Preparation and thermal properties of colloidal mixtures of capric acid and Na2HPO4·12H2O as a phase change material for energy storage

This paper presents a colloidal binary composite phase change storage material prepared using inorganic–organic phase-change materials (PCMs) to improve the thermal conductivity of the organic PCM and solve the supercooling and phase separation problems associated with inorganic PCMs. Samples of disodium hydrogen phosphate dodecahydrate (DHPD)/capric acid (CA) were prepared using a sol-gel method. These samples were characterized by differential scanning calorimetry (DSC), thermal conductivity analyses, Fourier transform infrared (FTIR) spectroscopy, and X-Ray diffraction (XRD). First, the eutectic mixture ratio for a DHPD-CA binary system was theoretically predicted. An analysis of the DSC curve and the two-phase diagram of the eutectic system reveals that the colloidal mixture is close to the eutectic state when the DHPD/CA ratio is 7:3. The results of FTIR spectroscopy and XRD revealed that DHPD and CA in the colloidal mixture did not undergo a chemical reaction, indicating the occurrence of physical mixing. Second, the melting temperature of the composite material was 33.8 °C, the degree of supercooling was 0.9 °C, the melting enthalpy of phase change was 168.8 J g−1, and the thermal conductivity was 0.468 W m−1K−1 at 15 °C. Moreover, after 300 accelerated cold–hot cycles, the thermal properties and morphology of the colloidal binary composite were stable. The addition of CA reduced the degree of supercooling of the DHPD. Furthermore, the addition of DHPD improved the phase-change latent heat and thermal conductivity of CA, thereby reducing the cost of the composite material. The material presented can be used in passive solar energy applications.