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Image courtesy / National Renewable
Energy Laboratory
Among three designs they considered for
floating giant wind turbines in the deep ocean, MIT research is focusing
on the tension-leg platform (center), a system that oil companies use
for deep-water rigs.
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An MIT researcher has a vision of four hundred huge, offshore, wind turbines
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. (Ref. 1)
Out of sight and out of earshot. And out of mind. Many of the usual
arguments against wind power evaporate right there. Some others are sustained. (Ref.
2)
Many offshore wind turbine proposals involve towers driven deep into the ocean
floor, an approach suitable only in water depths of about 15 meters or less.
Such installations are therefore typically close enough to shore to arouse
strong public opposition, citing interference with the beauty of an ocean view.
Whether or not the turbines can be heard past the crashing of breakers on the
shore, the anti-noise complaints are usually even louder.
Inspired by Oil Rig Technology
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 stimulated him to search for a different solution. "Wait a
minute., he found himself thinking, 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
flotation system. 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 anchored by its weight to
the ocean floor. The platform and turbine are thus supported not by an expensive
tower but by buoyancy. After the initial outlay for the floatation device,
"
you don't pay anything to be buoyant," said Sclavounos.
Image courtesy / Stephen Connors, MIT
This figure shows (from left to right) an
onshore wind turbine, a conventional offshore unit, and the experimental
unit used in MIT's concept for a deep-water floating turbine. In each
case, the disk indicates the area swept by the turbine blades. The
Washington Monument in Washington, DC, appears for comparison.
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According to their analyses, this type of floating turbines could work in water
depths ranging from 30 to 200 meters (approx. 650 feet). In the Northeast, for
example, such a floating windfarm 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 choice of a 5.0 megawatt (MW)
experimental turbine now being developed by industry. (Onshore units are 1.5 MW,
conventional offshore units, 3.6 MW.)
Stable enough for towing
Hiring of ships and crew capable of doing ocean-based assembly of such oversized
floating turbines would be prohibitively expensive because of their dimensions:
the wind tower is fully 90 meters tall, the rotors about 140 meters in diameter.
Therefore, researchers consider that onshore assembly -- probably at a shipyard
-- and towing out to sea by a tugboat is a more practical as well as less
expensive way to proceed.
To keep each platform stable during towing, cylinders inside it would be
ballasted with concrete and water. Once the unit arrives at its destined
co-ordinates, the platform is designed to be hooked to previously-installed
anchor and tethers. Ballast-water would then be pumped out of the cylinders
until the entire assembly lifts up far enough in the water to pull the tethers
taut.
Although tethers allow the floating platforms to move from side to side, they do
not permit the up-and-down motion typical of ships. This is a relatively 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 as the power of the ocean could drag the weighted anchor by that
much. However, the tips of the turbine blades passing their six o-clock
position would be expected to remain well above the peak of even the highest
storm-driven waves. By installing especially-designed dampers similar to those
used to steady the sway of skyscrapers during high winds and earthquakes,
researchers are hoping to reduce the sideways motion still further. Proposed
designs for wave-motion dampers have not yet been disclosed.
Competitors in the Race
 Stanbury Resources design. |
Another floating-power proposal from Stanbury Resources involves both electrical generation and hydrogen extraction by electrolysis. This one would be set up on a platform, and includes battery backup to enable the system to supply continuous energy even if the winds die down. Having hydrogen tanks onboard means these platforms would have to be large and expensive, with docking facilities for tankers to offload and transport hydrogen to its point of use. Being like a ship, the platform could be unhooked from its base and sailed out of range of an approaching hurricane. (Ref. 3) It is not certain at this point whether there will be enough of a market for hydrogen to justify the additional expense.
 Norsk Hywind design |
A Norwegian company has taken a different, more minimalist approach to floating huge turbines. The Norsk design is simply a huge floating pole standing upright in the ocean. It is not susceptible to wave motion. Being a vertical shaft, the waves pass by it, rather than causing pitching or tilting as with any broad-surfaced floating object. As shown in the case of the FLIP research ship (Ref. 4) that converts to a vertical orientation for long-term studies in a given location, the principle holds true even with its thicker body; the stability is reliable enough to enable scientists to work in laboratories without being troubled by perceptible pitching and rolling. The size of the turbine blades of the Norsk Hywind design is comparable to the MIT proposal, and the pole-based wind-farm can be placed in even deeper water of 300 meters (approx. 980 feet), allowing more flexibility for avoiding shipping channels. (Ref. 5.)
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Still another floating-windfarm proposal substitutes many very small rotors
for the huge blades of the giant turbine. Ganging up small blades on the same
shaft extracts more wind power from a smaller swept area. Lightweight and
small, this option offers lower cost of materials. Inventor Doug Selsam
describes his design as self-orienting and self-tilting in response to the wind.
Due to its smaller profile it would be possible to place these turbines much
closer to shore without affecting ocean vistas, thus reducing the length and
cost of the undersea transmitting lines linked to the shore. (Ref.
6) It remains to be seen whether these small turbines can withstand oceanic
conditions in the long term, but at least they would be cheap to replace.
Which version of the flotation system for far-offshore wind-turbine installation
will eventually win the technology race will depend on various factors such as
comparative cost, quality of engineering, and performance in pilot projects.
Given the level of expertise at MIT, they can be expected to be very much in the
running.
Cost Projections of MIT platform design
Installing anchors and tethers, the electrical components, and transmission
cable to shore is standard procedure for all ocean-based generators, fixed or
floating. Professor Sclavounos estimates that building and installing his
floating support system should cost a third as much as constructing the type of
truss tower planned for fixed deep-water installations. Cost details are not
available at this time to enable a cost comparison between the MIT design and
competing floating turbines described above. Because of the strong winds farther
off shore, the floating turbines should produce up to twice as much electricity
per year (per installed megawatt capacity) as the onshore wind turbines now in
operation.
And because the wind turbines are not permanently attached to the ocean floor,
they can be seen as a movable asset. If a company with 400 wind turbines serving
the Boston area needs to add more capacity quickly to its installation near New
York City, it can unhook some of the floating turbines and tow them south -- as
long as the Boston grid can spare them, that is.
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 and
could 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."
The MIT research was supported by the National Renewable Energy Laboratory.
# # #
REFERENCES
Ref. 1: Deep-sea
oil rigs inspire MIT designs for giant wind turbines - by Nancy Stauffer,
Laboratory for Energy and the Environment, in MIT News, Aug. 29, 2006. This
article, cited by permission, is the source of quotations and forms the basis of
understanding the MIT proposal.
Ref. 2: Offshore
Wind Drawbacks - Initial content on this page concerns wildlife impact of
wind turbines. Further content may be added as it comes to our attention. (PESWiki)
Ref. 3: http://pesn.com/2005/10/31/9600198_Offshore_Wind_Hydrogen
Ref. 4: FLIP
Marine Science Acoustic Ocean Research Vessel - The ship is designed to be
lived in for months at a time, whether vertical or horizontal. Series of photos
shows ship ballasts being emptied. Floors become walls; furnishings are designed
to be used in either orientation. FLIP has been serving as a stable working
platform for research for several years.
Ref. 5: http://pesn.com/2005/11/03/9600200_NorskHydro_deep-off-shore-wind
Ref. 6: http://www.selsam.com
- Numerous illustrations show the outside-the-box idea of allowing the turbine
shaft to lie at an angle to maximize wind exposure for the series of small
blades, and to swing around freely with the wind direction changes.
See also
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| Page posted by Sterling
D. Allan Sept. 18, 2006 Last updated September 20, 2006 |
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