Ocean Currents May Bring Italy More Tidal Energy

A 500-kilowatt (kW) tidal power prototype that could be scaled up for ocean use is set to be tested next August in the Strait of Messina, a stormy stretch of sea separating Italy's mainland from Sicily.

If the trial is successful, an even more powerful tidal power plant could be ready for mass production in about five years time, and installed not just in the seas off Italy but also in the Gulf of Florida and other coastal sites with stronger currents.

Tests so far of the Sea Power tidal device on open sea have been successful, said Werner Ebner from Fri-El Green Power, the company based Italy and developing the Sea Power power plant.

"These tidal power plants are an economical way of producing electricity. The system is comparatively inexpensive to build and also to maintain, not least because it is based on modules, which can also be easily transported," he said.

The tidal power plant has an electricity generator that is located on a floating platform, held in place by devices fixing it to the bottom of the sea.

Attached to the platform are four cables; each one of the four cables extends for the same length behind the platform and has five buoys placed at regular intervals. (See top view image, above.) These five buoys contain electrical and other equipment for the five turbines, churning below the water's surface, so keeping vital equipment dry. Each of the twenty turbines operating underneath the twenty buoys has a diameter of four meters.

Constantly rotating in the fast and ever shifting marine currents, the turbines generate power, which is transmitted via the cable to the platform floating on the sea surface. A generator on the platform transforms the kinetic and mechanical energy into electrical power. This electrical power is then sent to the mainland using a cable.

Each line of five submerged turbines will be able to produce 1.2 MW of electricity as they churn in the tidal currents in the Strait of Messina, a place so stormy that it is where the legendary Greek hero Odysseus got shipwrecked.

An additional turbine is to be attached directly to the platform: this turbine will be able to capture energy from slower moving currents, making the tidal plant more versatile in terms of the energy that it can capture from tides.  (See image of platform, above.)

The horizontal axis turbines are each equipped with three rotor blades that spin at right angles to the water, similar to the way wind turbines on land operate, and with a comparable level of efficiency. (See image of submerged turbines, below.)

"During the nine-month trial we will probably be able to feed 400 kW of electricity into the national grid through an underwater cable," said Josef Göstner, one of the co-initiators of the project and vice-president and CEO of Fri-El Green Power.

The currents in the Strait of Messina reach speeds of 2.5 m/s or 4 to 5 knots and change direction every six hours. However, a computer-operated system allows the rotor blades to be turned around 180 degrees as the currents change, ensuring a steady amount of power.

Because the power generated by tides is predictable, the device can be a reliable form of baseload power for a national grid.

Using cables to transmit electricity more than 100 miles has proved difficult and so the current large scale prototype tidal power plants need to be located close to the coast, said Göstner.

However, he believes that the tidal turbine technology will really come into its own when devices that are much larger can be built and placed far out in the ocean to tap powerful, deep level currents.

Göstner and his partners envision a future where fleets of tidal power plants anchored far out in oceans such as the Atlantic can generate as much as 8 terawatt-hours (TWh) of electricity a year.

According to one estimate, the wave and current energy potential that could be tapped could be enough to cover about 10 percent of the world's electricity needs using 2006 consumption levels of 19,900 TWh/yr.

To overcome transportation problems, the tapped ocean current energy would first have to be turned into hydrogen using electrolysis. The hydrogen would then be stored in tank ships and taken to the mainland. (See image of tank ship with hydrogen storage, left.)

The Sea Power device is being developed in cooperation with scientists from the University of Naples, led by project partner Professor Domenico Coiro from the Department of Aeronautical Engineering.

Jane Burgermeiser is a writer based in Austria.