**3.3 The enthalpy of phase change**

The enthalpy of fusion corresponds to the energy absorbed by the material during the solid/liquid transition. The liquid/solid transition is achieved by fully restoring this energy. Except in the case of significant supercooling, this is the enthalpy of crystallization. These enthalpies can be determined experimentally by DSC analysis, and like temperatures, differ by a few J g–1 depending on the purity and origin of the products. The literature considers that a suitable storage material must have a melting enthalpy, ∆Hm, higher than 180 MJ m<sup>−</sup><sup>3</sup> . Even if some reach 580 MJ m<sup>−</sup><sup>3</sup> , few are those to exceed 350 MJ m<sup>−</sup><sup>3</sup> . A change in volume accompanies a first-order transition, and for most materials, the volume expansion ∆V/V at fusion is positive except for ice and gallium and its alloys. Depending on the nature of the materials, it can vary from a few percent to 50%, and in particular from 5 to 15% for paraffin.
