[Editors] MIT designs 'invisible,' floating wind turbines

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT designs 'invisible,' floating wind turbines
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For Immediate Release
MONDAY, SEP. 18, 2006
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

GRAPHICS AVAILABLE


CAMBRIDGE, Mass.--An MIT researcher has a vision: 
Four hundred huge offshore wind turbines are 
providing onshore customers with enough 
electricity to power several hundred thousand 
homes, and nobody standing onshore can see them. 
The trick? The wind turbines are floating on 
platforms a hundred miles out to sea, where the 
winds are strong and steady.

Today's offshore wind turbines usually stand on 
towers driven deep into the ocean floor. But that 
arrangement works only in water depths of about 
15 meters or less. Proposed installations are 
therefore typically close enough to shore to 
arouse strong public opposition.

Paul D. Sclavounos, a professor of mechanical 
engineering and naval architecture, has spent 
decades designing and analyzing large floating 
structures for deep-sea oil and gas exploration. 
Observing the wind-farm controversies, he 
thought, "Wait a minute. Why can't we simply take 
those windmills and put them on floaters and move 
them farther offshore, where there's plenty of 
space and lots of wind?"

In 2004, he and his MIT colleagues teamed up with 
wind-turbine experts from the National Renewable 
Energy Laboratory (NREL) to integrate a wind 
turbine with a floater. Their design calls for a 
tension leg platform (TLP), a system in which 
long steel cables, or "tethers," connect the 
corners of the platform to a concrete-block or 
other mooring system on the ocean floor. The 
platform and turbine are thus supported not by an 
expensive tower but by buoyancy. "And you don't 
pay anything to be buoyant," said Sclavounos.

According to their analyses, the floater-mounted 
turbines could work in water depths ranging from 
30 to 200 meters. In the Northeast, for example, 
they could be 50 to 150 kilometers from shore. 
And the turbine atop each platform could be 
big--an economic advantage in the wind-farm 
business. The MIT-NREL design assumes a 5.0 
megawatt (MW) experimental turbine now being 
developed by industry. (Onshore units are 1.5 MW, 
conventional offshore units, 3.6 MW.)

Ocean assembly of the floating turbines would be 
prohibitively expensive because of their size: 
the wind tower is fully 90 meters tall, the 
rotors about 140 meters in diameter. So the 
researchers designed them to be assembled 
onshore--probably at a shipyard--and towed out to 
sea by a tugboat. To keep each platform stable, 
cylinders inside it are ballasted with concrete 
and water. Once on site, the platform is hooked 
to previously installed tethers. Water is pumped 
out of the cylinders until the entire assembly 
lifts up in the water, pulling the tethers taut.

The tethers allow the floating platforms to move 
from side to side but not up and down--a 
remarkably stable arrangement. According to 
computer simulations, in hurricane conditions the 
floating platforms--each about 30 meters in 
diameter--would shift by one to two meters, and 
the bottom of the turbine blades would remain 
well above the peak of even the highest wave. The 
researchers are hoping to reduce the sideways 
motion still further by installing specially 
designed dampers similar to those used to steady 
the sway of skyscrapers during high winds and 
earthquakes.

Sclavounos estimates that building and installing 
his floating support system should cost a third 
as much as constructing the type of truss tower 
now planned for deep-water installations. 
Installing the tethers, the electrical system, 
and the cable to the shore is standard procedure. 
Because of the strong offshore winds, the 
floating turbines should produce up to twice as 
much electricity per year (per installed 
megawatt) as wind turbines now in operation. And 
because the wind turbines are not permanently 
attached to the ocean floor, they are a movable 
asset. If a company with 400 wind turbines 
serving the Boston area needs more power for New 
York City, it can unhook some of the floating 
turbines and tow them south.

Encouraged by positive responses from wind, 
electric power, and oil companies, Sclavounos 
hopes to install a half-scale prototype south of 
Cape Cod. "We'd have a little unit sitting out 
there andŠcould show that this thing can float 
and behave the way we're saying it will," he 
said. "That's clearly the way to get going."

This research was supported by the National Renewable Energy Laboratory.

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Written by Nancy Stauffer, Laboratory for Energy and the Environment