The US Department of Energy (DoE) has invested in a novel turbine design to meet its goal of generating 20 per cent of US electricity from wind power by 2030.
In contrast to the flaming turbine that graced the front page of newspapers after storms in Scotland this winter, the new wind turbine operates below a cool -200ºC. Rather than conventional materials, the turbine uses superconductors, which have no electrical resistance below a critical temperature. These superconducting materials form the magnets inside the turbine, an essential component of any electricity generator. Because they have no electrical resistance, the superconductors don’t heat when electricity flows through them, helping to render the new turbines twice as efficient as current models.
Advanced Magnet Lab (AML) received a grant from the DoE in September to develop its turbine design, intended to be sited offshore on foundations buried deep in the ocean bed around the US. “In the US we don’t have any commercial wind farms,” said Vernon Prince, general manager of AML Clean Energy. The government needs to build around 30,000 of these new 10MW designs to meet its energy generation target.
The new models are much cheaper per megawatt than current designs. Because they are twice as powerful, half the number of concrete foundations needs to be dug miles under the sea for the same power. “The foundation is a high proportion of total cost, so this is significant,” said Martin Lyster, of American Superconductor (AMSC), who manufacture the superconducting components of the new models.
Despite the turbines’ efficiency, the entire cost of producing power will have to plummet to reach the DoE’s target price of 0.06 cents per kilowatt. The current price of wind power in the US stands at 5 cents per kilowatt, more than 80 times what the new designs hope to achieve. However, the total cost of production remains conjecture until they are built, and in particular because the cost of maintaining the turbines is still unknown.
Old technology, new application
Superconductors were discovered a century ago and have been used in hospitals worldwide in MRI scanners for decades, but recent advances have broadened their potential applications.
The AML turbine employs magnesium diboride as its superconductor. Researchers in Tokyo discovered this material’s superconductivity in 2001, and found that it had no electrical resistance below -240ºC.
The constituent elements – magnesium and boron – are cheap and abundant, and although the cooling system is expensive, it requires little maintenance. “The cryocoolers are ultra-reliable,” said Prince. “This tech has been used on the Hubble space telescope. If it breaks up there, it’s very expensive.”
AMSC’s components use a high-temperature superconductor, discovered in 1986, that needs cooling to a relatively toasty -200ºC. Both designs do away with the traditional gearbox and drivetrain, which AMSC say are the weak point that requires most maintenance in older models. Instead, the generator is directly attached to the turbine’s rotor.
Energy companies are frustrated by the unreliability of gearbox-driven current models, according to Dr Nick Atkins, a lecturer in turbomachinery at the University of Cambridge. In a wind farm near Calais earlier this year he saw “only one in three turbines that were working”.
But until the turbines are out at sea, it won’t be known exactly how reliable they are. If they cannot withstand stormy conditions, Atkins believes the cooling system could leak out the liquid coolant. He describes the buffeting forces the wind turbines experience at sea as “absolutely incredible”. Even though conditions in space are testing, the Hubble telescope was not built to withstand constant pummeling from air. There are other options besides superconductors, including hydraulic gearboxes, but “the superconducting technology is quite mature now”, said Atkins.
The current US energy secretary, Nobel Prize winner Dr Steven Chu, has made Prince confident about achieving the DoE’s targets. “[Chu’s] view of energy may have been different to previous [energy secretaries’]. Given the level of investment… [hitting the target] is quite possible.” But he concedes that the winner of the 2012 election could have different priorities. “The party in charge does have an impact on how much money gets spent.”
Nagamatsu J, Nakagawa N, Muranaka T, Zenitani Y, & Akimitsu J (2001). Superconductivity at 39 K in magnesium diboride. Nature, 410 (6824), 63-4 PMID: 11242039
Snitchler, G., Gamble, B., King, C., & Winn, P. (2011). 10 MW Class Superconductor Wind Turbine Generators IEEE Transactions on Applied Superconductivity, 21 (3), 1089-1092 DOI: 10.1109/TASC.2010.2100341