Rethinking Blade Design

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

The wind energy industry is in a rapid phase of change, growth, expansion and shifting market demands. The wind business is poised for growth, but at the same time has financial resource limitations. These factors lead to a climate of investment uncertainty and if there’s anything we’ve learned from history it is that investment uncertainly leads to economic uncertainty, which leads to further political uncertainty.

A great new resource is being unveiled that will propel the wind power industry to a new dimension: The mega turbine. Remember, we all started with 100 kW systems, then grew to 660 kW turbines. Recently we’ve jumped from 1 MW to 2 MW, 2.5 MW and then 3 MW turbines on 90 meter towers. Say hello to the new age of mega turbines: 10 MW wind turbines on a 100 meter tower. This new machine has a rotor diameter of 150 meters.

One of the first 10 MW wind turbines is being developed by Clipper Windpower, Carpentaria, CA, and is in R&D development in Scotland. Called “Brittania” it is scheduled to debut in 2012. Dilip Khatri, senior structural engineer with URS, spoke with Jim Dehlsen, founder of Clipper Wind.

Q. What prompted the development of the 10MW Turbine?

A. The 10MW turbine is a natural progression from our previous turbine sizes and brings together “scalability” for wind farms. The total development, operations and maintenance cost for 10 wind towers remains relatively constant, irrespective of the size of turbine. So the larger the turbine, the more power is generated and the cost of energy production is reduced because of the spread of these operational and development costs.

Q. What market will this turbine serve and what is the expected capacity factor?

A. Primarily offshore and near-shore markets. We expect an average capacity factor of 40 percent, although this depends on the wind regime.

Q. What is one of the key innovations in the Brittania?

A. Distributed generation, which is an innovation stemming from the Liberty Turbine. The Brittania will have four generators so as to avoid any service interruption cost. When a single generator is down, another generator will pick up the power delivery demand. The turbine is equipped with an onboard crane system to provide onboard access for machinery, equipment repair and serviceability.

Q. What type of global certifications are you pursuing in the final product delivery?

A. We are obtaining both Germanischier Lloyd and DNV Certifications. This will give us global acceptance for a worldwide market.

Q. Tell us about the investment from United Technologies.

A. United Technologies has bought a 49 percent share in Clipper, approximately $249 million, and is now a key partner in our success. We are pleased to have United Technologies as our partner because they bring a wealth of technology experience in Sikorsky Helicopters, cranes, and an assortment of manufacturing expertise to our team.

Q. What about the tower and foundation infrastructure?

A. Clipper is still researching this technology and examining various design solutions for 100 meter towers in different water depths.

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.