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Ocean thermal energy conversion, sometimes referred to as OTEC, is a form of renewable energy that utilizes the difference in ocean temperature at various depths and converts it to a heat engine. The heat engine converts this heat energy to a mechanical output. The types of ocean thermal energy conversion systems include closed, open, and hybrid.
French physicist Jacques Arsene d'Arsonval was the first person to consider taking thermal energy from the ocean water in 1881. It wasn't until 49 years later that Georges Claude, a student of d'Arsonval, actually constructed the first ocean thermal energy conversion plant in Cuba. His plant was able to produce 22 kilowatts of electricity, but weather destroyed the plant before it could become a net power generator, meaning that it could produce more energy than was required to run the plant. The United States did not pick up on this new method of renewable energy until 1974, when the Natural Energy Laboratory of Hawaii Authority was created.
Currently there are three types of ocean thermal energy conversion systems: closed, open, and hybrid. The closed cycle system uses a liquid that has a very low boiling point to move a turbine to create electricity. This is accomplished by sending warm ocean surface water through a heat exchanger where the fluid is vaporized. When the vapor expands, it moves the generator. Cold ocean water, drawn at much deeper depth, is sent through a different heat exchanger, which then reduces the vapor back to liquid form.
An open cycle system uses water from the ocean that is the closest to the equator, since it is typically the warmest. Warm ocean water that is placed inside of a low pressure container will begin to boil. As the steam expands and leaves the low pressure container, the salt remains. The steam powers a low pressure engine that is attached to a device that creates electricity. It is turned back to liquid form by exposing it to cold temperatures created by ocean water from a lower depth.
The hybrid system combines components of the first two methods. Warm ocean water is pulled into a container where it is turned into steam. The steam then turns a low boiling point liquid into vapor, which powers a turbine to create electricity.