Although wind power energy production in 2010 was estimated to be only about 2.5 percent of worldwide electricity usage, wind turbines are considered a mature technology with many experts suggesting that we're approaching the theoretical limit of individual wind turbine efficiency. For this reason, researchers are now looking at new approaches to wind farm design to increase the power output of wind farms. Researchers at the California Institute of Technology (Caltech) have been conducting a field study and claim the power output of wind farms can be increased at least tenfold by optimizing the placement of turbines on a given plot of land.

While most wind farms employing horizontal-axis wind turbines (HAWTs) - the standard propeller-like turbines most commonly found in wind farms around the world - space the individual turbines around seven rotor diameters apart, a recent study found that spacing of at least 15 rotor diameters apart produced the most cost-efficient power generation. But even though spreading the turbines out increases the cost-efficiencies by allowing for fewer individual turbines, it also lowers the power output of a given plot of land.

To compensate for the energy loss resulting from the wake generated from one turbine interfering with neighboring turbines, HAWT wind farms also resort to using bigger blades and taller towers that are capable of taking advantage of the more powerful gusts of wind found at greater heights. But these larger structures result in increased production and maintenance costs, visual, acoustic, and radar signatures problems, as well as more bat and bird impacts.

In an attempt to develop a more efficient wind farm design that maximizes its energy-collecting efficiency at heights closer to the ground, John Dabiri, Caltech professor of aeronautics and bioengineering, and his colleagues turned to vertical-axis wind turbines (VAWTs) - turbines like the Windspire that have vertical rotors and look like eggbeaters sticking out of the ground. VAWTs aren't more prominently used today because they tend to be less efficient individually and previous generations suffered from structural failures relating to fatigue.

But Dabiri says, "advances in materials and in predicting aerodynamic loads have led to new designs that are better equipped to withstand fatigue loads." Additionally, because VAWTs can be positioned very close together they are able to capture nearly all the energy of the blowing wind, and so the lower efficiency of individual turbines is not as much of an issue.

Dabiri carried out field tests in the summer of 2010 at an experimental farm known as the Field Laboratory for Optimized Wind Energy (FLOWE), which houses 24 10-meter-tall, 1.2-meter-wide VAWTs. In the field tests, which used six VAWTs, Dabiri and his colleagues measured the rotational speed and power generated by each of the turbines when placed in a number of different configurations. One turbine was kept in a fixed position for every configuration, while the others were on portable footings that allowed them to be shifted around.

They found that the aerodynamic interference between neighboring turbines was completely eliminated when all the turbines in an array were spaced four turbine diameters (roughly five meters or 16 feet) apart. In comparison, propeller-style HAWTs would need to be spaced 20 rotor diameters apart - which equates to a distance of more than one mile for the largest wind turbines currently in use - for the aerodynamic interference to be eliminated.

The six VAWTs generated from 21 to 47 watts of power per square meter of land area, while a comparably sized HAWT farm generates just two to three watts per square meter.

"Our results are a compelling call for further research on alternatives to the wind energy status quo," Dabiri says. "Since the basic unit of power generation in this approach is smaller, the scaling of the physical forces involved predicts that turbines in our wind farms can be built using less expensive materials, manufacturing processes, and maintenance than is possible with current wind turbines."

The researchers plan to scale up their field demonstration and are looking to improve upon the off-the-shelf wind turbine designs used for their field study.

The findings of the field study conducted by Dabiri and his Caltech colleagues appear in the July issue of the Journal of Renewable and Sustainable Energy.

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