The movements of fish and birds inspire wind power generation improvement
In 2008, John Dabiri, now a professor at Stanford but then at Caltech, was teaching a course on the mechanics of animal swimming and flight when inspiration hit. The lesson of the day was on collective behavior – groups of fish and birds moving in unison. Dabiri wondered whether such behaviors might apply in other fields.
“I was looking for a way to get into renewable energy and thought that I might use this knowledge to improve wind farms,” Dabiri recalls, adding: “Groups of animals have this whole-is-greater-than-the-parts effect where animals in front create air or water currents that make it easier for those behind to fly or swim. These currents actually pull them along.”
Examples are familiar enough in nature. A diamond-shaped school of fish slices through the sea. A V-shaped flock of geese makes its way south for the winter. The phenomenon is even familiar to fans of bicycle and auto racing. The leader cuts through the wind and those behind in the slipstream have an easier go of it.
In the years since his epiphany, Dabiri has nurtured his brainstorm and today he finds his work the object of much attention. Recently, he received a $2 million grant from the Gordon and Betty Moore Foundation to install a prototype wind farm in the lakeside village of Igiugig, in rural Alaska.
Igiugig (pronounced Ig-ee-Aw-gig) is a perfect test bed for a renewable energy project like this. The community currently powers its electrical grid with diesel generators whose fuel must be flown in at exorbitant expense. The cost of electricity in Igiugig is as much as five to six times that in the continental United States.
His wind farm won’t have to bear the entire power burden, Dabiri says, but will be complemented by a hydrokinetic generation system and supplemented by battery storage units. The long-term goal is for Igiugig – and eventually other remote villages across the world – to power itself exclusively with round-the-clock, round-the-calendar electricity from renewable sources of energy.
The wind farm Dabiri is planning for Igiugig will not be the variety most might envision when they think of wind energy with 300-foot-tall propeller-like turbines. Rather, he’s chosen to use much shorter 30-foot vertical axis turbines, whose blades resemble those of a wheat harvester turned on its side.
Visually speaking, vertical axis turbines are much less imposing, much less disruptive to the sort of pristine wilderness the Igiugigians call home. While appearance is an important consideration, vertical axis turbines have other advantages, too, Dabiri says.
“You don’t need to point them in the direction of the oncoming wind. Plus, they’re a lot simpler to operate and less expensive to fix, because the generator and components are a lot closer to the ground,” he says.
Despite these obvious advantages, vertical axis turbines have fallen out of favor due to their relatively low efficiency and the complex turbulence patterns they create.
Regardless of whether wind farms opt for horizontal or vertical turbines, all face a fundamental challenge: The windward turbines sap energy from the wind, reducing the efficiency of downstream turbines by as much as 40 percent. To maximize efficiency, most modern wind farms must place turbines quite far apart and come to dominate large tracts of land in the process.
This design conundrum is what Dabiri hopes to solve by turning to birds and fish. Though his Alaska project is still a work in progress, Dabiri’s team has tested a handful of turbines to see if they would survive the harsh Alaskan winter. This coming summer, they will set up a half-dozen turbines. Eventually, Dabiri hopes to install 30 or more turbines, all strategically placed according to his bioinspired models to gain every advantage the wind has to offer.
The science of turbulence is known as fluid mechanics, and the physics behind animal swimming and flight has been studied using concepts from fluid mechanics for decades. Each wash of a fish tail, each beat of a goose’s wings creates disturbances in the water or air – the fluid – through which the subject is moving. This causes the trailing water or air to spin off in small eddies, known as vortices.
The trailing fish and geese benefit first by not having to cut through undisturbed fluid. They not only experience less friction, but, if they are positioned just right, they actually find the swimming or flying easier as they are pulled along by the eddies. The swirling vortices in the water create an additional current that moves the fish (or the geese in the air) forward.
“At its basic level, fluid mechanics models show how, with an individual turbine, the air directly behind it may be slower because the turbine takes energy out of the air. In certain areas of the flow, however, the wind speed actually increases and that’s the spot where you want to place other turbines. They feed off one another,” Dabiri explains.
Searching for the ideal
As the field is still in its nascence, the ideal arrangement of a wind farm remains a matter of ongoing study, according to Dabiri. The ultimate goal is to reduce the cost of wind energy, which can be achieved in part by getting more power from a given area of land.
His initial designs were based on the diamond-shaped patterns common in schools of fish, but he and others in the field have since branched out. One layout pairs turbines that spin in opposite directions. Another arranges turbines in triangular triplets, all spinning in the same direction. Yet another arranges the turbines in a fractal pattern that generates very good results, according to Dabiri.
“There’s still a lot of work to be done. We’re just beginning to scratch the surface here,” he says.
Helping him along the way are Stanford Engineering faculty members Ram Rajagopal, Arun Majumdar and Catherine Gorle, who are working on energy-related solutions, including smart grid concepts, particularly in urban environs where large turbines won’t work.
Another close collaborator is Sanjiva Lele, who is an expert in computational modeling of turbulence. Lele is best known for modeling the noisy turbulence of jet engines, but has since brought his expertise to improve the design of wind farms. The collaborative, multidisciplinary spirit is one thing that attracted Dabiri to Stanford.
“Stanford has a strong expertise in computational science and other disciplines that are complementary to our theoretical and experimental research,” he said. “It’s really important to have these partnerships.
“For example, Sanjiva and I are looking at optimizing wind farms that integrate conventional tall turbines with new, shorter designs, using a combination of computational tools developed in his lab and new techniques we’ve created to conduct flow measurements at our field sites. The need for wind farm optimization will become more pronounced going forward as farms grow in size. Having that expertise so close by is a huge asset.”