It has been suggested that a heat engine could make use of the temperature difference between water at the surface of the equatorial oceans (27) and the cooler water several hundred meters below (4). If such an engine could be built with near-Carnot efficiency, how much water (in kg) would it need per day if it were to generate 600 MW of power? [The mass of all the earth's oceans is about 1.4 1021 kg.]