Examination of eutectic phase change materials composed of diols and ionic liquids

Abstract

This paper presents solid–liquid phase equilibrium studies of binary systems composed of pyridinium chloride monohydrates: hexadecylpyridinium chloride monohydrate, [PyC16][Cl] or dodecylpyridinium chloride monohydrate, [PyC12][Cl] with diols: 1,2-hexanediol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol and 1,12-dodecanediol. These systems were selected to obtain eutectic mixtures with a high melting enthalpy and lower volatility (thus flammability) than pure diols. Ten eutectic Phase Change Materials (ePCMs) with melting points ranging from 265 K to 333 K (-8 °C to 60 °C) and enthalpies of melting from 125.9 to 212.8 J·g−1 were obtained. Experimental data were then correlated with the Non-Random Two-Liquid (NRTL) equation, and mixtures of eutectic composition were subjected to physicochemical characterisation. Using DSC, the latent heats of pure components and eutectic mixtures were determined, as well as the viscosity and the thermal conductivity of ePCMs was measured as a function of temperature. The final objective of the study was to increase the thermal conductivity of ePCMs by adding carbon nanotubes or expanded graphite, and to characterise the performance of such composite systems. The stability of nanofluids containing no less than 1 wt% SWCNTs and from about 10 wt% EG was confirmed under both isothermal and energy storage operating conditions, i.e. during a series of at least 1000 phase transformations. The addition of an inexpensive EG at 10 wt% increases the thermal conductivity by up to 200 % and reduces the probability of ePCM leakage as the material retains its shape even after melting.