An engineer in the US has built a machine that can harness energy from the slow-moving currents found in oceans and rivers around the world. By exploiting the vortices that fish use to propel themselves forward, the device could, he says, provide a new kind of reliable, affordable and environmentally friendly energy source.

Turbines and water mills can generate electricity from flowing water, but can only do so in currents with speeds of around 8–10 km/h if they are to operate efficiently. Unfortunately, most of the currents found in nature move at less than 3 km/h.

The new device is called VIVACE, which stands for vortex induced vibrations for aquatic clean energy, and its inventor claims it can operate in such slow-moving flows.

VIVACE has been developed by University of Michigan engineer Michael Bernitsas, and in its prototype form exists as an aluminium cylinder (91 cm long with a diameter of 12.5 cm) suspended by a pair of springs inside a tank. The tank, located in the university’s marine renewable energy laboratory, contains water that flows across the cylinder at around 2 km/h. The device does not convert the energy of the flow directly into electricity but instead exploits the vortices that form on opposite sides of any rounded object placed in a flow (J. Offshore Mech. Arct. Eng. 130 041101).

Vortex-driven fish

As such, it works like a moving fish. Fish cannot propel themselves forward using muscle power alone; instead they curve their bodies so that they form a vortex on one side of their body, straighten out, and then curve the other way to form a vortex on their other side, in order to glide between vortices. VIVACE remains in a fixed position in the water but is pushed and pulled by the vortices on either side, and these vibrations are then converted into electrical energy (the current cylinder is smooth, but future versions will have scale-like structures on the surface to enhance vortices).

It dawned on me four years ago that I can enhance these vibrations to harness energy Michael Bernitsas, University of Michigan

Bernitsas explains that engineers usually do all they can to suppress such vibrations, which can occur in either water or air, as they can cause enormous damage. They were, for example, responsible for destroying the Tacoma Narrows bridge in the US in 1940. “But,” he says, “it dawned on me four years ago that I can enhance these vibrations to harness energy. My colleagues and I searched the scientific literature and patents and found out to our surprise that no one had done this before.”

The total amount of energy generated by the Earth’s slow-moving currents is vast, but the density of this energy is low. This means that the VIVACE technology, like any other ocean-based device, will only ever be part of the solution to the world’s energy needs. However, Bernitsas believes it has a number of advantages over alternative ocean-based sources, pointing out that, unlike wave devices, for example, it is unobtrusive, and should also pose no harm to marine life.

Tests in the Detroit River

The group is currently installing a 3 kW device in the Detroit River to provide energy that will light a new pier being built there. Bernitsas says that the technology could then be scaled up by constructing arrays of cylinders, either suspended from ladders or built upwards from the river or sea floor, in order to build power stations large enough to power tens of thousands of houses. The electricity from such a plant would be cheaper than many alternative renewable sources, he adds — some 5.5 cents per kilowatt hour, compared with 7 for wind and at least 16 for solar.

"The device is highly scalable", said Bernitsas. "It could be used to build small devices of 5 kW, medium of 50 kW, larger of 500 kW and put them together to build large stations of 10 MW", he said. "The next step up is 100 MW and finally huge offshore underwater stations of the size of 1 GW, the size of a nuclear power plant".

Stephen Salter of Edinburgh University, who has carried out research on tidal and wave energy, believes that low-velocity flows are an important potential source of renewable energy. He points out that there are several kinds of structure that could be used to harness this energy, including, for example, hydrofoils. But he believes that cylinders could turn out to be cheaper and more efficient than the alternatives, if they can be made to move with sufficiently high velocities.

Bernitsas has founded a company called Vortex Hydro Energy to commercialize the technology.