Oceans cover 70 percent of the Earth, and water is a natural solar energy collector.
OTEC, or ocean thermal energy conversion, aims to exploit this fact and use the temperature differences between surface water heated by the sun and water in the ocean's chilly depths to generate electricity.

OTEC plants generally fall into three categories:
Closed Cycle: A liquid with a low boiling point like ammonia is boiled using warm seawater. The resulting steam is used to operate an electricity-generating turbine; the vapor is then cooled using cold seawater.
Open Cycle: Similar to the closed cycle OTEC, except there is no intermediate liquid. The warm seawater is converted into low-pressure vapor that is used to generate electricity. The vapor is then cooled and turned into usable fresh water with cold seawater.
Hybrid Cycle: A closed cycle OTEC is used to generate electricity, which is then used to create the low-pressure environment necessary for the open cycle.
OTEC plants can double as fresh water sources and the nutrient rich seawater drawn from ocean depths can be used to culture marine organisms and plants. The major drawback of OTEC is that since they operate on such small temperature differences, generally about 36 degrees Fahrenheit (20 Celsius), they are only 1 to 3 percent efficient.