Ocean Thermal Energy

Ocean thermal energy generates power from the difference between the surface temperature of the ocean and its temperature at greater depths. The ocean's surface heats and cools in daily and seasonal cycles, while the depths of the ocean maintain a constant temperature. This temperature difference, though small, could be used to power a heat engine and generate useful power (electricity, for instance, or hydrogen.) The ocean's surface is heated by solar energy, so this is a sustainable energy source; however, the low efficiency makes it unlikely to be useful in practice.

The difference in temperature between the surface and the depths is generally around 20 degrees C ¹ , producing a maximum theoretical efficiency of about 6%. By comparison, the maximum theoretical efficiency of a steam turbine in a normal power plant is around 66% and the actual efficiency is around 40%. Additionally, seawater is extremely corrosive.  For these reasons it is difficult to build a steam turbine which will generate more energy than is used to build and maintain it.