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Ocean Thermal Energy Converters (OTEC), use the temperature difference between cold deep ocean water and the warm surface water to produce energy.

This temperature difference is used to drive a turbine and produce electricity.

Prototype OTEC systems have been in development since 1979. The first such system developed by the US was placed in a converted Navy barge, off Keahole Point in Hawaii. It produced 50 kilowatts of power.

Jacques D'Arsonval a French Engineer envisioned the first OTEC in 1881, he never actually built one however.

Later in 1983, a design for a 40-megawatt plant to be placed Kahe Point off the coast of Oahu were completed but the funds for constructions were never released.

Estimates indicate that a total of about 10,000,000 megawatts of power are potentially available in the oceans.

Hawaii built a prototype 210-kilowatt OTEC system in 1995. It was built by the Natural Energy Laboratory of Hawaii. This system also produces 7000 gallons of fresh water a day. The cold water uplifted from the ocean depths is also used to air condition the 800 acres of the OTEC industrial park. A 10 megawatt plant would produce about 3 million gallons of fresh water per day.

OTEC’s are probably the most environmentally friendly energy available on the planet today.

The State of Hawaii's official site lists the advantages and disadvantages of OTEC's.

The technology for generating electricity from different ocean temperatures is known as "ocean thermal energy conversion," or OTEC. OTEC makes use of the difference in temperature between the warm surface water of the ocean and the cold water in depths below 2,000 feet to generate electricity. As long as a sufficient temperature difference (about 40 degrees Fahrenheit) exists between the warm upper layer of water and the cold deep water, net power can be generated.

Advantages

1. OTEC uses clean, abundant, renewable, natural resources. Warm surface sea water and cold water from the ocean depths replace fossil fuels to produce electricity.
2.Suitably designed OTEC plants will produce little or no contribute to acid rain or global warming (the "greenhouse effect"). Extensive research indicates little or no adverse environmental effects from discharging the used OTEC water back into the ocean at prescribed depths.
3. OTEC systems can produce fresh water as well as electricity. This is a significant advantage in island areas where fresh water is limited.
4. There is enough solar energy received and stored in the warm tropical ocean surface layer to provide most, if not all, of present human energy needs.
5. The use of OTEC as a source of electricity will help reduce the state's almost complete dependence on imported fossil fuels.
6. The cold sea water from the OTEC process has many additional uses, including air-conditioning buildings, assisting agriculture, and growing fish, shellfish, kelp and other sea plants which thrive in the cold, nutrient-rich, pathogen-free water.

Disadvantages

1. OTEC-produced electricity at present would cost more than electricity generated from fossil fuels at their current costs. The electricity cost could be reduced significantly if the plant operated without major overhaul for 30 years or more, but there are no data on possible plant life cycles.
2. 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.
3. Although extensive and successful testing of OTEC has occurred in experiments on component parts or small scale plants, a pilot or demonstration plant of commercial size needs to be built to further document economic feasibility. Construction of OTEC plants and laying of pipes in coastal waters may cause localized damage to reefs and near-shore marine ecosystems.
4. Some additional development of key components is essential to the success of future OTEC plants (e.g., less-costly large diameter, deep sea water pipelines; low-pressure turbines and condensers for open-cycle systems; etc.).

The energy available in the Hawaiian Exclusive Economic Zone is about 80 quads, more than enough to meet all of the United States energy needs. Enough in fact to once again make the US an energy exporter.

The company India OTEC, currently has a ship based OTEC in operation, it is aboard the vessel Sagar Shakthi.

Aquaculture is a feature of many OTEC's, the cold mineral rich water is used to grow plants and fish for sale.

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