While
a single VAWT is not as energy-producing as an individual HAWT, the
wind flow synergies created in a closely-spaced array of VAWTs can
potentially generate up to 10 times more power per unit of land area
than an array of widely-spaced HAWTs.
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The sight of propeller-like rotating blades positioned high up the pole
of a tall horizontal-axis wind turbine (HAWT) may be familiar to many.
Often grouped in wind farms, HAWTs provide significant amounts of energy
for local communities. One drawback to HAWTs is the large space they
take up, needing to be placed far apart from each other. If placed too
close together, the turbulence and wind velocity deficit caused by one
HAWT can make a neighboring HAWT output much less power.
To address this, researchers are looking at vertical-axis wind turbines
(VAWTs), which could be either arranged in groups or interspersed within
HAWT arrays. A VAWT has an overall cylindrical shape, with the blades
aligned parallel to, and rotating around, the pole on which the rotor is
mounted. These "egg-beater" VAWTs tend to be much smaller than the
"propeller" HAWTs, typically about 10 times shorter in height, and
output only about 0.1 percent as much power per turbine.
Anna Craig, a mechanical engineering doctoral candidate at Stanford
University, and her research team recently studied modeling VAWT array
arrangements, the results of which they report this week in the Journal
of Renewable and Sustainable Energy, by AIP Publishing.
While a single VAWT is not as energy-producing as an individual HAWT,
the wind flow synergies created in a closely-spaced array of VAWTs can
potentially generate up to 10 times more power per unit of land area
than an array of widely-spaced HAWTs.
"For the vertical axis wind turbines, what you get, especially as you
place them in close transverse proximity to each other, is that they can
actually interact positively," Craig said. "Although it is still an
active area of research, we think that the VAWTs can have blockage
effects causing speedup around the turbines that helps downstream
turbines. They can also have vertical wind mixing in the turbine's wake
region, which assists in the wind velocity recovery."
Craig said researchers agree that there is more research to be done on
VAWTs before they can be deployed at an energy sector scale. However,
Craig and her colleagues provided significant insights into one central
VAWT challenge: how to research, test and develop insights for effective
array arrangements. They did this in a lab experiment because field
testing is currently very expensive, and computer simulations are not
yet refined enough or are too computationally expensive.
"Right now the majority of numerical simulations are either fully
two-dimensional or are three-dimensional, but use highly simplified,
effectively two-dimensional models for the turbines. Neither approach
can capture the vertical flows, which are critically important in the
energy dynamics of a VAWT system," Craig said.
Craig and her colleagues believe that this lab experiment and similar
follow-ups offer important possibilities both for in-field arrangements
and refining numerical simulations. They conducted the experiment in the
large water flume at the Bob and Norma Street Environmental Fluid
Mechanics Laboratory in the department of civil and environmental
engineering at Stanford, with the system's water flow effectively
representing the wind flow.
Craig set up roughly 1,300 1-inch gears between plates, which were
reconfigurable during the experiment. On top of these gears sat
approximately 300 rotating cylinders mounted to create a 10-foot-long
array, with the cylinders effectively representing VAWTs. They tested a
total of 10 different arrays with different configurations.
"The three variables I was looking at were spatial configuration,
rotational configuration, and height configuration of the elements,"
Craig said. "I wanted to find out how the interactions between elements
could set up larger scale flow patterns."
The experiment illuminated the VAWTs' time-space averaged vertical flow, which is significant for turbine arrangements.
"What I saw is this net vertical flow from above the array, down into
the array and out the sides of the array, which was somewhat
unexpected." Craig said. "These net vertical and transverse flows
eliminate horizontal homogeneity within the array and introduce a new
mechanism by which the energy resource within an array can be
replenished."
For future studies, Craig said this experiment offers important insights for both numerical and in-field testing.
"The three-dimensionality of the flow through the array is critical to
understanding the energy dynamics of the system," said Craig. "This
paper really focuses on allowing us to design appropriate numerical and
experimental studies."
Craig is optimistic about VAWT technology and its potential uses, noting
that in the future it might be interspaced within HAWT arrays and
brought to places that are not amenable to the much larger HAWTs, such
as islands and cities. She says that VAWTs could also potentially be
less environmentally impactful than HAWTs.
"We should consider numerical or even field experiments with larger
numbers of VAWTs because the laboratory experiments have shown that the
physical mechanisms are there for these larger arrays of turbines to
work," Craig said.
source: http://www.winddaily.com
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