Development of paraffinic phase change material nanoemulsions for thermal energy storage and transport in low-temperature applications

In this study, new phase change material nanoemulsions (PCMEs) were designed and characterized as possible storage and heat transfer media for low-temperature thermal uses. Water- and (ethylene glycol + water)-based emulsions with fine droplets of n-heptadecane and RT21HC commercial paraffin were produced by a solvent-assisted emulsification method. No phase separation or significant growth in PCM drops were observed for the prepared emulsions through storage, after freeze-thaw cycles and under mechanical shear. Phase change transitions were characterized and a significant sub-cooling was observed, with solidification temperatures up to 13 K below the melting point. One pure alkane and two commercial paraffin waxes with higher melting points were considered as nucleating agents to reduce sub-cooling effect. Although the emulsions exhibited diminutions in thermal conductivity up to 9% with respect to the carrier fluids used as base fluid, enhancements in energy storage capacity (considering an operational temperature interval equal to the sub-cooling) reached 26% in the case of RT21HC nanoemulsion based on the (ethylene glycol + water) mixture that contained 10% in mass of paraffin. In addition, the thermal reliability of the nanoemulsions was verified analyzing the changes in latent heat after storage and throughout 1000 thermal cycles.