Ocean thermal energy conversion, illustration. Cold seawater from the ocean depths (blue, right) and hot seawater from the ocean's surface (red, left) are passed through heat exchangers. A pump (lower centre) pumps ammonia around the system through pipes. The heating, expansion and cooling of this liquid is driven by the energy contained in the heat differential between the hot and cold seawater. A turbine (top centre) is used to convert this energy into electricity for use elsewhere. Ocean thermal energy is an environmentally friendly, sustainable and renewable source of power. For this image without labels, see C046/8147.