If heat (energy) is supplied to or removed from a substance, the temperature of the substance changes or the substance changes its aggregate state (solid, liquid, or gaseous) at certain temperatures (melting and boiling point) without further temperature changes. All substances and materials have this property, but at different temperatures and pressures. For ventilation technology, paraffin or salt hydrates with melting points between 20 and 25°C can be used as PCMs. In case of a change of aggregate state, a large volume of heat, the so-called latent heat, can be stored or released at a constant temperature.
A small temperature difference suffices to bring about the change of the aggregate state. Supposing that the mass of one kilogramme of concrete is cooled by 10 K at normal room temperature during nighttime cooldown, this thermal mass has the cooling potential to draw 10 kJ heat from the room during the day.

Since the PCM changes its aggregate state from liquid to solid under the same conditions during nighttime cooldown, this gives rise to a cooling potential of approx. 190 kJ (approx. 0.05 kWh) per kilogramme, i.e. 19 times greater than concrete.