Ocean - Thermal

 

The surface of the water is warmed by the sun. A liquid  such as ammonia can be evaporated by the surface water and condensed by the colder  water deeper down. The vapour turns a turbine.

 ( 2-27 deg C)

There are two systems, a closed cycle evaporating and condensing ammonia, and an open system evaporating seawater under vacuum and producing both electricity and fresh water. Ref.

Open system