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Since a substance emits heat as it transitions from a high energy state (gas) to a lower energy state (solid), is it possible to devise a method to capture this heat and convert it to usable energy?

I'm not sure how economical this would be, but consider the exothermic reaction of water freezing. The heat of fusion for water is $3.33 *10^5 J/kg$

So if a 1 cubic meter of water was frozen, there would be: $3.33 * 10^5 J/kg * 1000kg = 3.33 * 10^8 J$ of heat energy produced.

The heat value for the vaporization of water is $2.36 * 10^6 J/kg$ which would be even more efficient, though it is more difficult to capture 1000kg of water vapor.

I imagine this might be akin to how an air conditioner works, but I cannot imagine how this may be done or if it is already being attempted. So I pose the question here, would it be possible to harness the heat energy from the air and water concentrate it into a usable form of energy--a clean form of energy at that. Why, or why not?

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We already do harness it. –  Parth Vader Apr 10 at 7:04
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As you guessed in the end of your question: We already use the energy released or absorbed in phase transitions to transport energy. For example, when we put ice on a drink, and also in refrigerators and air conditioners we remove thermal energy from a region that is already cold and release it where it is already hot. The problem is that these processes do not generate energy, they only store energy that must be obtained from some source, such as burning fuel or running water downhill in a hydroelectric plant. In a geothermal vent we might find large amounts of water that is being transformed into vapor and this can be used to generate energy, as long as there is a natural source of heat to replenish the part that we used.

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Nothing, generates energy per se. But consider for a moment if the same principals of an air conditioning system were used to transfer heat from the ocean to a boiler which powered a steam engine to generate electricity. The evaporator unit in the ocean to store the thermal energy, the condenser unit in the boiler to gather the thermal energy and then concentrate it to power a boiler so the thermal energy is converted into usable energy. –  Klik Dec 9 '13 at 19:33
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