Electricity from offshore wind farms could become cost-competitive with fossil fuels through the introduction of superconductors to wind turbines. That approach is being used by some US and European companies and government research laboratories to double the power of existing wind-turbine generators. Several groups expect to have generator prototypes ready for testing within two years.
Wind power is produced when the spinning blades of a wind turbine rotate magnets around coiled wire in a generator to induce electrical flow. More powerful generators mean that fewer turbines will need to be built to provide a given amount of electricity. That’s especially important at sea, where frequent and expensive maintenance voyages have threatened to cripple the offshore expansion of wind farms. By using high-temperature superconducting (HTS) wire, which packs more than 100 times the current density of copper wire, such power increases can be achieved without the proportional size and weight gains that would accompany scaling up a conventional generator.
Super wind
“The manufacturing upper limit for conventional wind turbines is around 5 to 6 megawatts,” says Daniel McGahn, a vice president at American Superconductor Corp in Massachusetts. But HTS technology is surpassing those limits: AMSC and Texas-based TECO-Westinghouse Motor Co have teamed up on an estimated $6.8 million project to design components for a 10-MW HTS generator. Another HTS device manufacturer, Germany’s Zenergy Power Group, is working with Converteam Ltd in the UK to commercialize an 8-MW HTS wind-turbine generator. Because of the practical limitations to erecting large turbines, a generator’s size and weight do matter, says Larry Masur, a Zenergy vice president.
A less powerful, research-grade HTS wind-turbine is near completion at Denmark’s Risø DTU National Laboratory for Sustainable Energy. “We hope to use our [10-kW] prototype to model the properties of HTS generators and to learn how they would integrate with a turbine,” says Risø senior scientist Asger B. Abrahamsen, who coordinates the activities of Risø’s Superwind project. “We will not compete with industry,” adds Abrahamsen. “We will collaborate with them.” Denmark draws 20% of its electricity from wind power, more than any other nation, and is home to some of the wind industry’s leading companies.
The efficiency benefits of HTS wire have been demonstrated in other applications. In 2007 AMSC designed and tested a 36.5-MW HTS ship-propulsion motor for the US Navy. And compact HTS power cables have replaced bulky copper-based ones in electrical-grid demonstrations (see Physics Today, April 2005, page 41, and January 2008, page 30). For electrical equipment, the physics of HTS wire performance has been mostly solved, and only engineering optimization steps remain, says Bruce Gamble, AMSC’s director of engineering. “We feel this is a ready-to-go technology.”
But first engineers at the National Wind Technology Center in Colorado will factor performance, manufacturing, and operating costs into an evaluation of the cost of electricity from AMSC’s 10-MW HTS wind turbine. The resulting model will be helpful for developing reliable offshore wind farms, says NWTC engineer Walter Musial.
Not science fiction
“Half of the Superwind project is making the wires cheaper,” says Abrahamsen, whose colleagues are working on a more efficient process to deposit the layers of YBCO (YBa2 Cu 3O7) superconducting cuprates that form coated conductors. “The cost of offshore wind power is about €$1 million [$1.3 million] for 1 MW, and depending on the design, a 10-MW generator will require several hundred kilometers of HTS wire.” To compete with the cost of copper wire, which is around $50/kA·m, Zenergy’s Masur says that HTS wire manufacturing needs to ramp up, and the price of HTS wire needs to fall to $15–$30/kA·m—from values estimated by other sources to be as high as $100/kA·m at low-production volumes. That does not include the cost to maintain and operate the cryogenic equipment needed to cool the wire below its critical temperature.
The HTS generator project teams are also testing designs that eliminate the gearbox, which converts the low angular speed of a turbine’s blades to a higher rotor speed to match the electrical grid’s AC frequency. Gearboxes often break down, especially in the humid offshore environment, and that adds to the cost of maintenance. AMSC’s Gamble says that his team has already yielded a gearless design that increases the torque on the rotor, which makes it easier to control the speed of the blades and maintain constant power flow to the grid.
The promise of HTS wind-turbine generators has the support of sectors from environmental groups to governments. Musial says it may take 10–15 years for commercial 10-MW or greater HTS generators to take off. “This is not science fiction,” he adds, “but it is not a garage project either.”
Lightweight, superconducting generators will make possible 8- to 10-MW wind turbines (right) at a fraction of the weight and size that geared (left) and gearless (center) conventional copper-wire generators would have to be.
Lightweight, superconducting generators will make possible 8- to 10-MW wind turbines (right) at a fraction of the weight and size that geared (left) and gearless (center) conventional copper-wire generators would have to be.
