Swarms of marine turbines could 'tap the Gulf Stream'

World's largest tidal turbine
Image caption The current best-bet ideas for marine energy make use of large, fixed turbines

Darris White is a deep thinker.

The engineer at Embry-Riddle Aeronautical University in the US is currently finalising designs for a series of turbines that could be used to harness the immense energy of the Gulf Stream, flowing deep in the Atlantic Ocean.

The underwater stream roughly contains around 21,000 times more energy than the Niagara Falls and by some estimates, could potentially provide up to one-third of Florida's electricity needs.

"Hydrokinetic power from the Gulf Stream can provide enough power for over a million households in Florida," said Professor White.

But that is easier said than done: harnessing that energy needs to happen 50m below the surface of the ocean in turbulent and constantly changing conditions.

The "marine energy" industry has come up with a number of ideas to make use of the movement of water around the globe, be it from ocean waves, tides slipping into and out of inlets, or regular ocean currents like the Gulf Stream.

The more common solution to the problem has been to build large turbines, to be anchored to the seabed.

Image caption "Swarm intelligence" accomplishes a single goal even if no individual is in charge

But the nature of the Gulf Stream presents different challenges, said Professor White.

"Even though the Gulf Stream is constrained between two bodies of land, the flow rate and location of peak velocity will change, based on seasonal and weather conditions."

The solution, Professor White and his team suggest, are autonomous turbines with so-called "swarm intelligence" that can navigate through the ocean currents, similar to a school of fish searching for food.

"Swarm intelligence can achieve two goals. One is to find the 'sweet spot' of the Gulf Stream, which is the location where the array will achieve maximum power output," he said.

"The other goal is to find the array orientation and alignment that provides optimal efficiency."

Current thinking

A prototype is currently under construction and should be complete within the next 18 months, he said.

The team plans to equip the turbines with sensors that detect the change of hydrodynamics and the swarm's own movements, along communication mechanisms so that turbines can "talk" to one another and share their position.

The entire swarm will either be tethered to the sea floor with anchors, allowing them to migrate within a limited area, or be attached to a movable platform for fixing and transferring the power.

Power from all the turbines will be integrated into a single transmission line and transmitted to a substation on land through high-voltage power lines.

The idea of harnessing the power of the Gulf Stream is not entirely new.

Gulf Stream Turbines is a start-up company that holds several patents for water turbine designs; its founders hope to tie up with interested parties to develop the technology further and produce inexpensive energy continuously from the ocean currents.

But Professor White and his team believe their solution has several advantages over other approaches and current renewables, such as wind turbines.

"The best wind resources are in sparsely populated areas, which results in transmission challenges," he explained.

"Water turbines placed in streams, rivers and ocean currents provide a relatively constant source of power with fewer intermittence problems."

An array of 30 to 50 turbines is expected to generate around 15 to 20 million Watts of electricity at the sweet spot in the Gulf Stream, which is enough energy to meet the requirements of around 6000 to 8000 houses.

However, some experts are sceptical of the idea.

"It will require a herculean effort to tackle this approach," says Trey Taylor, president of renewable firm Verdant Power.

"Collectively, the industry does not know enough yet about all of the variables that need to be addressed in this effort."

Peter Fraenkel, technical director of leading UK marine energy firm Marine Current Turbines, agrees.

"The main disadvantage of this approach is that it does not sound very practical," he said.

In particular, he said, the forces involved with extracting energy form the Gulf Stream are huge.

"This force has to be carried through any moorings and anchors into the seabed."

Ordinary anchors would simply plough a furrow, rather than staying put.

As a result, he said, the turbines would need to be fixed to the seabed with solid anchors.

"How do you drill holes in the seabed 300 metres underwater?," he asked. "With considerable difficulty."

Spot on?

There's also a question as to whether the turbines need to move around at all.

"The location of the highest currents is very predictable in these cases, and there would little benefit in trying to move the turbines around 'intelligently' from day to day," says Chris Lawn, Professor of thermo-fluids engineering at Queen Mary, University of London.

Image caption Oil rig technology may figure into eventual solutions for the swarm

However, Professor White and his colleague Yan Tang are not put off.

They point to evidence - although limited - that shows the sweet spot can move by a distance of up to 1km and that fixing the turbines in place wouldn't allow for optimal operation during all seasons.

"Even if the currents don't migrate that much, the current direction may change. So we need to adjust water turbine orientation to achieve optimal performance," says Dr Tang.

The team also thinks that existing mooring systems developed for movable offshore rigs could be adapted to help the swarm operate, rather than developing an entirely new fixture.

They are also trying to head off other potential problems early, like the turbines' effect on sea life.

One solution could be in-built intelligence that either shuts down the turbines or move the whole swarm out of the way of sea life.

Right now, the team is busy constructing a prototype and hopes to begin tests in 2012.

Testing individual or swarm turbines in the ocean will require the team to gain permission and permits from several federal agencies including the US Army Corp of Engineers.

"We need to overcome certain major milestones but the largest issue we face is permitting," said Professor White.

"A permanent placement of water turbines in the Gulf Stream will also require an extensive and ongoing environmental impact study," he said.

However, he added, it will be worth it if they can prove the concept works.

"Greenhouse gases would be reduced by the same amount as removing over a million vehicles from the road.

"I have three young daughters and one reason we are working on renewable energy projects is to ensure that our children and future generations enjoy the same quality of life that we have today."

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