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Thermal Energy
What is Ocean Thermal Energy Conversion?
A
process called Ocean Thermal Energy Conversion (OTEC)
uses the heat energy stored in the Earth's 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 20°C (36°F). These conditions exist in tropical coastal
areas, roughly between the Tropic of Capricorn and the Tropic
of Cancer. To bring the cold water to the surface, OTEC plants
require an expensive, large diameter intake pipe, which is
submerged a mile or more into the ocean's depths.
Some
energy experts believe that if it could become cost-competitive
with conventional power technologies, OTEC could produce billions
of watts of electrical power.
History of OTEC
OTEC
technology is not new. In 1881, Jacques Arsene d'Arsonval,
a French physicist, proposed tapping the thermal energy of
the ocean. But it was d'Arsonval's student, Georges Claude,
who in 1930 actually built the first OTEC plant in Cuba. The
system produced 22 kilowatts of electricity with a low-pressure
turbine. In 1935, Claude constructed another plant aboard
a 10,000-ton cargo vessel moored off the coast of Brazil.
Weather and waves destroyed both plants before they became
net power generators. (Net power is the amount of power generated
after subtracting power needed to run the system.)
In
1956, French scientists designed another 3-megawatt OTEC plant
for Abidjan, Ivory Coast, West Africa. The plant was never
completed, however, because it was too expensive.
The
United States became involved in OTEC research in 1974 with
the establishment of the Natural Energy Laboratory of Hawaii
Authority. The Laboratory has become one of the world's leading
test facilities for OTEC technology.
Technologies
The types
of OTEC systems include the following:
 Closed-Cycle
These systems use fluid with a low-boiling point, such as
ammonia, to rotate a turbine to generate electricity. Warm
surface seawater is pumped through a heat exchanger where
the low-boiling-point fluid is vaporized. The expanding
vapor turns the turbo-generator. Cold deep-seawater-pumped
through a second heat exchanger-condenses the vapor back
into a liquid, which is then recycled through the system.
In 1979, the Natural Energy Laboratory and several private-sector
partners developed the mini OTEC experiment, which 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 km) 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 tested a 250-kW pilot
OTEC closed-cycle plant, the largest such plant ever put
into operation.
Open-Cycle
These systems use the tropical oceans' warm surface water
to make electricity. 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.
In 1984, the Solar Energy Research Institute (now the National
Renewable Energy Laboratory) developed a vertical-spout
evaporator to convert warm seawater into low-pressure steam
for open-cycle plants. Energy conversion efficiencies as
high as 97% were achieved. In May 1993, an open-cycle OTEC
plant at Keahole Point, Hawaii, produced 50,000 watts of
electricity during a net power-producing experiment.
Hybrid
These systems combine the features of both the
closed-cycle and open-cycle systems. In a hybrid system,
warm seawater enters a vacuum chamber where it is flash-evaporated
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.
Other Technologies
OTEC
has important benefits other than power production. For example,
air conditioning can be a byproduct. Spent cold seawater from
an OTEC plant can chill fresh water in a heat exchanger or
flow directly into a cooling system. Simple systems of this
type have air conditioned buildings at the Natural Energy
Laboratory for several years.
OTEC
technology also supports chilled-soil agriculture. When cold
seawater flows through underground pipes, it chills the surrounding
soil. The temperature difference between plant roots in the
cool soil and plant leaves in the warm air allows many plants
that evolved in temperate climates to be grown in the subtropics.
The Natural Energy Laboratory maintains a demonstration garden
near its OTEC plant with more than 100 different fruits and
vegetables, many of which would not normally survive in Hawaii.
Aquaculture
is perhaps the most well-known byproduct of OTEC. Cold-water
delicacies, such as salmon and lobster, thrive in the nutrient-rich,
deep seawater from the OTEC process. Microalgae such as Spirulina,
a health food supplement, also can be cultivated in the deep-ocean
water.
As
mentioned earlier, another advantage of open or hybrid-cycle
OTEC plants is the production of fresh water from seawater.
Theoretically, an OTEC plant that generates 2-MW of net electricity
could produce about 4,300 cubic meters (14,118.3 cubic feet)
of desalinated water each day.
OTEC
also may one day provide a means to mine ocean water for 57
trace elements. Most economic analyses have suggested that
mining the ocean for dissolved substances would be unprofitable.
Mining involves pumping large volumes of water and the expense
of separating the minerals from seawater. But with OTEC plants
already pumping the water, the only remaining economic challenge
is to reduce the cost of the extraction process.
Environmental and Economic Challenges
In
general, careful site selection is the key to keeping the
environmental impacts of OTEC to a minimum. OTEC experts believe
that appropriate spacing of plants throughout the tropical
oceans can nearly eliminate any potential negative impacts
of OTEC processes on ocean temperatures and on marine life
OTEC
power plants require substantial capital investment upfront.
OTEC researchers believe private sector firms probably will
be unwilling to make the enormous initial investment required
to build large-scale plants until the price of fossil fuels
increases dramatically or until national governments provide
financial incentives. Another factor hindering the commercialization
of OTEC is that there are only a few hundred land-based sites
in the tropics where deep-ocean water is close enough to shore
to make OTEC plants feasible.
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