29 September 2011—Think about wind energy and chances are that fish and whales aren’t the first things that pop into your head. But the marine environment is just where some researchers are looking for inspiration to improve wind energy. They’re applying the concepts of biomimicry—using nature’s designs in their own—and it’s working, as evidenced by big strides in efficiency and output.
Janine Benyus, an expert on biomimicry who has been writing, consulting, and teaching on the topic for more than a decade, says that aping nature’s designs generally occurs on three levels. We can mimic form (the shape of a plant or an animal), we can mimic a process, and we can mimic an ecosystem ("like having a city that functions as well as a forest," says Benyus).
With wind power, there are opportunities across that spectrum, and they are helping to address some of the biggest problems in the field: turbine efficiencies and space requirements.
The efficiency of wind turbines suffers largely because of the variability of wind speeds. At higher speeds, most new turbines function well, but just as an airplane loses lift as its air speed drops past a certain point, turbine blades stall when the wind speed drops. In order to handle a variety of wind speeds without stalling, most turbine blades are positioned well below the ideal angle for generating power. If there was a way to tilt those blades so that more of the wind’s energy could be captured—in effect, increasing the angle of attack of the wind on the blade—the output of the turbine could be increased.
Enter humpback whales. These massive mammals have small bumps known as tubercles along the leading edge of their fins. According to West Chester University biology professor Frank Fish, adding those tubercles to the edge of a turbine blade can provide the needed improvement in the angle of attack by delaying stall, and they do it without causing a severe increase in drag. For whales, this means improved efficiency in swimming; for wind turbines, it means more power. And this isn’t just theoretical: Fish helped start a company called WhalePower that is marketing the technology.
"You can modulate the pitch of your windmill so that it operates at higher angles of attack without fear of stalling," Fish says. In other words, by tilting the blades farther in relation to the wind angle, turbines can generate more power without the usual risk of a stalled blade causing an asymmetry and the turbine "shaking itself to death." As an added bonus, the tubercle turbines could potentially reduce a noise problem known as tip chatter, in which stalling at the blade’s tip causes a vibration.
What’s more, the tubercled turbine is more efficient. Using a 35-kilowatt test windmill (large, utility-scale wind turbines have a generating capacity of around 5 megawatts), researchers found that adding tubercles to standard turbine blades bumped up the efficiency by 20 percent. That is, the same amount of wind produced 20 percent more electricity on a windmill with tubercles than without.
So far, WhalePower is selling tubercle technology for industrial ventilation fans but not yet for mass-produced wind turbines.
Nature’s designs are also applicable further up Benyus’s three-level biomimicry scale. Somewhere between the function and ecosystem level lies an ongoing project led by John Dabiri, a professor of aeronautics and bioengineering at Caltech. Dabiri wondered if there could be a way to improve the spacing requirements of wind turbines, so he started looking at fish.
Standard horizontal-axis wind turbines create a lot of turbulence just behind the spinning blades. As a result, the turbines have to be fairly dramatically spaced out—10 turbine diameters apart or more. This means that large wind farms take up enormous amounts of space, and whether the target is otherwise pristine public lands or areas that encroach on homes and agricultural land, the space requirements tend to run afoul of wind opponents.
Dabiri’s group takes advantage of a totally different turbine design—the vertical-axis turbine. These already have substantially smaller spacing requirements than their horizontal-axis counterparts, but they do still create turbulence behind them. The biomimetic idea involves copying how schooling fish take advantage of eddies in the water caused by the other fish in the school and applying it to the specific spacing of the vertical-axis turbines.
Mathematical modeling suggested a tenfold improvement in power output by copying the fish positioning—high enough to warrant testing the theory with a real wind farm. Caltech purchased a plot of land now known as the FLOWE laboratory (for Field Laboratory for Optimized Wind Energy). Using actual turbines, Dabiri’s team confirmed the impressive upgrade in output when the fish-schooling model was used. An ideal configuration of a six-turbine test involved placing the turbines about four turbine diameters apart and allowed for an output of between 21 and 47 watts per square meter; a horizontal-axis wind farm of a similar size would yield no more than 3 W/m2.
"One of the principal things that comes out of the bioinspiration is that the vortices that the fish create don’t all rotate in the same direction," Dabiri says. The researchers decided to have the vertical axis wind turbines rotate in varying directions within the wind farm, instead of uniformly in one direction. "It turns out to have a real impact on the [output]."
Dabiri’s group plans to scale up the FLOWE tests to larger wind farms; after all, most schools of fish have more than six members. He points out, though, that biomimetics can take us only so far with wind power, or really any other field. The real issue with copying nature, to this point, is materials.
"Most natural systems have muscle and flexibility that is very difficult to replicate using engineering materials," Dabiri says. "If the materials were available, we’d be much further along. That’s the rate-limiting step."
And biologically inspired materials may follow soon enough. Benyus says that in her experience, many more technology and engineering fields have started thinking along biomimetic lines, and the WhalePower and FLOWE examples show how there are substantial gains to be made. "In the natural world, you have 3.8 billion years of reducing friction and drag…and taking advantage of free energy sources in the environment. Those same selection pressures are bearing down on us," Benyus says. "It’s the perfect opportunity for biomimicry."
About the Author
Dave Levitan is a science journalist who contributes regularly to IEEE Spectrum’s Energywise blog. In June he reviewed The Flooded Earth: Our Future in a World Without Ice Caps by Peter D. Ward.