Phase Change Energy Transfer

The Motion of Molecules

Phase change is also known as phase transition. This generally refers to the transition from a solid to a liquid (melting/freezing point) and between a liquid to a gas (boiling/condensation point). The difference in the motion of the molecules in a solid substance (almost no movement) and the motion of the same molecules when the substance is in a liquid state (much movement) creates a dramatic transfer of energy. When a liquid freezes, the energy that animates its molecules is released into its surroundings. The energy is no longer required because the molecules are nearly static. When a solid melts, it pulls energy in from its surroundings because so much is needed for the now animated molecules. This marked energy transfer corresponds to the pronounced change of state between liquid and solid.

phase change heat transfer

Phase change energy transfer used to as passive coolant.

Phase change is generally not a problem but rather a solution to other types of heat transfer. If you wish to absorb heat energy, a solid can be added to absorb the heat as it melts. Substances (phase changing materials) are created to have specific melting points in order to pull energy out of their surroundings at a specific temperature.
Example: A kilogram of phase changing material created to have a melting point of 25°C (slightly below the melting point of chocolate) can absorb as much heat energy as a kilogram of ice that turns to water. However, the ice creates unwanted condensation, and will not freeze again unless the ambient temperature drops to below 0°. The phase changing material with a higher melting point of 25° will not cause condensation, and if the night time temperature drops below 25° the phase changing material will re-freeze and be ready to absorb more heat the next day.

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