OTEC

Ocean thermal energy conversion is an electricity generation system.
Ocean Thermal Energy, also called Ocean Thermal Energy Conversion (OTEC), refers to using the temperature difference between the deep parts of the sea, which are cold and the shallow parts of the sea, which are cold, to run a heat engine and produce useful work.
The deeper parts of the ocean are cooler because the heat of sunlight cannot penetrate very deep into the water.
Here the efficiency of the system depends on the temperature difference.
Greater the temperature difference, the greater the efficiency.
The temperature difference in the oceans between the deep and shallow parts is maximum in the tropics, 20o C to 25o C.
Tropics receive a lot of sunlight which warms the surface of the oceans, increasing the temperature gradient.
The energy source of OTEC is abundantly available, free, and will be so for as long as the sun shines and ocean currents exist.