Powerful whirlpools

Illustration by Gabriel Silveira
Date:1 May 2009 Tags:

Scientists borrow from fish to create energy in slow-moving water.

Hydropower researchers at the University of Michigan are borrowing strategies from fish to maximise the energy that can be harvested from slow-moving water. For example, trout use spinning eddies that form on both sides of a stationary object, such as a rock, to help conserve energy as they swim upstream. The vortices that form alternate from one side of the rock to the other, so fish swimming upstream slalom between these whirlpools. Schools also use vortices created by the fish ahead to conserve energy when swimming upstream.

The University of Michigan team’s design harnesses these alternating vortices: aluminium cylinders joined to built-in electromagnets form a laddershaped device. As flowing currents swirl past a cylinder, the vortices that form above and below push and pull the cylinders to generate electricity. Inventor Michael Bernitsas estimates the bobbing action can yield 51 watts per cubic metre at water speeds of 5 km/h, depending on the number and size of the cylinders. The device, known as Vortex-Induced Vibrations for Aquatic Clean Energy, or VIVACE, attracted US Navy interest for its potential to generate power at flow speeds slower than the 9 km/h minimum of most turbines.

To maximise the system’s efficiency, Bernitsas draws insight from the anatomy of fish. Sandpaper-like surface roughness on the cylinders mimics scales to form more energetic whirlpools. Flexible plates designed like a fish’s tail could speed up cylinders in very slow-moving water or decrease the cylinder movement, which would help to protect aquatic life. “We may design a tail that’s adjustable and has sensors, but I’m not anywhere near that yet,” Bernitsas said. “Simply, we are not as smart as fish at this point.”

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