Green energy, a term used as an antidote to fossil fuels that have worsened global warming, takes on a new form as it suddenly seems plausible to harness the smallest of currents to produce a lot of energy. Scientists claim that a revolutionary device operating on slow-moving rivers and ocean currents could provide enough power for the entire world.
Existing technologies require an average current of five or six knots to operate efficiently, while most of the earth's currents are slower than three knots. The new technology device can generate electricity in water flowing at a rate of less than one knot - about one mile an hour - meaning it could operate on most waterways and sea beds around the globe.
The new device - inspired by the mechanism used by fish to swim against great currents - consists of a system of cylinders attached to springs and positioned horizontal to the water flow. Gushing water leads to vortices that push and pull the cylinder up and down. The mechanical energy in the vibrations can then be converted into electricity. Cylinders arranged over a cubic metre of the sea or riverbed in a flow of three knots produce up to 51 watts. Seemingly more efficient than similar-sized turbines or wave generators, the amount of power produced is likely to increase sharply if the flow is faster or if more cylinders are added.
A "field" of cylinders built on the seabed over a 1km by 1.5km area, and the height of a two-storey house, with a flow of just three knots, could generate enough power for around 100,000 homes. Just a few of the cylinders, stacked in a short ladder, could power an anchored ship or a lighthouse.
The technology has been developed in research funded by the US government and scientists say that potential costs would be as low as 3.5 pence per kilowatt-hour, compared to about 4.5p for wind energy and between 10p and 31p for solar power. They add the technology would require up to 50 times less ocean acreage than wave power generation. The system, conceived by scientists at the University of Michigan, is called Vivace, or "vortex-induced vibrations for aquatic clean energy".
Eddies or vortices, formed in the water flow, can move objects up and down or left and right. Fish curve their bodies to glide between the vortices shed by the bodies of the fish in front of them. Their muscle power alone could not propel them through the water at the speed they go, so they ride in each other's wake. Leonardo DaVinci first observed such vibrations 500 years ago in the form of ''Aeolian Tones''.
With about 0.1 per cent of the energy in the ocean, the energy needs of 15 billion people could be taken care of. The technology is less likely to be harmful to aquatic wildlife than dams or water turbines. And as the installations can be positioned far below the surface of the sea, there would be less interference with shipping, recreational boat users, fishing and tourism.
The engineers are now deploying a prototype device in the Detroit River, which has a flow of less than two knots. Their work, funded by the US Department of Energy and the US Office of Naval Research, is published in the current issue of the quarterly Journal of Offshore Mechanics and Arctic Engineering.