How Tides Could Power the Future

Tag : Stand By Power

The installation of the SeaGen marine current generator wascompleted this past May in Strangford Lough, a large inlet on thecoast of Northern Ireland. The system, designed and built by MarineCurrent Turbines Ltd., has two rotors that each span 16 meters (52feet) in diameter.
"The technology is very analogous to wind, except we are doingeverything underwater,"
said Peter Fraenkel, the company's technical director.
The maximum power output of the SeaGen will be 1.2megawatts—four times more than any other turbine. Operatingfor roughly 20 hours per day, it is expected to supply 1,000 homes.
Go with the flow and the ebb
The $20 million SeaGen is a single tower, moored to the seafloor aquarter mile (400 meters) from shore. It is designed to catch boththe incoming (flow) and outgoing (ebb) tides by rotating its tworotors 180 degrees.
No power is generated during tide changes, as the turbine onlyworks when the water is moving 2 knots (2.3 mph) or more.
The rotor blades are similar to those on wind turbines. That'sbecause the same basic physics applies to wind streams and tidalcurrents. The big difference is that water is much denser than air,so it puts more stress on tidal system structures.
"The bad news is tidal turbines can't be as big as windturbines," Fraenkel told LiveScience. "But the good newsis that they don't need to be," since water carries moreenergy per area than air.
As a case in point: a wind turbine would need to be 65 meters (210feet) in diameter to achieve the same 1.2 megawatts of power asSeaGen.
Just like clockwork
The other difference with wind is that tidal energy is completelypredictable.
"Tides are not driven by weather, but instead by the relativemotion of the moon, sun and Earth," Fraenkel explained."You can predict what the tides will be 20 years ahead,whereas you can't tell if it will be windy."
The site at Strangford Lough was chosen because it has very fasttidal flows that reach up to 8 knots (9 mph).
These strong currents caused some problems during the installationof SeaGen. Likewise, other tidal projects have had difficulty withthe unwieldy ocean.
"The whole point is to put these things in the most energeticenvironments," said Eoin Sweeney, head of the ocean energydevelopment unit of Sustainable Energy Ireland. "Bydefinition, they are going to be tricky places to work."
The Paris-based International Energy Agency has estimated that theglobal tidal energy potential is around 800 billion kilowatt-hoursper year (about 5 percent of current world demand). This isconsiderably less than the potential of wind and wave energy.
Making sushi?
Stand-alone turbines are not the only way to capture tidal energy.In fact, a 240-megawatt tidal barrage has been working in La Rance,France, for over 40 years. It's essentially a large hydroelectricdam that gets its water from the incoming tide.
Although other tidal barrages have been built over the years, theyhave fallen out of favor because they completely alter the flow ofwater in what are often sensitive coastal eco-systems.
Turbines are thought to be more environmentally friendly. Even so,there are concerns over the turbine blades "making sushi"out of unsuspecting marine life. Fraenkel thinks this is unlikelysince the blade tips move at most 15 meters per second (33 mph),slower than some boats.
Ongoing studies into the environmental impact and long-termreliability of tidal turbines may decide the industry's future.
"We're in a bit of a crystal-ball-gazing period rightnow," Sweeney said. But he predicts an increase of developmentover the next five years.