As most school science students know,there are two kinds of energy stored in water.First is the gravity potential energy.In a river dam,the water at a higher level has greater gravity potential energy than at lower levels.And when water from a river dam flows down the power tunnel rotating the water turbine by spending the gravity energy,we get electricity.The second is the heat energy in water,a liquid.Drain away the heat from water and it becomes ice(easier said than done).We can even calculate the entire heat energy of the entire water mass on the planet by multiplying mass with temperatures at different places(except the icy regions).But we are yet to hit upon an idea how to use this colossal amount of energy smoothly,which appears to be a sort of useless waste at present.
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Energy becomes useful when it is possible to transfer from a high energy level to a lower energy level which creates force for work. If the energy level of an environment is uniform no useful work can be accomplished.
Quite true,Sand.Energy transfer occures form high temperature source to lower ones.Also at room tmperature from high pressure air source. When released the expelled and expanded air can do lots of useful work by expending its internal energy and thus getting colder than the surroundings.
If we can control cloud water evaporation then we can put water's gravity energy potential to work. Sun heats and evaporates water from earth > clouds absorb then release water over high points on earth (mountains) > use hydro-power to operate turbines. Viola!