People think of wind as an energy source with few limits,
offering an unending power source with distinct capacity advantages over
sources that deplete, such as fossil fuel.
Yet, new research in mesoscale atmospheric modelling by UNC
Charlotte's Amanda S. Adams and Harvard University's David W. Keith, published
Monday in the journal Environmental
Research Letters, suggests that the power capacity of large-scale wind
farms may have been significantly overestimated.
With large-scale wind farms, as many as hundreds of turbines
mounted on tall towers and connected to the electrical grid capture the kinetic
energy of the wind. Each wind turbine creates a "wind shadow" behind
it, in which the turning blades slow the air. In an effort to reduce the impact
of the wind shadows, wind farms space the turbines apart, while still locating
as many turbines as they can on the land.
Estimates off target
Current estimates of the global wind power resource over
land range from 56 to 400 terawatts. Most of these estimates assume implicitly
that the turbines extracting the wind energy have little impact on the
atmosphere and, therefore, little effect on the energy production.
The new research says that scientists have underestimated
the impact that large numbers of wind turbines have on energy production within
large farms. Estimates of wind capacity that ignore the effect of wind turbine
drag on local winds have assumed that wind power production of 2 and 4 watts
per square meter could be sustained over large areas.
The new modelling results suggest that the generating
capacity is more likely limited to about 1 watt per square meter at wind farms
that are larger than 100 square kilometres.
"It's easy to mistake the term renewable with the term
unlimited when discussing energy," Adams said. "Just because you can
keep generating new energy from a source does not mean you can generate energy
in an unlimited amount."
The research suggests the potential for wind energy could be
significantly less than previously thought.
"It's important to take into account all factors
impacting the wind energy, so we can assess the capacity of this critical power
resource," Adams said. "One of the inherent challenges is how
harvesting the resource changes it, making it difficult to accurately calculate
how much energy can be produced. The modelling we have done provides
information that can help in the understanding of our ability to count on
renewable energy sources."
Low level warming
The research also considers the impact of wind energy
production on temperatures and by extension possibly climate. Wind farms change
the natural wind shear and produce various scales of turbulence. Higher
potential temperatures are mixed downward due to this turbulence and result in
low level warming, the research indicates.
"Our research suggests that how densely the turbines
are placed affects not only energy production but also environmental
impacts," Adams said. "We see this impact on average temperatures not
only at large-scale farms, but also in small-density wind farms. Some things to
consider are the magnitude of temperature changes and also the size of the area
affected. We think these findings indicate that additional research is needed
in these areas."
The Natural Sciences and Engineering Research Council of
Canada funded the research.
Adams' primary research interests focus on mesoscale
phenomena, processes, and modelling with an emphasis on phenomena that involve
boundary layer processes and/or topographic influences. In recent years, she
and her research group at UNC Charlotte have focused on the link between
small-scale processes and climate, particularly at the atmosphere and earth
surface interface. Her research group concentrates primarily on question at the
interface between energy, weather and climate.
Current questions her group is addressing include: How will
large scale wind energy development impact the Great Plains low level jet? What
are the meteorological conditions that lead to wind turbine icing? How does
temperature variability in urban areas impact electricity demand? Can we
quantify the risks of off shore wind turbines to hurricanes? The energy-related
research that Adams' group is conducting includes collaborations with San Diego
Gas & Electric, Xcel Energy, and the Weather Underground.