Thermal and Electrical Characterisation of Composite Phase Change Materials for Thermal Energy Storage in Nigerian Solar Applications

Background: Phase change materials for thermal energy storage can extend the usability of solar thermal systems into the evening hours, but their performance under Nigerian climate conditions, and composites using local encapsulation materials, have not been systematically studied. Aim: This study thermally and electrically characterised composite phase change materials based on paraffin wax embedded in expanded graphite for thermal energy storage applications in Nigerian solar systems. Methods: Composites were prepared by melt impregnation at graphite mass fractions of 5, 10, and 15 percent. Differential scanning calorimetry measured latent heat and phase transition temperatures. Thermal conductivity was measured by the transient hot wire method. Thermochemical stability was evaluated over 500 melt-freeze cycles. Electrochemical impedance spectroscopy characterised any ionic conductivity contributions. Results: Thermal conductivity improved from 0.26 W/mK for pure paraffin to 4.8 W/mK at 15 wt% graphite, an 18-fold improvement. Latent heat was reduced from 220 to 196 J/g at 15 wt% due to dilution. Phase transition temperature remained stable at 52 degrees Celsius across 500 cycles. Conclusion: Expanded graphite-paraffin composites offer dramatically enhanced thermal conductivity for solar thermal storage applications with acceptable latent heat retention. The 10 wt% composite provides the optimal balance and is recommended for Nigerian solar water heating applications. Keywords: phase change material, thermal energy storage, expanded graphite, paraffin, solar thermal.