The oceans, the nearest things to perpetual motion, are being increasingly explored for energy potential as we shed fossil fuel dependency. Now a team in Florida has released a comprehensive global map of potential ocean energy locations around the world and show how ocean current turbines could tap that power.

Out in the Bass Strait the waves are legend; southern ocean gales coupled with rocky outcrops make this one of the most treacherous stretches of water in the world. But all that raw wildness means renewable energy.

The Commonwealth Scientific Industrial Research Organisation (CSIRO) recently released Australia Wave Energy Atlas indicating such energy could provide up to 11 percent of the country’s power needs, that’s a Melbourne-sized city-worth, 5.3 million people. 

But the team from Florida Atlantic University (FAU) has explored the potential for capturing kinetic energy from ocean currents. In a paper published in Renewable Energy, they investigated currents around the world, focussing on power densities and their variation over time and location.

The study was based on 30 years of data from the National Oceanic and Atmospheric Administration (NOAA) Global Drifter Program which has 1250 satellite-tracked buoys measuring ocean currents and their positions.

Some 43 million data points were used, covering March 1988 to September 2021 and four regions were explored: South-East USA, Brazil and French Guiana, South Africa and East Asia.

It is  “the most comprehensive global assessment of ocean current energy to date” says lead author Mahsan Sadoughipour of FAU.    

High power densities were detected off South Africa and Florida’s East Coast, suggesting ideal locations for power generation — 2500 watts per square metre, 2.5 times more energy dense than what is formally described as an ‘excellent’ wind resource, he adds.

Sadoughipour says they found 75% of the total high-power density areas, covering around 490,000 square kilometres of the ocean, have energy levels between 500 and 1,000 watts per square meter.  “This suggests a lot of potential for sustainable ocean current energy harvesting.”

High power densities, 2,000 watts or more, per square metre, were found off the Southeast coast of the U.S. from Florida to North Carolina and along the Eastern and Southeastern coasts of Africa (Somalia, Kenya, Tanzania, South Africa and Madagascar). Lower power densities showed up in the eastern Pacific (Japan, Vietnam and Philippines), Northern South America (Brazil and French Guiana), and the Eastern coast of Australia.

Sadoughipour and his team also found seasonal variation, with summer providing relatively high power densities in inshore shallow waters off Florida, Japan and Brazil from June to August and off South Africa in December and February.  For Australia, it was winter that showed reasonable potential, with power densities exceeding 1000w per m².

Three key factors determine site suitability: current speed, distance from shore and depth.

On that basis, Sadoughipour concluded that Florida and North Carolina, and South Africa and Japan, the latter two with more variable current speeds, offered substantial potential for sustainable energy generation using ocean current turbines (OCTs.).

These OCTs are the same idea as wind turbines, with blades spinning in response to current movement — in this case inside cylinders, moored below the surface and anchored to the seafloor. South-western Japan’s Kairyu OCT project is currently scaling up to 2MW, with a full-scale system intended to have blades 40m in diameter.

Turbine design and placement relies on the relationship between depth and power, says co-author James Van Zwieten. “Strong ocean currents are located near the sea surface where the total water depth typically ranges from 250 meters to more than 3,000 meters,”

Which makes operation, mooring attachment and stability a challenge, he says, adding to complexity and cost. Marine fouling would also be a major issue.

Such developments, particularly those on the scale of Kairyu, may have significant ecological impacts.  The researchers do note environmental concerns, but issues such as changes in flow, animal collisions with the albeit slow-moving blades, noise and the potential for electromagnetic interference from power cable have not been fully explored for this new technology.

“Findings from this study highlight the need to carefully consider these variables, and the provided energy characteristics will help ensuring that ocean current energy can be efficiently integrated into the broader renewable energy landscape.” said Stella Batalama of dean of the College of Engineering and Computer Science at FAC.

Ocean currents and wave power