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

Novel UV-transparent 2-component polyurethane resin for chip-on-board LED micro lenses

In this work we present a novel optical polymer system based on polyurethane elastomer components, which combines excellent UV transparency with high thermal stability, good hardness, high surface tension and long pot life. The material looks very promising for encapsulation and microlensing applications for chip-on-board (CoB) light-emitting diodes (LED). The extinction coefficient k, refractive index n, and bandgap parameters were derived from transmission and reflection measurements in a wavelength range of 200-890 nm. Thermogravimetry and differential scanning calorimetry were used to provide glass transition and degradation temperatures. The surface tension was determined by means of contact angle measurements. As proof of concept, a commercial InGaN-CoB-LED is used to demonstrate the suitability of the new material for the production of microlenses. © 2020 Optical Society of America