A wind tunnel on the Homewood campus allows researchers to experiment with variables such as the correct spacing of wind turbines. Charles Meneveau and a colleague have devised a new formula for determining the optimal positioning. Photo: Will Kirk/Homewoodphoto.jhu.edu |
Large
wind farms are being built around the world as a cleaner way to
generate electricity, but operators are still searching for the most
cost-effective and efficient way to arrange the massive turbines that
turn moving air into power.
To
help steer wind farm owners in the right direction, Charles Meneveau, a
Johns Hopkins fluid mechanics and turbulence expert, working with a
colleague in Belgium, has devised a new formula through which the
optimal spacing for a large array of turbines can be obtained.
“I
believe our results are quite robust,” said Meneveau, who is the Louis
Sardella Professor of Mechanical Engineering in the university’s Whiting
School of Engineering. “They indicate that large wind farm operators
are going to have to space their turbines farther apart.”
The
newest wind farms, which can be located on land or offshore, typically
use turbines with rotor diameters of about 300 feet. Currently, turbines
on these large wind farms are typically spaced about seven rotor
diameters apart. The new spacing model developed by Meneveau and Johan
Meyers, an assistant professor at Katholieke Universiteit Leuven in
Belgium, suggests that placing the wind turbines 15 rotor diameters
apart—more than twice as far apart as in the current layouts—results in
more cost-efficient power generation.
The
study by Meneveau and Meyers was presented recently at a meeting of the
American Physical Society Division of Fluid Dynamics.
The
research is important because large wind farms—consisting of hundreds
or even thousands of turbines—are planned or already operating in the
western United States, Europe and China. “The early experience is that
they are producing less power than expected,” Meneveau said. “Some of
these projects are underperforming.”
Earlier
computational models for large wind farm layouts were based on simply
adding up what happens in the wakes of single wind turbines, Meneveau
said. The new spacing model, he said, takes into account interaction of
arrays of turbines with the entire atmospheric wind flow.
Meneveau
and Meyers argue that the energy generated in a large wind farm has
less to do with horizontal winds and is more dependent on the strong
winds that the turbulence created by the tall turbines pulls down from
higher up in the atmosphere. Using insights gleaned from
high-performance computer simulations as well as from wind tunnel
experiments, they determined that in the correct spacing, the turbines
alter the landscape in a way that creates turbulence, which stirs the
air and helps draw more powerful kinetic energy from higher altitudes.
The
experiments were conducted in the university’s wind tunnel, located on
the Homewood campus, which uses a large fan to generate a stream of air.
Before it enters the testing area, the air passes through an “active
grid,” a curtain of perforated plates that rotate randomly and create
turbulence so that the air moving through the tunnel more closely
resembles real-life wind conditions.
Air
currents in the tunnel pass through a series of small three-bladed
model wind turbines mounted atop posts, mimicking an array of full-size
wind turbines. Data concerning the interaction of the air currents and
the model turbines is collected by using a measurement technique called
stereo particle-image-velocimetry, which requires a pair of
high-resolution digital cameras, smoke and laser pulses.
Further
research is needed, Meneveau said, to learn how varying temperatures
can affect the generation of power on large wind farms. He has applied
for continued funding to conduct such studies.
