**3. Properties of phase change materials**

The use of latent heat from a phase change material is not in itself a new technique. At the end of the nineteenth century, to overcome the inconvenience of changing hot water bottles too often to heat railway cars, water was replaced by sodium acetate. This salt can store a large amount of heat which it releases entirely after a few hours. While a few patents were filed before 1973, it was not until the first oil shock that many specialized laboratories began research in this field. Taking into account the thermodynamic can only make the choice of good storage material, kinetic, chemical and economic criteria considered essential for the proper functioning of the application in question [19, 20].
