December 7, 2011

By Dan Haugen

K.K. Choi has spent much of his research career studying how subtle design changes affect the durability of large military vehicles.

Now, the University of Iowa mechanical engineering professor is working with the wind industry to design a tougher turbine.

Improving wind turbine reliability has become a national priority in recent years. The U.S. Department of Energy highlighted reliability in its July 2008 report on increasing wind generation to 20 percent of the nation’s electricity supply by 2030.

If the nation is to reach that goal, it’s believed that the number of costly, unplanned repairs to wind turbines will need to be reduced, which will bring down the overall cost of wind power.

“To beat or to be competitive with fossil-fuel based energy sources, the biggest challenge in wind energy is reliability,” said Choi, whose research revolves around fatigue analysis using computer modeling technology.

Choi’s computer models have been applied to everything from the Ford Taurus to Stryker tanks. A tool he developed for the U.S. Army predicts how slight design changes would affect the cost, durability and reliability of vehicles and their components.

The modeling tool created by Choi is more advanced than others because it takes into account uncertainties such as tiny variations in the materials and manufacturing precision, as well as the conditions in which the vehicles are driven.

The process is called reliability-based design optimization. In the Army’s case, Choi identified a change to a Stryker tank part that made it ten times more durable and 20 percent lighter at the same time. The military is considering whether to incorporate the new part.

Tanks to turbines

The leap from tanks to turbines isn’t as big as it might seem.

The military and wind industry both need reliable equipment that’s going to hold up under extreme conditions, whether it’s a desert battle zone or the blustery lower atmosphere. It’s also important that it’s not heavier or more expensive than it needs to be.

“While the components may look different, obviously, and serve different purposes, inherently they have the same technical challenges,” said University of Iowa Provost Barry Butler.

Butler is head of the Iowa Alliance for Wind Innovation and Novel Development (IAWIND), a multi-university, public-private research partnership, and he’s also the one who first connected the dots between Choi’s work and the wind industry.

After attending a wind reliability conference a few years ago, Butler asked Choi about his vehicle reliability tools and concluded they were a “natural fit” for turbine research. Butler approached Clipper Windpower about partnering on a project, and soon they were on a plane to the manufacturer’s California headquarters.

“They spent an entire day with us and gave us time to present, and they presented what their challenges are, and that’s when I think everybody around the table started realizing there’s some connectivity here,” Butler said.

How thick? How precise?

K.K. Choi

What’s unique about the type of computer modeling Choi does is that it helps judge how precise the manufacturing process needs to be in order to achieve reliability gains.

“If you make 10,000 blades, none of them are identical,” said Choi. Same goes for cars. They’re not exactly snowflakes, but even two vehicles of the same year, make and model will have slight differences — fractions of millimeters here and there.

It’s a way for car makers to keep their costs down. They can get a better price on sheet metal, for example, if it just has to be roughly one millimeter thick instead of precisely one millimeter.

A tool like Choi’s can help automakers calculate the ideal thickness of a piece, but also how much leeway is acceptable before it starts to noticeably affect the reliability results.

Choi is working on translating the tool to work with turbines. IAWIND awarded him a three-year, $300,000 research grant, which is being matched by Clipper Windpower. The goal is to create a design that doesn’t cost more but requires substantially less maintenance.

“What happens is in lots of cases is we over-design, which means that our products cost more than they should, which increases the cost of energy,” said Clipper engineering manager Rob Budny. “In some cases we under-design, which means that the product is not reliable and that you have large warranty costs.”

Clipper is asking Choi to design a new rotor hub and suggest changes to its blades that it can compare to its existing ones. And the new parts can’t cost more to manufacture.

“What I’m saying is: Guys, we can make it even cheaper, and yet improve reliability,” Choi said.

And that may lead to turbines that are as tough as tanks.

Dan Haugen is an Energy Journalism Fellow at Midwest Energy News. Contact him at

Photo by Christiaan Conover via Creative Commons

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