The power output of wind farms can be increased by an order of magnitude—at least tenfold—simply by optimizing the placement of turbines on a given plot of land.
The results of field tests conducted by John Dabiri, Caltech professor of aeronautics and bioengineering —appears in the July issue of the Journal of Renewable and Sustainable Energy.
Dabiri’s experimental farm, known as the Field Laboratory for Optimized Wind Energy (FLOWE), houses 24 10-meter-tall, 1.2-meter-wide vertical-axis wind turbines (VAWTs)—turbines that have vertical rotors and look like eggbeaters sticking out of the ground.
Modern farms generally employ horizontal-axis wind turbines (HAWTs)—the standard propeller-like monoliths.
In such farms, the individual turbines have to be spaced far apart—not just far enough that their giant blades don’t touch. With this type of design, the wake generated by one turbine can interfere aerodynamically with neighboring turbines, with the result that “much of the wind energy that enters a wind farm is never tapped
Designers compensate for the energy loss by making bigger blades and taller towers, to suck up more of the available wind and at heights where gusts are more powerful. But this brings other challenges, such as higher costs, more complex engineering problems, a larger environmental impact. Bigger, taller turbines, after all, mean more noise, more danger to birds and bats, and—for those who don’t find the spinning spires visually appealing—an even larger eyesore.
The solution is to focus instead on the design of the wind farm itself, to maximize its energy-collecting efficiency at heights closer to the ground. While winds blow far less energetically at, say, 30 feet off the ground than at 100 feet, the global wind power available 30 feet off the ground is greater than the world’s electricity usage, several times over. That means that 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.
VAWTs are ideal, because they can be positioned very close to one another. This lets them capture nearly all of the energy of the blowing wind and even wind energy above the farm. Having every turbine turn in the opposite direction of its neighbors, the researchers found, also increases their efficiency, perhaps because the opposing spins decrease the drag on each turbine, allowing it to spin
The tests showed that an arrangement in which all of the turbines in an array were spaced four turbine diameters apart (roughly 5 meters, or approximately 16 feet) completely eliminated the aerodynamic interference between neighboring turbines. By comparison, removing the aerodynamic interference between propeller-style wind turbines would require spacing them about 20 diameters apart, which means a distance of more than one mile between the largest wind turbines now in use.
The six VAWTs generated from 21 to 47 watts of power per square meter of land area; a comparably sized HAWT farm generates just 2 to 3 watts per square meter.