Marine Current Turbines Ltd. (MCT) has been developing tidal stream turbines for almost 13 years and is now underway with the third and final test to prove the technology. Off the coast of Northern Ireland in the Strangford Lough, MCT is preparing to deploy a tidal stream turbine — the last step in its SeaGen project.
“It’s essentially like a wind mill underwater,” says Peter Fraenkel, technical director for SeaGen and MCT. According to the company, the turbines harness the power of a constant, predictable and powerful tide. The turbine works by using the force of the tide to spin a rotor, which then transfers that force back into a gearbox and then into the generator. “We’ve got a fairly novel gearbox,” says Fraenkel. “It’s a three-stage gearbox so it basically jumps the speed up in three stages. Two of them are what are called planetary gears. You’ve got little planet wheels running around a sun wheel. And the third one is the spur gear. It’s fairly conventional, similar to what’s used on wind turbines.”
Since the tide goes both in and out, the SeaGen bi-directional rotor blades change their pitch depending on the state of the tide. “We have a patented arrangement where we can reverse the pitch of the rotor blades rather like an old-fashioned aircraft or propeller-driven aircraft. When they want to reverse thrust after landing they can turn the pitch back to front and blow forwards rather than backwards,” says Fraenkel.
Previous generations of this project successfully proved their capability of producing electricity, but the electricity generated was grounded and never used. Now, once installed, these tidal turbines will be connected to the power grid in order to use the electricity produced. A substation will be built on the shore near the turbines and, using a directionally drilled hole, a cable will be fed to connect the turbine station to the substation and then to the power grid.
Between tides the rotors will shut down as they require a velocity of 1.5 knots in order to efficiently produce energy. After this downtime, which lasts between 10 and 20 min, the turbine will draw energy out from the grid in order to start itself back up again. According to Fraenkel, “it might take 20 or 30 kW out for a minute or two, but it then puts up to one MW in.”
The station in the water is made mostly of standard construction steel coated in marine epoxy paint to reduce corrosion. Other methods for avoiding corrosion include cathodic protection, where there is a small voltage passed through the structure and the placement of zinc anodes around the station.
While developing the technology, MCT has had to prove its minimal impact to the marine environment that surrounds the turbines. Studies have been performed by Queen’s University in Belfast, Northern Ireland as well as by the Sea Mammals research unit at the University of St. Andrews in Scotland. “Those studies continue and will continue for the duration of the project,” says Paul Taylor, public relations representative for MCT. “There’s been a very comprehensive environmental monitoring program, which is overseen by an independent body.”
The installation of this final stage of SeaGen has been postponed from its intended August 2007 date due to complications with the third party A2Sea, operators of the offshore marine installation barge the Jumping Jack. A2Sea has experience installing wind turbines over the past few years.