Superconducting cable technology is expected to be used in electric trains

The Japan Railway General Technology Research Institute (Railway Research Institute), located in Kokubunji, Tokyo, opened the world's first electric train running test using superconducting cables on July 24th.

Superconductivity refers to the phenomenon that when the specific metals and compounds are cooled to ultra-low temperatures, the resistance becomes zero. The superconducting cable is made by storing the wire material manufactured by this technique in a heat insulating tube or the like.

By maintaining a superconducting state by flowing a refrigerant such as liquid nitrogen in the tube, power loss during power transmission can be greatly reduced as compared with a cable using a common copper wire or the like. Since the development of "high-temperature superconductors" that could reach superconducting state at higher temperatures in 1986, superconducting power transmission that countries all over the world have been promoting has finally become a reality.

Transmission tests using superconducting cables that are publicly implemented on experimental lines in railway research institutes
Applied for a number of superconducting related patents

The superconducting cable developed by the railway research institute was used for direct current transmission and the high-temperature superconducting wire was enclosed in a tube approximately 10 cm thick. The liquid nitrogen at minus 196 degrees is circulated and cooled in its interior, so that the resistance is reduced to zero.

In the case of superconducting power transmission, in order to circulate the liquid nitrogen, two parallel cables are generally provided in parallel, or the pipe is formed in a ring shape. However, in consideration of the long-distance use and the degree of freedom of installation, the railway research institute has improved and adopted a multi-layered structure to allow liquid nitrogen to flow back and forth in a single pipe.

In addition, it is also conceived to use liquid nitrogen cooling and circulation systems on railways to develop autonomous technologies that fully consider safety. Railway Research Institute is currently applying for a number of patents.

Dr. Fu Tianyou, director of the Research Institute of the National Academy of Advanced Research and Development and the director of the Superconducting Applied Research Office, said: "Superconducting cables not only reduce transmission losses, but are also very effective in the effective use of renewable energy."

When the electric train is stopped, the motor is used as a generator to recover the kinetic energy as electricity, which is regenerative energy.

The energy regeneration system has been put into practical use, and its generated energy can be supplied to other electric trains through electric wires. However, the resistance of the existing system transmission lines is large, and electricity can only be supplied to nearby electric trains.

However, if superconducting cables are used, on principle, no matter where the electric trains are located, they can integrate electricity with each other. The overall power reduction effect is expected to reach about 5%. Moreover, through the realization of efficient and average power supply to the vehicles, it is also expected to reduce and concentrate the substations that now need to be installed every few kilometers.

This time, a 31-meter cable was used for the driving test. The obtained data was verified and it was planned to use the 310-meter cable after the fall to implement more practical empirical experiments. Dr. Tomita said with full confidence: “We also imagine that we will build on the actual railway network and make breakthroughs in practicality.” It is estimated that it will take about five years to complete the practical application.

The railway research institute led Dr. Tomita from the superconducting technology development team. The temperature of the liquid nitrogen used for cooling and the like are displayed on the display in real time.

Japanese manufacturers dominate

The practical application of superconducting power transmission is already in place. Therefore, there is a huge business opportunity for Japanese companies that lead the world in superconducting cable and wire development.

There are several types of high-temperature superconductors. For example, in the field of superconductors using rare earth metals, Sumitomo Electric Industries is leading the way. In addition, other Japanese electric wire manufacturers such as Furukawa Electric Industries Co., Ltd. and Fujikura Co., Ltd. are currently developing cables using rare earth niobium.

In the field of mass production and lengthening technology, germanium superconductors are in the leading position, but it is said that the germanium superconductors with less silver content are dominant in the final cost.

Tomita said: "We are currently negotiating with a number of Japanese manufacturers. Due to their different characteristics, we will discuss which wire is suitable for use according to the location and purpose."

Easy to realize "superconducting" railway network

Superconducting power transmission will realize its true value by combining with the HVDC transmission network. Tomita said: "If AC transmission is used, the resistance will not be zero in principle."

The structure of the AC superconducting cable is to set three core wires in one tube. Wires will affect each other and generate power loss called "AC loss." On the other hand, DC will not produce the same loss as exchange. Moreover, only one core wire DC cable can be thinner than the AC cable, and the cooling cost is lower, which is also its characteristic.

Existing power supply networks used in homes and the like are mainly based on AC power supply. The disadvantage of DC power supply is that it requires power conversion equipment when connected.

At present, the railway sector has taken the lead in implementing DC. It is said that in the electrification section of Japan's existing national railway lines (Japanese passenger railway companies and non-governmental railways), about 70% of the routes have been DC-based, and new routes have also basically adopted DC systems.

The future of superconducting power transmissions will be hopeful if reliability can be ensured in the railway sector where safety is extremely important. The demand for superconducting power that can contribute to energy conservation will increase globally in the future. Superconducting transmission grids are also likely to become the detonators for infrastructure exports that Japan has not yet seen progressing. In addition to basic technologies, Japanese companies that are leading in this field should also actively establish laying and application methods and obtain relevant patents.