Ocean Thermal Energy Conversion (OTEC)
The temperature difference between the surface water of tropical oceans and their deep water is roughly 30 degree centigrade. This temperatur difference can drive a turbine to provide electricity via a generator.
Using the small temperature difference of 30 degree centigrade means a low efficiency for the energy conversion of 5-10 percent. To establish a rankine cycle at the given temperatures of 35 / 5 degree centigrade the preferred working medium is ammonia (NH3).
OTEC systems may play a role for small scale power demand in rural regions near tropical oceans. A further function of OTEC systems might be cooling (with the cold water) and water deslination by condensating water on tubes cooled by the deep ocean water.
A main drawback is the high constructive effort to supply enough cold deep ocean water to the plant - OTEC systems are scientifically interesting but in the long run pv devices with cheap rechargeable batteries (which have to be developed) will be much more effective and versatile.