Supercooled, superconducting power cables have long held the potential to deliver power efficiently, since they offer no resistance losses. Now they're being examined as a way to add redundancy in the cramped quarters of Manhattan's local power grid, potentially protecting against natural disruptions and terrorist attacks.

In a city, superconducting cables offer an advantage: they are far more compact than copper wires. And some of the requisite surge protection can be engineered directly into the cables--a feat not possible with copper wires--reducing the need for bulky mechanical circuit breakers in city substations.

To develop the concept, the U.S. Department of Homeland Security and New York's major utility, Consolidated Edison (Con Ed), announced last week that they would invest $39 million over the next three years to connect two substations at undisclosed locations in Manhattan, allowing each to take over for the other in the event that one burns out. The effort will use technology from American Superconductor, of Devens, MA, which makes so-called high-temperature superconducting cables ("high temperature" means that they can operate at 90 degrees kelvin) and associated control systems.

"It's not a panacea for every system problem, but it would give us more reliability and flexibility and asset sharing," says Steve Kurtz, project engineer at Con Ed, a utility whose grid flaws became evident last summer when a power outage darkened parts of Queens for 10 days. "It would make the grid more resilient."

Part of the power grid's shortcoming in New York City--as in many other parts of the country--is the lack of Internet-like cross connections, which would add reliability. The grid's endpoints are substations that typically serve tens of thousands of customers apiece. A single lightning strike or errant squirrel can burn out a substation, leaving tens of thousands of people in darkness until the utility can get the substation back online.

The solution is to add cross connections between substations so that others can quickly step in to supply power. Such redundancy could also prove useful during a terrorist attack. But if a city adds connections, it also needs to add more equipment to stop faults from propagating through these new connections. "It sounds pretty good, so why don't they do it?" asks Greg Yurek, CEO of American Superconductor. "The answer is that there is not enough real estate under the streets of Manhattan."

To be sure, all this is possible using tried-and-true copper wires and mechanical equipment. But in places like Manhattan, there is no room under the ground for all the extra copper cable--which has to be given some air space to dissipate heat--and no room in cramped midtown substations for new breakers.

Superconducting cables are one-tenth the size of copper wires. What's more, they can handle surge protection within the cable itself without requiring new sets of bulky breakers. Superconducting materials, which are based on a flexible ceramic called yttrium barium copper oxide, can tolerate a certain power load. At every point up to that maximum, they carry current with zero loss. But the moment the maximum is exceeded, the superconductors become highly resistive and stop propagation of surges. "Depending on the number of wires you put in, you can carry more current without losses, and you can design the cable in terms of having the right amount of resistance" in the event of power surges, says Alexis Malozemoff, chief technology officer at American Superconductor.

Of course, if the superconducting cable becomes resistive, it can burn up, so some hardware is needed to prevent such damage. The company says that unspecified associated control systems will handle this problem effectively. "Necessity is the mother of invention," says Yurek. "You can put [fault protection] in the cable, with some other proprietary technology," which he says is more space efficient.

However sensible this might be, it won't be simple. It will take more than a year just to put all the pieces together in laboratories. "You have to design it, then build it, then develop testing protocols, then [do] full prototype tests, then analyze the data, then develop a design specification for installation," says Kurtz. The hope is that the concept will be proved by August 2008. Actual construction won't happen before 2010.

High-temperature superconductors were born two decades ago. American Superconductor found a way to commercialize the material by making a flexible version of the ceramic and nudging the temperature requirements up to a more-manageable 90 degrees kelvin. That temperature can be reliably maintained using liquid nitrogen for cooling.

While superconducting cables--including some transmission lines in Columbus, OH, Albany, NY, and Long Island--are slowly making headway, the Manhattan project marks a new milestone, says Yurek. "Con Ed is deciding they are willing to put these superconductors in their grid. It's a heck of a validation point, to say the least."