High-temperature superconducting wire successfully tested in liquid hydrogen from liquefaction to energization
Development of integrated one-stop system from hydrogen liquefaction to energization testing.
image: Hydrogen liquefied using a refrigerator accumulates in the chamber. High-temperature superconducting wire carries current while immersed in liquid hydrogen.
Credit: National Institute for Fusion Science
At the National Institute for Fusion Science, we have been developing high-temperature superconducting coils for magnetic-confinement fusion that use liquid hydrogen refrigerant. This makes it possible to reduce power consumption and cost, ensuring a stable supply of refrigerant for future magnetic-confinement fusion plants. Furthermore, carbon free hydrogen produced using high temperatures from fusion is fed into the supply chain. This will contribute to realizing a hydrogen-fueled society and achieving carbon neutrality.
Ohya’s group in Kwansei Gakuin University are conducting research into superconducting generators for hydrogen-power generation that is carbon free. The efficiency of superconducting generators is improved by superconducting field coils which need to be kept at a cryogenic temperature. This utilizes the cold energy of liquid hydrogen, eliminating the need for refrigeration, thereby reducing operating costs as the power consumption is reduced. Consequently, energy-intensive liquefaction costs in the supply chain can be recovered at the power-generation stage.
Nevertheless, the transport-current capacity of superconducting wire in flammable liquid hydrogen is not fully understood because of limited global development. The practical application of high-temperature superconducting devices in liquid hydrogen cooling is necessary to promote fundamental research such as the cooling characteristics of liquid hydrogen.
The experimental apparatus used currently for carrying out tests on high-temperature superconducting wire submerged in liquid hydrogen was developed at the National Institute for Fusion Science for the investigation of electrical characteristics of high-temperature superconducting wire in liquid hydrogen.
Our research group has achieved the world’s first successful transport-current test of superconducting wire submerged in liquid hydrogen. In this apparatus, hydrogen is liquefied by a cryocooler supplying the chamber. The liquid hydrogen and cooling conditions of high-temperature superconducting wire under load can be observed through a viewing window, as shown in Fig. 1.
In previous similar experiments, off-site supplied liquid hydrogen was vented after the experiment, which could only be conducted when liquid hydrogen was available. This apparatus we developed can improve experimental efficiency due to our ability to liquefy hydrogen on our own and re-liquefy it repeatedly.
The inductive energization method was adapted to minimize the required powersupply current capacity. By this method, as shown in Fig.2, with a power source supply to the primary coil and an induced current flow in the secondary test coil made of a high-temperature superconducting wire, it was possible to carry a large current in a small space and small amounts of liquid hydrogen.
The secondary current was measured with a Rogowski coil. The same REBCO type of 4 mm wide wire was used for the primary and secondary coil. In our experiment, a current of over 1,100 A, corresponding to the estimated critical current of the high-temperature superconducting wire, was demonstrated at a liquid hydrogen temperature of 20 K. Then we confirmed the cooling stability of the high-temperature superconducting wire in liquid hydrogen when applying a current over the critical level of 1,000 A.
Our group has been consolidating research into the cryogenic stability of high-temperature superconducting wire in liquid hydrogen by using this apparatus we have developed. In particular, the essential data, including the cooling conditions of high-temperature superconducting wire and the behavior of liquid hydrogen have been verified by an experimental varying of the temperature and pressure of liquid hydrogen.
To apply the results obtained on the development of high-temperature superconducting coils for fusion reactors and hydrogen-power generators, we aim to commercialize carbon free and sustainable power- generation technologies.