Ocean Thermal Energy

Ocean thermal energy is also a potential source of energy. Covering more than 70% of Earth's surface, the oceans are the world's largest solar collectors, absorbing enormous amounts of Sun energy in the form of thermal energy. A process called Ocean Thermal Energy Conversion (OTEC) uses this heat stored in the oceans to generate electricity. It exploits the difference in temperature between the sea's upper layer, which is warmed by the sun, and the colder deep water. The warmer water at the surface is used to vaporize a working fluid, or is transformed to steam under vacuum, to run a turbine/generator system producing electricity. Cold water pumped typically from water depths of around 1000 m is then employed to recondense the vapor and close the cycle. To produce significant amounts of power, the temperature between the upper and lower water layer should differ by at least 20 °C; these conditions are encountered in tropical seas. Tapping the thermal energy of the oceans was initially proposed in 1881 by the French physicist Jacques Arsene d'Arsonval. It was, however, his student Georges Claude who built the first OTEC land-based system in Cuba in 1930, followed by a floating model off the coast of Brazil. Due to poor location selection and technical difficulties these projects were abandoned before they actually produced more power than was necessary to run the systems. In the late 1970s, interest in OTEC rose again and a few experimental units were constructed in Hawaii, Japan and the island republic of Nauru in the Pacific Ocean. Besides electricity production, the spent cold deep seawater rich in nutrients from an OTEC plant could also be used for the culture of both marine and plant life near the shore, or on land. In case water is used as the working fluid, fresh water obtained by vacuum distillation could be produced from seawater. High construction costs are still an obstacle for this technology, however. The fact is that OTEC is trying to produce electricity from a temperature difference which would be typically considered unusable for power generation. To compensate for this, massive amounts of water from the ocean's surface and their depths have therefore to be pumped through the plant to generate reasonable quantities of energy. The challenge is to achieve this at a reasonable cost. Thus, even if the potential resources are very large, OTEC is still in the research phase and is likely to remain there in the foreseeable future. If developed and economically viable, the first market for this technology would most likely be in tropical island nations for the combined production of energy and desalinated water.

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