A Model of Efficiency
By David Brill
Standing at the base of an operating wind turbine, most of us would be awestruck by its immensity, looming nearly 400 feet above with its 150-foot blades rotating slowly in the breeze. But to Kivanc Ekici, this behemoth represents a towering opportunity for improvement.
Ekici, an assistant professor in UT’s Department of Mechanical, Aerospace, and Biomedical Engineering, is intent on making turbines less prone to failure and capable of wringing ever more energy from the wind. He believes new designs for turbine blades hold the key to those gains.
Turbine blades—a composite of fiberglass, carbon fiber, and epoxy resin—are sculpted, much like the wings of an aircraft, to create lift, which causes the rotor to spin as the wind nudges the blades. Viewed from the distance of a mile or two, a gently revolving turbine may appear to be the picture of serenity and calm.
But the wind itself is a turbulent swirl of eddies that are constantly in flux, as is the direction of the wind, which forces the turbine to constantly yaw from side to side to face windward. Winds near the ground are roiled by friction with the earth’s surface, while those higher aloft tend to be less chaotic, which subjects turbine blades to varying forces depending on their point in rotation. These combined forces act in particular on the turbine blades, which bend, twist, and vibrate under the wind’s assault. These unwanted motions can put undue stress on the blades and can cause them to fail.
A wind turbine idled by a blade failure can quickly become a $3- to $4-million white elephant. Cumulative annual losses across the nation’s entire wind-power network due to unplanned shutdowns measure in the billions of watts.
“In theory, wind turbines are designed to operate for years without much upkeep,” Ekici says. “But unplanned maintenance and unscheduled shutdowns remain significant problems.”
Before the dawn of the computer age, Ekici’s efforts would have relied on a slow, iterative march toward optimization based on a wind tunnel, construction of prototypes, and a time-consuming process of trial and error to assess the effects of fluid flow—yes, air is a fluid—over the blades.
Today’s high-speed computers have dramatically eased the design process through a discipline known as CFD, or computational fluid dynamics. CFD allows Ekici and other engineers to create virtual blades that display aerodynamic properties, chiefly lift and drag, and that also vibrate and deform as the wind flows around them.
Traditional CFD techniques calculate, in tiny time increments, aerodynamic effects of wind turbulence at thousands of test points on the computerized grid representing the turbine blade. These techniques can be highly accurate in evaluating the effects of design changes on blade performance, but they also come at “extremely high computational costs,” according to Ekici.
For instance, using a time-accurate CFD technique to model the effects of structural fatigue on a blade over two months of operation might take two months of processing time, making the technique impractical for most applications. To avoid this dilemma, Ekici is developing a new suite of software tools that may result in a 10- to 100-fold reduction in the time required to optimize blade design. Instead of solving the equations that model the fluid flow in the time domain, Ekici’s tools focus on solution techniques in the frequency domain, allowing measurement over much larger time increments. “If we’re considering some kind of vibration of the blade, the frequency would correspond to that of the vibration,” Ekici explains.
Data produced by these calculations will lead to better blade designs at a fraction of the computational time.
Stretching the Limits
Ekici’s advancements will not only bolster turbine durability and reliability, but they’ll also improve efficiency, as blades and turbine structures continue to grow in scale and operate at higher wind speeds.
Modern turbines are relatively efficient, extracting about 47 percent of the energy carried by the wind, but they fall short of the theoretical top-end efficiency of about 59 percent, a threshold established by German physicist Albert Betz. The strategy to reduce that 12 percent efficiency gap consists of increasing some other numbers.
As the diameter of wind turbine blades—known collectively as blade sweep—doubles, available energy increases by a factor of eight. Intent on capitalizing on that energy boost, manufacturers are steadily increasing the size of their turbines. The world’s largest currently stands 720 feet tall with 260-foot blades.
Most modern turbines are designed to shut down at wind speeds in excess of 55 miles an hour. Turbines capable of operating in higher wind speeds promise significant gains in efficiency. But as turbine size and operational wind speeds increase, so do the aerodynamic forces that bluster the blades. Designs produced by Ekici’s computational tools will lead to configurations better able to withstand those forces.
Ekici’s computer-based work may occupy him miles from the nearest operational wind turbines—TVA’s eighteen units on Buffalo Mountain near Oak Ridge, Tennessee. Nevertheless, the imprint of his research will likely be manifest in the future silhouettes of enormous rotor blades that, one sweep at a time, will power our homes and propel our nation toward increased reliance on an energy resource that’s clean, renewable, and affordable.