Hydro-Electric Dams hold water above its natural level creating a head pressure needed to power generators. Turbines have been designed to take advantage of the Mississippi River and the Gulf Stream. Ocean Thermal gradients are the temperature difference between surface and deep waters. In some parts of the world, heat engines can be powered this way. Tidal power can harness ocean movement resulting from gravitational forces of the moon.
Dams can trap water to be released through turbines.
Turbines that take advantage of water movement in both directions are more efficient. Cooling with water is possible by exposing it to the clear night sky where heat is given up by radiation. Ponds on flat roofs are used in this manner. Heat can also be lost through evaporation. Water sprayed on roofs is beneficial even in humid areas, but more efficient in arid areas.