A Sandia team completes installation in the late 1980s of a vertical axis wind turbine test platform in Bushland, Texas. Photo by Randy Montoya |
Sandia
National Laboratories’ wind energy researchers are re-evaluating
vertical axis wind turbines (VAWTs) to help solve some of the problems
of generating energy from offshore breezes.
Though
VAWTs have been around since the earliest days of wind energy research
at Sandia and elsewhere, VAWT architecture could transform offshore wind
technology.
The
economics of offshore windpower are different from land-based turbines,
due to installation and operational challenges. VAWTs offer three big
advantages that could reduce the cost of wind energy: a lower turbine
center of gravity; reduced machine complexity; and better scalability to
very large sizes.
A lower center of gravity means improved stability afloat and lower gravitational fatigue loads.
Additionally,
the drivetrain on a VAWT is at or near the surface, potentially making
maintenance easier and less time-consuming. Fewer parts, lower fatigue
loads and simpler maintenance all lead to reduced maintenance costs.
Elegant in their simplicity
Sandia
is conducting the research under a 2011 Department of Energy (DOE)
solicitation for advanced rotor technologies for U.S. offshore windpower
generation. The five-year, $4.1 million project began in January of
this year.
Wind
Energy Technologies manager Dave Minster said Sandia’s wind energy
program is aimed at addressing the national energy challenge of
increasing the use of low-carbon power generation.
“VAWTs
are elegant in terms of their mechanical simplicity,” said Josh
Paquette, one of Sandia’s two principal investigators on the project.
“They have fewer parts because they don’t need a control system to point
them toward the blowing wind to generate power.”
These
characteristics fit the design constraints for offshore wind: the high
cost of support structures; the need for simple, reliable designs; and
economic scales that demand larger machines than current land-based
designs.
Large
offshore VAWT blades in excess of 300 m will cost more to produce than
blades for onshore wind turbines. But as the machines and their
foundations get bigger—closer to the 10–20 megawatt (MW) scale—turbines
and rotors become a much smaller percentage of the overall system cost
for offshore turbines, so other benefits of the VAWT architecture could
more than offset the increased rotor cost.
Challenges remain
However, challenges remain before VAWTs can be used for large-scale offshore power generation.
Curved
VAWT blades are complex, making manufacture difficult. Producing very
long VAWT blades demands innovative engineering solutions. Matt Barone,
the project’s other principal investigator, said partners Iowa State
University and TPI Composites will explore new techniques to enable
manufacture of geometrically complex VAWT blade shapes at an
unprecedented scale, but at acceptable cost.
VAWT
blades must also overcome problems with cyclic loading on the
drivetrain. Unlike horizontal axis wind turbines (HAWTs), which maintain
a steady torque if the wind remains steady, VAWTs have two “pulses” of
torque and power for each blade, based on whether the blade is in the
upwind or downwind position. This “torque ripple” results in unsteady
loading, which can lead to drivetrain fatigue. The project will evaluate
new rotor designs that smooth out the amplitude of these torque
oscillations without significantly increasing rotor cost.
Because
first-generation VAWT development ended decades ago, updated designs
must incorporate decades of research and development already built into
current HAWT designs. Reinvigorating VAWT research means figuring out
the models that will help speed up turbine design work.
“Underpinning
this research effort will be a tool development effort that will
synthesize and enhance existing aerodynamic and structural dynamic codes
to create a publicly available aeroelastic design tool for VAWTs,”
Barone said.
Needed: aerodynamic braking
Another
challenge is brakes. Older VAWT designs didn’t have an aerodynamic
braking system, and relied solely on a mechanical braking system that is
more difficult to maintain and less reliable than the aerodynamic
brakes used on HAWTs.
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HAWTS
use pitchable blades, which stop the turbine within one or two
rotations without damage to the turbine and are based on multiple
redundant, fail-safe designs. Barone said new VAWT designs will need
robust aerodynamic brakes that are reliable and cost-effective, with a
secondary mechanical brake much like on modern-day HAWTs. Unlike HAWT
brakes, new VAWT brakes won’t have actively pitching blades, which have
their own reliability and maintenance issues.
VAWT technology: A long history at Sandia
In
the 1970s and 1980s, when wind energy research was in its infancy,
VAWTs were actively developed as windpower generators. Although strange
looking, they had a lot going for them: They were simpler than their
horizontal-axis cousins so they tended to be more reliable. For a while,
VAWTs held their own against HAWTs. But then wind turbines scaled up.
“HAWTs
emerged as the predominant technology for land-based wind over the past
15 years primarily due to advantages in rotor costs at the 1 to 5
megawatt scale,” Paquette said.
In
the 1980s, research focused more heavily on HAWT turbines, and many
VAWT manufacturers left the business, consigning VAWTs to an “also ran”
in the wind energy museum.
But the winds of change have blown VAWTs’ way once more.
Sandia
is mining the richness of its wind energy history. Wind researchers who
were among the original wind energy engineers are going through decades
of Sandia research and compiling the lessons learned, as well as
identifying some of the key unknowns described at the end of VAWT
research at Sandia in the 1990s.
The
first phase of the program will take place over two years and will
involve creating several concept designs, running those designs through
modern modeling software and narrowing those design options down to a
single, most-workable design. During this phase, Paquette, Barone and
their colleagues will look at all types of aeroelastic rotor designs,
including HVAWTs and V-shaped VAWTs. But the early favorite rotor type
is the Darrieus design.
In
phase two researchers will build the chosen design over three years,
eventually testing it against the extreme conditions that a turbine must
endure in an offshore environment.
In
addition to rotor designs, the project will consider different
foundation designs: Early candidates are barge designs, tension-leg
platforms and spar buoys.
The project partners will work on many elements.
Another
partner, the University of Maine, will develop floating VAWT platform
dynamics code and subscale prototype wind/wave basin testing. Iowa State
University will develop manufacturing techniques for offshore VAWT
blades and subscale wind tunnel testing. TPI Composites will design a
proof-of-concept subscale blade and develop a commercialization plan.
TU-Delft will work on aeroelastic design and optimization tool
development and modeling. Texas A&M University will work on
aeroelastic design tool development.
“Ultimately
it’s all about the cost of energy. All these decisions need to lead to a
design that’s efficient and economically viable,” said Paquette.
Source: Sandia National Laboratories
