Turbine placement can boost power
The Field Laboratory for Optimized Wind Energy (FLOWE), an experimental farm, houses 24 10-meter-tall, 1.2-meter-wide vertical-axis wind turbines (VAWTs), essentially turbines with vertical rotors that look like eggbeaters sticking out of the ground, said John Dabiri, California Institute of Technology (Caltech) professor of aeronautics and bioengineering. He used six turbines in the 2010 field tests.
Despite improvements in the design of wind turbines that have increased their efficiency, wind farms are rather inefficient, Dabiri said. Modern farms generally employ horizontal-axis wind turbines (HAWTs), which are the standard propeller-like monoliths that you might see slowly turning, all in the same direction, in the hills of Tehachapi Pass, north of Los Angeles.
In such farms, individual turbines must have significant space between them. 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,” Dabiri said. He compares modern farms to “sloppy eaters,” wasting not just real estate (and thus lowering the power output of a given plot of land) but much of the energy resources they have available to them.
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 said, 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, Dabiri said. While winds blow far less energetically at 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 said. That means you can obtain enough energy with smaller, cheaper, less environmentally intrusive turbines — as long as they’re the right turbines, arranged in the right way.
VAWTs work because you can position them very close to one another, Dabiri said. 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.
In the summer 2010 field tests, Dabiri and his colleagues measured the rotational speed and power generated by each of the six turbines placed in a number of different configurations. One turbine stayed in a fixed position for every configuration; the others were on portable footings that allowed them to shift around.
The tests showed an arrangement in which all of the turbines in an array, 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.
“Dabiri’s bioinspired engineering research is challenging the status quo in wind-energy technology,” said Ares Rosakis, chair of Caltech’s Division of Engineering and Applied Science and the Theodore von Kármán Professor of Aeronautics and professor of mechanical engineering.
“We’re on the right track, but this is by no means ‘mission accomplished,’” Dabiri said. “The next steps 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.”
– Edited by Chris Vavra, Plant Engineering, www.plantengineering.com