The mechanisms study of the effect of sludge hydrolysis residual preparation on the performance of composite phase change materials

Abstract

Composite phase change materials composed of hydrated salts for heat storage have become a hot research topic, in which the carrier plays a decisive role in preventing phase separation. Little sawdust was mixed into municipal sludge when directly hydrolyzed in an autoclave for the preparation of residual. The separated and dried hydrolysis residue was used as a carrier for loading sodium acetate trihydrate (SAT) with a mass ratio of 1:6 to prepare composite phase change materials (CPCM) for heat storage. The influence of hydrolysis temperature and pressure on the characteristics of the residue carrier and the heat storage performance of CPCM was then investigated. Multiple cycles of melting and solidification were undergone with CPCM in the plastic tube immersed in a water bath. The thermal stability of CPCM was evaluated by the multi-cycle step-cooling curves, DSC, and XRD. The results show that the residue hydrolyzed at 264 °C and 5 MPa (residue R5) has the highest pore volume and uniform particle size distribution compared with the residues hydrolyzed at other conditions. The latent heat storage density of the CPCM-R5 is 256.3 kJ/kg. After 100 melting-solidification cycles, the difference between the latent heat of CPCM-R5 and its theoretical value is only −2.5 %–4.4 %. The XRD patterns of CPCM-R5 are consistent before and after multiple cycles, indicating that CPCM-R5 is of better thermal and chemical stability, and its thermal conductivity is increased.