Can concrete replace steel for wind turbine towers?
25 June 2013
Grant Schmitz, eyes just inches away from a seven and a quarter square meter panel of ultra-high performance concrete, studies the smooth surface for tiny cracks. He and other research engineers at Iowa State University carefully mark every one with black marker pens.
Schmitz, an Iowa State graduate student of civil, construction and environmental engineering, and Sri Sritharan, Iowa State’s Wilson Engineering Professor and leader of the ’s College of Engineering's Wind Energy Initiative, are trying to answer some basic questions about using concrete panels and columns to build wind turbine towers using prefabricated, easily transportable components.
Could assembled concrete towers be a viable alternative to the steel towers now used for wind turbines? Could concrete towers be a practical way to raise turbine towers from today’s 80m to the steadier winds at 100m and taller? Which of three ways to connect the columns and panels works best for wind turbine towers?
“We have definitely reached the limits of steel towers,” Sritharan said. “Increasing the steel tower by 20m will require significant cost increases and thus the wind energy industry is starting to say, ‘Why don’t we go to concrete?’”
And so, Sritharan and Schmitz were keen to watch as Doug Wood, an engineering specialist and manager of Iowa State’s Structural Engineering Research Laboratory, typed in the commands for the lab’s hydraulic equipment to push or pull with bigger loads on a full-size test segment of a 100m concrete wind turbine tower.
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 a 68,000kg load - some 20 percent above the extreme loading that can occur at that height.
Sritharan and Schmitz 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 the 68,000kg of lateral load. The researchers also tested the segment with the grout connections under a 77,000kg load, some 36 percent beyond the extreme load. In each test, the segments performed well with no sign of distress at the operational load of 45,360kg.
Some distress to the test segments was visible at the extreme load and beyond. Panel cracking was expected at very high loads, although this closes upon removal of the load. Sritharan believes this, too, can be avoided if the industry requests it.
As a result of the test programme, Schmitz is confident that the industry is getting closer to being able to use this technology. This is particularly important as the industry seeks to move from 80m to 100m towers for their larger turbines.
The advantages of concrete
The concrete tower design offers several advantages over today’s steel towers. Apart from improving upon steel’s 20-year tower life, thanks to the use of ultra-high performance and high-strength concrete, the sections are much easier to transport, the pieces being small enough for loading on standard haulage vehicles.
Moreover, the smaller precast pieces can be assembled on site in multiple ways. The concept is versatile and towers can be tailored for any turbine size - even a height beyond 100m. Sritharan concludes:
“As turbine size increases, the need for taller towers will be inevitable. A lot of people are talking about taller, concrete wind turbine towers, and we’ve already established a new versatile concept with multiple construction options. What we have shown is that this system can potentially be deployed to a 100m height for a 2.5 to 3MW system.
“Wind conditions at 100m are steadier and less turbulent; taller towers also allow for longer turbine blades - and studies indicate all of that can increase energy production by 15 percent.”
Meanwhile, studies of concrete turbine towers continue apace at Iowa State, thanks to state funding and support from industry partners such as California based Clipper Windpower and interested parties from the cement manufacturing and concrete structure fabrication industries.
And Schmitz, who’s describing the project for his master’s thesis, can now breathe a little easier after the successful testing. “There is a lot of preparation for this,” he says. “We started coordinating the tests back in August . We had to arrange for the precast and transportation and assembly through the fall. It’s definitely a relief when you see it handling the capacity it has to meet.”