The Power of Vertical Axis Wind Turbines

by Sara Goudarzi

The use of vertical-axis wind turbines could increase wind farm efficiency and cut costs, scientists suggest in a new study.

Wind farms are typically equipped with horizontal axis turbines (HAWTs)—where the propellers rotate around a horizontal axis. These types of turbines must be placed far apart from each other so the air each unit stirs up doesn't interfere with the adjacent units.

"If you put them close together, the efficiency of the individual turbines goes down," says study author, John Dabiri, a researcher from California Institute of Technology, Pasadena.

Because of this, HAWT's require generous land resources and must be placed at higher altitudes, both of which increase costs.

Dabiri investigated using counter-rotating vertical-axis wind turbines (VAWTs), propellers that rotate on a vertical axis and look like an eggbeater sticking out of the ground.

Vertical axis wind turbines were first developed in the early 20th century. Researchers soon realized that VAWTs offer some advantages, such as collecting wind from any direction. However, because each individual vertical axis unit was found to be less efficient than a horizontal axis wind turbine, VAWTs haven't yet found use in utility scale power plants. But as a collective, VAWTs could prove to be more efficient.

"Usually when people talk about efficiency, they're referring to the individual wind turbine—how much wind can an individual wind turbine convert to electricity?" Dabiri says. "We're talking about the efficiency of the wind farm, a group of turbines, and asking, of the wind that enters into the wind farm, how much of that total wind is converted to electricity?"

"So if your definition of efficiency is the wind farm efficiency, then this new approach, using vertical access turbine can be up to a factor of 10 times more efficient than horizontal access turbines," he adds.

And because groups of vertical axis wind turbines are so much more efficient than their horizontal axis counterparts, researchers believe they could get away with capturing the wind at lower altitudes.

"From an engineering perspective, it's easier to design, install and maintain a 30-foot tall turbine as opposed to a 300-foot tall turbine or many 30ft turbines as opposed to a few 300-foot tall turbines," Dabiri says.

Dabiri, whose research is detailed in the July 19 issue of the Journal of Renewable and Sustainable Energy, is currently testing the technology with 24 VAWTs on a wind farm in California. He hopes to soon assess a larger group of turbines.

"In wind energy, the question is always 'does this work if you had a big—hundred megawatt, five hundred megawatt—facility?'" he says.

To do that, he needs better vertical axis turbines. Currently, off the shelf units have been designed with the assumption that a steady wind is always hitting the blades. But because the units will be used as a group, the design has to be adjusted to account for interacting wind fields and forces.

"We're working on prototypes that we hope within the next couple of years will be available," Dabiri says.

Sara Goudarzi is a freelance writer based in New York City.
Her website is saragoudarzi.com

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