The oceans are huge reservoirs of warmth, especially in the tropics, where the temperature differential between warm surface water and cool deep water can reach 40 or more degrees Fahrenheit. This differential can be harnessed to run a heat engine, using some principles from utility plants, plus some from refrigeration. Warm surface water goes into a heat exchanger, which vaporizes an ammonia solution; the resulting steam drives a turbine, and the ammonia is re-condensed using cold ocean water. The water is then pumped back into the ocean, a few degrees warmer than when it came in. But to reach the energy production scales of a traditional fossil fuel-fired power plant, 100 megawatts or more, you need a whole lot of water.