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.