14 July 2011
Experimental wind-farm produces tenfold power increase
by Kate Melville
Caltech researchers say the power output of wind-farms can be increased by an order of magnitude - at least tenfold - simply by optimizing the placement of vertical wind turbines on a given plot of land. Details of the experimental wind-farm, located in northern Los Angeles County, appear in the Journal of Renewable and Sustainable Energy.
The experimental wind-farm houses two-dozen 1.2-meter-wide vertical-axis wind turbines (VAWTs). Vertical turbines that have rotors and look like eggbeaters sticking out of the ground. Each turbine is 10 meters tall.
Caltech's John Dabiri, who leads the research, said that despite improvements in the design of conventional propeller-type wind turbines, wind-farms remain inefficient. In such farms, the individual turbines have to be spaced far apart so they don't interfere aerodynamically with neighboring turbines, with the result that "much of the wind energy that enters a wind-farm is never tapped," says Dabiri.
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," Dabiri says, such as higher costs, more complex engineering problems and a larger environmental impact.
The solution, according to Dabiri, 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," he says. "The key is to ensure they're the right turbines, arranged in the right way."
VAWTs provide an important advantage in that 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 faster. Dabiri got the idea for using this type of constructive interference from his studies of schooling fish.
Dabiri measured the rotational speed and power generated by each of the six turbines when placed in a number of different configurations. One turbine was kept in a fixed position for every configuration; the others were on portable footings that allowed them to be shifted around.
The tests showed that an arrangement in which all of the turbines in an array were spaced four turbine diameters apart (roughly 5 meters) 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.
Impressively, the six VAWTs generated from 21 to 47 watts of power per square meter of land area; a comparably sized conventional wind-farm generates just 2 to 3 watts per square meter. The next steps, says Dabiri, "are to scale up the field demonstration and to improve upon the off-the-shelf wind-turbine designs used for the pilot study. I think these results are a compelling call for further research on alternatives to the wind-energy status quo."