What is Ocean Thermal Energy Conversion

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What is Ocean Thermal Energy Conversion

• The oceans cover a little more than 70 percent of
  the Earth's surface. This makes them the world's
  largest solar energy collector and energy storage
  system.
• On an average day, 60 million square kilometers
  (23 million square miles) of tropical seas absorb
  an amount of solar radiation equal in heat
  content to about 250 billion barrels of oil.
• If less than one-tenth of one percent of this
  stored solar energy could be converted into
  electric power, it would supply more than 20
  times the total amount of electricity consumed in
  the United States on any given day.

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There are three ways the OTEC method can produce
electricity.

• Closed-Cycle
• This system relies on low-boiling point fluids
  such as ammonia. The warm ocean water is used to
  heat up and boil the ammonia, which will turn a
  turbine. The turbine then produces electricity.

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Open-Cycle

• This system relies on a low-pressure environment
  to actually boil the ocean water and create
  steam. When warm ocean water moves into a
  low-pressure environment it will boil. The steam
  is almost pure water as the salts and other
  impurities are left behind. The steam then
  recirculates to deeper and colder areas of the
  ocean and condenses back to water. Just like the
  ammonia gas, the steam will turn a turbine.

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Hybrid System

• This system simply combines both Closed and Open
  Cycle systems.

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Ocean Thermal Energy Conversion

• The ocean thermal energy can be harnessed by
  means of a thermodynamic cycle, which uses the
  temperature gradient between the cold deep waters
  and the warm surface waters. It is estimated
  that, the amount of solar energy absorbed
  annually by the oceans is equivalent to 4000
  times the world energy demand in the same period

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• The main problem with this renewable energy is
  the necessity of a temperature gradient higher or
  equal to 20ºC, between the hot and cold
  reservoir. The higher the temperature difference,
  the better efficiency it will have.
• This requirement is only fulfilled in tropical
  and equatorial zones during the whole year.

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• The OTEC systems efficiency is not quite high
  because of the little temperature difference used
  in the thermodynamic cycles. Although the ideal
  energy conversion using 26 ºC and 4 ºC warm and
  cold seawaters is 8, due to several losses final
  3-4 efficiency is get

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Advantages

• Damaging environmental effects are minimized if
  the cold water is discharged to the ocean at
  enough depth.
• Highly available energy resources.

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Disadvantages

• The small land based OTEC plants need kilometres
  of piping to move a high volume of cold water fro
  deep ocean. Its cost could be up to the 75 of
  the total power plant costs. Therefore,
  researches show that power plants with a rated
  power lower than 50 MW can not compete with other
  energy sources.
• As an example, a 50 MW power plant would require
  a 3 km long pipe with a 8 m diameter to pump 150
  m3/s of cold water The suitable locations to
  harness this kind of energy are reduced to
  equatorial and tropical zones.
• Low thermal efficiency due to the low
  temperature difference between the cold and hot
  reservoir (around 22 ºC).
• Although floating OTEC plants could apparently
  be as olution, maintenance and repair costs would
  also be high.
• Floating plants and piping of land based plants
  must withstand high stresses during storms.

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Comparative table of different sea energy
resources