0713MEM_Wind copy.indd - The American Society of Mechanical

0713MEM_Wind copy.indd - The American Society of Mechanical
ENGINEERS ARE WORKING TO
DESIGN EVER-LARGER
WIND TURBINES
THAT CAN EXTRACT MORE POWER
WITH GREATER EFFICIENCY.
BY MARK CRAWFORD
0713MEM_Wind copy.indd 41
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W
MECHANICAL ENGINEERING | JULY 2013 | P.41
Wind turbines dotting the landscape may seem
futuristic, but the roots of the technology are ancient.
Early sailors were probably the first to exploit the power
of the wind, and by around 100 A.D. a Greek mathematician and engineer named Heron devised a wind-driven
wheel to power an organ in ancient Alexandria.
Soon windmills were being used across Europe, the
Middle East, and Asia to process grain and to pump
water. In the United States, 19th century sodbusters
used wind-powered pumps to pull water from wells,
helping to transform vast acres of semi-arid prairie into
irrigated farmland. Today, the multi-bladed wind pump
is an icon of the American heartland.
The basics of wind power haven’t changed much over
the centuries. The force of the wind turns airfoils—originally sails, now blades—around a rotor, which is connected to the main shaft. The shaft then spins a generator to generate electricity.
“A wind turbine seems easy enough to build, but
it’s actually very complicated,” said Paul Veers, chief
engineer at the National Renewable Energy Laboratory’s
National Wind Technology Center in Golden, Colo.
“Engineers must integrate aerodynamics, structural
dynamics, and material fatigue with gear boxes, electric
generators, and other components, and connect it all to
the grid.”
A lot of engineers, however, don’t have much experience with such large-scale projects. Wind turbines are
the largest rotating structures in the world. The entire
process—from design to installation to operation and
maintenance—deals with very large components.
“Wind turbines are immense machines,” said Douglas
Adams, professor of mechanical engineering at Purdue
University in West Lafayette, Ind., and director of the
Purdue Center for Systems Integrity. “Onshore, utilityscale wind turbines stand nearly 300 feet off the ground
and have rotor diameters approaching 300 feet. It is
fascinating that the operation of such immense machines
Wind farms are rising against the horizon in Arizona
(opposite), Colorado (left), Oklahoma (near left), and elsewhere.
Photos: Iberdrola Renewables, Inc.(opposite); Jenny Hager
Photography (left); Todd Spink (near left).
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6/4/13 3:38 PM
depends critically on very small things—
such as the lubricating layer 1-micronthick between the gear teeth in the gear
box.”
Rotors have increased in size from
about 150 feet in diameter 20 years ago
to about 400 feet in diameter today, with
towers over 300 feet in height.
A bigger turbine diameter means a
larger area can be swept; a taller tower
allows turbines to catch faster-blowing
winds at greater distances from the
ground. When combined, these trends
enable the turbine to extract more power
from the wind.
“As a result, utility-scale turbines can
produce as much 5 MW of power today,”
said Fotis Sotiropoulos, a civil engineer
NEW TURBINES AREN’T JUST BIGGER—
THEY ARE GETTING SMARTER, TOO. Smart
wind turbines are one of the most important
developments in wind energy technology. With
their embedded sensors and data processing algorithms, smart wind turbines can recognize when wind
conditions, such as direction and speed, have changed,
so the turbine control system can quickly adapt to those
changes.
“This adaptive capability is a game-changer because
it allows the turbine to maximize the power it produces, while simultaneously ensuring the reliability
of the turbine is maintained,” Adams said. “In other
words, smart wind turbines are a key technology for
minimizing the cost of wind energy. When wind energy costs come down, more wind farms will go up.
“Historically, wind turbines have been designed
to operate optimally in specific wind conditions,”
Adams added. “Therefore, smart wind turbines
should help expand the operating envelope of
wind turbines so that they operate optimally, even
To catch the most powerful wind,
turbines towers have grown taller
and their rotors can reach 400 feet
in diameter.
Photo: Klaus Obel
To erect a wind turbine tower,
workers must stack and anchor
enormous tubes of high strength steel.
Photo: Siemens AG
and director of St. Anthony Falls Laboratory and the EOLOS Wind Consortium at
the University of Minnesota in Minneapolis. “This is the result of major advances
in rotor aerodynamics, wind turbine
controls, and materials, all of which enable lighter and structurally more reliable
turbine designs that can operate safely
and efficiently under high wind speeds.”
“We haven’t hit the barrier yet for how
large these machines can be,” Veers said.
“The only restriction for size on land,”
Veers added, “is the difficulty in transporting parts. Offshore, wind turbines
continue to get larger because components can be brought in by barge.”
when wind conditions vary.”
The U.S. Department of Energy’s wind research focuses on
integrated, systems-level optimization of the entire wind plant.
Researchers are interested in understanding the multi-scale physics
that impact the performance and reliability of wind plants, ranging
from mesoscale atmospheric flow to the microscale flow over the
blade surface.
“Research has shown that the performance of the downstream
wind turbines is significantly degraded when operating in the wakes
This research turbine at the University of Minnesota’s EOLOS
Wind Consortium is bristling with strain gauges that will
provide data to inform updated computational models.
Photo: EOLOS Wind Consortium/University of Minnesota
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MECHANICAL ENGINEERING | JULY 2013 | P.43
of the upstream turbines,” said Shreyas Ananthan,
wind program aerodynamicist for the Department
of Energy in Washington, D.C. “Understanding the
evolution of the wind turbine wakes within the wind
plant, and its dependence on the atmospheric stability and turbulence, are keys to improving performance and reducing the cost of energy in the coming
years.”
Wind turbines on a wind farm actually interact
with one another through these “wake effects,”
which occur when wind passes through the rotor of
a wind turbine and passes downstream. The wake
consists of a volume of lower velocity air that creates fluctuations downstream which then encounter
the next rotors. Detailed computer modeling shows
how turbulence in the atmosphere and complex
topography affect the energy-capture ability of wind
turbines and impact structural loads on the turbine
blades.
“With computer models we actually see the
wakes and how they affect the downstream turbines,” Veers said. “By aligning rotors with the wind
in specific ways, curved wakes can be created that
can be steered away from downwind turbines.”
Another “smart” improvement is passive loadshedding capability. Dynamic controls allow each
blade to pitch at its own ideal angle, independent
of the other blades, depending on the location of
the wind shear. “This can reduce load by 20 to 25
percent,” said Veers.
Load shedding is also enhanced by feed-forward controls using light detection and ranging,
or LIDAR. Such systems shoot laser beams in
front of the turbines; the light reflects off particles carried in the wind, allowing calculation of
both velocity and changes in velocity before the
wind gets to the turbine. The LIDAR data provide
advanced information that allows for the most
efficient blade positioning.
The University of Minnesota’s EOLOS wind energy research consortium uses innovative computational approaches to study offshore wind farms—especially how turbines respond to the combined effect
of turbulence induced by wind and ocean waves.
“Experiments in our wind tunnels are helping
us understand how the relative arrangement of
wind turbines impacts the power production of
wind farms,” said Sotiropoulos. “We have found,
for example, that staggering the turbines can be
more efficient than aligning them in rows. We
have also learned that using variable size turbines, and mixing together larger and smaller
turbines, can be quite beneficial for optimizing
wind farm energy extraction.”
0713MEMp041.indd 43
Engineers at Purdue University constructed a wind turbine
dynamics and control test bed to enable experiments using
inertial sensors inside the rotor. The goal is to learn the best
direction to point the turbines in any given wind.
Photo: Purdue University
When the machinery in the nacelle malfunctions, workers must climb into the towering
structure (below and inset) to effect repairs.
Research is looking into designs that
elimination the complicated gearbox
mechanism altogether.
Photos: Dennis Schroeder / NREL(inset);
First Wind (below).
6/4/13 3:40 PM
THE ECONOMICS—AND
POLITICS—OF OFFSHORE
WIND ALLOW FOR
EXTRAORDINARILY LARGE
TURBINES. THOSE WITH
RATED POWER IN THE
10 MW TO 20 MW RANGE
AND ROTOR DIAMETERS
AS GREAT AS 500 FEET
ARE NOW BEING
DEVELOPED.
Offshore wind turbines, such as
this BARD 5.0 installed in the
German Bight, can be bigger
and more powerful than
land-based turbines.
Photo: BARD Holding GmbH
COMPUTATIONAL MIGHT IS ALSO
BEING EMPLOYED TO STUDY HOW TO
MAKE THE TURBINE BLADES LARGER
AND MORE EFFICIENT. Todd Griffith,
offshore wind technical lead for the Wind and
Water Power Technologies Department at Sandia National
Laboratories in Albuquerque is investigating potential
technology barriers for future large blades. Recent work
includes study of carbon materials for large blades and an
associated manufacturing cost analysis. Researchers are
also investigating the impact of flatback (or thick) airfoils
on the weight and performance of very large blades.
“Research aimed at improving operations and maintenance processes is another active area of research at
Sandia,” Griffith said. “We are developing a roadmap for
structural health and prognostics management applied
to wind plants, as this technology has great
potential to reduce operational and maintenance costs and increase energy
Workers prepare the blade assembly for
lifting into place atop a 2 MW Gamesa
wind turbine being installed at
the National Wind Technology Center.
New, less expensive blade designs are
being investigated.
Photo: Dennis Schroeder / NREL
0713MEMp041.indd 44
capture. We are also developing tools for evaluating the
techno-economic feasibility of enhanced blade sensing and
smart load management.”
Purdue University is collaborating with
Sandia National Laboratory to develop
these sensing strategies for wind turbine
Birds and turbines
blades, as well as the data processing algocan coexist. A great
rithms that work along with these sensors
horned owl perched on
to reduce the cost of wind energy. For ina non-operating 65 kW
turbine at Altamont
stance, inertial sensors are mounted inside
Pass in California.
the blade cavity to detect small amounts of
Photo: Shawn
aerodynamic imbalance due to phenomeSmallwood
na such as pitch error in the blade setting,
soling of the blade, or ice accretion.
“Sources of imbalance like these can substantially reduce
the power produced by the turbine and degrade the reliability of the blades and driveline,” said Adams. “If a turbine
is not available, it cannot produce power, which drives up
the cost of energy. By detecting aerodynamic or mass imbalances, we can adapt the operation of the turbine
to ensure that it remains productive.”
Also, if sensors indicate that the blade
has been damaged, the turbine can be
operated to produce less power so that
it does not over-exert itself, leading to a
major blade failure.
Safety, efficiency, and power production can intersect in other ways. One of the
biggest recent developments in wind energy
is that grid operators have been successful
in finding ways to integrate large quantities of
variable, location-constrained energy resources at
reasonable costs, without compromising the overall requirements of a safe and reliable electricity supply system.
This balance has been accomplished through new wind
turbines with more grid-friendly electrical characteristics,
better wind forecasting techniques, advanced grid-wide
communications and control, and regulatory changes to
eliminate uncertainty in technical requirements.
“Turbine designs for onshore applications appear to have
reached some practical limits for size—around 3 to 3.5 MW
for power rating and 300- to 400-foot rotor diameters,”
said Steve Williams, senior engineer for S&C Electric Co.
in Milwaukee. S&C Electric works with many renewable
plant owners to facilitate interconnection and control of
wind plants.
“We are also seeing more applications of energy storage systems to integrate wind and solar energy sources,
for both large, centralized power systems and microgrids,”
Williams said.
Energy storage helps maximize wind plant output and
mitigate inherent intermittency issues in order to make
renewables a more stable and reliable source. This makes
it more profitable for project owners and much easier for
utilities to manage.
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MECHANICAL ENGINEERING | JULY 2013 | P.45
WHILE MOST OF THE WIND POWER DEVELOPMENT IN THE UNITED STATES IS IN ONSHORE
WIND FARMS, INTERNATIONALLY SOME MAMMOTH PROJECTS ARE BEING BUILT OFFSHORE.
For instance, an offshore wind farm with over 2
GW capacity is currently being developed off the
coast of South Korea.
“I believe the wind energy industry will increasingly look at offshore opportunities because of the
desirable aspects of the offshore wind resource
and that wind energy technology will increasingly
involve global research collaborations,” Adams said.
“As new technologies continue to reduce the cost
of manufacturing, installing, and maintaining turbines, I think we’ll see more onshore wind energy
installations in North America.”
The economics—and politics—of offshore wind
allow for extraordinarily large turbines. Turbines
with rated power in the 10 MW to 20 MW range
and rotor diameters as great as 500 feet are now
being developed.
“Larger and taller turbines are critical for helping the industry achieve economies of scale, making
wind farms cheaper to install and maintain since
fewer turbines are required,” Sotiropoulos said.
Bigger turbines also mean engineers must deal
with bigger mechanical stresses.
“I recently read that one blade designer was
moving away from the traditional spar design, as
they increased machine size due to inherent limitations of spars,” Williams said. “Other designers are
looking to improve reliability by eliminating the
gearbox, and instead using a high-pole-count, lowspeed electrical machine directly connected to the
turbine shaft. Another example is reducing tower
mass by incorporating active damping control sys-
0713MEMp041.indd 45
Photo: Aeolus
tems. All of these improvements point to modern analysis and optimization
techniques, instead of brute-force solutions.”
Researchers are also exploring the aerodynamic design of blades,
advanced approaches to control the turbulent flow around the blades,
and lighter and structurally more resilient materials for blades, gear box
technology, and power transmission systems. The integration of advanced
computational approaches with laboratory and field-scale experimentation
is helping researchers understand the very complex interaction between
turbulence in the atmosphere and the machine.
“Such knowledge, which was not available to the engineers who
designed the current generation wind turbines, will revolutionize wind
turbine design,” Sotiropoulos said. “Most importantly, this knowledge and
advanced research tools will enable engineers to tackle in the near future
the problem of wind plant scale optimization, something that has never
been possible in the past.” ME
MARK CRAWFORD is a geologist and independent writer based in Madison, Wis.
TODD SPINK
“These solutions make wind more viable and
attractive, which is especially important as the
U.S. seeks energy independence and cleaner
sources of power,” Williams said. “Many states
have renewable portfolio standards and rigorous
goals to meet.”
Turbulence coming from
wind turbine array in Denmark
is clearly seen in the fog. Such
turbulence reduces the power
available to downwind turbines.
ccording to Douglas Adams, professor
of mechanical engineering at Purdue
University, wind farms are taking a
heavy toll on bats. Bat mortality rates
are as high as 50 bats per turbine per year.
“This significant ecological impact has
been successfully addressed by wind farm op-
BATTERED BATS
erators through the process of curtailment—
choosing not to operate the turbine during the
peak bat hours/season,” Adams said.
That does, however, decrease revenues
from the wind farm, so operators are trying
to find new ways to solve the bat mortality
problem. One of the technological solutions
being considered is using the blade as a
loudspeaker to produce an acoustic fog
that will repel bats.
6/4/13 3:41 PM
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