JESSICA MARSHALL, Discovery.com News, November 30, 2009
“For the fish, they are trying to minimize the energy that they consume to swim from Point A to Point B,” said John Dabiri of the California Institute of Technology in Pasadena, who led the study. “In our case, we’re looking at the opposite problem: How to we maximize the amount of energy that we collect?”
“Because both of these problems involve optimizing energy, it turns out that the model that’s useful for one is also useful for the other problem.”
Both designs rely on individuals capturing energy from their neighbors to operate more efficiently.”If there was just one fish swimming, it kicks off energy into the water, and it just gets wasted,” Dabiri said, “but if there’s another fish behind, it can actually use that kinetic energy and help it propel itself forward.”
The wind turbines can do the same thing. Dabiri’s wind farm design uses wind turbines that are oriented to rotate around the support pole like a carousel, instead of twirling like a pinwheel the way typical wind turbines do.
Like the fish, these spinning turbines generate a swirling wake. The energy in this flow can be gathered by neighboring turbines if they are placed close enough together and in the right position. By capturing this wake, two turbines close together can generate more power than each acting alone.
This contrasts with common, pinwheel-style wind turbines where the wake from one interferes with its neighbors, reducing the neighbors’ efficiency. The vortexes occur in the wrong orientation for the neighboring turbines to capture them.
For this reason, such turbines must be spaced at least three diameters to either side and 10 diameters up — or downwind of another, which requires a lot of land.
Although individual carousel-style turbines are less efficient than their pinwheel-style counterparts, the close spacing that enhances their performance means that the amount of power output per acre is much greater for the carousel-style turbines.
Dabiri and graduate student Robert Whittlesey calculated that their best design would generate 100 times more power per acre than a conventional wind farm.
The model required some simplifications, however, so it remains to be seen whether tests of an actual wind farm produce such large gains. That will be the team’s next step. “Even if we’re off by a factor of 10, that’s still a game changer for the technology,” Dabiri noted.
In the end, schooling fish may not have the perfect arrangement. The pair found that the best arrangement of wind turbines did not match the spacing used by schooling fish.
“If we just mimic the fish wake, we can do pretty well,” Dabiri said. “But, as engineers, maybe we’re smarter than fish. It turns out that for this application there is even better performance to be had.”
This may be because fish have other needs to balance in their schooling behavior besides maximizing swimming efficiency. They seek food, avoid predators and reproduce, for example.
“I think that this is a very interesting possibility,” said Alexander Smits of Princeton University, who attended a presentation of the findings at a meeting of the American Physical Society Division of Fluid Dynamics in Minneapolis last week.
But a field test will show the idea’s real potential, he noted: “You have to go try these things. You can do a calculation like that and it might not work out. But it seemed like there was a very large reduction in the land usage, and even if you got one half of that, that would be pretty good.”