Covering about three quarters of the earth, our continuously flowing oceans hold the potential to be one of the biggest - and cleanest - renewable energy sources of the future.
Compared to wind and solar power, tidal power is still very much in its infancy. The past decade has seen its growth from initial concepts and companies are now in the early stages of developing ocean current technology.
The US is one nation that is heavily investing in the potential of marine and hydrokinetic (MHK) energy, with the Department of Energy (DOE) predicting that the Gulf Stream alone has the potential to supply nearly 30% of the power consumption in North Carolina, South Carolina, Georgia and Florida - the equivalent of $15bn a year in sales of electricity.
World's first ocean current energy test site
One of the biggest developments in recent months comes from Florida Atlantic University (FAU), which has been granted permission by the US Department of Interior's Bureau of Ocean Energy Management to install the world's first ocean current energy test site in the outer continental shelf.
The five-year lease, which permits FAU's Southeast National Marine Renewable Energy Centre (SNMREC) to install multiple anchored floating test berths offshore Florida, will allow marine energy technology companies to test their ocean current turbine technology in waters 13 miles offshore from Broward County.
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Minesto, whose Deep Green power plant recently became the first known marine power plant to generate electricity from low velocity currents, will work, together with the University, to examine the technical, environmental and economic feasibility to install demonstration and commercial power plants in the Florida current.
Exploring deep ocean current as an energy source
Whereas renewable energy sources such as wind and solar energy face intermittency issues, deep oceans stand out as a reliable energy source because of their continuously flowing current, according to Minesto managing director Anders Jansson, who says that the Gulf Stream has the equivalent of two million nuclear power plants of energy passing through it all the time.
Susan Skemp, executive director of SNMREC, adds that the 24/7 nature of deep ocean current sets it apart from shallower waters, where tidal energy depends "on the waves being strong enough and the winds being forceful enough to move those waves".
"Wind energy is only produced when the wind blows, so you don't know exactly what it's going to produce, whereas with ocean power you know exactly when is being produced and how much, making it much easier to transform the old energy mixed on coal and gas to renewable," adds Jansson.
"The importance of this is that we can have a stable renewable source producing electricity into the grid."
Developing ocean current technology in Florida's waters
As with any industry in the early stages of development, there are challenges to overcome, the biggest one being consent, says Skemp, who describes the five-year lease awarded to FAU's SNMREC as "a breakthrough".
Work on establishing the world's first offshore ocean current turbine test site began in 2007, and included sea trials of the first test berth buoy, as well as preliminary tow tests of a small-scale research turbine. An environmental assessment concluded that "no significant impact" was expected from the installation of the site, which has received close to $20m in funding from the US DOE, the state of Florida and private companies.
"The biggest hurdle in effect, which we've just overcome, was actually getting through the regulatory environment here in the US because that process was not in place for marine renewable energy," Skemp says, adding that the environmental assessment carried out in the area was also a first.
Jansson adds that consent can delay an installation more than the actual design and manufacturing of the technology. "It might be rather easy to get a prototype in the water, but when you start building larger arrays, you might need to close down certain parts of the ocean", which can lead to objections and defence issues, he says.
Challenges of MHK energy regeneration
Minesto is also prepared for challenges that may be encountered with its Deep Green technology as the project progresses. Although the power plant is currently undergoing ocean trials off the coast of Northern Ireland, and plans are in place to install a 10MW marine energy array in Wales in 2016, its partnership with FAU marks the first time it has looked into developing the technology in a deep ocean current in this part of the world.
Jansson says a warmer climate could have a different effect on materials, while a higher level of salinity and different types of species will also need to be considered. "The challenge is how to produce electricity from these currents in a safe way both from an environmental perspective such as seals, dolphins, whales and sharks, and also without interfering with these currents, which play an important part of the ecosystem."
FAU's SNMREC has outlined the potential impact on Florida's marine mammals, and shipping, recreational boating and fishing as important environmental issues to consider, but Skemp says studies into these will be ongoing throughout the overall project.
"Many of the environmental effects analyses will have to be conducted once the mooring buoy is deployed and testing of a device is initiated. For example, what we are doing now is establishing baseline information for species distribution and migratory patterns for sea turtles and marine mammals. From that we will be able to determine if there are any affects due to deploying the infrastructure and testing devices," she says.
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"We need to understand what happens when we start putting these types of devices into the water," Jansson adds. "We have a fairly good idea that not too much will happen but it's up to us to prove that it doesn't happen and that's where the university will play a very important role to underpin this with scientific facts."
Ocean power as a future clean energy source
When it comes to the power plant's suitability for energy regeneration in Florida's current, Jansson is very confident.
Equipped with a hydrodynamic wing and a gearless turbine anchored to the ocean bed with a tether, Deep Green is said to produce more electricity than any other ocean current technology because of the rate at which water hits the turbine. Water current creates a hydrodynamic lift force on the wing, which pushes the kite forward; as the kite moves, water flows through the turbine and electricity is produced in a gearless generator.
Now the priority for Minesto is to work with FAU over the coming autumn to start looking into the technical feasibility of the test area and explore ways of mounting the technology to a sea bed or floating structure. "The only way we can really find out if it's going to be as good as we believe is to get the device into the water and test it," Jansson says.