*4.1.5.3 Global enthalpy*

The global enthalpy represents the enthalpy balance of all transformations from the solid state to fusion. Its variation is practically linear as a function of the number of carbon atoms [25]. It, therefore, seems that this balance is independent of the nature of the low-temperature phase and the parity of the n-alkane.

#### *4.1.6 Improvement of thermal conductivity*

Much research has attempted to compensate for the poor thermal conductivity of paraffin by adding fins or expanded metal to the material or by dispersing it in a porous conductive material such as natural expanded graphite [26]. This method makes it possible to obtain composites with high thermal conductivity, high paraffin mass content (65–95%) trapped by capillarity [27]. A more common solution is to divide paraffin by volume by encapsulating it in PE spheres or by dispersing it as an emulsion in water. However, such a solution has the effect of considerably reducing (50%) the effective volumetric heat capacity of the process. In other words, its power is improved, but it takes up twice as much space.

#### *4.1.7 Binary systems of n-alkanes*

By combining alkanes in pairs, it is possible to develop systems of various structural types, some of which have very interesting thermal characteristics for energy storage [28]. The study of the behavior of binary mixtures of n-alkanes through phase diagrams or phase equilibrium has generated growing interest in the design of latent heat storage materials since it allows the prediction of the system's behavior during phase changes. Nevertheless, this behavior is governed by some general rules of thermodynamics on the possibility of forming solid solutions.

#### *4.1.7.1 Gibbs rule*

The solid solution of two n-alkanes is favored if the Gibbs free energy of the mixed crystal is lower than that of each of the pure constituents; otherwise, there is no miscibility in the solid state, but a eutectic.
