Liquid Nitrogen operated Cooling Systems for Superconducting Power Lines

HTS power cables or high current bus bar systems can be cooled by circulating subcooled liquid nitrogen through the cryostats in which they are integrated. The heat impact which has to be removed is mainly caused by heat transfer through thermal insulations and by the circulation pumps. To minimize the required cooling energy it is necessary to optimize pressure drop and surface area of cable cryostats and operate the circulation pumps with low dissipation. The circulating liquid nitrogen is cooled in a vacuum subcooler which uses liquid nitrogen as refrigerant. Vacuum pumps allow an operation pressure of 150 mbar to achieve a vaporisation temperature of 64 K (13 K below the atmospheric boiling point of nitrogen). This is advantageous because the ampacity of superconductors is dependant on the difference between their transition- and operation temperature. Lower temperatures are not practical, because at 63 K nitrogen becomes solid. Messer has developed a cooling system with an adequate vacuum subcooler, a liquid nitrogen circulation system and a storage vessel for the German AmpaCity project of Innogy SE (formerly RWE Deutschland AG), Nexans and KIT (Karlsruhe Institute of Technology). Within this project in 2014 the worldwide longest superconducting power cable was installed in the city of Essen, Germany. After more than three years of practical operation many important figures from cable and cooling unit are available. The system can easily be adapted for the operation of high current bus bars. These installations are normally much shorter than power cables, so pressure drop through the cryostat and heat impact of the circulation pumps are comparatively low. On the other hand the heat impact of the current leads is quite high and an efficient cooling system affords a combination of mechanical refrigeration and liquid nitrogen cooling