Cooling of an FCL can be done by submerging it into boiling liquid (i.e. nitrogen, neon) since the formation of bubbles around the FCL is not an issue. Therefore a gravity feed system can be considered.
The FCL is placed in a bath (cryostat) with liquid that is at equilibrium (saturation temperature). Heat production of the FCL like AC losses and thermal load of the cryostat will cause the liquid to boil. The evaporated gas rises to the top of the cryostat where the Cryogenerator is connected. The boil off gas is returned to the cold head where it is reliquefied and fed back to the cryostat bath by gravity. This set-up is visualized below:
Schematic configuration typically used for Superconducting Fault Current Limiters.
The most common cryogenic fluid used is liquid nitrogen with a boiling temperature of 77K at 0 barg and a freezing temp of 63K. As the superconductor tends to work better at lower temperatures the systems are often operated below atmospheric pressure to reduce the boiling temp (LN2 at 0,17 bar boils at 65K). Other cryogenic liquids such as neon (boiling temperature of 27K, argon 87K) can also be used.
To handle a variable heat-load due to variable operational conditions, the re-liquefaction systems are equipped with a capacity control. Based on the pressure of the nitrogen bath, the Cryogenerators vary their refrigeration output keeping this pressure and therefore the liquid temperature constant.
Stirling can design and offer a customized system for your specific project, please contact us to discuss your requirements.HTS leaflet Stirling Cryogenics