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.

RITE stuff
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.

Water-to-wire efficiency
Trey Taylor, Verdant's president and head of marketing and business development, estimates his company's turbines have a "water to wire" efficiency of about 40 percent, meaning that 40 percent of the overall water-flow energy turning the turbines is converted into electrical power on the grid. For the RITE demonstration the power generated was provided to two locations on Roosevelt Island—a local supermarket and a parking garage.

Taylor likens his work at Roosevelt Island to another famous first step in the history of technology: "The East River is like our Kitty Hawk," he says, "but eventually it will lead to our 747."

Verdant would like to place additional turbines in the waters around New York City. The U.S. Coast Guard has even mentioned possibly placing turbines in the United Nations's security zone in the East River, Taylor says. "The U.N. seemed to like that because the turbines might discourage boats from driving into that zone, where there aren't supposed to be any boats," Taylor says. Verdant has obtained a preliminary permit from FERC to study the site for an installation that could produce up to five megawatts. The water depth in that area could allow Verdant to install turbines that are seven meters in diameter, which Taylor estimates would translate into 110 kilowatts of power per turbine. "Once we have studied the site and have applied project and economic modeling to the site," he says, "we will engage the U.N. in discussions about building a project in its security zone.

The East River is just the beginning, according to Taylor, who says that the deeper, faster-flowing waters of the Saint Lawrence River that forms part of the border between New York State and Ontario, Canada, have the potential to produce three times as much energy as the RITE project. A key distinction is that the Saint Lawrence is a steadily flowing river, unlike the East River that depends on a tide that flows either north or south depending on the time of day.

Taylor is hoping to be able to sell river-based power to Ontario in 2012, giving his company two successful projects that might attract additional investors. In a third key project, Verdant will work with the U.S. Navy in Puget Sound. The Navy is studying Verdant's triframe turbines for their potential to provide energy to naval bases worldwide, Taylor says. Longer term, Taylor would like to install his company's technology in developing countries, something that will require additional capital. All together, Verdant expects to be producing 22,000 megawatts of power from its various installations by 2018.

A test at a lock-and-dam system
Houston-based Hydro Green Energy likewise make hydrokinetic power systems that generate electricity from river currents, tidal currents and ocean currents. The company struck a deal with City of Hastings, Minn., in late 2008 to test Hydro Green Energy's turbine technology at a U.S. Army Corps of Engineers lock-and-dam system on the Mississippi River.

Hydro Green CEO Wayne Krouse's goal was to develop a technology that could generate electricity without the need for more dams and that could be done at a cost comparable with electricity produced by burning fossil fuels. That meant coming up with a compact, low-cost technology that required a minimal number of parts. "In an offshore or saltwater environment, the more parts you have the more that could go wrong," Krouse says. Making the blades from plastics and carbon-fiber components rather than metal alloys was one solution.

Hydro Green is still working toward its goals. The company's prototype turbine operating in the Mississippi is about 3.7 meters in diameter with blades made from cast aluminum, which Krouse says is fine for freshwater but could pose a problem in salty ocean water.

The Hydro Green turbine uses flow from the nearby hydroelectric lock-and-dam system to simulate the marine tidal environment where the company wants to eventually operate. About 30 cubic meters of water per second flow through the turbine at any given time.

At this point, Krouse is trying to keep his company's turbine in the water as long as possible, logging the precious hours required to validate the technology's design and durability. Green Hydro received its FERC permit in December 2008, and the 35-kilowatt turbine went into the water in February 2009. Krouse, who says that getting the FERC permit is the biggest barrier to funding, estimates it will take anywhere from 10,000 to 25,000 hours of cumulative operation to attract the investment he needs to further develop the technology. Hydro Green has applied for grants to get financial assistance to start up additional projects in Alaska and Mississippi.

Other tidal projects
OpenHydro Group, based in Dublin, Ireland, in November 2009 successfully deployed a 400-ton in-stream tidal turbine in Canada's Bay of Fundy, which has the world's highest tides, on behalf of its customer, Nova Scotia Power. The turbine, which rests at the bottom of the bay's Minas Passage, is capable of producing one megawatt of power (pdf).

East Yorkshire, England–based Lunar Energy in May 2009 announced that a one-megawatt commercial prototype of its turbine (which was developed with the help of Rotech Tidal Turbine) successfully synchronized to a simulated grid and produced electricity in testing on land. Lunar Energy in 2008 began working with Korea Midland Power  to create a 300-turbine field in the Wando Hoenggan Water Way off the South Korean coast. The plant is expected to provide 300 megawatts of renewable energy to Korea Midland Power by December 2015. A commercial prototype unit is also expected to be deployed at the European Marine Energy Center (EMEC) in Orkney over the next year.

Similarly, Sandia National Laboratories in Albuquerque, N.M., announced in December that it will receive more than $9 million over three years from a U.S. Department of Energy competitive laboratory solicitation for the development of advanced water power technologies. Sandia researchers are expected to evaluate new device designs and conduct basic research in materials, coatings, adhesives, hydrodynamics and manufacturing to assist industry in bringing efficient technologies to market. Sandia will also evaluate environmental factors including rates of sediment transport, water flow, water quality and acoustic changes.

Regulatory challenges
Even though a lot of the testing taking place involves proving the turbines can work efficiently, the biggest roadblocks to making tidal energy commercially available are on the regulatory front. "That's not to say there are no engineering problems," Bedard says, "but you need to put these devices in the water to test them."

The money that start-ups must spend on environmental studies is a major barrier to getting a FERC license that would allow companies to test their turbines in a real body of water, as opposed to a laboratory tank. Verdant has spent at least $9 million on its East River project, one third of which was expended on studies to gauge the potential impact of the turbines on ships traversing the channel, aquatic life and fish migration, according to Taylor. The company has received some help from the government along the way in the form of state and federal funding.

Green Hydro had to study, among other things, the birds, fish and water quality (including turbidity and dissolved oxygen) around its City of Hastings installation, Krouse says, adding that the amount of research needed to get the FERC license was "onerous." The fish study cost $400,000, whereas for the bird research it was another $45,000, he says.

"A true commercial project cannot afford to deal with the government," Bedard says. "For these projects to be real and supply real power, there needs to be a process that will allow for the economical development."