Superconductivity becomes affordable

Despite the hype about the ability of 'high temperature' superconductors to carry large currents without resistance losses at liquid nitrogen temperatures, another material is much more economically attractive. It is cheaper to make, easier to work with and superconducts alternating current at temperatures of 25 deg K to 30 deg K, well within the range achievable by commercially available single-stage cryocoolers. Interesting potential applications under investigation include pod mounted ship drive motors, induction heaters, fault current limiters and compact high-powered generators. Unlike so called high temperature superconductors, magnesium diboride can be made into cable by pouring its ingredients, magnesium and boron, into a copper or stainless steel tube followed by swaging, drawing and sintering. Fabrications can also be rolled flat, which assists compaction. Current price of high temperature superconductor material is around $200/kA-m, whereas according to Philip Sargent of Cambridge based Diboride Conductors, magnesium diboride, costs around $20/kA-m. American Superconductor claims that they expect to bring their price for the high temperature product down to $50/kA-m but the corresponding figure for magnesium diboride is $5/kA-m, half the $10/kA-m generally considered to be the breakeven price for most commercial applications. The economics of using superconductors are complicated. In theory, they should require little cost to keep cold, once they have reached superconducting temperatures. Unfortunately, this is only true if they carry DC, since AC does suffer losses, converted into heat, caused by the magnetic flux changes in each cycle. Energy required for refrigeration is about three times higher at 25 deg K than it is at 77 deg K and 70 times higher at 4.2 deg K. Some microwave antennae already use superconductors. Despite the cost, use of superconductivity triples 'Q', a measure of antenna effectiveness. Sargent considers that for industrial electric motors, use of superconductors doubles capital costs, but halves electrical losses. Payback time would then be five years which is not commercially attractive. Used in ship propulsion pods, however, use of superconductors greatly reduces size and thus, water resistance. It is said that a cruise ship with superconducting motors would have room for 14 extra luxury cabins. The other big field of interest is in fault current limiters. The idea is that when a fault voltage comes along, current goes greater than critical, initiating electrical resistance and blocking the pulse. When the fault condition passes, the device cools to normal working temperature again. The idea is to do away with the present need to oversize transformers to cope with fault conditions. Diboride Conductors is funded by £50,000 from the Carbon Trust. Its main US competitor, Hyper Tech based in Columbus Ohio, has so far received $800,000 from the state of Ohio and $920,000 from the USAF. Their interest is for light-weight power systems for radar and energy weapons. The material was originally discovered by Jun Akimitsu at Tokyo Methodist University, who, for some reason, failed to patent it. Pointers Superconductors carry very large currents without resistance losses Traditional niobium titanium superconductors are cheapest, but require cooling to liquid helium temperature, 4.2 deg K. Magnesium diboride is more expensive, but still affordable, and superconducts up to 39 deg K. Useful working temperatures are in the range 25 to 30 deg K. It has one third the density of copper. So called 'high temperature' superconductors may be used at liquid nitrogen temperatures, but they are very expensive, come as difficult to work with ceramics, and the best ones use highly noxious substances such as thallium and mercury. Diboride Conductors Philip Sargent