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Capturing Ocean Heat

Ocean thermal energy conversion offers a timely renewable alternative.

Fortnightly Magazine - October 2009

According to the National Renewable Energy Laboratory, 1 60 million square kilometers (23 million square miles) of tropical oceans daily absorb solar radiation equal in heat content to about 250 billion barrels of oil ( e.g., 1,450 Quads). This is an order of magnitude greater than the expected total U.S. energy consumption through 2030, and twice the projected global energy demand. Even using present maximum estimates for steady-state sustainable energy harvesting 2 ( e.g., 3 to 5 TW, or 90 to 150 Quads), this resource still provides enough clean and non-GHG emitting energy to supply 15 to 20 percent of the global energy demand in 2030.

Ocean thermal energy conversion (OTEC) technologies convert the solar radiation that heats the surface of the ocean into electrical power by exploiting the thermal gradient temperature differences between the surface and the depths. This temperature gradient in the Tropics ( see Figure 1 ) can be 20 degrees C (36 degrees F) or more between the warm surface water and the cold deep seawater, which is sufficient to produce usable power, albeit not very thermodynamically efficiently ( i.e., 3 to 5 percent). It should be noted that lying within the Tropical zone——the area most favorable for OTEC—are some 29 territories and 66 developing nations, as well as portions of Australia and Hawaii, all of which are natural markets for OTEC-generated energy and other side-products.

This enormous resource merits a closer look as policy makers consider alternative technologies for serving future energy demands. Achieving viability, however, will require more supportive and stable regulatory policies as well as funding for research and development.

Ocean Power’s History

The science behind OTEC was first described in 1881, when a French physicist, Jacques Arsene d’Arsonval, proposed using what came to be known as a closed-cycle plant to tap the thermal energy of the ocean; 3 however, it took almost 50 years for the concept to be applied. In 1930, Georges Claude, a student of d’Arsonval, built the first open-cycle OTEC plant in Matanzas Bay, Cuba, which produced 22 kilowatts (kW) of electricity before being destroy-ed by inclement weather and waves. Undeterred, Claude constructed another open-cycle plant aboard a 10,000-ton cargo vessel moored off the coast of Brazil in 1935; however, it too was destroyed by weather and waves before he could produce net power. 4 Twenty-one years

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