Vertical-axis wind turbines are more effective in turbulent winds like those occurring near the ground, have a simple design with no gearbox or yaw drive, and features a lower profile that reduces environmental impacts. |
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
& 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.”
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 |
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.
