19 July 2012
At dusk outside Limon, an isolated interstate interchange on Colorado’s vast eastern frontier, a string of active wind turbines lines the southwest horizon — their red aviation warning lights almost keeping time with their rotating blades. And more are on the way; the local hotels are packed with summer wind farm construction workers pulling three shifts a day.
In fact, such high desert landscapes are an obvious choice for wind power technology — but what about the mountains and hillsides that never seem to top the wind energy list?
These steeper gradient sites are getting a second look. Hui Hu, an aerospace engineer at Iowa State University, and colleagues have been studying how hillside gradients and hilly terrains impact wind energy models for wind farms. Hu argues that current models are based on wind energy efficiencies for turbines on a flat terrain, without taking into account the dynamics of turbines on hillsides.
Until now, there’s been a data gap when engineers consider the effects of atmospheric winds on uneven terrain. But the engineers at Iowa State used scale model mini-turbines in a large on-campus wind tunnel to study the effects of hilly terrain and turbine placement on power production.
“Tools for predicting how existing wind turbine farms will work aren’t well developed, so people resort to rules of thumb,” said Richard Wlezien, Iowa State’s Chair of Aerospace Engineering. “But you can’t just look at an array of turbines and say this is a good or bad way to align them.”
Hu’s argument is that by modeling turbines on hillsides we can get a more accurate picture of energy potential on hilly or mountainous terrain, since wind farm designs are still based on flat terrains.
Hu’s group is studying how the steep [20 to 40 degree] gradients of a hill impact the turbine efficiency. The results show that turbines on hilly terrain experience higher wind loads than their flat terrain counterparts.
As a result, wind making its way over hilly terrain recovers its power potential more quickly as it moves from turbine to turbine. Hu reports that his results show that on hillsides, turbine rows can be more closely spaced than previously thought.
“That means for the same acre of land you can put more wind turbines, and thus [harvest] more energy, out of a given project,” said Hu.
The conventional wisdom, says Wlezien, is that you stagger the turbines to keep one out of the wake of another. But what Hu is showing is that on hilly terrain, the effect of the upstream turbine may disappear faster than people expect. So, on hillsides turbine rows can be more closely spaced.
“Putting one behind the other may not be as bad as it seems even in a flat terrain,” said Wlezien. “It’s all a matter of how far down stream. Hu’s work [shows] that when you put one turbine downstream of another, you can actually get enough mixing in the wake of the upstream turbine that [mountainous sites] become desirable.”
Does this mean the wind energy industry should suddenly head for the hills?
To meet the Department of Energy’s wind energy production targets of 20 percent of the U.S.’ electricity output by 2030, developers may have to.
In order to meet its goal, the U.S. would need to install more than 300 GW of wind energy capacity. If each turbine had a minimum of capacity of 2 MW, the U.S. would still need to install 150,000 more commercial wind turbines over the next two decades. Thus, to meet such goals, wind energy developers may have to consider unconventional mountain sites.
“There’s been an awful lot of confusion about what’s a good location and bad location for turbines,” said Wlezien. “But at higher altitude, you’re going to get higher winds.”
In fact, Jim Manwell, Director of the Wind Energy Center at the University of Massachusetts at Amherst, says the site of the world’s first modern wind farm in 1981 was on New Hampshire’s Crotched Mountain. He notes that, more recently, wind turbines have since been installed on mountainside ski areas in Vermont and Massachusetts. Indeed, the country’s highest winds are in the Intermountain West, the Appalachians, and mountainous parts of the Northeast.
At first blush, placing wind turbines on mountaintops does seem like a good idea, says Manwell. That’s because mountains serve as natural towers and so the wind speed is higher than it is on the surrounding lowlands.
“Wind flow over mountainous terrain may be relatively turbulent and may be outside the range for which commercial turbines have been designed,” said Manwell. “That could accelerate fatigue damage and premature failure and the need for replacement of some components. Thus reputable manufacturers may be loathe to sell turbines for such locations. There may also be problems getting financing or insurance.”
It’s most cost effective to set up wind farms in plains regions and flat mesas, says David Minster, the Manager of the Wind Energy Technologies department at Sandia National Labs in Albuquerque. That is, places that have known winds and predictable directions. Although most wind turbines are designed to be able to change direction to face the wind, Minster says in mountainous areas they would have to change direction frequently.
Darrell Pepper, a professor of mechanical engineering at the University of Nevada at Las Vegas, has studied potential mountainous wind farm sites all over the state.
“Mountainous spots in Nevada have very high wind potential, but are so isolated they are not cost effective,” said Pepper. “What’s really going to kill the deal though is an absence of electrical transmission lines, each of which might cost a million dollars a mile.”
What would have to change to make commercial wind energy commonplace in mountainous or very hilly terrain?
“The cost of electricity would have to go up significantly to make it profitable,” said Minster. “And wind technology would have to be cheaper with more reliable turbine blades.”
Until then, wind farms, in the U.S. at least, will likely continue to only dominate the high desert and Great Plains.