Vertical axis wind turbines. Credit John Dabiri/Caltech.

Wind farm power output can be increased at least tenfold by optimizing how turbines are placed on a given plot of land, say researchers at the California Institute of Technology, who have been conducting a field study at an experimental two-acre wind farm in northern Los Angeles County.

A paper describing the findings—the results of field tests conducted by John Dabiri, Caltech professor of aeronautics and bioengineering, and colleagues during the summer of 2010—appears in the July issue of the Journal of Renewable and Sustainable Energy.

Dabiri's Field Laboratory for Optimized Wind Energy houses 24 10-meter-tall, 1.2-meter-wide vertical-axis wind turbines. The turbines' vertical rotors look something like eggbeaters sticking out of the ground. Half a dozen turbines were used in the 2010 field tests.

Improvements in wind turbine design have increased their efficiency, but wind farms remain rather inefficient, Dabiri said. Modern farms generally use horizontal-axis wind turbines.

In such farms, turbines have to be spaced far apart. With horizontal turbine design, the wake generated by one turbine can interfere aerodynamically with neighboring turbines. The result is that much of the wind energy that enters a wind farm is never tapped. Dabiri said this is akin to "sloppy eaters" and wastes not just real estate but much of the energy resources.

Designers compensate for the energy loss by making bigger blades and taller towers. "But this brings other challenges," Dabiri said, such as higher costs, more complex engineering and a larger environmental impact.

The solution, said Dabiri, is to focus on the wind farm design itself, to maximize its energy-collecting efficiency closer to the ground. He said enough energy can be obtained with smaller, cheaper, less environmentally intrusive turbines as long as they're the right turbines, arranged in the right way.

Vertical axis turbines are ideal, Dabiri said, because they can be positioned close to one another. This lets them capture nearly all of the wind's energy even from above the farm. Having every turbine turn in the opposite direction of its neighbors also increases their efficiency.

In the 2010 field tests, Dabiri and his colleagues measured the rotational speed and power generated by each of the six turbines when placed in different configurations. One turbine was kept in a fixed position for every configuration; the others were on portable footings and could be moved.

The tests showed that an arrangement in which all of the turbines in an array were spaced four turbine diameters apart (roughly 5 meters) eliminated the aerodynamic interference between neighboring turbines. By comparison, removing the aerodynamic interference between propeller-style horizontal wind turbines would require spacing them about 20 diameters apart, a distance of more than one mile between the largest wind turbines now in use.

The six vertical axis turbines generated from 21 to 47 watts of power per square meter of land area; a comparably sized horizontal axis turbine farm generates 2 to 3 watts per square meter.

Dabiri said the next steps are to scale up the field demonstration and improve upon the off-the-shelf wind-turbine designs used for the pilot study.

The world's turbine manufacturers, who have built markets based on horizontal turbine designs, presumably also will need convincing.