Ocean Thermal Energy Conversion (OTEC) is a process which utilizes the temperature difference between warm surface seawater and cold deep ocean water to drive turbines to generate electricity. The OTEC process requires a temperature difference of approximately 20°C to 24°C, unlike geothermal, coal, nuclear and diesel power plants which employ a gaseous medium with a temperature difference of several hundred degrees. This is achieved by using concentrated solar energy that has been absorbed by warm 24° to 29°C tropical surface seawater in combination with cold 6°C seawater pumped from a depth of at least 600 meters. Since the temperature of both the tropical surface seawater and the cold deep ocean water stays fairly constant daily and throughout the year, the 20°C temperature difference is always available both day and night and from season to season. Using the sun for its heat source, the OTEC process is free of any kind of emissions and is, therefore, non-polluting.
Basically, there are two different OTEC systems for the extraction of thermal energy from the oceans to generate electricity: the open-cycle system and the closed-cycle system.
Open Cycle OTEC exploits the scientific principle that water boils at low temperature in a vacuum. In this system, warm surface seawater (about 25°C) is pumped into a vacuum chamber. The low pressure (1,400 pascals) of the chamber causes the seawater to boil and partially vaporize at that low temperature. The resulting low-pressure steam turns a turbine that drives an electrical generator. After it has passed through the turbine, the steam is then condensed either by ?direct contact? with cool seawater or by passing it over a heat exchanger through which cold ocean water is pumped, thereby providing a
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OTEC ? Open cycle diagram ? Image adapted from NELHA
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cold surface to reliquefy the steam. The resultant condensate is a bonus byproduct ? desalinated water that has the purity of distilled water and a crisp taste and can therefore be used without treatment for drinking and irrigation. A 250 kW experimental open-cycle plant was built at the Natural Energy Laboratory of Hawaii Authority (NELHA) in the early 1990s. This facility has used 150 kW to operate the plant, resulting in 100 kW of usable electrical power and 7,000 gallons of fresh (desalinated) water per day.
Georges Claude, a French scientist who also invented the neon sign, first
designed an open-cycle OTEC system, which he tested at Matanzas Bay in northern Cuba in 1930. The turbine generated 22 kW of gross power, but consumed more than that in operating, partly because of poor site choice.
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Jacques-Arsène d?Arsonval, a French scientist, first proposed the concept of a closed-cycle OTEC system in 1881. In the closed-cycle system, warm surface seawater is pumped into a heat exchanger (the evaporator) containing a low-boiling point ?working? fluid such as ammonia. Heat transferred from the warm surface water via the heat exchanger vaporizes the working fluid. The expanding vapor turns a turbine driving an electrical generator. Cold deep ocean water pumped to a second heat exchanger (the condenser) provides a cold surface to reliquefy the ammonia vapor. The fluid is then returned to the first heat exchanger.
A hybrid OTEC plant can be used for desalinated water production; the hybrid plant includes a closed cycle component for electricity generation and a second stage, consisting of a separate vacuum ?flash? evaporator and a surface condenser,
for fresh water production.
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OTEC ? Closed cycle diagram ? Image adapted from NELHA
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It is estimated that a 1 MW hybrid cycle plant can produce
one million gallons of fresh water per day , while a 50
MW plant can produce as much as 16 million gallons of
water per day. That is sufficient to support a community
of approximately 300,000 people in the developing world.
The first OTEC plant to yield more electrical power than it consumed was ?Mini-OTEC?. It was a closed-cycle system built in 1979 by mounting it on a barge moored about two kilometers off Keahole Point where NELHA is located.
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