Ocean thermal energy conversion is a process that uses the temperature difference between the warm surface water of tropical oceans and the cold deep ocean water to run a heat engine. The graph shows a typical decrease of temperature with depth below the surface in tropical oceans. In the heat engine, the warmer surface water vaporizes a low-boiling-point fluid, such as ammonia. The heat of vaporization of ammonia is 260 cal/g at 27°C, the surface-water temperature. The vapor is used to turn a turbine and is then condensed back into a liquid by means of cold water brought from deep below the surface through a large intake pipe. A power plant producing 10 MW of useful power would require a cold seawater flow rate of about 30,000 kg/s.

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5 10 15 20 25 30 °C 100 200 300 400 500 600 700 800 900 1000 Depth (m)