The technology uses sea water from the ocean depths to generate electricity, and various pilot projects are already under way in the developing world.
For example, French defence firm DCNS Group aims to start building an OTEC plant on the Caribbean island of Martinique this year, to be completed by 2016. The company already has a prototype plant on Reunion in the Indian Ocean.
Oceans store solar energy on and near their surface, while the water 1,000 metres below is much colder, remaining at a nearly constant four to five degrees Celsius.
OTEC technology exploits this temperature difference. To create energy, warm water is sucked from near the surface and run through a heat exchanger containing a liquid with a low boiling point, such as ammonia. As this liquid changes to gas, it is used to turn a turbine and generate electricity. Then cold water pumped from the deep ocean is used as a coolant, causing the ammonia to condense into a liquid again. And the cycle is repeated.
The technology could be a particular boon for island nations near the equator, says Rémi Gruet, policy and operations director at Ocean Energy Europe, a trade association for ocean renewables, based in Brussels, Belgium.
“The cost of generating electricity through OTEC systems could rapidly come close to competitiveness.”
Rémi Gruet, Ocean Energy Europe
“That’s where the temperature difference is going to be highest, and thus the advantage of using that technology is going to be highest,” he says. “You need a good temperature difference for OTEC to be cost efficient and in developing countries such as Indonesia and African countries all across the equator you have an average [surface] water temperature of around 25 to 30 degrees, and that gives you a really good differential that can enable good electricity generation.”
In addition, many island nations in the tropics rely on expensive energy imports, so OTEC could help make them energy independent, says Linus Hammar, who is doing a PhD on the ecological risk assessment of renewable ocean energy at Chalmers University of Technology in Sweden.
But to fully benefit from the technology, such islands need to have sizeable populations because a full-scale OTEC plant would supply about a million people with electricity and drinking water made from sea water using the plant’s electricity, he says.
Last October, Hammar took part in the OTEC Africa Conference 2013 in Borås, Sweden. He says participants were told that the technology was likely to take off on a mass scale within 30 years.
The technology was first dreamt up in the late nineteenth century and experimented with in the 1970s, says Gruet. Recent technological developments have finally enabled us to develop it properly, he says.
“OTEC must be deployed in warm waters and countries, with generally high electricity prices. This means that the cost of generating electricity through OTEC systems could rapidly come close to competitiveness,” he adds.
But challenges remain. For example, firms are still experimenting with the pipes that are used to pump cold water to the surface. These need to be strong and flexible enough to withstand currents, and yet big enough to carry large volumes of water, says Gruet.
Various projects around the world are testing the OTEC concept. A 50 kilowatt test facility is already operating on Kume Island, Japan. Last year, US defence and technology firm Lockheed Martin announced plans to build a ten megawatt plant off the coast of southern China. And in the Bahamas, US company Ocean Thermal Energy Corporation is constructing a pair of commercial, ten megawatt plants.
In addition, Makai Ocean Engineering is constructing a 100 kilowatt plant in Hawaii, United States. And Bluerise, a spin-off company from Delft University of Technology in the Netherlands, aims to build a 500 kilowatt OTEC plant near the Caribbean island of Curacao.
While most leading OTEC manufacturers are from Europe, Japan, North America and South Korea, Gruet says he expects them all to test their technology in the developing world.
“You develop the technology at home because you have the technical capacity to do that, but then you go and test it somewhere else, where the temperature difference is higher,” he says.
See below for a video by Lockheed Martin on OTEC: