We have already posted on the
huge improvement in general area efficiency to be attained through ‘schooling’
propeller type wind turbines. Here they
show vertical axis type turbines are able to improve energy density ten fold in
quite low build outs.
It is certainly time to see if we
can winkle out more energy from established operations and these are two good
approaches. As we replace old hardware
in prime locations this all works.
New projects are often driven by
financing expediency and little engineering finesse. Of course, these approaches are in their
infancy and we do not have a manual as yet.
Wind-turbine placement produces tenfold power increase
by Kathy Svitil
These are vertical-axis wind turbines at
the Field Laboratory for Optimized Wind Energy (FLOWE)
facility in northern Los Angeles
County
The power output of windfarms can
be increased by an order of magnitude-at least tenfold-simply by optimizing the
placement of turbines on a given plot of land, say researchers at the
California Institute of Technology (Caltech) who have been conducting a unique
field study at an experimental two-acre wind farm in northern Los Angeles
County.
A paper describing the findings-the results of field tests conducted by
John Dabiri, Caltech professor of aeronautics and bioengineering, and
colleagues during the summer of 2010-appears in the July issue of the Journal
of Renewable and Sustainable Energy.
Dabiri's experimental farm, known as the Field Laboratory for Optimized
Wind Energy (FLOWE), houses 24 10-meter-tall, 1.2-meter-wide vertical-axis wind
turbines (VAWTs)-turbines that have vertical rotors and look like eggbeaters
sticking out of the ground. Half a dozen turbines were used in the 2010 field
tests.
Despite improvements in the design of wind turbines that have increased
their efficiency, wind farms are rather inefficient, Dabiri notes. Modern farms
generally employ horizontal-axis wind turbines (HAWTs)-the standard
propeller-like monoliths that you might see slowly turning, all in the same
direction, in the hills of Tehachapi Pass , north of Los
Angeles
In such farms, the individual turbines have to be spaced far apart-not
just far enough that their giant blades don't touch. With this type of design,
the wake generated by one turbine can interfere aerodynamically with
neighboring turbines, with the result that "much of the wind energy that
enters a wind farm is never tapped," says Dabiri.
He compares modern farms to "sloppy eaters," wasting not just
real estate (and thus lowering the power output of a given plot of land) but
much of the energy resources they have available to them.
Designers compensate for the energy loss by making bigger blades and
taller towers, to suck up more of the available wind and at heights where gusts
are more powerful.
"But this brings other challenges," Dabiri says, such as
higher costs, more complex engineering problems, a larger environmental impact.
Bigger, taller turbines, after all, mean more noise, more danger to birds and
bats, and-for those who don't find the spinning spires visually appealing-an
even larger eyesore.
The solution, says Dabiri, is to focus instead on the design of the
wind farm itself, to maximize its energy-collecting efficiency at heights
closer to the ground. While winds blow far less energetically at, say, 30 feet
off the ground than at 100 feet, "the global wind power available
30 feet off the ground is greater than the world's electricity usage, several
times over," he says.
That means that enough energy can be obtained with smaller, cheaper,
less environmentally intrusive turbines-as long as they're the right turbines,
arranged in the right way.
VAWTs are ideal, Dabiri says, because they can be positioned very close
to one another. This lets them capture nearly all of the energy of the blowing
wind and even wind energy above the farm.
Having every turbine turn in the opposite direction of its neighbors,
the researchers found, also increases their efficiency, perhaps because the
opposing spins decrease the drag on each turbine, allowing it to spin faster
(Dabiri got the idea for using this type of constructive interference from his
studies of schooling fish).
In the summer 2010 field tests, Dabiri and his colleagues measured the
rotational speed and power generated by each of the six turbines when placed in
a number of different configurations. One turbine was kept in a fixed position
for every configuration; the others were on portable footings that allowed them
to be shifted around.
The tests showed that an arrangement in which all of the turbines in an
array were spaced four turbine diameters apart (roughly 5 meters, or
approximately 16 feet) completely eliminated the aerodynamic interference
between neighboring turbines.
By comparison, removing the aerodynamic interference between
propeller-style wind turbines would require spacing them about 20 diameters
apart, which means a distance of more than one mile between the largest wind
turbines now in use.
The six VAWTs generated from 21 to 47 watts of power per square meter
of land area; a comparably sized HAWT farm generates just 2 to 3 watts per
square meter.
"Dabiri's bioinspired engineering research is challenging the
status quo in wind-energy technology," says Ares Rosakis, chair of
Caltech's Division of Engineering and Applied Science and the Theodore von
Karman Professor of Aeronautics and professor of mechanical engineering.
"This exemplifies how Caltech engineers' innovative approaches are
tackling our society's greatest problems."
"We're on the right track, but this is by no means 'mission
accomplished,'" Dabiri says. "The next steps are to scale up the
field demonstration and to improve upon the off-the-shelf wind-turbine designs used
for the pilot study." Still, he says, "I think these results are a
compelling call for further research on alternatives to the wind-energy status
quo."
This summer, Dabiri and colleagues are studying a larger array of 18
VAWTs to follow up last year's field study. Video and images of the field site
can be found here.
Bold new approach to wind 'farm' design may provide efficiency gains
by Staff Writers
Research at
the Caltech Field Laboratory for Optimized Wind Energy,
directed by John Dabiri, suggests that arrays of closely spaced vertical-axis
wind turbines produce significantly more power than conventional wind farms with
propeller-style turbines. Credit: John Dabiri, Caltech
Conventional wisdom suggests that because we're approaching the
theoretical limit on individual wind turbine efficiency, wind energy is now a
mature technology.
But California Institute of Technology researchers revisited some of
the fundamental assumptions that guided the wind industry for
the past 30 years, and now believe that a new approach to wind farm design-one
that places wind turbines close together instead of far apart-may provide
significant efficiency gains.
This challenges the school of thought that the only remaining advances
to come are in developing larger turbines, putting them offshore, and lobbying
for government policies favorable to the further penetration of wind power in
energy markets.
"What has been overlooked to date is that, not withstanding the
tremendous advances in wind turbine technology, wind 'farms' are still rather
inefficient when taken as a whole," explains John Dabiri, professor of
Engineering and Applied Science, and director of the Center for Bioinspired
Engineering at Caltech.
"Because conventional, propeller-style wind turbines must be
spaced far apart to avoid interfering with one another aerodynamically, much of
the wind energy that enters a wind farm is never tapped. In effect, modern wind
farms are the equivalent of 'sloppy eaters.' To compensate, they're built
taller and larger to access better winds."
But this increase in height and size leads to frequently cited issues
such as increased cost and difficulty of engineering and maintaining the larger
structures, other visual, acoustic, and radar signatures problems, as well as
more bat and bird impacts.
Dabiri is focusing on a more efficient form of wind 'farm' design,
relegating individual wind turbine efficiency to the back seat. He describes
this new design in the American Institute of Physics' Journal of Renewable and
Sustainable Energy.
"The available wind energy at 30 feet is much less abundant than
that found at the heights of modern wind turbines, but if near-ground wind can
be harnessed more efficiently there's no need to access the higher altitude
winds," he says.
"The global wind power available at 30 feet exceeds global
electricity usage several times over. The challenge? Capturing that
power."
The Caltech design targets that power by relying on vertical-axis
wind turbines (VAWTs) in arrangements that place the turbines much closer
together than is possible with horizontal-axis propeller-style turbines.
VAWTs provide several immediate benefits, according to Dabiri,
including effective operation in turbulent winds like those occurring near the
ground, a simple design (no gearbox or yaw drive)
that can lower costs of operation and maintenance, and a lower profile that
reduces environmental impacts.
Two of the primary reasons VAWTs aren't more prominently used today are
because they tend to be less efficient individually, and the previous
generation of VAWTs suffered from structural failures related to fatigue.
"With respect to efficiency issues, our approach doesn't rely on
high individual turbine efficiency as much as close turbine spacing. As far as
failures, advances in materials and in predicting aerodynamic loads have led to
new designs that are better equipped to withstand fatigue loads," says
Dabiri.
Field data collected by the researchers last summer suggests that
they're on the right track, but this is by no means 'mission accomplished.' The
next steps involve scaling up their field demonstration and improving upon
off-the-shelf wind turbine designs used for the pilot study.
Ultimately, the goal of this research is to reduce the cost of wind
energy. "Our results are a compelling call for further research on
alternatives to the wind energy status quo," Dabiri notes.
"Since the basic unit of power generation in this approach is
smaller, the scaling of the physical forces involved predicts that turbines in
our wind farms can be built using less expensive materials, manufacturing
processes, and maintenance than is possible with current wind turbines."
A parallel effort is underway by the researchers to demonstrate a
proof-of-concept of this aspect as well.
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