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How does the nitrogen in a superconducting cable stay cold enough?

 

Answer

Kühlschema_Kabel_SFCL_en

Illustration: Nexans Deutschland GmbH

RWE, a German electric utilities company, is currently testing a superconducting cable developed and built by Nexans. The trials, which are being carried out at RWE’s base in Essen, involve keeping the cable at roughly -200 °C. That’s around 20 °C colder than the point at which the electrical resistance in the high-temperature superconductor drops to around zero.
The cable being trialled is a coaxial cable. The core is surrounded by a corrugated, double-walled stainless steel tube known as a cryostat. The ridges in the cryostat mean the cable remains flexible and can be wound around a reel. The cryostat contains layers of insulating sheets, and the space between them is a vacuum. This means the cable is extremely well insulated and can maintain a stable temperature over several hours even if the cooling system breaks down.
This diagram shows the basic layout of a cooling system. Liquid nitrogen circulates through a closed loop that runs into the cable, back out and then around again. Nitrogen is plentiful, cheap and environmentally friendly.
A cooling device keeps the liquid nitrogen at the necessary temperature. Nitrogen flowing out of the cable is warmer than when it went in. When it reaches the heat exchanger, the coolant in the cooling device chills it again. The heat gets transferred to the coolant, and the cooled liquid nitrogen flows back out into the superconducting cable.
Different types of cooling devices exist that draw on different thermodynamic processes and achieve different levels of efficiency. All a “closed” system needs, once it has been cooled and filled with a one-off supply of liquid nitrogen, is access to the electricity supply. An “open” system, which is the kind RWE is testing in Essen, uses a bath of super-cold liquid nitrogen to re-cool the closed cable loop. Vacuum pumps reduce the pressure and keep the temperature of the bath at around 65 K (degrees Kelvin), which is well below nitrogen’s boiling point (77 K). The bath continually loses liquid nitrogen, so the level has to be topped up from a storage tank.
Although cooling obviously costs money, superconducting cables are still significantly more efficient than conventional medium-voltage power cables. This is because almost none of the electricity traveling through them is lost, which saves on energy as well as money.

This question was answered by Dr Joachim Bock, Director of Sales and Market Development for HTS systems at Nexans Deutschland GmbH.