Phase Change Material

A Phase change material (PCM) is a substance with a high heat of fusion which, melting and solidifying at certain temperatures, is capable of storing or releasing large amounts of energy.
The only phase change used for PCMs is the solid-liquid change. Liquid-gas PCMs are not yet practical for use as thermal storage. Although they have a high heat of transformation, the increase in volume during the phase change from liquid to gas makes their use impractical.
Initially, the solid-liquid PCMs perform like conventional storage materials; their temperature rises as they absorb solar heat. Unlike conventional (sensible) storage materials, however, when PCMs reach the temperature at which they change phase (their melting point) they absorb large amounts of heat without getting hotter. When the ambient temperature in the space around the PCM material drops, the PCM solidifies, releasing its stored latent heat. PCMs therefore absorb and emit heat while maintaining a nearly constant temperature. Within the human comfort range of 20° to 30°C, latent thermal storage materials are very effective. They store 5 to 14 times more heat per unit volume than sensible storage materials such as water, masonry, or rock.

Phase change materials are employed in different fields of thermal engineering:

• energy storage
• thermal conditioning of buildings
• waste heat recovery
• off peak power utilisation
• heat pump systems
• space applications

Technology

The most commonly used PCMs are salt hydrides, fatty acids and esters, and various paraffins (such as octadecane).

Phase change materials perform best in small containers, which therefore are usually divided in cells. The packaging material should conduct heat well; and it should be durable enough to withstand frequent changes in the storage material's volume as phase changes occur. It should also restrict the passage of water through the walls, so the materials will not dry out. Packaging must also resist leakage and corrosion. Steel and polyethylene are common packaging materials.