Environmental Engineering Reference
In-Depth Information
A process called ocean thermal energy conversion (OTEC) uses heat energy stored in the oceans
to generate electricity. OTEC works best when the temperature difference between the warmer,
top layer of the ocean and the colder, deep ocean water is about 36°F (20°C). These conditions
exist in tropical coastal areas, roughly between the Tropic of Capricorn and the Tropic of Cancer.
To bring cold water to the surface, ocean thermal energy conversion plants require an expensive,
large-diameter intake pipe, submerged a mile or more into the ocean's depths. Some energy experts
believe that if ocean thermal energy conversion can become cost-competitive with conventional
power technologies, it could be used to produce billions of watts of electrical power (USDOE
2011). It has a long way to go before this happens. There are three kinds of OTEC systems: closed-
cycle, open-cycle, and hybrid.
Closed-Cycle OTEC
Closed-cycle systems use fluids with a low boiling point, such as ammonia, to rotate a turbine and
generate electricity. Warm surface seawater is pumped through a heat exchanger, where a low-
boiling-point fluid is vaporized. Expanding vapor turns a turbo-generator. Cold, deep seawater—
pumped through a second heat exchanger—then condenses the vapor back into a liquid that is
recycled through the system (Robinson 2006; Avery and Wu 1994).
In 1979 the Natural Energy Laboratory and several private-sector partners developed a mini
OTEC experiment that achieved the first successful at-sea production of net electrical power from
closed-cycle OTEC. The mini OTEC vessel was moored 1.5 miles (2.4 kilometers) off the Hawaiian
coast and produced enough net electricity to illuminate the ship's light bulbs and run its computers
and televisions. In 1999 the Natural Energy Laboratory successfully tested a 250-kilowatt pilot
OTEC closed-cycle plant, the largest such plant ever put into operation (USDOE 2011).
Open-Cycle OTEC
Open-cycle systems use the tropical oceans' warm surface water to make electricity, produc-
ing fresh water as a by-product. When warm seawater is placed in a low-pressure container, it
boils. The expanding steam drives a low-pressure turbine attached to an electrical generator. The
steam, which has left its salt behind in the low-pressure container, is almost pure, fresh water. It
is condensed back into a liquid by exposure to cold temperatures from deep-ocean water (Avery
and Wu 1994).
Hybrid OTEC
Hybrid systems combine the features of closed- and open-cycle OTEC systems. In a hybrid system,
warm seawater enters a vacuum chamber where it is flash-vaporized into steam, similar to the
open-cycle evaporation process. The steam vaporizes a low-boiling-point fluid (in a closed-cycle
loop) that drives a turbine to produce electricity (Avery and Wu 1994). An advantage of open or
hybrid-cycle OTEC plants is the production of fresh water from seawater. Theoretically, an OTEC
plant that generates 2 megawatts of net electricity could produce about 14,118 cubic feet (4,300
cubic meters) of desalinated water each day (USDOE 2011).
Ocean waters are constantly on the move. Ocean currents flow in complex patterns affected by
wind, water salinity and temperature, topography of the ocean floor, and the earth's rotation. They
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