Full steam ahead?

Jun 3rd 2004
From The Economist print edition


Superconductors are starting to become useful

A WAG once said of Brazil that it is tomorrow's country, and always will be. Superconductivity frequently suffers similar jibes. On the face of things, a technique that allows electricity to be transmitted without resistance should be ubiquitous. But the obstacles have proved insurmountable for all but niche applications, such as medical imaging.

Three things stand in the way of the wider use of superconductors. First, they need to be cooled well below room temperature. Second, those that need the least cooling are the most difficult to work with. Third, the passage of an electric current creates magnetism, and magnetism tends to destroy superconductivity.

But these obstacles are being overcome. As a result, superconductivity is starting to appear in applications as diverse as telecoms and electric motors. There is even talk of reaching superconductor nirvana: using them for controlling and transmitting large amounts of electrical power.

The future's taped

Superconductor Technologies, a company based in Santa Barbara, California, is building superconducting filters for mobile-phone base stations. In common with all the other putative mass-market applications of superconductivity, the materials in question are so-called high-temperature superconductors (HTSs). High-temperature is a relative concept. These materials need to be cooled with liquid nitrogen (a mere -196°C). But that is cheap compared with the liquid helium (-269°C) used to cool traditional superconductors, such as those employed in medical imaging. There is, however, a price to pay. HTSs are ceramics, and thus much harder to shape and work than traditional superconductors, which are metals. This difficulty is an especially important factor when what is required is something that will do the same job as a traditional wire.

Superconducting Technologies' devices, though, are not wires. They are chips with superconducting surfaces. No flexion is involved, so using rigid ceramics is not a problem. The superconductor's lack of resistance allows the firm to build filters that can chop a signal into much narrower frequency bands than a traditional filter. This, in turn, translates into improved performance. In a recent test on a commercial network, using superconducting filters increased that network's capacity by half.

Mobile-phone networks are an important market, but a specialised one. The hope in the industry is that HTSs will be taken up in more traditional, but bigger electrical-engineering applications. This is the sort of thing that American Superconductor, a firm based in Westborough, Massachusetts, is working on.

American Superconductor believes it has cracked the most difficult fabrication problem of HTSs, turning them into the equivalent of wire. Ceramics cannot be drawn in the way that metal is. Instead, American Superconductors and its rivals powder the stuff and pack it into silver tubes. The tubes, being metallic, can be drawn. They can also be plaited together to form cables, heated to improve the contact between the superconducting grains, and rolled flat. The result is flexible tapes that can be used in a similar way to wires.

One application of these tapes is in electric motors. And, as is often the way with new technologies in America, the government has conveniently created a market opportunity. It did so by announcing four years ago its intention to convert the country's navy to a fleet powered by electric motors.

Accordingly, American Superconductor has been using its tape to build superconducting motors. The first, a small one producing 5 megawatts, was delivered to the navy a year ago, and has completed its tests successfully. As a result, a 36.5 megawatt motor will be delivered next year. This motor is easily big enough to propel a destroyer (it is just a tad less powerful than were the engines of the Titanic). But, according to Greg Yurek, American Superconductor's boss, it will be a third of the weight of its conventional alternative.

American Superconductor is also starting to infiltrate another area of heavy electrical engineering: power transmission. The first application the firm has in mind is a niche in this market, but it is an important one: stabilising the grid in order to prevent power failures. In January, the firm installed a prototype “synchronous condenser” designed to provide this stability to an electrical sub-station in Tennessee. Large-scale electricity transmission usually uses alternating current, in which both current and voltage oscillate. Crashes occur when the two get out of sync. The condenser's job is to stop this happening.

American Superconductor and its competitors are also engaged in a race to make superconducting tape that could be used to carry power on the grid itself. The challenge here is twofold. The first is to create tapes that can support large currents undisturbed by the magnetic fields those currents create. The second is to scale up the processes by which the tapes are made, to allow mass production.

The current record-holder for current, as it were, is SuperPower, a firm based in Schenectady, New York. In March, it transmitted 120 amps over a distance of 50 metres—three times better than the best previous result. The trick in this tape—and other so-called “second-generation” tapes—is that a thin layer of the superconducting ceramic is sandwiched between two layers of metal. Such a thin layer alters the current flow in a way that reduces the current's susceptibility to magnetic interference. It also results in a tape that costs a third to a fifth as much to make.

SuperPower is now scaling up its production capacity. By next year it should be able to make cables a third of a kilometre long. It plans to use one of these to connect two sub-stations in Albany, New York. Shorter, experimental segments have already been tried elsewhere, but the significance of the Albany project is that it will be using superconducting cables in a completely commercial environment. Then, perhaps, superconductivity's tomorrow will truly have arrived.