Phase-change material (PCM)
A PCM is a substance that is capable of storing and releasing large amounts of energy when it melts and solidifies. Heat is either absorbed or released when the material changes from solid to liquid and vice versa, thus, PCMs are classified as latent heat storage units and can be used for thermal storage in buildings.
Latent heat storage can be achieved through a solid–solid, solid–liquid, solid–gas or liquid–gas phase change, depending on the material. However, for practical reasons, the only phase change used in thermal storage PCMs is the solid–liquid change. Initially, a solid–liquid PCM’s temperature rises when it absorbs heat. When the PCM reaches the temperature at which it changes phase (its melting point), it absorbs large amounts of heat but its temperature remains almost constant.
When the ambient temperature around the now liquid PCM falls, the PCM solidifies and it releases the stored heat to its surroundings. Within the human comfort range of 20–30°C, PCMs are very effective for thermal storage. They store four to five times more heat per unit volume than conventional storage materials such as water, masonry or rock.