"Whirlpools created by currents as they flow over obstacles are powerful enough to tear apart bridges and offshore rigs. So why not use them as a source of renewable power?
Previous attempts to harness energy from the flow of the world's rivers and oceans have had limited success, at best. Tidal flow can only be tapped at certain times of day, while underwater turbines are only viable if they are mounted in rapid currents.
Now researchers led by Michael Bernitsas at the University of Michigan, Ann Arbor, are preparing for the first outdoor trials of a technology that makes use of the slow-moving currents down rivers and across the ocean.
When water flows over an underwater obstacle, whirlpools or vortices form alternately above and below it. The vortices create a tugging effect, so the result is an alternating force that yanks the object up and down
In his lab, he took a cylinder 10 centimetres in diameter and 91 centimetres long with the same average density as water and suspended it horizontally in a bath. Then he generated currents of between 0.5 and 1.0 metres per second - speeds that are common in rivers. The vortices generated by the flow moved the cylinders up and down, and by attaching the cylinders to springs that turn an electric generator he was able to convert the motion into 10 watts of electrical energy. Bernitsas calls the technology Vortex Induced Vibrations Aquatic Clean Energy, or VIVACE, and plans to commercialise it with his company Vortex Hydro Energy.
He has also come up with an idea for squeezing more energy from VIVACE. At the Offshore Mechanics and Arctic Engineering conference in Estoril, Portugal, in June this year, he will show how roughening the surface of the cylinders allows them to capture more energy. The idea was inspired by the fact that fish that use energy from vortices to help propel themselves forward also have rough skin.
VIVACE's big test will come next year, when the team plans to deploy a larger version in the Detroit river. They expect it to generate 3 kilowatts, enough to power lights on a nearby pier, and claim that still larger versions could produce megawatts of power at a cost of around 5 cents per kilowatt-hour. This would make it competitive with coal and gas-fuelled generators.
These projections are contested, however, by commentators who point out that the performance has yet to be tested in the fluctuating current of a real river. They also have doubts about the claimed cost of the power it produces, since it is not yet clear how much the system will cost to maintain. "It is very new and very different to existing devices," says Walter Musial of the National Renewable Energy Laboratory in Golden, Colorado. "There are a lot of questions still to be answered."