Types of Ocean energy - Ocean energy resource
Types of Ocean energy
Published on Ocean energy  |  April 30, 2010, 22:43
Offshore Wind Energy | Ocean Current Energy | OTEC Energy | Tidal Energy | Wave Energy

The ocean can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves.

Oceans cover more than 70% of Earth's surface, making them the world's largest solar collectors. The sun's heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy. Just a small portion of the heat trapped in the ocean could power the world.

Ocean thermal energy is used for many applications, including electricity generation. There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems.

Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves are driven primarily by the winds. As a result, tides and waves are intermittent sources of energy, while ocean thermal energy is fairly constant. Also, unlike thermal energy, the electricity conversion of both tidal and wave energy usually involves mechanical devices.

A barrage (dam) is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator. For wave energy conversion, there are three basic systems: channel systems that funnel the waves into reservoirs; float systems that drive hydraulic pumps; and oscillating water column systems that use the waves to compress air within a container. The mechanical power created from these systems either directly activates a generator or transfers to a working fluid, water, or air, which then drives a turbine/generator.

source : http://www.renewableenergyworld.com/rea/tech/ocean-energy

Offshore Wind Energy

Wind energy is an indirect form of solar energy. It is estimated that 1-2% of the solar radiation that reaches the earth is converted to wind energy. In general, wind results from an unequal heating of different parts of the earth, causing cooler, dense air to circulate to replace warmer light air.

While some of the sun's energy is absorbed directly by the air, most of the energy in the wind is first absorbed by the surface of the earth and then transferred to the air by convection.

Ocean Current Energy

Areas that typically experience high marine current flows are in narrow straits, between islands and around headlands.

Entrances to lochs, bays and large harbours often also have high marine current flows (EECA,1996). Generally the resource is largest where the water depth is relatively shallow and a good tidal range exists.

In particular, large marine current flows exist where there is a significant phase difference between the tides that flow on either side of large islands.

OTEC Energy

Ocean thermal makes use of temperature gradients in a thermal (Rankine) cycle process.
OTEC plants must be located where a difference of about 40° Fahrenheit (F) occurs year round.
Ocean depths must be available fairly close to shore-based facilities for economic operation.
Floating plant ships could provide more flexibility.

Only small-scale versions have been made.
The largest so far is near Japan, and it can create 100 kilowatts of electricity.
Another small-scale OTEC is off the coast of Hawaii, producing 50 kilowatts of electricity.
If a successful OTEC is built, it is planned to produce 2 megawatts of electricity.
However, a full scale OTEC would cost many millions of dollars, and it would be very difficult to build.

Tidal Energy

Tidal energy is one of the oldest forms of energy used by humans. Indeed, tide mills, in use on the Spanish, French and British coasts, date back to 787 A.D.. Tide mills consisted of a storage pond, filled by the incoming (flood) tide through a sluice and emptied during the outgoing (ebb) tide through a water wheel. The tides turned waterwheels, producing mechanical power to mill grain.

Tidal power is non-polluting, reliable and predictable.Tidal barrages, undersea tidal turbines - like wind turbines but driven by the sea - and a variety of machines harnessing undersea currents are under development.

Unlike wind and waves, tidal currents are entirely predictable.

Wave Energy

 Wave energy is an irregular and oscillating low-frequency energy source that can be converted to a 60-Hertz frequency and can then be added to the electric utility grid. The energy in waves comes from the movement of the ocean and the changing heights and speed of the swells. Kinetic energy, the energy of motion, in waves is tremendous. An average 4-foot, 10-second wave striking a coast puts out more than 35,000 horsepower per mile of coast.
Waves get their energy from the wind. Wind comes from solar energy. Waves gather, store, and transmit this energy thousands of miles with little loss. As long as the sun shines, wave energy will never be depleted. It varies in intensity, but it is available twenty-four hours a day, 365 days a year.
Ocean wave energy technologies rely on the up-and-down motion of waves to generate electricity. The first wave-power patent was for a 1799 proposal by a Parisian named Monsieur Girard and his son to use direct mechanical action to drive pumps, saws, mills, or other heavy machinery. Installations have been built or are under construction in a number of countries, including Scotland, Portugal, Norway, the U.S.A., China, Japan, Australia and India.

source : http://www.oceanenergycouncil.com/

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