Optimizing Paraffin Wax PCMs: A Comparative Study of SWCNT and MWCNT Additives for Solar Thermal Energy Storage

Paraffin wax is a promising phase change material (PCM) for thermal energy storage (TES), but its low thermal conductivity usually hinders its usefulness. While the addition of nanoparticles like carbon nanotubes (CNTs) is a known enhancement strategy, knowledge gaps exist in how CNT types and concentrations affect the performance of solar thermal storage under the same processing conditions. This study systematically compares paraffin composites with three distinct CNTs—2 SWCNTs from different synthesis methods and 1 MWCNT—at concentrations of 0.25, 0.5, and 1.0 wt%. We evaluated microstructure, rheology, thermal properties, as well as performance under simulated solar charging and heat recovery. Results showed that while all CNTs can increase thermal conductivity, performance of the charging rate varied greatly. SWCNTs could accelerate the charging rate by 45% with 16% conductivity gains, but MWCNT composites can hinder the charging rate (−20%) even with 9% conductivity gain. Higher CNT concentrations also led to additional challenges, including processability and localized degradation. This work demonstrates that optimal material selection for solar storage systems requires a holistic approach, balancing thermal conductivity, charging rate, and long‐term stability.