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February 5, 2013 — Worldwide, the low efficiency of wind farms is a mystery, considering it seems logical that the momentum of the wind moving from one wind turbine to the next -- much like a domino effect -- should actually increase energy efficiency.
Couple that with the expensive process of creating accurate computer simulations -- many of which can provide only limited efficiency data on one wind turbine or blades of a turbine rather than multiple turbines -- and answers about low wind farm efficiency become even more difficult.
Stefan Heinz, a professor in the University of Wyoming Department of Mathematics who has studied turbulence and combustion for more than 20 years, says he can perform computer simulations of wind farms that are more accurate and less expensive than existing models.
“Right now, no one really understands how a wind farm works. No one knows how one turbine affects the efficiency of another,” says Heinz, who conducts research at UW’s School of Energy Resources’ Wind Energy Research Center. “We need computer simulations that are accurate and as inexpensive as possible to help businesses that build wind farms or turbines.”
Catch the wind
The basic problem of calculating wind turbine efficiency via computer simulations is that the grid applied in numerical simulations has to use a very small cell size and cover a large domain, Heinz says. However, that “makes good simulations extremely expensive,” he says.
Theoretically, he says, a wind farm’s performance should exceed the production of the same number of isolated wind turbines. However, in reality, the overall efficiency of wind farms is below this value because the first turbine disturbs the energy efficiency of the next turbine, and so on. In other words, two plus two does not necessarily equal four in wind farm reality.
“With a wind farm, you have one turbine after another. Each turbine rotates air and creates a wake behind it,” Heinz says. “This air will heavily affect the next turbine. Efficiency of wind farms is relatively low.”
Due to the wakes, the efficiency of wind turbines is reduced by 20 percent to 50 percent compared to turbines in isolation, according to generally accepted research in the field.
Heinz says that turbine performance reduction could possibly be due to one or a combination of factors, including: the distance between the wind turbines, the height of the turbines, variations in topography between the turbines, and rotor size and shape. Heinz plans to study an enclosed wind turbine that spins horizontally, much like a weather vane, to see whether such a model would be more efficient.
With computer simulations, such variables can be changed to determine the impact on wind turbine performance and provide more clues to which wind farm arrangement is most efficient, Heinz says. As a result, it would be theoretically possible for a wind farm to operate with a higher efficiency than the same number of isolated wind turbines, he says.
“We want to not only change the (wind turbine) interaction, but also to understand the interactions,” he says.
Harnessing wind energy through technology
With the added tool of the National Center for Atmospheric Research (NCAR)-Wyoming Supercomputing Center (NWSC) at his disposal this winter, Heinz hopes to create even more accurate and detailed computer-model simulations of wind turbine action. His project is titled “An Order-Magnitude Enforcement of Wind Farm Power Density.”
Heinz has developed novel simulation methods that are more accurate than existing simulation methods -- and roughly 1,000 times faster. The new simulation methods enable complex simulations that were previously not feasible. Nevertheless, such simulations definitely require a supercomputer to deal efficiently with the extreme complexity of realistic wind farms, Heinz says.
“Wind energy will contribute significantly to renewable energy. Every performance increase, by 1 percent or so, will have a significant impact on the energy supply,” Heinz says. “The point is, these wind farms are way behind the performance of future wind farms. In 20 years, we should know how to build wind farms so they (wind turbines) don’t disturb each other, but interact.”
Heinz applied (along with Jayanarayanan Sitaraman and Dimitri Mavriplis, both faculty in UW’s Department of Mechanical Engineering) for and received $508,000 in research funding from NASA. The NASA project is not directly related to wind energy, but does provide an opportunity to develop revolutionary new computational methods that will influence future wind-energy studies, Heinz says.
“The project is part of NASA’s Revolutionary Computational Aerosciences Solicitation,” Heinz says. “The proposal was the second-best proposal in the United States. Only four proposals were funded.”
NWSC is the result of a partnership among the University Corporation for Atmospheric Research (UCAR), the operating entity for NCAR; the University of Wyoming; the state of Wyoming; Cheyenne LEADS; the Wyoming Business Council; and Cheyenne Light, Fuel & Power. The NWSC is operated by NCAR, which is sponsored by the National Science Foundation (NSF).
The NWSC contains one of the world's most powerful supercomputers (1.5 petaflops, which is equal to 1.5 quadrillion mathematical operations per second) dedicated to improving scientific understanding of climate change, severe weather, air quality and other vital atmospheric science and geo-science topics. The center also houses a premier data storage (11 petabytes) and archival facility that holds historical climate records and other information.
Stefan Heinz, an associate professor of mathematics, is heading one of six UW computational research projects that will use the NWSC this winter. Heinz will use the supercomputer to create more detailed and less expensive simulation models in an effort to make wind farms more efficient.