The industry standard turbine has grown in the last two years from 1.5 megawatts to 2.5 megawatts, which is power enough for 600 U.S. homes. Turbine makers are now in a sizzling competition to perfect turbines in the 3-to-6 megawatt class. And long-term planners are thinking about the limits of turbine size. A recent National Renewable Energy Laboratory (NREL) paper speculated about a 15-megawatt machine and the European Wind Energy Association (EWEA) has called for research and development on a 20-megawatt turbine.

At least two U.S. companies are deeply engaged in the development of the world's first 10-megawatt turbine, a single windmill capable of powering 2,400 homes. The two highest-profile undertakings are by two very differently structured companies, Clipper Windpower and American Superconductor, approaching the challenge from different points of view.

The SeaTitan will come from American Superconductor (AMSC), a more than 20-year old company that emerged from the Massachusetts Institute of Technology in the 1980s. AMSC's edge is its 2-decade experience with "superconductors and superconductor rotating machines," said Dan McGahn, the company's President and COO. "The SeaTitan program going to require our core competencies in superconductors, power electronics and wind turbine design. We have complete control over the design from the superconductor wire to the power conductors that will be used to how we develop the supply chain."

Though AMSC will have control over the SeaTitan's conception and design, its business model calls for licensing the technology to partners. Its current partner roster includes major turbine manufacturers like China's Sinovel and important emerging industry players like Korea's Hyundai Heavy Industries. AMSC is looking for a U.S. partner.

McGahn is convinced SeaTitan will achieve the 10-megawatt ambition because of the AMSC superconducting material's capabilities. "Superconductor wire is 150 times more power dense than traditional copper. If you take that into the format for our 10-megawatt system, it will be about the same size as today's 5 megawatt generators from a size and weight standpoint."

Hail Brittania!

Clipper Windpower, a manufacturer of its own turbines, did not respond to interview requests. It is a matter of public record that the company is designing its Britannia 10-megawatt concept machine in the U.K. with partial funding from the U.K. Department of Energy and Climate Change (DECC). Britannia will be an elaboration of Clipper's widely praised Liberty 2.5-megawatt machine, perfected by Clipper engineers in conjunction with NREL.

Clipper reportedly aims to build a machine with a size and weight comparable to a 5-megawatt turbine but with twice the power rating, in order to take advantage of the powerful yet accessible offshore winds in the U.K. and Europe. Britannia's permanent magnet generators and sophisticated 4-generator system will, Clipper reportedly promises, have a longer life span and require significantly less maintenance.

Brittania is reported to have a 150-meter rotor diameter with 72-meter, 30-ton blades. The company has said it plans to operate a 7.5-megawatt prototype by the middle of 2011 and to have a full-scale model in service by the middle of the following year.

These 10-megawatt ambitions are understandable to Walt Musial, NREL's principal engineer and offshore wind leader and one the foremost U.S. wind turbine experts. "The advantages are theoretical but I think we can quantify them," Musial said, "If you build a bigger machine, you have to put in fewer machines. If you're trying to build a 100-megawatt wind farm, you only have to put in 10 machines and it you stick with the 5-megawtt size, you have to put in 20 machines."

Half the machines mean half the costs in a variety of different ways, Musial said. "10 machines [are] going to be easier to maintain. There will be fewer electrical hook-ups and fewer trenches to dig. The cable to shore costs the same. The construction to install 10 foundations versus 20 foundations is cheaper."

AMSC calls this "more power per tower." McGahn said. AMSC plans to have a SeaTitan prototype around 2013 and intends to begin commercial scale activity by the 2014-to-2016 period. He did not seem concerned about Clipper's more accelerated timeline, perhaps because there are challenges ahead for both companies.

"In my opinion," NREL's Musial said, "it's the rotor technology that is the most risky, the most uncertain, in going from one size to the next."

A turbine's energy output is proportional to the size of the blades and the total area "swept" by the rotor-blade complex. "If you take a RePower 5-megawatt machine and you scale it to 10 megawatts," Musial said, "you end up with a rotor diameter that's something like 175- to-180-meters."

But Clipper's rotor diameter is reportedly going to be about 150 meters and the AMSC rotor is designed at 164 meters.

"Both the 10-megawatt projects are using smaller rotor diameters than conventional scaling would suggest in going from a 5-megawatt class to a 10-megawatt class," Musial noted. "It will require higher wind speeds to reach their rated power."

The break with convention is justifiable because both turbines are intended for offshore, where winds are stronger. "They are both using conservative scaling convention to keep the rotor relatively small compared to the generator size. This could be to allow deployment in a higher wind regime offshore," Musial acknowledged.

McGahn said AMSC is designing a whole new turbine concept and depending on cutting-edge superconductor materials to allow for a new scaling convention. Clipper seems to be depending on the success of its Liberty design to outperform convention.

They are both so far ahead of the curve that there are not adequate testing facilities for them to use. The U.S. Department of Energy (DOE) is building a new testing facility for 5MW-to-15MW machines at the Charleston Naval Complex in South Carolina with Clemson University. Colorado's National Wind Technology Center will soon be ready to test 50-meter blades but the 70+-meter blades in the 10-megawatt designs will have to wait for a DOE blade testing facility being built in Massachusetts to be available in 2011. It will be able to test 90-meter blades.

The trend may benefit vertical wind turbine makers too, who have proposed putting verticals in the spaces between the jumbos on wind farms to maximize power production at the same site.