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Researchers at Iowa State University are working to design and test taller wind turbine towers using concrete panels that could reach more robust winds at 100 meters and above.

Sri Sritharan, Iowa State's Engineering Professor and leader of the College of Engineering's wind energy initiative, is trying to answer some basic questions about using concrete panels and columns to build wind turbine towers using pre-fabricated, transportable components.

"We have definitely reached the limits of steel towers," Sritharan said. "Increasing the steel tower by 20 meters will require significant cost increases, and thus, the wind energy industry is starting to say, 'Why don’t we go to concrete?'"

The goal was to test three column-and-panel segments for the expected loads at the top of a turbine tower. The engineers wanted to see if the segments could handle 150,000 pounds of load, 20% over the extreme load at that height.

Sritharan and Grant Schmitz, an Iowa State graduate student of civil, construction and environmental engineering, designed the concrete towers to be built in hexagon-shaped segments, with six panels connected to six columns. They tested three methods to connect the panels and columns: bolted connections; horizontal, pre-stressed connections with cables running through the tower pieces; and a grout connection using ultra-high performance concrete poured into the joints between panels and columns. In addition, the concrete columns were attached to a foundation using pre-stressing methods.

All three versions of the test segments withstood 150,000 pounds of lateral load. The researchers also tested the segment with the grout connections under 170,000 pounds of load, 36% beyond extreme load. In each test, the segments performed well with no sign of distress at the operational load of 100,000 pounds. Some distress to the test segments was visible at the extreme load and beyond.

According to Sritharan, the concrete tower design offers several advantages over today’s steel towers, such as easier transportation and less reliance on imported steel. He adds that the concept is versatile, and towers can be tailored for any turbine size or even a height beyond 100 meters.

"What we have shown is that this system can potentially be deployed to a 100-meter height for a 2.5 MW to 3 MW system," Sritharan said.

Moving from 80-meter to 100-meter towers is important for wind energy producers. Sritharan said wind conditions at 100 meters are steadier and less turbulent. Taller towers also allow for longer turbine blades. Taken together, higher wind speeds and taller towers can increase energy production by 15%, he notes.

The project was supported, in part, by a $109,000 grant from the Grow Iowa Values Fund, a state economic-development program. Industry partners in the experimental program are Clipper Windpower with a turbine design and manufacturing facility in Cedar Rapids; Iowa and Calgary-based Lafarge North America Inc.; and Bellevue, Neb.-based Coreslab Structures.

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