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Energy

263: Oceanic thermal energy collectors (OTECS)

Problem:

The oceans cover a little more than 70 % of the Earth’s surface: this makes them the world’s largest solar energy collector and energy storage system. On an average day, 23 million mi.² (60 million km²) 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 % 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.

Solution:

OTEC (Ocean Thermal Energy Conversion) is a form of Solar Thermal Energy technology that essentially uses the ocean as a solar collector. OTEC takes advantage of the small temperature differential that exists between the warm surface of the sea and the cooler water at the bottom.


In deep waters in excess of 3280 ft (1000 m) this difference is as much as 20°C. In 1881, Jacques Arsene d’Arsonval, a French physicist, proposed tapping the thermal energy of the ocean.

D’Arsonval’s student, Georges Claude, built the first OTEC plant, in Matanzas, Cuba in 1930. The system generated 22 kW of electricity with a low-pressure turbine. The plant was later destroyed in a storm.

His idea called for a closed-cycle system.Thanks too recent technological advances in heat exchangers, D’Arsonval’s concept was demonstrated in 1979, when a small plant mounted on a barge off Hawaii (Mini-OTEC) produced 50 kW of gross power, with a net output of 18 kW. Subsequently, a 100 kW gross power, land-based plant was operated in the island nation of Nauru by a consortium of Japanese companies.

The economics of energy production today have delayed the financing of a permanent, continuously operating OTEC plant. However, OTEC is very promising as an alternative energy resource for tropical island communities that rely heavily on imported fuel.

OTEC plants in these markets could provide islanders with much-needed power, as well as desalinated water and a variety of mariculture products. One of the disadvantages of land-based OTEC plants is the need for a 1.86 mi (3 km.) long cold water pipe to transport the large volumes of deep seawater required from a depth of about 3,280 ft. (1,000 m).

Currently the world’s only operating OTEC plant is in Japan, overseen by Saga University. Japan is a major contributor to the development of OTEC technology. Beginning in 1970 the Tokyo Electric Power Company successfully built and deployed a 100 kW closed-cycle OTEC plant on the island of Nauru.

The plant became operational on October 14, 1981, producing about 120 kW of electricity; 90 kW was used to power the plant and the remaining electricity was used to power a school and other places. This set a world record for power output from an OTEC system where the power was sent to a real (as opposed to an experimental) power grid. (ioes.saga-u.ac.jp)

1981 also saw a major development in OTEC technology when Russian engineer, Dr. Alexander Kalina, used a mixture of ammonia and water to produce electricity. This new ammonia-water mixture greatly improved the efficiency of the power cycle.

In 1994 Saga University designed and constructed a 4.5 kW plant for the purpose of testing a newly invented Uehara cycle, also named after its inventor Haruo Uehara. This cycle included absorption and extraction processes that allow this system to outperform the Kalina cycle by 1-2 %.

Currently, the Institute of Ocean Energy, Saga University, is the leader in OTEC power plant research and also focuses on many of the technology’s secondary benefits. The laboratory located at lmari bay area, some 50km to the north from the administrative office in Saga City, functions as our study center of fundamental and practical aspects of OTEC technology.

The Ocean Thermal Energy Corporation with offices in Pennsylvania, Virginia, Hawaii, The Bahamas, and Cayman Islands been preparing clean hydrothermal energy plants worldwide using the proven technologies of Ocean Thermal Energy Conversion (OTEC) and Seawater Air Conditioning (SWAC).

Since 1988, OTE has established a noteworthy pipeline of projects with a signed energy services agreement (ESA), four signed memoranda of understanding (MoU) and proposals to the United States Department of Agriculture (USDA) and United States Department of Defense (USDoD).

In 2017, new Rankine power cycle utilising a combination of ocean thermal energy and geothermal waste energy, called a GeOTEC (Geo-Ocean Thermal Energy Conversion) power cycle/plant. The potential geothermal waste heat, which exists in the form of raw hot natural gas would be continuously pumped from a shallow water Malaysia-Thailand Joint Authority (MTJA) gas production platform, and the supply data is estimated based on the output of the platform.

A thermodynamic model derived from an energy balance calculation is used to simulate the proposed GeOTEC cycle with Matlab. With higher superheated ammonia temperature, GeOTEC power plant efficiency increases, while the net power output decreases. A maximum net power produced by the proposed GeOTEC is 32.593 MW.

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