Energy 2.0

OCEAN THERMAL ENERGY CONVERSION

Posted on: June 23, 2008

Energy is a crucial input in the process of economical, social and industrial development. High energy consumption has traditionally been associated with higher quality of life, which in turn is related to the Gross National Product (GNP). Variation in magnitude of energy resources , differing mix of energy resource profiles , lack of adequate resources of fossil fuels in many nations, dispersed geographical location of energy resources within nations and in the world are some of the complexities that characterize the global energy scene.Sources that are replenished more rapidly are termed as ‘renewable’.These include solar,wind and ocean which are inexhaustible.

 

Significance of Ocean Energy

Oceans cover more than 70% of earth’s surface, making them the world’s largest solar collectors. The sun’s heat warms the surface of 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 can produce two types of energy: thermal energy from the sun’s heat and mechanical energy from the tides and waves. 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. Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activities, 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.

 

OCEAN THERMAL ENERGY CONVERSION

Ocean Thermal Energy Conversion (OTEC) utilises the temperature difference between the warm surface sea water and cold deep ocean water to generate electricity. For OTEC to produce a net output of energy, the temperature difference between the surface water and water at a depth of 1000m needs to be about 20oC.

Temperature difference between surface and sub surface (1000m) sea water

 

The concept of OTEC is envisioned by Jacque’s D’Arsonval in 1881. However, D’Arsonval did not live to see his idea to fruition, and the task was completed by his student Georges Claude in 1930. Although the theoretical efficiency of OTEC is small (~2%), there are vast quantities of sea water available for use in power generation. It has been estimated that there could be as much as 107 MW power available worldwide.

 

Otec Systems are Classified into Three Categories

Closed Cycle Otec

D’Arsonval’s original concept used a working fluid with a low boiling point, such as ammonia, which is vapourised using the heat extracted from the warm surface water. The heated working fluid is used to turn a turbine to produce electricity. Cold deep sea water is used to condense the working fluid in a second heat exchanger prior to being recirculated to the first heat exchanger.

Open Cycle Otec

Open cycle OTEC is very similar to the closed cycle one. The only difference is that an open cycle OTEC does not use intermediate fluid with low boiling point but uses the sea water as working fluid that drives the turbine. The warm sea water on the ocean surface is turned into low pressure vapour under a partly vacuumed environment.The steam is then condensed either by a second heat exchanger, as in the closed cycle, or by mixing with the deep cold water.

 

Hybrid Otec System

Hybrid Cycle OTEC is a theoretical method of maximizing the use of ocean thermal energy.

There are two concepts. The first one is to use a closed cycle OTEC to generate electricity to create the necessary low-pressure environment for the open cycle OTEC. The second concept is to integrate two open cycle OTEC (one is used to create the vacuumed environment) so that there will be twice the amount of the original desalinated water.

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