Engineers are giving wind power a boost—literally—by testing ways to increase energy production by making the towers 20 percent taller.
Researchers at Iowa State have set out to determine if concrete could be a viable alternative to steel towers for wind turbines and if using concrete could raise turbine towers from 80 meters (262.5 feet) to 100 meters (328.1 feet), where winds are steadier.
Iowa State / Mike Krapfl
Grant Schmitz looks over a concrete test panel for signs of cracking under heavy loads as part of his research into taller concrete wind turbines.
The research is being carried out by Grant Schmitz, and Iowa State graduate student of civil, construction and environmental engineering, and Sri Sritharan, Iowa State's Wilson Engineering Professor and leader of the College of Engineering's Wind Energy Initiative.
Concrete vs. Steel
"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?'"
Sritharan said that as turbine size increases, there will definitely be a need for taller towers, adding that wind conditions at 100 meters are steadier and less turbulent. Taller towers would also allow for longer blades, and studies have shown that these changes could increase energy production by 15 percent.
According to the researchers, concrete could offer several advantages over steel, such as increasing tower life, making the industry less reliant on imported steel, and easing tower transportation with smaller, precast pieces that fit on standard trucking and can be assembled on site.
Testing Concrete Towers
The researchers designed concrete towers in hexagon-shaped segments, with six panels connected to six columns. To connect the panels and columns, they tested three methods: bolted connections; horizontal, prestressed connections with cables running through the tower pieces; and a grout connection using ultra-high performance concrete poured into the joints between the panels and columns.
The concrete columns were attached to a foundation using prestressing methods, and, with help from Doug Wood, the engineering specialist and manager of the university's Structural Engineering Research Laboratory, the lab's hydraulic equipment was programmed to push and pull larger loads on the 100-meter concrete wind turbine towers.
All three columns reportedly withstood 150,000 pounds of lateral load, which is 20 percent over the extreme load at that height. The researchers also tested the tower with the grout connection at 170,000 pounds of load.
The segments performed well in each test, with no signs of distress at the operational 100,000-pound load. However, some signs of distress were found at the extreme load and higher.
"Panel cracking was expected at very high loads and will be closed upon removal of the load," Sritharan said. "This can also be avoided if this is requested by the industry."
Schmitz added, "I definitely think we're getting close to being able to use this technology in the industry."
The project was partially supported by a $109,000 grant from the Grow Iowa Values Fund, a state economic development program. Industry partners in the program include Clipper Windpower, Lafarge North America Inc. and Coreslab Structures Inc.