29 February 2012—The U.S. energy agency devoted to big dreams and blue-sky ideas is funding research on something really big that could stir up that blue sky.
In September, the Advanced Research Projects Agency for Energy (ARPA-E) awarded US $31.6 million in grants to groups looking for alternatives to rare earth materials, which are critical to energy technologies. Two of those groups are trying to develop next-generation superconducting wire that can replace rare earth permanent magnets in the rotors of wind turbines. This research could lead to smaller, lighter, and more-powerful rotors that could enable giant offshore wind turbines capable of generating 10 megawatts of power each.
Despite their name, rare earth elements are not particularly rare in the Earth’s crust. They have become more scarce only since most companies mining them closed down their rare earth mines decades ago because of pollution and unprofitability, leaving Chinese mining companies with a near monopoly. When China began reducing its exports of rare earth minerals in 2009, industrial companies around the world began to worry. They feared that their ability to make a host of products, including electric motors in hybrid vehicles, disk drives, and compact fluorescent lighting, would be dramatically curtailed.
Among their many high-tech applications, rare earth elements are used to make powerful permanent magnets that generate magnetic fields inside turbine rotors. Generating the fields with coils of superconducting wire would let turbine makers do away with those large, heavy permanent magnets. That would allow wind companies to keep wind turbines at a reasonable size but get more power per tower.
It makes sense to use magnets in turbines that generate less than 6 MW, says Jason Fredette, vice president of communications and marketing for the Devens, Mass., green-energy company AMSC, a member of one of the two groups funded by ARPA-E. “But when you get above 6 MW, these turbines just get massive,” he says. “They’re very hard to transport, and they use a lot of rare earth. That’s when it starts to look very compelling to use superconducting technology.” AMSC has already designed a 10-MW wind turbine called the SeaTitan, which uses superconductors instead of magnets.
The two research consortiums working on superconductor wire, led by materials scientists at the University of Houston and Brookhaven National Lab, in Upton, N.Y., have the same goal: to engineer a fourfold increase in the current-carrying capacity of their wire. Because a higher current would generate a more powerful magnetic field, such an advance would decrease the amount of superconducting wire needed in the rotor, which would cut down on cost.
Both groups include industry partners that have already cranked out state-of-the-art wire in significant quantities: The group led by the University of Houston includes SuperPower of Schenectady, in New York, while the Brookhaven group includes AMSC.
Manufacturers have already overcome significant materials science challenges. Today’s superconductor of choice, yttrium barium copper oxide (YBCO), is a brittle ceramic that doesn’t readily lend itself to the construction of bendable wire; the YBCO crystals also need to be aligned precisely in order to carry the current. Sandwiching the superconducting material between flexible layers to create tape-like wires and controlling the growth of crystals during the chemical-vapor deposition process make for a commercially viable superconducting wire.
“This is not just a laboratory curiosity,” says Venkat Selvamanickam, who heads the Houston group. He notes, for example, that SuperPower’s superconducting wire was installed in an electricity grid in upstate New York in an experiment that ended in 2008. ( A set of AMSC’s superconducting wires was energized that same year on Long Island.) “It’s been around for some years now. The next step is to increase the performance of these wires,” says Selvamanickam.
But to get four times the current-carrying capacity of today’s wires, researchers are struggling to overcome the limitations imposed by magnetic flux lines. These lines are essentially the lines of magnetic force seen in the classic science experiment in which iron filings are sprinkled on a piece of paper above a magnet. When a superconducting wire generates a strong magnetic field, the lines of flux don’t just radiate away from the wire; they also penetrate back into the wire. This interferes with the superconductor’s ability to carry current.
But a solution is at hand: It’s possible to guide those flux lines to regions of the superconducting wire where they can do no harm. “It has been known for a long time that if you introduce defects similar in magnitude to the flux lines, then the flux lines will reside in the defects and not the superconductor material, so your current-carrying capacity increases,” explains Selvamanickam.
Selvamanickam’s team is working on ways to introduce nanoscale defects into the superconducting layer within the wire to “pin” the flux lines in place. This can be done by doping the material with other chemicals during the vapor-deposition process. “But,” says Selvamanickam, “we’re still learning a lot about how to place the defects and how to control them.”
At Brookhaven, team leader Qiang Li says his group is also working to embed nanostructures within the superconducting material. Neither group would disclose the full details of its approach, citing the proprietary nature of the technology.
David Cardwell, a professor of superconducting engineering at the University of Cambridge, in England, says that even if these groups figure out how to uniformly distribute the flux-pinning structures and thus increase the amount of current that can flow through the wire, there will still be economic hurdles to overcome. “You have this long spool of wire that’s undergoing a lengthy deposition process inside a vacuum. It’s expensive, and it’s slow,” he says.
“Superconducting wire can carry hundreds of times more current than copper wire, but you have to make it for no more than hundreds of times the cost,” Cardwell continues. “If it’s thousands of times the cost, it doesn’t necessarily make sense. These researchers are talking about making the material properties even better, which would be a good thing, but they still have to keep the cost down.”