Directionally conductive and flexible phase change composites enabled by graphite alignment and silicone rubber encapsulation for high-efficiency solar thermal storage

Phase change materials (PCMs) are one of the primary methods for solar energy storage and utilization. However, PCMs can't be used as photothermal conversion and storage materials because of their poor photothermal conversion capacity, low thermal conductivity and thermal stability. In this study, thermoplastic styrene-ethylene-butylene-styrene (SEBS) and expanded graphite (EG) are combined with paraffin wax (PW) to enhance the flexibility of the PCM and to increase the thermal conductivity, as well as to reduce leakage. Furthermore, directional thermal conduction channels are successfully formed through a pressing process, leading to the thermally conductive anisotropy of the composite PCM. Silicone rubber (SR) is introduced as a light-absorbing layer, sealing layer, and thermal insulation layer to encapsulate the outer layer of DTC-FCPCM, successfully fabricating a new type of directionally thermally conductive flexible composite phase change material, denoted as SDTC-FCPCM. Among SDTC-FCPCM, photothermal conversion and thermal storage mainly occur in DTC-FCPCM. We successfully increase the thermal conductivity of DTC-FCPCM from the initial value of 0.2 W/(m·K) to 6.049 W/(m·K), and improve the anisotropy index to 1.91. The combined effect of the high directional thermal conductivity of the inner layer and the high absorbance of the outer layer enables an excellent photothermal conversion efficiency of SDTC-FCPCM (93.19%). After 100 cycles, SDTC-FCPCM exhibits high enthalpy retention rate (~99.68%) and mass retention rate (~99.47%). Based on these advantages, the SDTC-FCPCM can be applied in thermal management fields in most cases of outdoor environment with solar energy.