Rethinking Blade Design

Researchers at the Georgia Tech University Research Institute, in collaboration with California-based PAX Streamline, are working on a new wind blade design that could mean more energy efficiency and lower costs with help from a $3 million grant from the Advanced Research Projects Agency-Energy, or ARPA-E.

Bob Englar, principal research engineer at the Georgia Tech institute, is working on a blade using circulation control technology to create more power using a smaller blade. He got the notion from something called the “Coanda Effect,” which says a moving gas or liquid stream from a jet will flow along the shape of a surface, even if it curves or bends, instead of continuing to move in a straight line out of the jet. The Coanda Effect can also increase wind turbine efficiency and electricity production by increasing the lift to the wind turbine blades.

The hope is that, even in places with little to no wind available, redesigned blades could generate the same amount of wind power with the same efficiency and at lower costs compared with more common wind turbines by using less wind.

Making It Work

Englar said he had been perfecting the design for more than 40 years while working on U.S. Navy aircraft, including helicopters, airplanes and jets. Englar said the blade design works just like a helicopter’s rotors but in reverse. (Left: Trailing edge of the circulation control airfoil in research tunnel with cotton strings showing flow patterns. This trailing edge affords high lift generation and drag control. Photo courtesy GTRI and NASA.)

“When you put wind in a rotor blade, it turns the blade that turns the turbine that creates power,” Englar said. Unlike with a chopper blade, which turns the power into lift, the turbine blade turns lift into power.

Preliminary tests show the blades could produce 30 to 40 percent more power than conventional blades at the same speed.

“If I can generate a 40 percent power increase, then I can use some of that (leftover power) toward the compressor,” Englar said.

More Good Ideas?

Georgia Tech is not the only university coming up with a new wind blade design. Dr. Majid Rashidi, chairman of the Engineering Technology Department and Betty L. Gordon distinguished professor of the Mechanical Engineer Department at Cleveland State University, is also working on a new wind turbine design with support from the U.S. Department of Energy.

Rashidi’s design includes incorporating four wind turbines on a cylindrical structure and placing it on a building’s rooftop.

Test results show a Rashidi-designed turbine with a 15-foot diameter cylinder and a wind speed of 18 mph produced about 1,580 Watts, while a turbine with a more conventional general design produced 580 Watts of energy.

At Georgia Tech, Englar is working to change the wind blade’s lift coefficient by blowing pressurized air out of thin slots on the back side of the blade. This is done to change the angle of attack without having to twist the blade.

“Think of a plane going down the runway for take off,” Englar said. “The pilot increases the angle of attack by pulling back on the stick. It increases the lift coefficient and the plane takes off.”

A second way to get the technology to work is to change the area of the blade by changing the length of the “chord,” which is the distance from the blade’s front to its back.

“When the blade is rotating, each section is going at a different speed because of the radius,” Englar said. “Different speeds mean a different outboard of the chord, which means a different angle of attack for each section.”

Englar said doing that requires finding the correct rotation for the blades. The biggest problem is trying to get wind turbines with these new blades to generate the same amount of power at lower speeds, such as in parts of the United States where the wind does not blow very strongly.

“It turns out these can generate up to 10 times the lift coefficient and I don’t need the angle of attack to do it,” Englar said. That would be similar to a plane taking off without the pilot having to pull back on the stick. “If I can increase the lift coefficient by 10, I only have to change the blowing or make it give me the same torque at a lower wind speed.”

Englar said he could get the lift by turning on the blowing or getting the same amount of torque at a lower wind speed, creating the ability to run the wind turbine in places where they could not be run because of a lack of wind.

But how do you get the right amount of power to create the blowing needed to make a wind turbine turn without a lot of energy?

“With planes, they have jet engines, but turbines don’t have jet engines, so I have to come up with something that takes a little air and gives out power,” Englar said. The solution came by designing an airflow section that requires a minimum amount of blown airflow while producing the same amount of power.

Business Case

PAX CEO John Webley said the research and development of the Georgia Tech project could help get one of the redesigned turbines on the market in a short amount of time.

“This is a significant validation of our established turbine R&D,” said Webley. With the help of the DOE grant, the company expects to rapidly accelerate its research program and, within the next two years, deploy a prototype wind turbine.

Englar said the technology could easily be applied to hydropower developments and has already been tested on semi trucks to test vehicle fuel efficiency by reducing drag. “The pressure was less than 1/4 psi,” Englar said. “It shows you don’t need a huge amount of psi to put out power.”

No matter where the technology is used, Englar and PAX hope it can help the country at a time it is looking for ways to save more energy.

“I’m looking at ways to generate an economic and efficient device,” Englar said.

Sidebar: Toward a 10 MW Wind Turbine

Trailing edge of the circulation control airfoil in research tunnel with cotton strings showing flow patterns. This trailing edge affords high lift generation and drag control. Photo courtesy GTRI and NASA.

With a 100-meter-tall tower, the 10 MW Brittania wind turbine would have a 150 meter blade sweep. Graphic courtesy Clipper Windpower.

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