<p>Rising variability in electricity supply and increasing power density in electronics created a need for thermal storage to protect devices and support grid flexibility. Biodegradable phase change materials provided latent heat storage but often exhibited low thermal conductivity and a risk of leakage. Previous studies rarely quantified durability, ageing and design optimization of biodegradable expanded-graphite composites. This work aims to develop and optimize biodegradable polymer/expanded graphite phase change composites that deliver high thermal conductivity, stable latent heat storage, and controlled ageing for smart-grid applications. Composite panels of PHB, PHBV, and PCL were impregnated into expanded graphite scaffolds, and their structures and thermal responses were examined using microstructural and thermal analysis techniques under both fresh and aged conditions. Molecular dynamics simulations and response surface methodology were used to characterise interfacial transport and optimise graphite loading and processing temperature. The composites achieve strong in-plane thermal transport while preserving latent-heat capacity, as increased graphite content within 9–17 wt% raised conductivity by 30% with modest reductions in melting enthalpy. Latent heat retention during 500 thermal cycles remains high, with PCL- and PHBV-based panels retaining 94–97% of their initial enthalpy, and ageing in air produced mainly surface-localized oxidation while the internal function remained intact. Response surface analysis identifies expanded-graphite loading as the dominant design variable, with conductivity sensitivity near 0.031 W·m⁻¹·K⁻¹ per wt%. These findings support the use of biodegradable fixed-form composites as durable thermal buffers for demand-side management, waste-heat utilization and thermal regulation of power electronics within smart grids. Future work targets life-cycle assessment and scale-up of the manufacturing route for industrial deployment.</p>