Rice husk hydrochar/paraffin composite as a phase change material for a solar-driven water desalination system

Rice husk hydrochar/paraffin composite phase change materials (PCMs) were developed to improve thermal energy storage for solar-driven desalination. Rice husk hydrochar was synthesized by hydrothermal carbonization and KOH activation, incorporated into paraffin at different loadings, and characterized using SEM, FTIR, thermal analysis, and thermal conductivity measurements. Thermal conductivity exhibited a significant nonlinear dependence on hydrochar loading, with a quadratic-model-predicted optimum near 1.15 wt.% attributed to conductive network formation. The selected composite showed enhanced thermal stability while preserving the chemical structure of paraffin. When integrated into a double-slope solar still, the composite provided more stable basin-water and glass-temperature profiles than paraffin alone, resulting in freshwater production of 102.52 ± 16.12 g and a thermal efficiency of 9.25%. These values represent improvements of 43.5% and 46.3%, respectively, over the paraffin-only system, although the conventional solar still remained the best-performing configuration. The study establishes structure–property-performance relationships for biomass-derived composite PCMs and demonstrates the potential of rice husk hydrochar to improve thermal regulation in solar desalination. Hydrochar–paraffin PCM improves thermal conductivity of paraffin. Optimal ~1.15 wt.% hydrochar loading forms conductive networks in paraffin. Composite stabilizes temperatures and extends evaporation in solar stills. DSSSHP outperforms paraffin-only in yield and efficiency.