The adoption of Phase Change Materials (PCMs) like Octadecane (C18, C18H38, RT27, etc.) enhances the thermal storage capacity of Solar Thermal Collectors due to their high energy storage density. This is primarily attributed to the latent heat absorbed or released during the phase transition between solid and liquid phases at constant temperature. In this regard, this study focuses on the comparison between two PCMs: C18 and RT27 from the perspective of their thermo-physical properties, primarily their thermal conductivity, with the aim of selecting the most suitable paraffin with the highest thermal conductivity for use in an Air-PCM heat exchanger to store or release more thermal energy. The first part involves classifying PCMs and their thermo-physical properties. Additionally, this research examines the rationale for choosing RT27 paraffin, followed by a comparison between the two paraffins C18 and RT27.