• A new control logic for the SCMSHPD system under low meteorological conditions was proposed. • Performance of the SCMSHPD system operating under the new control logic were analyzed. • The effects of the waste heat recovery on the combined drying system under different meteorological parameters were obtained. • Drying process of the combined drying system was optimized. To solve the problem that the high drying temperature demand could not be met when combined drying system operated under low ambient temperature and weak solar irradiance, a new control logic of the solar combined multi-heat source heat pump drying (SCMSHPD) system was proposed and its simulation model was established. By modifying its operating strategy, the SCMSHPD system achieved a higher drying temperature under low environment temperature and weak irradiance, which ensured the implementation of the drying process. The simulation results was verified through experimental study. Additionally, to address the issue of significant heat loss of the exhaust air, waste heat recovery was implemented and its impact on the system’s operating characteristics was analyzed. At last, the drying process was optimized due to the significant improvement of the specific moisture extraction rate ( SMER ) and the system performance was further improved. The results showed that the errors in coefficient of performance ( COP ), energy consumption, and SMER between the experimental and simulation results were 4.04%, 1.84% and 1.53% respectively, which verified the reliability of the simulation model. In October, the COP of the SCMSHPD system without waste heat recovery was 3.66% higher than that of the ASHPD system, but with the decrease of ambient temperature and solar irradiance, the COP of the combined drying system increased by 5.86% in December, demonstrating the advantage of the SCMSHPD system with the new control logic under low meteorological parameters. The waste heat recovery could not improve the system performance of the SCMSHPD system operating with the new control logic proposed, which caused a decrease of 1.06% and 1.19% in October and December respectively. However, the SMER was significantly increased by 70.67% and 12.78% respectively, and the energy consumption was significantly decreased by 41.33% and 10.94%. The results indicated that although waste heat recovery resulted in a slight decrease in system COP , the significant reduction in energy consumption and notable increase in SMER demonstrated a marked improvement in the overall system performance. Due to the significant increase of the SMER , the drying time was shortened, which caused an increase of 1.77% in COP in October. With meteorological parameters became lower, the COP get a bigger increase of 3.34%. Therefore, it could be concluded that the combination of waste heat recovery and shortened drying time could effectively improve the system performance, and the lower the meteorological parameters, the greater the improvement in the system performance. This study can provide important guidance for the selection of control logic and the optimization of the drying process for the SCMSHPD system under low meteorological parameters.

• Novel FFF of HDPE/Expanded Graphite composites for PCM applications. • Melting range of 50–60 °C with a latent heat capacity of 70 J/g. • Thermal conductivity enhanced up to 0.8 W/m·K via expanded graphite addition • Excellent leakage control, shape stability, and enhanced mechanical stiffness. • Demonstrated suitability for thermal management in electronic devices. Organic Phase Change Materials (PCMs) are essential for passive cooling, yet their poor shape stability and low thermal conductivity limit their integration into advanced manufacturing techniques. Addressing this, we report a novel, high-performance functional composite filament developed for the first time for Additive Manufacturing (AM) via Fused Filament Fabrication (FFF). The composite leverages a non-microencapsulated PCM (fatty acid mixture, T m = 55 °C) confined within a high-density polyethylene (HDPE) matrix, chosen for its low processing temperature (140–160 °C) to prevent PCM degradation. Expanded graphite (EG) was preliminarily vacuum-impregnated with the PCM and subsequently melt-compounded with HDPE, serving as a multifunctional filler to enhance both thermal transport and structural stability. The resulting filament was successfully processed into 3D-printed geometries, achieving a high PCM loading of 36 wt% and a melting enthalpy of 70 J/g. Comprehensive analysis revealed a significantly enhanced thermal conductivity, reaching 0.8 W/m·K, and a Figure of Merit of 0.71 · 10 4 W/(m 2 ·K 0.5 ·s 0.5 ), effectively balancing thermal response with energy density. Crucially, the synergistic architecture formed by the HDPE matrix and the EG functional filler ensures exceptional PCM shape stability and robust leakage resistance, while retaining superior mechanical integrity across the entire temperature range of the phase transition. This work demonstrates a viable and scalable manufacturing route for producing multifunctional, high-loading PCM composites with exceptional potential for thermal management in demanding applications like battery and electronic cooling.