OTEC (Ocean Thermal Energy Conversion)

Ocean thermal energy conversion (OTEC) generates power from the temperature difference between warm tropical surface water (about 25 to 28 C) and cold water pumped from 800 to 1,000 m depth (about 4 to 6 C), a gradient near 20 C. A closed-cycle plant boils a low-boiling working fluid such as ammonia to drive a Rankine-cycle turbine, condensing it against the cold water; open-cycle plants flash-evaporate the seawater itself. Thermodynamic efficiency is low, roughly 3 to 5 percent, so plants need very large water flows. Demonstrations include the 100 kW Okinawa plant and Makai Ocean Engineering’s Hawaii unit.