Preparation of Cellulose Nanofibrils/Salt Hydrate Composite Phase Change Materials for Thermal Energy Storage

Abstract

Salt hydrate phase change materials (PCMs) have been intensively used for thermal energy storage (TES) due to their sharp melting points, high energy storage density, small volume change and low cost. However, the problems of phase separation, supercooling and relatively low thermal conductivity of salt hydrate PCMs need to be addressed for high-efficiency TES. In this research, cellulose nanofibrils (CNFs) and CNFs-based composites were used to improve the TES performance of sodium acetate trihydrate (SAT). The effect of CNFs on the phase stability of SAT was investigated and the involved mechanism was explored by the rheological study. CNFs/graphene nanoplatelets (GNPs) composites and CNFs/silver nanoparticles (AgNPs) composites were prepared and used to improve the TES efficiency of SAT. Results indicate that adding 0.8% of CNFs to SAT increased the viscosity, enhanced solid-like rheological behaviors by entangled nanofiber network, and successfully eliminated phase separation of SAT. Owing to the excellent dispersing capability of CNFs, the aggregations of GNPs and AgNPs were avoided in the prepared CNFs/GNPs and CNFs/AgNPs composites. The resulting SAT-based composite PCMs were phase-stable and exhibited improved thermal conductivities over pure SAT due to the thermal conductivity enhancers, GNPs and AgNPs. Besides, with the combined use of sodium phosphate dibasic dodecahydrate and CNFs/AgNPs0.02 composite, the supercooling degree of SAT decreased to 1.2 °C. The prepared composite PCMs exhibited reasonable phase change temperature and enthalpy, and improved thermal stability. In summary, green and versatile CNFs based composites were prepared, and they successfully overcame the drawbacks of salt hydrate PCMs for TES applications.