Phase Change Materials, commonly referred to as PCMs,  are hydroscopic materials or organic, inorganic, eutectic compounds with a high heat fusion which melts and solidifies at a certain temperature. PCMs latent heat storage can be achieved through solid–solid, solid–liquid, solid–gas and liquid–gas phase change. However, the only phase change used for PCMs is the solid–liquid change. Liquid-gas phase changes are not practical for use as thermal storage due to the large volumes or high pressures required to store the materials when in their gas phase. Liquid–gas transitions do have a higher heat of transformation than solid–liquid transitions. Solid–solid phase changes are typically very slow and have a rather low heat of transformation.

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PCMs products can store and release thermal energy during the processes of melting and freezing. PCMs are capable of storing and releasing large amounts of energy and are classified as latent heat storage (LHS) units. Latent heat is the heat released or absorbed by the PCM during a change of state, without change in temperature.  PCM can be packaged in various forms for use: storage tube, storage balls, storage plates and in metallic pouch forms and are typically  made with paraffin wax.