Unlike conventional storage materials, however, when PCMs reach the temperature at which they change phase (their melting point) they absorb large amounts of heat without a significant rise in temperature. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat. Within the human comfort range of 20° to 30°C, some PCMs are very effective. They store 5 to 14 times more heat per unit volume than conventional storage materials such as water, masonry, or rock.

PCMs can be broadly grouped into two categories; ”Organic Compounds“ (such as polyethylene glycol) and “Salt-based Products” (such as Glauber’s salt). The most commonly used PCMs are salt hydrates, fatty acids and esters, and various paraffins (such as octadecane). Recently also ionic liquids were investigated as novel PCMs.

Phase change materials perform best in small containers, which therefore are usually divided in cells. The cells are shallow to reduce static head - based on the principle of shallow container geometry. 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 (or water-out, if the material is hygroscopic). Packaging must also resist leakage and corrosion. Steel and polyethylene are common packaging materials.

Indian Telecom Industry has made use of this technology to keep their electronics of BTS Sites cool using Phase change material with melting points in range of 20 Deg C to 35 Deg C. Some Telecom sites are kept cool using the low ambiant night Temp to charge the Phase Change material.