Graphene-integrated bamboo biochar with enhanced anisotropic heat transfer for directional solar-thermal energy storage

The study of anisotropic composite phase change materials (CPCMs) is crucial for enhancing the efficiency of solar energy storage and conversion. However, existing directional CPCMs exhibit limitations including complex fabrication processes, low heat transport and poor stability, constraining their development and application. In this work, a simple and mechanized method based on flattening, slicing, spraying, self-assembly and carbonization technology is developed for the preparation of lamellar carbonized bamboo skeleton materials with enhanced directional thermal transport, which realizes the directional bonding arrangement of graphene and avoids agglomeration. The skeleton material exhibits a macroscopic, mesoscopic, and microscopic pore structure and excellent radial mechanical compression properties, and the porosity reaches 76.6 % without the need for chemical reagents. An anisotropic high-efficiency photothermal lamellar carbonized bamboo-derived CPCM is successfully developed by vacuum impregnation using paraffin as phase change material (PCM), and the heat storage density is as high as 108.25 kJ·kg⁻¹. Based on this method, the longitudinal thermal conductivity and anisotropy of oriented CPCM are also synergistically improved, reaching 1.39 W·m⁻¹·K⁻¹ and 2.017, respectively. The significant anisotropy improves the directional transport and storage of solar heat and reduces heat loss. Additionally, the addition of graphene increases the light absorption of the CPCM to 98 %, resulting in a solar thermal storage efficiency of 91.25 % under 1.6 solar radiation intensities. Meanwhile, the shape-stable CPCM's excellent permeability resistance and stability promote its application in practice. The work has broad application potential in the application of medium and low temperature solar heat storage.