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TIDAL TURBINE: Verdant Power's kinetic hydropower underwater turbines are designed to generate electricity from the water currents of tides, rivers and man-made channels.

For more images, view our Extreme Tech Slideshow.
Courtesy of Verdant Power

Additional images of tidal and wave energy technologies can be found in our Extreme Tech Slideshow.

Thirty feet (nine meters) below Manhattan's East River, next to Roosevelt Island, six turbines—each 16 feet (five meters) in diameter, churning at a peak rate of 32 revolutions per minute—stand at attention on the riverbed. The turbines—which belong to New York City-based Verdant Power, Inc., —are built on a swiveling platform that keeps their nose cones facing the tide, whether it's coming in or going out. Resembling an underwater wind farm, these kinetic hydropower systems use gearboxes and speed increasers—which convert the slower rotating rotor into a faster rotating generator—to transform each turbine's mechanical power into electricity.

Verdant's turbines require tides that move at least six feet per second in order to generate enough energy for them to be cost-effective, and the East River is more than obliging. "The East River is a good tidal channel that links the Long Island Sound to the ocean," says Trey Taylor, the company's president and head of market development. "Plus, New York is an expensive place to buy power, so it would be easier here to prove that this could help."

A few dozen feet away from the closest turbine, an onshore control room gets a feed of the energy created by the entire cluster. To prove that this energy could be usable for local businesses, Verdant last year sent a test transmission of electricity to a supermarket and parking garage on Roosevelt Island that were willing to participate in the Roosevelt Island Tidal Energy project.

The Earth's oceans, pushed by wind and tugged by the moon and sun, ebb and flow over more than 70 percent of the planet, but only recently has technology emerged to finally harness some of that kinetic energy as usable power for us landlubbers. Underwater turbines, submerged "wind" farms and wave-riding electrical generators are being tested around the world, with new advances in technology promising relief for overworked energy utilities. "We consider wave energy to be more predictable than wind," says Phil Metcalf, CEO of Edinburgh-based Pelamis Wave Power, Ltd., a company taking a different approach than Verdant in developing ocean power–utilizing devices. "You look at the ocean 1,000 miles out, you'll get a good idea of what to expect over the next 24 to 48 hours. We think it's actually going to be easier to dispatch to the grid."

Pelamis's devices are big red tubes, each 426.5 feet (130 meters) long, 13 feet (about four meters) in diameter, weighing around 750 tons (635 metric tons), and with a life expectancy of up to 20 years. They flex as the ocean swells around them. The wave-induced motion of the tubes' joints is resisted by hydraulic rams, which pump high-pressure fluid through hydraulic motors that drive electrical generators to produce electricity. Power from all the joints is fed down a single umbilical cable to a junction on the seabed. Three of the tubes, which work best at a depth of 165 to 230 feet (50 to 70 meters) and roughly 3.7 miles (six kilometers) from the shore, can produce up to 2.25 megawatts.

Pelamis—which until September had been called Ocean Power Delivery—has taken its prototype through about 2,000 hours of testing at the European Marine Energy Center's wave test site near Scotland's Orkney Islands. Three additional machines will form the initial phase of Agucadoura, the world's first commercial wave farm, in April off the coast of Portugal, a project developed by Portuguese utility Enersis, a subsidiary of Babcock and Brown. Pelamis is negotiating with other utilities and governments as well, with future deployments depending on how well the Portuguese project is able to turn waves of water into currents of electricity.