Evaluation and enhancement of crystallization performance in xylitol-based eutectic phase change materials for low-temperature solar thermal storage

Low-temperature solar thermal storage using a phase change material (PCM) at 70–90°C enables highly efficient water heating toward user-side decarbonization. In this work, the crystallization properties of six xylitol-based eutectic PCMs (EPCMs) were investigated, with the other component being isobutyramide, butyramide, N,N-dimethylurea, glutaric acid, heptanedioic acid, or citric acid. The results indicated that the xylitol-isobutyramide EPCM exhibits the highest latent heat of melting and the best crystallization performance, with melting enthalpy of 245.8 J/g and crystallization enthalpy of 54.5 J/g, and a supercooling of only 4.4°C in the temperature-history test. We then selected different nucleating agents to further improve crystallization and heat release. The findings revealed that 0.7 wt% 3,3,5,5-tetramethylbenzidine (TMB-5) and 0.7 wt% graphene nanosheets (GNS) provide the most effective crystallization triggering, increasing the crystallization enthalpy by 35.7% and 29.9%, respectively, while achieving almost no supercooling. The specific heat capacities of the modified EPCMs in both solid and liquid states exceed 1.6 J/g·K and 3.0 J/g·K, respectively. The thermal conductivity of the EPCM with TMB-5 remained almost unchanged, while the thermal conductivity of the GNS-enhanced EPCM increased by 57.6%, reaching 0.52 W/m·K. The EPCMs obtained in this study also exhibited good thermal and cyclic stability, highlighting their great potential for water heating applications.