Overcoming the challenges of wind energy, and learning how to make it safe and efficient

James Buchholz (right) with two members of his student research team, Craig Wojcik and Jordan Null, in the lab.

Can wind energy provide 20 percent of U.S. electricity needs by 2030? That’s the goal set by U.S. Department of Energy in 2007. More than five years later, a great many questions still need to be answered to make wind turbines work better, more efficiently, and more safely.

A team of researchers at Iowa’s three public universities is working to help make Iowa a major player in energy-related research, with support from a $20 million, five-year grant from the National Science Foundation. The grant will help build Iowa’s research capacity in renewable energy and energy efficiency, with four different focus areas: bioenergy; wind energy; energy utilization; and energy policy.

The grant is part of NSF’s Experimental Program to Stimulate Competitive Research (EPSCoR), which is designed to improve the research capacity of eligible states and help them become more competitive for future grants.

IIHR Assistant Research Engineer James Buchholz is focusing on wind turbine blade reliability and performance (part of the wind energy focus area). He explains that many uncertainties persist about wind turbines loads — wind speed and direction can change abruptly, and both are difficult to predict with accuracy. These uncertainties can bring about very dramatic results.

“Turbines have fallen down for no apparent reason,” Buchholz says. “Blades will break, towers will collapse, drivetrains can fail. It can be dangerous and costly.”

Dynamic Stall

Buchholz, who is also an assistant professor of mechanical and industrial engineering, is particularly interested in the aerodynamics of the blades. Difficulties often begin, he says, when air flow separates from the surface of the blade instead of following it closely. What results is a phenomenon called dynamic stall, which can lead to very high unsteady forces and moments on the blade, due to the strong vortices that are shed. These loads are difficult to predict, Buchholz explains, and can be detrimental to the blades and the entire wind turbine structure.

The rotation of the blades further complicates the problem, since it causes enhanced resistance to flow separation and increased stability of the vortices created by the blade – a phenomenon called rotational augmentation.  Surprisingly, this phenomenon is also found at small scales, such as bird and insect flight, a topic of previous and ongoing research for Buchholz. Investigators want to understand the vortex structures formed in the air flow around wind turbine blades and flapping wings, with the goal of predicting how and when they occur, where they will go, and how strong they are.

With financial support from the Iowa Power Fund, Buchholz is designing a closed-loop recirculating wind tunnel to be constructed in one of the IIHR Oakdale Annexes to allow the team to further study these phenomena. It’s not feasible to build a wind tunnel large enough to study a utility-scale wind turbine, so Buchholz is designing a scale model.

Researchers can compensate for the reduction in size by making the turbine turn faster — within limits. To actually spin fast enough, the turbine would approach the speed of sound, causing still more complications. The wind tunnel model will not yield performance measurements that can be scaled to utility-scale turbines, but they are expected to go a long way to improve our understanding of the fundamental mechanisms governing real aerodynamic loads.

Salad Dressing, or Science?

Buchholz says the model study will include the use of particle image velocimetry (PIV) to visualize the air flow in the wind tunnel. A key ingredient in the PIV process is actually more commonly used in salad dressing: olive oil. The scientists will build an atomizer to generate very small particles of olive oil, which follow the flow of air in the wind tunnel. A laser sheet will reflect and scatter light off the oil particles, allowing researchers to see the particles and track their motion.

It’s inexpensive and very effective, Buchholz says. “A bottle of olive oil will last for several PhD students.”

The long-term goal is to improve the accuracy of the aerodynamic design codes to predict wind turbine performance and loads. “Once we have a good handle on predicting the formation and evolution of the unsteady flow structures or vortices,” he says, “then I think we can address the question of how to optimize the blades in a new way.”

After Fossil Fuels, What Then?

Despite their great size, utility-scale wind turbines are highly engineered and graceful in design, with all the precision of a Swiss watch. “It’s quite an elegant piece of machinery,” Buchholz says.

Buchholz remains excited by the prospects of wind energy. There are challenges ahead, and many of them, he freely admits. But wind turbines offer a clean source of energy in a world where fossil fuels will eventually be used up.

“We can’t stop doing things because there are challenges,” Buchholz says. “We have to overcome those things, and figure out how to do this better.”