Iowa State Aerospace Engineers Anupam Sharma and Hui Hu are exploring the possibility of adding a smaller, secondary rotor to wind turbines. The engineers studied the base of existing turbines and found two major problems. First, they are big round structural pieces that don’t harvest any wind energy because they are not shaped like an airfoil. Second, the large base of the blades actually disrupt the wind, causing a wake behind them which reduces the energy harvesting capacity of any downwind turbines. Hu says that a turbine in the slipstream of another “can lose 8 to 40 percent of its energy production, depending on conditions.”

Their solution? Add a second, smaller rotor. “To try to solve these problems, we put a small rotor on the turbine,” Hu said. “And we found that with two rotors on the same tower, you get more energy.” Lab tests and computer simulations found the extra blades and increase the energy harvest by up to 18 percent. “These are fairly mature technologies we’re talking about – a 10 to 20 percent increase is a large change,” Sharma said.

Using a one-year, $116,000 grant from the Iowa Energy Center, the pair is currently using wind tunnels and computer simulations to study the dual rotor idea and measure power outputs and wind loads. The questions they hope to answer are: How is the wake distributed? Where are the whirling vortices? How could the wake be manipulated to pull down air and recharge the wind load?

They plan to use the research results to find the best aerodynamic design for a dual-rotor turbine. The goal is to find out where the second rotor should be located, how big it should be, what kind of airfoil it should have, and if it should rotate in the same direction or in the opposite direction.

The above image (courtesy of Anupam Sharma) shows air flowing through a dual-rotor turbine. Read more details about the research over at the Iowa State website.

What do you think? Could a second rotor make wind turbines more efficient? Leave your comments below…

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