grid connection test in combination with a superconducting flywheel. It is planned that the collection of basic data for demonstration testing will began after the plant starts operating.

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