The water comes in, the water goes out, propelled by the perpetual engine of the sun and moon. With 70% of the earth's surface covered by the restless tides and currents of the oceans, the idea of harnessing that movement to serve the planet's energy needs is too tempting to ignore.

Since the Middle Ages people have built tidal mills, trapping an incoming tide in a storage pond to turn a wheel as the water ebbs. But the dream has always been to tap the power of the ocean itself — to harness the force of tides mighty enough to erode and shift entire coastlines. And so later this year a small South Wales company called Tidal Hydraulic Generators (THG) plans to lower a steel frame supporting five 6-m diameter turbines to the floor of the Severn Estuary. The tubes will translate the power of the tide as it ebbs and flows into one megawatt of electricity, enough to power about 500 homes. The project will be a test run for much larger rigs, with up to 50 turbines apiece, that could produce enough electricity for a small town.

The inspiration for the turbines came on a calm day in 1997 when Richard Ayre, managing director of THG, was working for the marine national park in St. Bride's Bay, Pembrokeshire. Trying to place buoys in the water, he realized the current was dragging the boat sideways. "The energy here is absolutely astronomical," thought Ayre, who started wondering how to generate power from it without damaging the bay's pristine environment. He came up with the answer — a 6-m turbine blade, with a drive/gear system and a generator — and tested it on a small, experimental rig in the sea off Milford Haven. The amount of energy produced by Ayre's test rig so impressed the government that it has promised €2.23 million to fund the construction of the five-turbine unit in the Bristol Channel.

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Because water is 800 times more dense than air, tide power is a more productive energy source than wind power, now commonly used throughout the developed world. Where a wind turbine may be up to 80 m in diameter, a tidal stream turbine need be only 10 m across to produce 50% more energy. Though the blades turn slowly, just 10 revolutions per minute, the rotation generates a great deal of torque — a force that causes an object to rotate. That is then turned into energy via a hydraulic accumulator, which condenses the power into a motor that drives the generator. David Baird, managing director of Babtie, the engineering group that is working with Ayre, thinks tidal stream power has a big future. "Twice a day the North Sea drains in and out of the Pentland Firth," he says. "We estimate that using tidal stream generators we could meet all of the renewable energy needs of the U.K."

The Severn Estuary was chosen to test the new technology because it has the world's highest range between high and low tides, around 12 m, and during the Severn Bore, at the times of the spring and autumn equinoxes, tides of up to 15.4 m rush at 24 km/h up the funnel-shaped channel. But Ayre stresses that huge tidal flows are not essential to the system. "We will tailor the units to be site specific," he explains, "So that in areas with lower tidal flow we'd use larger blades and more of them than we'd need in areas with greater flow." Even with a modest tidal flow, the technology could still be used to desalinate sea water and pump it ashore — a self-propelled water purifier. Babtie has already sent a team of engineers to the Middle East to discuss the possibilities of setting up tide-driven desalination plants. Ayre is expecting that once the technology is proven, THG could be supervising the construction of rigs to generate "many thousands of megawatts."

But no one expects the new system to be entirely smooth sailing. As any boatsman knows, the moment something goes into seawater it starts corroding, and soon becomes covered in barnacles and weed. The THG engineers are working on several ideas to combat corrosion, including protective coatings and cathodic protection — attracting corrosive chemicals to electrically charged plates of a dissimilar metal, known as sacrificial anodes, by running a weak electric current through the framework. The blades themselves will be cleaned with every rotation, to avoid a build-up of animal and plant life.

Even with its unresolved issues, the system is a huge improvement over past methods of harnessing tidal energy, which aren't cost efficient and are also clumsy — they tend to create problems of silting and interfere with shipping and wildlife. Though tidal stream generation is still in its infancy, Baird estimates that with the coming of commercial-sized installations within the next five years, the cost of producing electricity for the national grid would be about €.05 per kilowatt — slightly more than gas or coal at current prices, but about the same as wind power. Perhaps the tide is finally starting to turn on fossil fuel consumption.

• KATE NOBLE
• Has a Welsh company turned the tide in sustainable energy?