Iowa State engineers are using wind tunnel tests to study the effects of hilly terrain and turbine placement on power production. (ISU/Bob Elbert)
A miniature turbine measuring just 10 inches high is helping a research team led by Hui Hu - an Iowa State University associate professor of aerospace engineering - understand how hills, valleys, and the placement of turbines affect the productivity of onshore wind farms.

The turbine's three blades are 10 inches in diameter and is a perfect 1:320 scale reproduction of the 80-meter diameter wind turbines spinning across Iowa, the country's second-ranked state in installed wind power capacity.

While the wind industry has data about offshore turbine performance over flat water - especially from European studies - Hu said there's little data about the effects of uneven ground on wind turbines. Hu and his research team have created the mini turbines and started running tests in Iowa State's $1.25 million Aerodynamic/Atmospheric Boundary Layer Wind and Gust Tunnel.

The team is using wind tunnel tests to quantify the characteristics of surface winds over hilly terrains, determine the best placement of wind turbines on hilly terrains, and find the best design for large wind farms on hilly terrains.

The 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 said.