Ocean Power

Interest in ocean power, the process of generating electricity through ocean temperature differentials between surface and deep water, was the subject of a modest amount of research effort in the 1970s and early 1980s.
By Anne White-Clergerie
May/June 1980
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Mini-OTEC is the first ocean power plant to produce more electricity than it uses. The barge is moored off Hawaii's coast.
PHOTO: ANNE WHITE-CLERGERIE
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Today—as energy shortages and escalating prices force us to accept the indisputable fact that man has nearly depleted the planet's store of fossilized solar power—it's apparent that we must rapidly change our methods of energy production and our habits of consumption. Windmills, biomass conversion centers, hydroelectric plants, and solar collectors are already being used to help us redirect our energy needs toward renewable sources. And, late last year, another benign method of harnessing the sun's power became a reality off the coast of Hawaii.

OTEC—or Ocean Thermal Energy Conversion—is a hundred-year-old idea that was brushed aside during the years of fossil fuel abundance. However—under the pressure of our recent energy shortages—the concept has undergone a renaissance which could eventually make ocean power a significant source of both economical and environmentally sound power. By taking advantage of the natural difference in temperature between sun-warmed surface water and the chilly depths, OTEC is tapping our planet's largest solar collectors ... the seas.

Electricity is generated in an ocean thermal plant through a Rankine cycle (condensation/vaporization cycle) ... the same basic technique that air conditioners employ. However, OTEC combines warm and cold water to produce electricity instead of using electricity to separate warm and cold air. The theory is rock solid, but until recently OTEC was nothing more than a good idea.

Then, in early August of 1979, a converted U.S. Navy dump scow—moored in 3,000 feet of water off Keahole Point, Hawaii—became the world's first ocean thermal plant to produce more electricity than it consumed. So far, the barge isn't putting out much juice by power plant standards: Its total capacity is only 50 KW, and 10 KW of that wattage goes to plant operation ... but it is a beginning! The floating power producer—designed and built by the State of Hawaii, Lockheed Missiles and Space Company, and the Dillingham Corporation—has demonstrated that OTEC is more than theory.

Admittedly, like Edison's first light bulb, Mini-OTEC's operation is feeble and sometimes cantankerous, but the real economical application of the concept is almost entirely a matter of upping the prototype's scale. From the titanium plate-type heat exchangers (donated by Alfa-Laval of Sweden) to the Rotoflow turbine/generator system, all of Mini-OTEC's hardware is off-the-shelf, state-of-the-art equipment. Similarly, her moorings consist of common materials such as polyethylene pipe ... and her anchor is made of concrete.

Mini-OTEC pumps up about 2,700 gallons of 40°F water per minute from 2,500 feet, and employs an equal amount of 80°F surface water. The warm liquid evaporates ammonia ... which is circulated through pipes arranged in heat exchanger fashion. Pressure—developed by the vaporization of the fluid—then spins a turbine and power generator. Upon exiting from the turbine, the ammonia flows into another heat exchanger (located in the cold water inlet) which condenses the gas and readies it for reuse.

One of OTEC's great advantages is that it doesn't pollute. The temperature of the liquid involved in the process is changed by only 2° to 3°F ... so the warm water that's returned to the sea tends to remain near the surface, and the cold—in turn—descends to find a layer of approximately its own density. Thus OTEC merely exploits the natural ocean patterns of sun-warmed surface water and deep, cold, arctic currents.

Of course, as you'd anticipate from any prototype, there are problems to be worked out. Extensive research has been going on for the past several years to develop methods to prevent efficiency-robbing marine organisms—such as barnacles—from attaching themselves to the heat exchangers. And there's also the question of how to transmit the energy generated at an OTEC plant to the mainland. One promising solution involves the "grazing" concept, which suggests that the barges be directed by satellites to the warmest pools of water. The plants would then use their electricity to produce hydrogen ... which could be shipped in a liquid state.

Whatever form the energy potential of the oceans takes, OTEC will play a role in our shift from nonrenewable to renewable fuels. With 70% of the earth's surface area covered by water, the capacity of our seas to hold valuable solar heat is a resource we cannot afford to ignore.








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