Producing current from tidal currents

Posted on 01 November 2010 Written by Jim Flannery
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n_13_coastwiseA turbine in Maine's Cobscook Bay is producing electricity to power systems on a Coast Guard utility boat

Down East Maine's enormous tidal range - a bane to mariners - one day could be a boon to the region as a renewable source of electrical energy.

A 60-kW turbine suspended 20 feet beneath the surface of Maine's Cobscook Bay is producing electricity to recharge a bank of batteries that supplies power to a 41-foot utility boat at the Coast Guard station in nearby Eastport. The 13-ton turbine, which looks like a paddle wheel, is in a rectangular frame 14 feet wide, 14 feet deep and 46 feet long. Called a cross-flow turbine, its composite blades - actually carefully sculpted foils - spin in the tidal current, which flows at a speed of up to 6 knots four times a day during the 18- to 20-foot tidal changes into and out of the Bay of Fundy.

The turbine drives a fixed-magnet generator, which powers the unit's monitoring systems and recharges the battery bank - eight 12-volt batteries stored on a 20-foot skiff tied to the turbine's 60-foot barge, Energy Tide 2. Once the batteries are recharged, the skiff is towed to the Coast Guard station and swapped out for a second skiff loaded with batteries, which go back to the barge for recharging.

"This is the largest ocean energy device deployed in U.S. waters," says John Ferland, vice president of project development for Ocean Renewable Power Co., which switched on the power Aug. 18 for the Coast Guard. The beauty of tapping the tides for energy is that they are predictable. "They are predictable every day of the week," Ferland says. "You've got two high tides and two low tides" - and a precise daily schedule of when they occur.

The battery banks supply electricity to the 41-footer's heater, its battery charger, radio chargers and other systems while it is dockside, says Capt. Jim McPherson, commander of Coast Guard Sector Northern New England. "We're the first federal facility to use tidal power. It's just a modest first step." But not the last.

In addition to supplying power to the Coast Guard, the turbine - still in a test phase - has shown it can be a reliable source of power for the electrical grid. Ferland says the turbine generator unit is showing that its electrical output is compatible with the grid and predictable in quality and quantity.

Because Ocean Renewable Power already planned to test the tidal generator in Cobscook Bay, the Coast Guard awarded the company a $100,000 grant to put it to work supplying electricity to its station. When linked to an inverter, the battery banks supply a third to half of the daily shoreside power needs of the 41-footer, Ferland says.

This isn't Sector Northern New England's first foray into alternative energy. The station at Southwest Harbor, Maine, uses wind and solar power to augment the electricity it consumes from the grid and wood pellets to power a furnace for heat in the winter. "We've been producing enough electricity [from these other energy sources] that we've been able to put some power back on the grid [from time to time]," McPherson says.

He hopes to also bring solar, wind and wood-pellet power to the Eastport station and take alternative energy to other stations in Maine. "We hope to have a station one day that's completely off the grid," he says.

The Coast Guard's Research and Development Center in New London, Conn., is evaluating Sector Northern New England's use of tidal power for potential application in other regions - possibly Alaska and the Pacific Northwest - where there are big tidal changes and fast-running rivers, Ferland says.

Ocean Renewable Power also aspires to increase its electrical output in Maine. "This is the best location on the East Coast of the United States for tidal-generating potential," Ferland says.

The power company wants to scale up to a 150-kW turbine generator unit in Cobscook Bay to connect to the electrical grid. Once that has tested out, it plans to add four more 150-kW units, all interconnected in a cluster, which should deliver enough output to power a few hundred homes in the area, Ferland says.

Ferland says the test unit cost $2 million to $3 million to build and install. The first 150-kW unit likely will cost $7 million, including connection to the grid, he says.

Late next year, Ferland says, Ocean Renewable Power expects to be the first tidal power generation company to connect to the grid. "We consider what's happening in Eastport to be to tidal engineering what Kitty Hawk was to flight," he says.

This article originally appeared in the November 2010 issue.