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How to build a better wind farm

IOWA STATE (US) — A turbine only ten inches high is helping researchers understand how hills, valleys, and tower placement can affect the productivity of onshore wind farms.

The tower is a perfect 1:320 scale of the 80-meter wind turbines in Iowa, the nation’s second-ranked state for installed wind power capacity. While the wind industry has data about offshore turbine performance over flat water—especially from European studies—there is little data about the effects of uneven ground on wind turbines.

So researchers created the mini turbines and started running tests in the Aerodynamic/Atmospheric Boundary Layer Wind and Gust Tunnel at Iowa State University.

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“We want to work with the wind turbine industry to transfer some of our findings,” says Hui Hu, associate professor of aerospace engineering. “We can help boost total energy capture. And we can lengthen the lifetimes of wind turbines, making them more efficient.”

The wind tunnels are being used to quantify characteristics of surface winds over hilly terrains, determine the best placement of wind turbines on hilly terrains, and to find the best design for large wind farms on hilly terrains.

Experiments include:

  • Mini-generators mounted inside the mini turbine nacelles measure power production.
  • Sensors mounted at the base of the mini-turbines measure the wind loads placed on turbines and turbine towers.
  • Advanced flow measurements such as particle image velocimetry (which uses a laser and camera to take nearly simultaneous images that show the movement and velocity of individual particles) to measure wind flow fields, the wind vortices created by the tips of turbine blades and the total wind energy captured by the blades.

Preliminary results indicate that wind turbines on hilly terrain are hit with much higher wind loads than turbines on flat terrain. The experiments also show that, compared with turbines on flat ground, wind flowing over hilly terrain recovers its power potential more rapidly as it moves from turbine to turbine.

Data from the wind tunnel indicate a turbine on flat ground in the wake of another turbine at a distance equal to six times the diameter of the turbines loses 13 percent of power production.

A turbine in the wake of another with the same downstream distance on hilly ground loses 3 percent of power production. “That means you can put wind turbines closer together in hilly terrain,” Hu says.

The research is reported online in the Journal of Visualization. The project’s next steps include building a nine-turbine array in Iowa State’s big wind tunnel to study power production and wind flows through a mini-wind farm.

Hu says the data from the experiments will be valuable as more wind farms are built across Iowa and the country. “These studies are telling us things we didn’t know before,” he says. “And this will help optimize the design of wind turbine layouts with consideration of the terrain.”

Co-authors of the paper are Zifeng Yang, a former Iowa State post-doctoral researcher and now an assistant professor at Wright State University in Dayton, Ohio; and Partha Sarkar, a professor of aerospace engineering, of civil, construction, and environmental engineering and a director of Iowa State’s Wind Simulation and Testing Laboratory.

The studies are supported by the National Science Foundation and the Iowa Alliance for Wind Innovation and Novel Development.

More news from Iowa State University: www.iastate.edu/

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