Regulating the Pore Structure of Wheat-Straw Biochar to Enhance Energy Storage in Paraffin/Stearic Acid/Polyethylene Glycol Phase Change Materials

Phase change materials (PCMs) have attracted significant attention for thermal energy storage, but the development of shape-stabilized organic composite PCMs with suitable leakage resistance and thermal performance remains challenging. In this study, wheat straw was used as the precursor and pretreated with KH-570 before carbonization at four different temperatures to prepare modified biochar supports. The effects of KH-570 modification and carbonization temperature on the pore structure of wheat-straw-derived biochar and the thermal performance of three representative organic PCMs, namely paraffin, stearic acid, and polyethylene glycol, were systematically investigated. The results showed that KH-570 modification improved the pore characteristics of wheat-straw biochar. The sample carbonized at 550 °C exhibited the highest specific surface area of 38.4 m 2 /g, representing a 9.4% increase compared with the corresponding unmodified sample. Nitrogen adsorption–desorption analysis revealed hysteresis behavior, and the average pore diameter of the modified samples increased by 19.00–30.49% compared with the unmodified samples. The melting and crystallization enthalpies of the composite PCMs increased by 5.64–7.99% after modification, with the paraffin-based system showing the most notable improvement. Thermogravimetric analysis showed lower mass-loss rates and higher residual masses for the modified composites, with residual mass increases of 14.11%, 27.29%, and 23.15% for the paraffin-, stearic acid-, and polyethylene glycol-based composites, respectively, indicating improved thermal stability under the tested conditions. Overall, under the present experimental conditions, KH-570 pretreatment was associated with incremental improvements in pore accessibility, support-PCM compatibility, and latent-heat utilization of wheat-straw-biochar-supported shape-stabilized composite PCMs.