Ice as made from pure water provides a very good illustration of what phase change materials are. In its solid phase, water exists as "ice" at sub-zero temperatures. When exposed to heat, the temperature of ice starts rising in correlation to its specific heat capacity, until at 0°C, it starts liquefying. This is the phase change temperature for ice, and two very important observations are noteworthy during its phase change;
As a result, if the requirement for a given application is to maintain temperatures invariably at 0°C, ice can be an excellent choice of energy storage. For example, for maintaining temperature of perishable sea-food over long journeys at ambient temperatures, the product can be kept in contact with sufficient amount of ice, which would absorb the ambient heat and maintain the temperature quite efficiently.
PCMs: Ice for every temperature: While it is always convenient to use ice for cold storage, an important point to note is that it absorbs large amounts of energy only at 0°C and would not be the most efficient solution at any other temperature requirement. This is where PCMs comes in use. Phase Change Materials can be tailored for use at melt/freeze point. In other words, one can have an ice-type thermal storage at whatever temperatures they require, or as we like to call it, an ice for every temperature.