Experimental investigations on the thermal performance and phase change hysteresis of composite phase change material Na2HPO4·12H2O/SiO2

Integrating hydrated salt phase change material (PCM) into building envelopes is a promising method for achieving building energy saving. However, the hydrated salt suffers from the issues of supercooling, phase separation, leakage and phase change hysteresis. In this work, a novel form-stable composite phase change material (CPCM) with mesoporous silica (SiO 2 ) encapsulating Na 2 HPO 4 ·12H 2 O(DHPD) was synthesized by porous matrix composite technology. Thereinto, Na 4 P 2 O 7 ·10H 2 O(TPD) used as the nucleating agent suppressed the supercooling phenomenon, 3 % sucrose can solve the issue of the water molecules loss of DHPD and phase separation.18 % SiO 2 was favored for preventing leakage. The modified CPCM exhibited good chemical compatibility, higher charge-discharge efficiency, and good thermal stability via DSC, SEM, XRD, FT-IR and TG tests. According to experimental results, the surface area and pore volume of CPCM decreased, and the weight loss rate was lower than that of pure DHPD. The FT-IR and XRD tests indicated that only physical interaction occurred between the DHPD core and the SiO 2 shell. Moreover, it displayed excellent thermal reliability after 500 melting-freezing cycles. Furthermore, the phase change onset temperature, supercooling, and hysteresis degree of CPCM at different melting states during partial melting phase transition were studied. It was found that the hysteresis degree of CPCM decreases with the increase in melting ratio, which may be related to the volume-free energy of PCM. • DHPD is adsorbed by SiO2 via hydrogen bond, van der Waals force and capillary effect. • The higher the melting ratio, the smaller the hysteresis of CPCM. • The magnitude of hysteresis is related to volume free energy. • It assumes that the hysteresis temperature will not increase infinitely with the sample mass.