Fantastic energy-saving superconducting transmission electric train is coming

On July 24, the Japan Railway Institute of Integrated Technology (KISI), located in Kokubunji City, Tokyo, publicly conducted the world's first electric train driving test using superconducting cables.

Superconductivity refers to the phenomenon that the resistance becomes zero when a specific metal and a compound are cooled to an extremely low temperature. The superconducting cable is manufactured by housing a wire produced by this technique in a heat-insulated pipe or the like.

By maintaining the superconducting state by flowing a refrigerant such as liquid nitrogen in the tube, the power loss at the time of power transmission can be greatly reduced as compared with a cable using ordinary copper wire or the like. Since 1986, HTS has been developed to reach the superconducting state at a higher temperature, the superconducting transmission developed all over the world has finally come to a reality.

Transmission test using superconducting cables publicly implemented on the experimental line in the Comprehensive Railway Research

Apply for a number of superconducting related patents

The superconducting cable developed by the railway research institute is used for direct current transmission, and the high-temperature superconducting wire is sealed in a pipe approximately 10 cm in diameter. Liquid nitrogen at minus 196 degrees is circulated within it for cooling, reducing the resistance to zero.

In the case of superconducting power transmission, in order to circulate the liquid nitrogen, generally two parallel cables are arranged in parallel, or the pipe is made into a ring shape. However, considering the long-distance use and the freedom to set up the railway track research, improvements have been made in using multi-layer structural pipes to allow liquid nitrogen to flow back and forth in a single pipe.

In addition, it is also envisaged that liquid nitrogen cooling and circulation systems will be used on railways to develop autonomous technologies that take full account of safety. Railway Research Institute is currently applying for a number of patents.

"The superconducting cable can not only reduce the transmission loss, but also be very effective in effectively utilizing regenerative energy," said Dr. Tomita Tomita, director of the National Institute of Advanced Research and Development in Railway Research and Development Division.

When stopping the electric train, the motor is used as a generator and the kinetic energy is recovered as electricity, which is regenerative energy.

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

However, if superconducting cables are used, the principle is that the electric trains can be electrically connected to each other no matter where they are located. The overall power reduction is expected to reach about 5%. In addition, it is also expected to reduce and concentrate the substations that now need to be installed every few kilometers by realizing more efficient and average power supply to vehicles.

The 31-meter driving test will validate the data obtained and plan to use the 310-meter cable for a more practical demonstration later in the fall. Dr. Tomita said confidently: "We also think that laying on the actual railway network will make a breakthrough in practical application." It is estimated that it will take about five years to complete its practical application.

Unit for connecting superconducting cables and catenaries

Japanese manufacturers dominate

The commercialization of superconducting electricity is now well underway, and there has been a tremendous business opportunity for Japanese companies that are leading the world in the field of superconducting cables and wire development.

High-temperature superconductors are divided into several types, for example, Sumitomo Electric Industries is a leader in the field of superconductors that use rare-metal bismuth. In addition, other Japanese wire manufacturers such as Furukawa Electric Co., Ltd. and Fujikura Co., Ltd. are currently developing cables that use rare earth yttrium.

Bismuth-type superconductors are leading in the field of mass production and lengthening technology, but the yttrium-type superconductors, which are said to contain less silver, are said to be more costly in terms of final cost.

Tomita said: "We are currently negotiating with a number of Japanese manufacturers and due to their different characteristics, we will explore which wire will be suitable for the place and purpose."

Easy to implement "superconductive" railway network

Superconducting power transmission through the combination with the DC transmission network, will play its true value. Tomita said: "If the AC power transmission, in principle, the resistance will not be zero."

The structure of the AC superconducting cable is to set three cores in one pipe. The wires affect each other, resulting in a power loss called "AC loss." On the other hand, DC does not produce the same loss as AC. What's more, DC cables that require only 1 core are thinner than AC cables and offer lower cooling costs.

The existing power supply network for homes and the like is mainly AC power supply. The disadvantage of DC power supply is that power conversion equipment is required for the connection.

At present, the railroad has taken the lead in achieving DC. Allegedly in the electrification section of Japan's existing railway lines (Japan's passenger railways and private railways), approximately 70% of the routes have been DC-enabled and the new route has basically adopted the DC system.

The future of superconducting transmission will be promising if reliability can be assured in railways where safety is paramount. The demand for superconducting power that can contribute to energy saving will increase globally in the future. It is also possible that the superconducting transmission network will become the detonating agent for the export of infrastructure that Japan has not seen any progress so far. In addition to the basic technologies, leading Japanese companies in this field should also actively establish the laying and operating practices and obtain relevant patents.