A eutectic mixture of galactitol and mannitol as a phase change material for latent heat storage
Graphical abstract
Cyclic stability of the eutectic mixture of galactitol and D-mannitol.
Introduction
High-density thermal energy storage (TES) has been identified as a promising approach for realizing a sustained use of energy for heating and cooling, solar energy harvesting, and other energy-related applications [1]. To be practical, TES systems require storage materials that display (1) high energy densities, (2) a practical phase change temperature, and (3) high durability. Phase change materials (PCMs), which absorb and release thermal energy when they melt and solidify at nearly constant temperature during the phase transition, offer the potential for meeting the aforementioned requirements. Indeed, paraffin, fatty acids, sugar alcohols and salt hydrates have been used or investigated as PCMs for thermal storage applications [2], [3], [4], [5], [6], [7], [8]. The sugar alcohols such as threitol, allitol, iditol, erythritol, mannitol, dulcitol and their eutectic mixtures are particularly promising candidates for use in TES applications due to their broad range of melting temperatures, high volumetric energy densities, non-corrosive nature, and high thermal stabilities [9]. As one of the widely studied sugar alcohol based PCMs, erythritol melts between 117 and 120 °C and displays a heat of fusion that ranges between 340 and 344 J g−1 [7], [10], [11], [12]. The stereoisomers, galactitol and mannitol, have also been investigated as PCMs (see Table 1 for a summary of their thermal data).
Recently, Solé et al. explored the potential of using myo-inositol, galactitol, and mannitol as PCMs via differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy [13]. As part of that study, myo-inositol was found to undergo polymorphic changes between 50 °C and 260 °C, and displayed a relatively high cyclic thermal stability when analyzed between 150 °C and 260 °C. For comparison, galactitol showed relatively poor thermal cyclic stability as the crystallization temperature decreased to 60 °C from its initial value of 120 °C after 18 cycles, and eventually no crystallization was observed after the 19th cycle of analysis. Similarly, the enthalpies of heating and crystallization displayed by D-mannitol decreased by 30% and 50% after 20 and 50 thermal cycles, respectively. FT-IR spectroscopic analysis of galactitol and D-mannitol after the aforementioned thermal cycles, which were performed under air, indicated that the sugars underwent undesired oxidation to derivatives that displayed relatively low heat capacities.
In general, sugar alcohols with high heats of fusion often exhibit high melting temperatures, which limits the choice of heat transfer fluids to be used in conjunction with sugar alcohol based thermal storage media. Hence, there is a need to develop sugar-based PCMs that display relatively low melting temperatures without reducing the heats of fusion.
Herein, we report the thermophysical properties of galactitol, mannitol, and their mixtures, and explore their potential for use as PCMs in medium temperature range TES systems. We also study the cyclic, thermal, and chemical stabilities of a eutectic mixture of galactitol and mannitol using differential scanning calorimetry (DSC) and FT-IR spectroscopy. The use of various additives to suppress the subcooling of the eutectic mixture was also explored.
Section snippets
Preparation of sugar alcohol mixtures
Galactitol (or Dulcitol) (99+%, Alfa aesar) and β-D-mannitol (99%, Acros) were used as received. In general, galactitol and mannitol were combined such that the resulting mixture contained 10, 20, 30, 40, 50, 60, 70, 80, 90, or 100 mol% of mannitol. In a typical procedure, a round-bottomed flask containing a mixture of galactitol and mannitol was immersed in a pre-heated silicone oil bath maintained at 200 °C. The mixture was then stirred for 15 min to melt both sugars. The resulted liquid mixture
Thermophysical properties of galactitol, mannitol, and their mixtures
The DSC thermograms of pure mannitol and galactitol as well as their mixtures are shown in Fig. 1. The amount of galactitol in the mixture decreases as the data are viewed from top to bottom in Fig. 1. Analysis of the data recorded for pure galactitol and mannitol revealed sharp endothermic signals that corresponded to the melting process of the respective sugar alcohols. The heat of fusion and the melting point were determined from the melting curve data. For galactitol, the onset of the
Conclusions
Mixtures of mannitol and galactitol were explored as potential phase change materials for use in medium-temperature thermal energy storage applications. Based on a series of DSC studies, galactitol was found to display a Tm = 187 °C and ΔHfus = 354 J g−1 whereas mannitol exhibited a Tm = 166 °C, ΔH = 300.6 J g−1. The sugars were found to form a eutectic mixture at galactitol to mannitol molar ratio of 30:70, which displayed a sharp melting point of Tm = 153.4 °C, which is lower than that of either sugar
Acknowledgements
We thank Dr. Steve Vienna for his help on X-ray powder diffraction measurements. We would also like to thank Dr. Iskandar Kholmanov for providing oxygen plasma treated ultrathin graphite foam and Dr. Dan P. Sellan for conducting thermal diffusivity measurements on sugar mixtures. This work is supported by the US Department of Energy ARPA-E HEATS program, Award # DE-AR0000178. CWB acknowledges the Institute of Basic Science (IBS-R019-D1) as well as the BK21 Plus Program as funded by the Ministry
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