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Smaller Rotor Added To Wind Turbines Increases Efficiency
April 2015

Hui Hu picked up a 3-D printed model of a typical wind turbine and began explaining two problems with the big, tall, three-bladed machines.

First, said the Iowa State University professor of aerospace engineering, check out the base of each blade. They’re big, round structural pieces. They’re not shaped like an airfoil. And so they don’t harvest any wind, reducing a turbine’s energy harvest by about 5 percent.

Iowa State aerospace engineers, left to right, Anupam Sharma and Hui Hu are using wind tunnel testing and computational modeling to improve the performance of wind turbines and wind farms. Photo by Christopher Gannon/Iowa State University.

Second, the big blades disturb the wind, creating a wake behind them and reducing the energy harvest of any downwind turbines. Prof. Hu said a turbine sitting in the slipstream of another can lose 8 to 40 percent of its energy production, depending on conditions.

Those losses prompted Prof. Hu and Anupam Sharma, an Iowa State assistant professor of aerospace engineering, to look for a solution. Their data suggest they’ve found one.

What they’ve done is add a smaller, secondary rotor. One model had three big blades and three mini-blades sprouting from the same hub. The other had a small, secondary rotor mounted in front of the big rotor, the two sets of blades separated by the nacelle that houses the generating machinery on top of the tower.

“To try to solve these problems, we put a small rotor on the turbine,” Prof. Hu said. “And we found that with two rotors on the same tower, you get more energy.”

Using lab tests and computer simulations, they have found those extra blades can increase a wind farm’s energy harvest by 18 percent.

“These are fairly mature technologies we’re talking about – a 10 to 20 percent increase is a large change,” Prof. Sharma said.

The Iowa Energy Center awarded Profs. Hu and Sharma a one-year, $116,000 grant to launch their study of dual rotors. (The two won the energy center’s 2014 Renewable Energy Impact Award for the rotor project.) The National Science Foundation is supporting continued studies with a three-year, $330,000 grant.

Prof. Hu is using experiments in Iowa State’s Aerodynamic/Atmospheric Boundary Layer Wind and Gust Tunnel to study the dual-rotor idea. He’s measuring power outputs and wind loads. He’s also using technologies such as particle image velocimetry to measure and understand the flow physics of air as it passes through and behind a rotating turbine.

How, for example, is the wake distributed? Where are the whirling vortices? How could the wake be manipulated to pull down air and recharge the wind load?

Prof. Sharma is using advanced computer simulations, including high-fidelity computational fluid dynamics analysis and large eddy simulations, to find the best aerodynamic design for a dual-rotor turbine. Where, for example, should the second rotor be located? How big should it be? What kind of airfoil should it have? Should it rotate in the same direction as the main rotor or in the opposite direction?

Prof. Hu said Prof. Sharma’s computer modeling will drive the design of next generation experimental models he’ll take back to the wind tunnel.

The idea to look for better performance by adding a second rotor to wind turbines came from a previous study. Prof. Hu and his research group used wind tunnel tests to see how hills, valleys and the placement of turbines affected the productivity of onshore wind farms.

They learned was that a turbine on flat ground in the wake of another turbine loses a lot of power production. And that presented Prof. Hu and his collaborators with another problem to study.

Article reprinted from materials provided by Iowa State University. ##

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