Instead, it stretches across at least 23 million square miles of earth's tropical oceans; the uppermost layers of which make a prime natural source of thermal energy.

Regardless of time of day or cloud cover, Ocean Thermal Energy Conversion (OTEC) promises to harness this thermal sea-based resource year round.

OTEC production converts heat energy from seawater into kinetic energy using the ocean’s naturally steep temperature gradient.  It’s this juxtaposition of tropical (and sometimes subtropical) subsurface seawater at temperatures typically above 80 degrees F. and below 40 degrees F. that makes OTEC possible.

An OTEC plant literally pumps the warm surface seawater through a heat exchanger connected to a closed circuit filled with several hundred tons of liquid ammonia.  Since ammonia boils at lower temperatures and at lower pressures than water, once the warm seawater hits the heat exchanger, it causes the ammonia to vaporize and expand in volume.  As this ammonia vaporizes, it creates pressure to run a turbine coupled to a generator.  In most cases, the resulting electricity would be delivered onshore via an undersea cable.

Once this ammonia vapor exits the turbine, it flows through a second heat exchanger that is connected to a cold water pipe carrying tons of seawater pumped from depths of 3000 ft.  This cold seawater, in turn, condenses the spent ammonia vapor back into liquid and the whole OTEC process begins again.

But despite the fact that the idea for the technology is more than a century old; to date, OTEC has only been successfully demonstrated on small scales of less than a quarter of a megawatt (MW) and has yet to produce utility-scale power.

Source: Renewable Energy World - via zeitnews.org