Hazelnut shell-based activated carbon/carbon nanotubes/palmityl alcohol as new form-stable phase change material with enhanced energy storage capacity and thermal conductivity

Recently, high thermal properties of phase change materials (PCMs) with low costs, and low density have been highly recommended for solar thermal energy storage (STES) in residential buildings. However, they suffer from leakage issues and must be encapsulated within porous materials. Among the porous materials used, biomass-based activated carbon is the most suitable. This study used waste hazelnut shells from the agricultural industry as biomass materials to produce activated carbon. The hazelnut shell-derived activated carbon (HSAC), incorporating carbon nanotubes (CNTs) and palmityl alcohol (PAl), was used in various ratios to manufacture form-stable PCMs (FSPCMs) with enhanced thermal properties. Several tests, such as XRD, FTIR, DSC, and IR thermal camera, were used to measure the chemical stability, thermal storage properties, the change in surface temperature of the proposed FSPCMs, and thermal reliability before and after the thermal cycling process (1000th cycle). The designed FSPCMs' TC values, enhanced by CNT, were carefully assessed. The results show that the inclusion of the PCMs into HSAC and CNT hybrid matrix, HSAC/PAl (43 %), HSAC/CNTs (4 %)/PAl (52 %), and HSAC/CNTs (8 %)/PAl (65 %) resulted in improved final values to 0.29, 0.57, and 0.86 W/m·K, respectively, compared to 0.17 W/m·K obtained with pure PAl. The 8 wt% CNT additive also boosted the energy storage capacity of the composite from 121.82 J/g to 184.92 J/g due to the increased impregnation ratio of PAl into the hybrid supporting matrix from 43 wt% to 65 wt%. Furthermore, utilizing HSAC in PCM production brings several environmental advantages, such as reducing landfill and carbon dioxide emissions, and can contribute to cleaner production of green construction elements for STES buildings implementations.