Clean Energy From An Unlikely Source: Lukewarm Oceans

The sun pours down a lot of energy onto the surface of the ocean every day. Now scientists are figuring out a way to get that energy onto the grid, with only seawater as a waste product. It just requires a really, really long pipe.

Clean Energy From An Unlikely Source: Lukewarm Oceans
Flickr user David Hopkins Photography

There is a lot of seawater on the planet. And that seawater is warmed, daily by the sun. If only we could tap it, warm seawater offers a virtually limitless source of pollution-free energy. Good news, we can.


Ocean thermal energy conversion (OTEC) converts the energy stored in warm seawater into electricity. It works much like any other power plant that burns coal or gas to create steam and turn turbines. OTEC just works at much lower temperatures and pressures, with seawater as the only heat source and the only by-product.

The mechanics are relatively simple. Typically, warm surface water is pumped into a heat exchanger to vaporize a fluid, such as ammonia, with a low boiling point. The vapor rushes through a network of sealed pipes to spin turbines and generate power before it is condensed back into a liquid using cold water pumped up from more than 1 kilometer below. Theoretically, OTEC works anywhere ocean temperatures differ by more than 36 degrees.

After decades of rock bottom oil prices, OTEC is looking profitable as prices rise, says Robert Varley at Lockheed Martin, one of several companies pursuing the technology. “The challenge is to pick the best and most efficient technologies together in one system, and then you’ll have the birth of a new industry,” he says. 

Most of the technology to build a 100-megawatt (MW) plant already exists. Deep-water floating ocean platforms, pioneered by the oil and gas industry, are common, and rugged aluminum heat exchangers exist. The holdup is the massive cold-water plumbing. The process requires a 10-meter diameter tube at least one kilometer long and able to survive the open ocean to pump billions of gallons of cold water. No such pipe has been manufactured, although most engineers say this is just a matter of time.

For now, there are a handful of organizations such as OTEC International, Ocean Thermal Energy Corporation, and a joint effort between Makai Engineering and Lockheed Martin–to build smaller or demonstration plants on their way to a full scale of 100 MW or more. “There is no more R&D needed to build a power plant of 10 megawatts,” says Jeremy Feakins, CEO of Ocean Thermal Energy Corporation, which plans to build the world’s first commercial OTEC plant for the Bahamian government. “The company that is first to market with OTEC will be the market leader.”  

Now it’s about who can find the investment, and refine the technology first. Governments from Africa to the Caribbean, as well as the U.S. military, are all keen to see the first megawatts generated by nothing more than the clear, blue ocean.

About the author

Michael is a science journalist and co-founder of Publet: a platform to build digital publications that work on every device with analytics that drive the bottom line. He writes for FastCompany, The Economist, Foreign Policy and others on science, economics, and the environment.