Riding Underwater Currents Like a Kite  

There seems to be plenty of ocean power news out this week. Scientific American leads off with a report on some new technology in Scotland - The Tide Is Turning: Turbine Rides Underwater Currents Like a Kite.

There is no market yet for turbines that turn the tides into a source of energy from deep beneath the sea. But that has not stopped mechanical engineers at the University of Strathclyde's Energy Systems Research Unit (ESRU) in Scotland from developing one that will ride the tide while latched to the seabed by a cable—like a kite flying on a windy day.

The ESRU team's goal: create a device that literally goes with the flow rather than resting on the sea bottom like an underwater windmill—a model already being developed by a handful of companies. The kite and cable model is designed to facilitate placing tidal turbines in deep water, where the stronger current has the potential for providing greater power but also makes it extremely difficult to plant a turbine in the seabed.

"The problem with regular turbines is the bigger they get, the harder they work, and the more likely the force of the water is to damage the turbine," says Andrew Grant, an ESRU mechanical engineer. "Our turbine can fly like a kite in the water." Instead of planting the base of a turbine in the seabed, researchers need only plant an anchor for the tether.

Another key difference in ESRU's design is that the turbine has two rotors attached—one in front of the other that turn in opposite directions on a single axis. The rotors' blades are made of either solid aluminum alloy or glass-reinforced plastic, depending on their sizes. By having the rotors turn in opposite directions, Grant and his team are trying to cut down on reactive torque (which pushes the turbine in the opposite direction) so that the unit can be attached to a relatively simple mooring system even in very deep water.

This "contra-rotating" design has been tested on wind farms since the 1980s but did not provide an advantage (in terms of generating more energy with less wind) in the open air, Grant says. ...

Lunar Energy, a U.K. tidal power company, in March began working with Korean Midland Power Company to create a giant 300-turbine field in the Wando Hoenggan Waterway off the South Korean coast. The plant is expected to provide 300 megawatts of renewable energy to Korean Midland Power by December 2015. Utilities interested in tapping into tidal power will have to spend money to create the energy-delivery infrastructure, or at least convince government to pay for it. One thing working in favor of new energy sources: the cost of oil is not getting any cheaper.

Tidal power technology developer Pulse Tidal have said its hydrofoil device could be operational next month in the Humber Estuary. The Sheffield company has installed piles ready for the main structure of the system to be put in place this week in water depths of about five metres.

The Pulse system is designed to generate power from tidal currents in shallower waters than tidal systems that use rotors to drive turbines. It uses two 12 metre long hydrofoils - which are like large underwater aeroplane wings that rise and fall in a tidal current to generate power.

The 100kW test system is to be connected to the grid at the Millennium Inorganic Chemicals plant on the south bank of the Humber.

Howard Nimmo, executive director at Pulse Tidal, was speaking at the Renewable Energy Association's annual Wave and Tidal Technology Symposium in Cardiff on Tuesday. He said: "The main structure is going to go offshore later this week. We hope to have the machine operational some time this October, so it's quite an exciting time." ...

In the location of the current Beatrice Offshore Wind Farm, the wind turbines can produce 5MW, but in the 45 sea depths a subsea rotor system could produce just 1.5MW - or 3MW from a twin-rotor system, he said. The Pulse system in similar depths could generate more than 5MW, Mr Nimmo claimed.

"For Pulse, the length of the foils are not dependent on depth, we can get a long and big swept area, which gives us an advantage against that key development barrier," Mr Nimmo said.

While the system being installed at the moment will generate only enough power for the equivalent of 70 households, the Pulse director said the company is already working on a 1.5MW demonstration project, and has hopes of scaling up to a 5MW system.

The company is aiming to develop oscillating hydrofoil columns that could stretch across a tidal stream.