Cool Current

High-temperature superconductors (HTS) sure sound like a good idea. When fashioned into wires and cooled to about 77K, which passes for balmy in superconductivity circles, these ceramic materials can carry at least a hundred times more current than comparably sized copper wires. Yet HTS wires have been so difficult and expensive to make that design engineers have viewed them as little more than a promising research project. Two American companies have recently taken big strides in taking the latest generation of superconducting wires out of the lab.

American Superconductor (Westborough, MA; http://rbi.ims.ca/3850-523) in March announced that it produced multiple lengths of 10-m HTS wire with a capacity of 250 A/cm of width. And IGC Super Power (Schenectady, NY; http://rbi.ims.ca/3850-524) the same month announced that it produced a 57-m length of HTS wire that carried 105 A/cm. Though each company chalked up very different combinations of electrical performance and wire length, both of these efforts broke world records for second-generation HTS wire, as opposed to the first-generation wire now in commercial production.

Generational differences

It's an important distinction. Commercial first-generation wire, which uses bismuth-based superconductor materials, already carries up to 140 times as much current as copper wire with the same cross-sectional area, according to Venkat Selvamanickam, materials program manager at IGC Super Power. But making the wire requires a slow, labor-intensive batch process that involves encapsulating the HTS material in a high-purity-silver matrix and then rolling this composite into a flat wire. The process can take days or weeks to make commercial useful lengths of wire, keeping costs high now and unlikely to fall much in the future.

Second-generation wire technology promises big productivity gains because its yttrium-based superconductor materials and other functional layers can be applied to a flat metal substrate using reel-to-reel manufacturing methods. Selvamanickam estimates that Super Power's continuous processing methods will be from 10 to 30 times faster than those used to make first generation wire.

The second-generation wire may not offer much improvement over first-generation wire in terms of its current-carrying capacity—at least at first. Selvamanickam expects that second-generation wire will very gradually improve to about 200 to 500 times greater than copper. American Superconductor's Chief Technical Officer, Alexis Malozemoff, predicts a similar trajectory for second-generation wire.

Leading pioneers

The superconducting material and basic architecture that define second-generation wire don't really differ all that much from one manufacturer to another manufacturer. American Superconductor and Super Power both employ an HTS made chiefly from yttrium, barium and copper oxide, or YBCO for short. So do a handful of Japanese and European companies. "YBCO has been around since the 80's, and a lot of research has been done on it," says Malozemoff. YBCO becomes superconductive above the temperature of liquid nitrogen, or 77K.

Aside from the YBCO, all second-generation wire includes a variety of other functional layers such as inert buffer materials as well as silver and copper to add electrical stability and improve mechanical properties. The new generation of wire will also be more robust than the first generation. Tensile properties should be about 10 times higher, according to Selvamanickam. The whole structure would typically measure about 100 microns thick with the HTS representing one or two microns of the total thickness and the substrate representing about 50 microns.

The biggest differences—and ones that will ultimately influence cost and quality—relate to the continuous deposition processes that grow a thin, epitaxial YBCO film on a metal substrate. American Superconductor, for instance, has paired liquid-phase HTS deposition with a mechanical deformation process to texture the substrate crystal growth. Super Power uses proprietary chemical vapor deposition methods to create a textured buffer layer on a smooth substrate and then to add the YBCO.

Whatever the manufacturing method, both suppliers believe that it's just a matter of time before second- generation wire can vie with copper on a cost basis. "I believe we'll be able to compete head-to-head with copper by the end of the decade," Selvamanickam predicts. First-generation HTS wire may ultimately cost as little as $50 per kA-meter—or the cost of carrying one thousand amps one meter. "We don't see much room for substantial cost reduction with the first-generation wire," says Selvamanickam. Second-generation wire, though, could do much better. Both Selvamanickam and Malozemoff predict that second-generation wire will cost two to five times less than first-generation wire, or about $10-25 per kA-meter. At that price, it would be cost competitive with copper, excluding the cost of cryogenic equipment and consumables.

Applying conductivity

Even before HTS wire can go head-to-head with copper, HTS wire has some technical advantages compelling enough to justify a price premium. "We don't need to wait until we can go head-to-head on price," says Philip Pellegrino, Super Power's president. He argues that the wire's current density advantage alone, makes first-generation HTS wire attractive right now for a variety of power transmission, generation, and regulation equipment.

These include power transmission cables. With the ability to carry more current than copper, potentially at lower voltages, HTS wire could play a role in shoehorning more capacity into a power grid that increasingly struggles to keep up with demand. According to Pellegrino, a single HTS power transmission cable takes the place of five conventional copper cables. And that savings can allow the existing power delivery infrastructure to carry more capacity. "We can do something to keep the lights on in a society with an insatiable appetite for electric energy," Pellegrino says.

HTS wire has also shown potential in transformer applications where its superior current density edge cuts down on the amount of wiring needed, saving some weight and space. Even bigger weight savings, as well as environmental advantages, come form the elimination of transformer oil. Pellegrino estimates that transformers made with HTS wire and their related cryogenic components weigh about 50 percent less and take up two-thirds less space than a conventional oil-filled transformer.

Malozemoff adds that HTS wire has some "special properties" that allow it to perform power regulation tasks. As an example, he notes that HTS materials become "normally resistive" above a certain current load. This capability allows them to function well in fault current limiters. HTS cables inherently have very low impedance, allowing them to "off-load" adjacent circuits with higher impedances. This property can allow HTS circuits to help "direct" power flows away from overloaded lines, extending the life of conventional circuit components.

Motoring

Large motors and other kinds of rotating machines have also taken advantage of HTS wire, which Malozemoff says "will change the name of the game in rotating machines." When used in motor windings, the power density advantage of HTS wire helps cut the weight and size substantially without reducing torque. For example, American Superconductor has designed and will soon help build a ship propulsion motor for the U.S. Navy. Malozemoff reports that this 36.5-MW motor will weigh about 75 tons, versus 300 tons for a comparable conventional motor. The switch to HTS will likewise produce a fourfold size reduction, important on ships.

American Superconductor has built other large rotating machines, including dynamic synchronous condensers—large motor-like pieces of equipment that stabilize power grids by absorbing or injecting reactive power. The company delivered just such a system to the Tennessee Valley Authority in 2003.

And both American Superconductor and Super Power report that HTS wire makes economic sense in large industrial motors. "Above 5,000 hp, HTS has major advantages," Malozemoff says, explaining that the cost of the cryogenics "pales" in comparison to the cost of the windings in motors this big. The move to second generation promises to make HTS suitable for even smaller motors. As the cost of the wire falls, Pellegrino envisions HTS will soon go into motors in the 1,000-hp range. And ultimately, he believes motors as small as 300 hp will take advantage of HTS wire.

Beyond cost, second-generation wire can help overcome two technical hurdles that affect first-generation wire. For one, motors currently use HTS only in the rotor and retain a conventional "warm" stator. Malozemoff points out that superconductors suffer from ac losses—such as hysteresis and eddy current losses— that prevent their use in the stator. Both American Superconductor and Super Power have developed proprietary ways to minimize these ac losses by altering the structure of the wire's superconducting material. In dc systems, HTS materials have no such problem since they don't suffer from electrical losses at all.

For another, second-generation wire also has an advantage over its predecessor when it comes to operating in high magnetic fields. "First-generation wire performs poorly in high fields," Selvamanickam says, explaining that the wire loses some of its current-carrying capacity as field density increases. To get that capacity back, the wire must be cooled to lower temperatures than they would otherwise need, a potentially expensive and bulky proposition. As an example, Selvamanickam says first-generation wire's capacity drops by a factor of more than 100 as a field increases from 0 to 1 Tesla at a temperature of 77K. To regain that performance, the first-generation wire would require temperatures under 35K. Second-generation wire exhibits a factor of five current drop from 0 to 1 Tesla at 77K, allowing it to operate at higher cryogenic temperatures.

Almost to market

Second-generation HTS wire may have poked its head out of the lab, but wire makers still have work to do. Malozemoff believes second-generation wire will have to carry more current to reach its full potential. American Superconductor has achieved the best performance on this score, but only at 10-M. And noting that commercial applications require high-quality wire in lengths best measured in the thousands of meters, Malozemoff says that "no one is there yet."

But HTS wire makers are getting close. Both American Superconductor and Super Power have started to scale up their second-generation manufacturing lines and expect next year to offer 100-M lengths of wire—long enough for testing, prototyping, and limited commercial use. Wire of commercially useful lengths and current-carrying capacities may start to appear as early as next year.

NextGeneration: First-generation wire packs its high temperature superconducting materials into a silver matrix using a slow, expensive manufacturing process. Second generation wire applies a different type of superconductor on a metal substrate in a less costly continuous deposition process.