Super-thin Superconducting Cables
New compact cables show promise for power transmission and high-field magnets.
Researchers at the National Institute of Standards and Technology (NIST) have found a way to make high-temperature superconducting power cables that are a tenth the diameter of existing superconducting cables but can carry just as much current. The thin, flexible cables could open up new applications in electrical power transmission and could lead to powerful new magnets.
The cables could provide a lightweight, compact replacement for copper power cables, says NIST researcher Danko van der Laan, who led the work. Superconducting magnets made with the cables would generate much higher magnetic fields than are possible today. Such high fields would be useful for high-energy physics and for proton therapies used in cancer treatment.
Superconductors conduct high electric currents without heating up or losing power when they are cooled. The superconducting magnets found in medical imaging devices and particle accelerators typically use niobium alloys that turn superconducting below 10 K (-263 °C). But certain compounds made of rare earth elements, barium, copper, and oxygen also become superconducting at higher temperatures of over 70 K (-203 °C), at which point they can be cooled using liquid nitrogen or helium gas.
High-temperature superconducting cables have been touted as a promising alternative to copper cables for electric power transmission in urban settings and compact spaces. That’s because just one superconducting cable could replace more than10 copper cables, cutting weight by over 95 percent and eliminating heating loss.
Cryogenic superconducting power cables are typically made using superconducting “tapes” wound around solid or hollow metal cores. The tapes are thin strips of metal coated with a micrometer-thick layer of superconductor and films of ceramic insulators. Superconducting cables have recently been used in small power-grid demonstrations. A bismuth-based cable was installed at a utility substation in Columbus, Ohio, in 2006, for instance. It has a diameter of seven centimeters and can carry 3,000 amperes.
In comparison, van der Laan has made a cable 7.5 millimeters wide that can carry 2,800 amperes. Another is 6.5 millimeters in diameter and can carry 1,200 amperes. The cables can be bent around a cable with a diameter of less than a quarter of a meter.
Van der Laan starts with a core made of multiple copper strands sheathed in nylon insulation. Then he winds superconducting tapes made of gadolinium barium cuprate in alternating directions around the core. His experimental results were recently published online in the journal Superconductor Science and Technology.
Conventional superconducting cables are lighter than those made of copper, but they are still so heavy that they have to be buried underground, van der Laan says. “Researchers are looking at options for using them as overhead lines instead of underground,” he says, “but conventional cables have been too heavy to use overhead. One benefit of our cables is they’re much more lightweight.”
Until now, it was assumed that you could not make superconducting cables so thin, says Venkat Selvamanickam, a mechanical-engineering professor and high-temperature superconductivity expert at the University of Houston. “The concern was whether the tapes could be bent at such small diameter cores and still maintain high current carrying capacity without any damage.”
David Larbalestier, a scientist at the National High Magnetic Field Laboratory in Tallahassee, Florida, says the new cables are a perfect example of good engineering. “There’s no new rocket science here. They have applied perfectly standard techniques to make a cable.” Larbalestier does not think the new cables will easily find their way into power transmission, though. “Many people would love to use high-temperature superconductors to revolutionize the electric utility industry,” he says. “But the industry is relatively conservative and not used to cryogenics. On the other hand, the big multibillion-dollar market for superconductors is making magnets that consume very little power.”
Today’s superconducting magnets contain niobium-titanium wires wound into coils that can provide at most 25 Tesla magnetic fields. Magnets made using the new high-temperature superconducting cables could give higher fields while potentially requiring less power for cooling. .
The low weight and flexibility are especially appealing to the military as a replacement for the bulky copper cables that carry large amounts of power from generators to weapons and devices on board aircraft and ships. “If you look at replacing standard copper cables on a Navy ship, you have to be able to pull the cable through existing conduits with many sharp bends,” van der Laan says. He is now making a demonstration cable for the U.S. military. Researchers at CERN (the European Organization for Nuclear Research) in Switzerland are also interested in using the thin cables to feed the several thousands of amperes of current to the magnets used at the Large Hadron Collider.