The quest to turn the motion of the world's waterways into a significant source of energy may still be in its nascent stage, but several tidal power projects are making headway. Whether they operate in lakes, rivers or the oceans, projects attempting to harness the tides share the same mission: to improve the technology and offer an economical alternative to fossil fuels.
Renewable hydrokinetic power comes from a number of different sources, including the up-and-down motion of waves and the smooth flow of the tides caused by the sun and moon's gravitational forces on Earth's bodies of water. Tidal power is seen as a promising source of energy because of its predictability and from the potential to draw it from ocean currents and estuary channels that connect rivers with the sea.
There are only a handful of tidal energy projects in place around the world, and none is producing commercially available electricity at this time. Most of these projects use some sort of turbine to capture the tide's kinetic motion. In general, as the turbines slowly spin, they turn the gears in an attached gearbox to create electricity. Cables connected to those gearboxes carry that electricity ashore.
Although it is unclear just how much electrical energy that the tides have the potential to generate, the Electric Power Research Institute (EPRI) has studied several tidal power project sites. In 2008 EPRI estimated those sites together have the potential for generating as much as 115 terawatt-hours of electricity annually, although the practical potential for energy generation from those sites is about 14 terawatt-hours per year. (Total electricity consumption in the U.S. is about 4,000 terawatt-hours per annum, according to EPRI.) Much of that energy would come from Alaska, thanks to high power density and large-size sites in southeast Alaska, Cook Inlet and the Aleutian Islands. Other locations studied were in Maine, San Francisco and Washington State's Puget Sound. Although New York City and the Chesapeake Bay were not studied for the 2008 report, EPRI concluded these sites could also make use of tidal hydrokinetic energy resources.
One of the more advanced tidal power operations in the U.S. is taking place in New York City's East River, where the Roosevelt Island Tidal Energy (RITE) project has been testing windmill-like turbines since 2006. Led by Verdant Power, the project installed six windmill-like turbines—each five meters in diameter and anchored to the bottom of the East River, about nine meters in depth—in the water next to Roosevelt Island, a sliver of land 3.2 kilometers long by 240 meters wide in the river between the boroughs of Manhattan and Queens.
"Verdant went with a design that looks like a conventional wind machine—an open rotor with three blades," says Roger Bedard, an EPRI researcher who has studied water current–based energy generation. This was a calculated move, given that wind is very commercially mature in terms of renewable energy sources, he adds.
After logging about 9,000 operational hours since being installed, all six original turbines were removed earlier this year and are being disassembled so Verdant can study their seals, bearings and other components for signs of wear. In the meantime, Verdant is developing its next-generation turbines that will be very different from their predecessors.
Whereas Verdant's original tidal turbines sat anchored individually to the riverbed, looking something like a field of underwater windmills, the new design will have three turbines operating on a triangular frame positioned on (not anchored to) the bottom of the river. The company plans to place 10 triangular frames—a total of 30 turbines—on the river bottom. Each of the new turbines will produce 35 kilowatts of power at the rated water speed, meaning that the 10-frame installation should produce up to about one megawatt of power (enough to provide electricity to roughly 800 homes).
This is, of course, if the company can get permission from the Federal Energy Regulatory Commission (FERC). Verdant has been operating with a preliminary FERC license and by August plans to apply for its full license, which the company needs in order to produce, deliver and sell one megawatt of commercial power.