High-temperature superconductors can be cooled simply and energy-efficiently with liquid nitrogen. Current limiters, for example, are installed in a vacuum-insulated cryostat, which is supplied via a level control with small amounts of liquid nitrogen to compensate the small but inevitable cold losses.
However, the cost of cooling power cables is considerably greater. Here, supercooled liquid nitrogen is pumped through the cable cryostat to remove the incoming heat. For this purpose, the liquid nitrogen must be cooled down to -206 °C (10 °C below its boiling point at atmospheric pressure).
Messer has developed a cooling system for cooling superconducting power supply cables according to the following scheme:
The plant consists essentially of a subcooler, a cooling circuit and a reservoir for liquid nitrogen. The outside of the subcooler is supplied by an expansion valve with liquid nitrogen from the storage container. Here, the nitrogen evaporates, thereby creating the cold in the subcooler.
If the nitrogen is allowed to flow directly into the atmosphere from the outside of the subcooler, it evaporates at its boiling point (-196 °C). However, this cooling temperature is not sufficient. Therefore, a vacuum pump is connected to the subcooler, so that the nitrogen evaporates in the vacuum (at 150 mbar). In this way, the evaporation temperature can be lowered to -209 °C. Further temperature reduction is not possible because nitrogen freezes at -210 °C.
Inside the subcooler is a heat exchanger. Through this, liquid nitrogen is pumped (acting as a refrigerant), which thereby cools to a temperature of -206 °C. Refrigeration and cold transmission is thus carried out with the same equipment.
The supercooled liquid nitrogen in the heat exchanger flows through the superconductor cable to dissipate the heat that has entered. The nitrogen heats up slightly, but it always remains liquid and does not evaporate. Then it returns to the pump and then to the subcooler, where the recooling to the cooling temperature of -206 °C takes place. This creates a closed cooling circuit, which is connected via a compensation line to the liquid nitrogen reservoir to compensate for volume and pressure fluctuations.
All components of the cooling system (except for the storage container) are installed in a frame made of steel profiles and completely cased, wired and insulated. A separate frame is provided for the vacuum pumps. These "skids" are fully functional units that are fully tested at the factory. As a result, the installation effort at the site is low, and the commissioning is done quickly and efficiently.
Function test of cooling unit