Stable, low-cost phase change material for building applications: The eutectic mixture of decanoic acid and tetradecanoic acid

Highlights: • Decanoic/tetradecanoic acid eutectic at 0.82 ± 0.02 mole fraction (78 ± 2 mass%) decanoic acid. • Melting of eutectic at 20.5 ± 1.5 °C, useful for building applications. • High enthalpy change, 153 ± 15 J g⁻¹, is promising. • Negligible change in stability after 3000 melt–freeze cycles. - Abstract: We present a thorough characterization of the thermal properties and thermal reliability of the eutectic mixture of decanoic acid with tetradecanoic acid, as a phase change material (PCM) of potential interest for passive temperature control in buildings. From the temperature-composition binary phase diagram we found that the eutectic composition is 0.82 ± 0.02 mole fraction (78 ± 2 mass%) decanoic acid. We thoroughly characterized the thermal properties of the eutectic mixture. The eutectic composition has a high latent heat of fusion Δ_fusH = 153 ± 15 J g⁻¹ and a melting temperature T_onset = 20.5 ± 1.5 °C. The heat capacity measured as a function of temperature for the solid and liquid phases just below and above the melting point is 1.9 and 2.1 ± 0.2 J K⁻¹ g⁻¹, respectively. The average value of the thermal conductivity of the solid phase measured between −33 and 9 °C is κ_s = 0.20 ± 0.02 W m⁻¹ K⁻¹ and for the liquid phase, the thermal conductivity is κ_l = 0.23 ± 0.03 W m⁻¹ K⁻¹ for 28 and 38 °C. The mixture has a good long-term thermal stability as indicated by negligible changes in Δ_fusH and T_onset after 3000 melt–freeze cycles. The parameters determined in this work allow more accurate modeling and optimization of the behavior of the eutectic mixture in preparation for implementation as a thermal energy storage PCM.