Almost 100 years after Dutch physicist Heike Kamerlingh Onnes found that mercury has an electrical resistance of zero when cooled in liquid helium, superconductors are finally being rolled out for use in national electricity grids.
Superconductive wiring carries about ten times as much power as the same volume of conventional copper wiring. Although some of that power is lost and liquid nitrogen must be used to keep the superconducting cables cool, such cables are still more efficient than copper wiring, which loses 7-10% of the power it carries as heat. Because of this, countries such as South Korea that wish to 'green' their electricity networks and build more efficient and robust 'smart grids' are interested in the technology.
Now, LS Cable, a South Korean company based in Anyang-si near Seoul, has ordered three million metres of superconducting wire from US firm American Superconductor in Devens, Massachusetts.
American Superconductor have not disclosed the value of the deal. But Jason Fredette, managing director of corporate communications at the company, says that LS Cable will use the wire to make about 20 circuit kilometres of cable as part of a programme to modernize the South Korean electricity network starting in the capital, Seoul.
The superconducting wire is made using the ceramic compound yttrium barium copper oxide (YBCO), part of a family of 'high-temperature' superconducting ceramics that were first discovered in 1986. It remains a superconductor up to 93 kelvin (–180 ºC), meaning it can be cooled using liquid nitrogen.
This is in contrast to low temperature superconductors, which are made of metal and must be cooled to below about 30 kelvin (–243 ºC) with liquid helium. That makes them too expensive to be used in large-scale commercial applications.
Superconductors also lose their remarkable properties when current above a critical value is passed through them, so the search for a commercially viable superconductor has focused on materials that operate at a high temperature relative to low temperature superconductors and can carry large currents. At the moment, YBCO is the most promising material available.
Evolution not revolution
David Cardwell, professor of superconducting engineering at the University of Cambridge, UK, says it has taken nearly 25 years for YBCO to be commercialized because a brittle ceramic "is exactly what you don't want for making wires".
Most previous attempts to commercialize YBCO have used a powder made of crushed ceramic crystals enclosed in a metal tube. But the YBCO crystals must be almost perfectly aligned for the wire's resistance to remain low, an expensive and time-consuming process. Fredette says American Superconductor have been working to make the process cheaper. "We've managed to get costs down — the wire is much less expensive to produce as we scale up our manufacturing," he says. Now, after a number of successful pilots, both in the United States and in Europe, interest in YBCO wire is increasing.
American Superconductor makes its wire using a core of YBCO coated with copper, stainless steel or brass to provide strength. Yttria (Y2O3) nanodots dispersed through the YBCO layer stabilize the current flow, improving the current carrying capability of the wire by helping to control the magnetic fields in and around the wire.
The deal did not come after any particular technical breakthrough. "It's really been an evolution rather than an overnight change," says Fredette. Cardwell says that the ability to make long cables at affordable prices is what is truly revolutionary here.
Replacing an existing grid with superconducting cable is less complicated than it sounds because cooling systems are already required for the copper cables currently in use. "You basically just need a different fridge," says Antony Carrington, professor of physics at Bristol University, UK.
Fredette explains: "The real difference is at the electricity substation, where a small piece of pumping equipment keeps the liquid nitrogen circulating in the cable, but from the outside you wouldn't be able to tell conventional and superconducting cables apart. The reason it has taken so long to get electric utilities to adopt it is because it sounds very different to a conventional cable, but it really isn't."
Fredette expects that other nations will soon be turning to YBCO as a superconductor. "China is looking at a project of is own, so the market is really finally starting to develop," he says. The United States is also interested in using superconducting cables, supplied by LS Cable, to connect the country's three main power networks.
South Korea's move to develop a smart grid with these superconducting wires is evidence of its emergence as a world leader in green technologies — as a proportion of its total fiscal stimulus during 2009, the nation spent more than any other country on energy-efficient or alternative energy technologies.
South Korea's president, Lee Myung-bak, has implemented a five-year plan for green growth, which includes the building of the new grid. But James Hoare, a teaching fellow at the Centre of Korean Studies, part of the School of Oriental and African Studies in London, says he is not convinced by President Lee's green credentials. "He is known as 'the bulldozer'," says Hoare, who is also an ex-head of the UK Foreign Office's North Asia and Pacific Research Group, "because he goes in for projects that destroy everything in their way. I would take the green policy with a pinch of salt."
Even if Korea's green growth programme is not all it is cracked up to be, superconductors are likely to become much more widespread in coming years and, according to both Carrington and Cardwell, YBCO is the material most likely to be used to rejuvenate power grids across the world.