Thermal energy power plants use the temperature difference in the ocean. The surface of the ocean is heated by the sun. Below the surface the ocean gets colder and colder.

The way of using this energy is called OTEC (Ocean Thermal Energy Conservation). Only a temperature difference of at least 15 kelvin guarantees a significant efficiency. This means that this way of generating power can only be used in tropical regions.

There are two different types of OTEC plants, with an open cycle and with a closed cycle.

Open cycle system:

Power plants with an open cycle system put the surface water into a vacuum chamber where it evaporates. The vapour is used to drive generators. A further advantage of these power plants is that the condensed water, which has driven the generators, is not salty any longer. It can be used as fresh water.

Closed cycle system:

Power plants with a closed cycle system use a heat exchanger. This plant does not vaporize the water, but uses a low level vaporization fluid like ammonia or propane. The advantage of the closed cycle system is that it can use pressure turbines to turn a generator. After the fluid has driven the turbine it is cooled and condensed by cold seawater.