Mechanical 'fish' could tap turbulence for energy

Devices that harvest energy from swirling wakes surrounding buildings are being developed by US researchers. Their novel designs – inspired by fish – could generate electricity using eddies, something that conventional turbines cannot do.

Fish use their bodies to get an energy boost from surrounding vortices, which may be created by other fish in the same shoal, or by stationary objects in the water. But this kind of turbulent flow cannot be used by conventional wind or water turbines, which instead need a steady flow.

John Dabiri at the California Institute of Technology, Pasadena, US, realised that it might be possible for a mechanical system to extract energy from vortex wakes and has developed a mathematical model, based on the way fish move, to help put this into practice.

"In fish we have a working example of a system that extracts energy from vortices, now we need to work out how to build our own," he says.

Lateral drift

Fish move from side to side in order to exploit the way wakes in flowing water produce vortices that alternately spin clockwise and anticlockwise, as shown in the videos on this page.

For a mechanical device to pick up energy from an eddy, Dabiri's model shows that it must also change its angle in a similar way to a fish, to pick up as much energy as possible.

"We need a device that can be aware of what is happening upstream and position itself relative to incoming flows accordingly," says Dabiri.

"To exploit this we [also] need to find locations with naturally occurring fluctuations," says Dabiri. "This is exactly the situation in urban environments." Buildings constantly produce turbulent wakes making many rooftops unsuitable for normal wind turbines, but perfect for harvesting eddies.

Dabiri and colleagues have begun designing prototypes devices for use in the air and in the water. "They look less like a fish than you might think – we aren't trying to copy them directly, just to uncover and use the underlying dynamics of what they do."

Slow but steady

Once operational such devices will generate less power than a normal wind turbine in full flow, but should still compete over the long term, Dabiri says. "Turbines need wind to get over about 10 metres per second to work," he says. "But we should be able to extract energy all the time. It's like the tortoise and the hare." Over about a year, Dabiri says, the two may harness the same total energy.

James Liao a biologist at Cornell University, US, first demonstrated in 2003 that fish in shoals use vortices to save energy, something long suspected.

Liao says Dabiri's model will help to push the concept forward. "It could make this idea accessible to a wider audience," he says. Dabiri's model is a simplified version, he adds, but a full model of the fluid dynamics involved is impractical, because of the complexity of modelling both a wake and the effect of a moving fish.

Laio has previously collaborated with other engineers interested in harvesting energy from vortices and is now looking at how fish use their lateral lines to sense vortices and guide the way they move. "The question is, what do you need from the environment to be able to do this," he says. "That is also important for any man-made uses of vortices."

Journal reference: Bioinspiration & Biomimetics (DOI: 10.1088/1748-3182/2/3/L01)

Energy and Fuels - Learn more about the looming energy crisis in our comprehensive special report.