Cold thermal energy storage is an issue of increasing importance on a global scale particularly in the format of passive thermal protection. This study presents three eutectic Phase Change Materials (ePCMs) composed of n-alkanes, which provide passive temperature control (their operation is automatically induced by exceeding the limit temperature without the need for a control system) around 4 °C (277.2 ± 2 K) and are chemically neutral. The solid-liquid equilibrium (SLE) in the following binary systems was investigated: {n-tetradecane + n-heptadecane}, {n-tetradecane + n-nonadecane}, {n-tetradecane + n-heneicosane}, allowing the determination of two ePCMs with enthalpies close to 220 J g-1 and one significantly lower (155.5 J g-1). Moreover, two solid-liquid-liquid equilibrium (SLLE) phase diagrams were determined for systems: {n-tetradecane + 1,6-hexanediol} and {n-tetradecane + 1,12-dodecanediol}. In addition, the work provides a systematic analysis of the problem of designing ePCMs with specific properties and the aspects that need to be considered. The possibility of predicting the parameters of eutectic mixtures using the UNIFAC (Do) equation and the equation of ideal solubility was verified. A method for predicting the enthalpy of melting of eutectic was also proposed and confronted with the results of DSC analysis. Thermodynamic studies have been supplemented with measurements and correlation of experimental data of ePCMs density and dynamic viscosity as a function of temperature. The final issue is the improvement of the thermal conductivity of paraffins by the addition of nanomaterials such as Single Wall Carbon Nanotubes (SWCNTs), Expandable Graphite (GIC) or Expanded Graphite (EG). The possibility of forming a long-lasting composite material composed of ePCMs and 1 wt% of SWCNTs with a thermal conductivity significantly higher compared to pure ePCMs has been proven via stability testing under operating conditions.