The next big thing in wind: Slow wind, huge turbines

Offshore wind farms have been discussed frequently in the U.S. lately, but the trend among the turbine manufacturer is to build even larger turbines for land use.

With politicians pushing adoption of renewable energy in the United States and Europe, the last few years have seen a surge in plans for wind farms--both on land and sea. But wind power isn't viable everywhere--and prime coastal spots are often already developed.

So some wind-turbine makers are shifting their focus toward building bigger wind turbines that can harvest the lower-speed winds that are more readily available. This next generation of wind turbines is no small matter: their rotors have a diameter the size of a football field.

In general, wind turbines get more powerful and efficient with taller turbine towers and larger areas swept by the blades, according to the American Wind Energy Association. A turbine's swept area is a key indicator in how much power output potential the turbine has.

"Lower wind-speed turbines certainly open up more land for development," said Rich Reno, platform leader for General Electric's new 2.5-megawatt wind turbine. "Larger turbines open up the opportunity to get more megawatts out of a given piece of land."

Siemens SWT-2.3-101 wind turbine
The Siemens SWT-2.3-101 has a maximum power output that's enough to power approximately 700 homes. Siemens

Finding new sites for wind farms is essential to the sector's growth because many of the places with the most attractive wind conditions tend to be developed already. More efficient turbines also make wind power a more realistic option for countries like Germany, which is not very windy and has relatively limited shoreline, where winds tend to be stronger. Despite those obstacles, the country has the world's second largest installed base of wind power.

Wind farm development continues to push into new regions--particularly in the U.S. and parts of Europe, where politicians are encouraging renewable-energy R&D.

"We see continued growth in Europe, notably because of EU legislation to derive 20 percent of its energy from renewable sources," Victor Abate, vice president for the renewables arm of GE Energy, told Reuters in an interview. The European Union issued a directive in December 2008 to have 20 percent of its energy come from renewable sources by 2020. Europe contains more than half of the world's installed wind power capacity.

The U.S. is now the country with the world's largest installed base of wind power, according to the World Wind Energy Association. More than 8,300 megawatts of wind power was installed in 2008, expanding the nation's total wind power generating capacity by 50 percent in a single calendar year.

Although that growth is expected to slow in 2009, according to both the AWEA and investment bank HSBC, the wind industry is still getting support stateside. As part of the economic stimulus plan signed in February, President Obama extended tax credits for wind and increased the amount the government will spend on those credits by 30 percent.

Industry watchers predict much of the growth in the wind market will be in the low- to medium-wind segments. And as that trend continues, wind turbines in general have been increasing in size--from typical 18-yard rotors 25 years ago to about 110 yards in recent years. The average wind turbine installed in 2007, with a capacity of 1.6 megawatts, is twice as powerful as the average wind turbine installed in 2000 (0.76 MW), according to AWEA.

Several manufacturers are rolling out new low-speed turbines to help meet that demand. Siemens, Vestas, and Nordex all recently released new, large wind turbines directed at low-wind markets. Last year, General Electric released its largest turbine for low-wind use.

Siemens predicts that the low-wind market alone should represent one-third of the total global wind-power market in the coming years. Its new turbine, the SWT-2.3-101, has a "nominal" (that's maximum in wind-engineering speak) power output of 2,300 kW, enough to power approximately 700 homes, depending on wind conditions. The turbine is enormous, with a 110-yard rotor diameter--larger than an American football field.

A turbine blade convoy passes through Edenfield, U.K., with blades 42 yards long. Nordex's newer low-wind model, the N100, has a 53-yard blade--11 yards longer than the ones pictured. Transportation might prove to be one limit to how big land turbines can get. Paul Anderson

Designed to harvest weaker winds, SWT-2.3-101 has a swept area of 86,111 square feet, 17 percent larger than Siemens' previous 2,300 kW turbine.

"The new wind turbine will open up many potential new sites for our customers," Andreas Nauen, CEO of Siemens' Wind Power Business Unit, said in a statement.

Danish wind-power company Vestas recently unveiled its V 112, a 3 MW low-wind turbine the company claims is the world's largest mass-produced wind turbine for low- and medium-wind-speed sites.

The wind turbine has a nominal power output of 3,000 kW, the same as the company's previous largest turbine. But because of its larger blade diameter, it's able to reach that potential with lower speed winds. It can generate electricity at wind speeds as low as 6.7 miles per hour and reach maximum power at about 31 mph. The previous model required 8.9 mph winds to deliver power, and 34 mph to attain maximum output.

Three factors have made it possible to make modern turbines so big, according to General Electric: controlling the load on the grid, new blade design, and new materials to reduce the weight of the turbine.

"As the technology increases in those areas it allows advancement," said Reno, of GE Energy.

New materials and construction techniques make it possible to build even larger wind turbines. Siemens says it uses a proprietary manufacturing process, which casts the 53-yard, fiberglass-reinforced epoxy blades in one piece. The process eliminates weaknesses from gluing parts together, as is done with traditional blades and thus makes it possible to make turbines this size.

Vestas is increasingly using composite materials and has a low-weight-design philosophy. In March, Vestas teamed up with U.S. aerospace company Boeing for projects in areas such as aerodynamics and composite-materials fabrication processes, design, and analysis.

But there are limits to how big these turbines can get. GE says the grid's ability to handle more powerful loads is one limiting factor. And in the future, problems with transportation could limit the size of new land-based turbines. Going around road corners and narrow bridges with 50-yard blades is already a problem, and the only thing that could alleviate that would be technology for making multiple-piece blades. Offshore this transport problem doesn't exist so the turbines could be bigger there. GE thinks the size increase of the turbines will plateau, but that won't happen for a while.

"I think 70 meters (77 yards) was a pretty big rotor 10 years ago. Today 110 is out there," Reno said. "Can they grow another 30 meters (33 yards) in 10 years? I think it is possible."

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