Ocean Thermal Energy Conversion
Ocean Thermal Energy Conversion(OTEC) is the utilization of the energy stored in the world's oceans by the thermal gradient created. The thermal gradient is the difference of the surface temperature of the ocean that is heated via solar energy and the water at depths of significantly colder temperatures. This thermal gradient is the energy source that is converted to usable energy by OTEC plants. The idea for using the oceans thermal energy is credited to several visionaries including Jacques D'Arsonval, a French Engineer, in 1881. The temperature difference needed for an OTEC plant is about 36 deg F (20 deg C). Temperature differences of this amount are readily available in many locations in the world. The areas depicted in red on the above map produce the best areas for OTEC sites due to the stablility of warm weather throughout the year.
The conversion of thermal energy to electricity is done by using the differences in both heat and pressure. There are three types of energy converters producing electricity: closed cycle, open cycle, and a hybrid system combining the previous two systems. Closed cycle OTEC systems use the warm surface water to boil a fluid that has a low boiling point (e.g. ammonia). The pressure caused by the expanding vapor is then used to turn a turbine. The turbine then turns a generator to produce electricity. Cold water is then used to recycle the vapor back to a condensed state and the cycle is repeated. An open cycle system directly boils the warm surface water. Operating at low pressure, the steam that is created is channeled in turn to a turbine engine. The water vapor is condensed again by cold water. If the condensed water is kept seperate from the cold seawater it can be used for consumption or agriculture. Other by-products produced by the open cycle OTEC are a variety of minerals. Both cycles produce electricity through the use of solar warmed water.
Closed Cycle OTEC & Open Cycle OTEC
(Image adapted from National Energy Laboratory Hawaii (NEHLA))