Explain in short :
Ocean thermal energy

Explanation

Ocean thermal energy : The water at the surface of the sea is heated by the sun while the water in deeper sections is relatively cold.
The difference in temperature of surface and depth of sea water is exploited to obtain energy in ocean thermal energy conversion plants. These plants can operate if there is difference between the water at the surface and a temperature water at depths up to 2 km is `20^(@)C` or more.
The warm surface water is used to boil a volatile liquid like ammonia. The vapours of the liquid are then used to run the turbine of generator. The cold water from the depth of the ocean is pumped up and it condenses vapour back to the liquid.