Screening of sugar alcohols and their binary eutectic mixtures as phase change materials for low-to-medium temperature latent heat storage. (Ⅰ): Non-isothermal melting and crystallization behaviors

Towards latent heat storage in the low-to-medium temperature range (70–250 °C), screening of sugar alcohols and their binary eutectic mixtures as potential phase change materials was carried out by focusing on the non-isothermal melting and crystallization behaviors. A preliminary screening shortened the long list of isomers from common four-carbon to six-carbon sugar alcohols to only six affordable candidates, i.e., xylitol, d-sorbitol, erythritol, d-mannitol, d-dulcitol and inositol (ordered with increasing the melting point). Based on the six pre-screened sugar alcohols, a total of 15 binary eutectic mixtures were prepared to manipulate the melting points for more flexible match with real applications. Non-isothermal tests were then performed on a differential scanning calorimeter at various ramping/cooling rates up to 10 °C/min. In addition to determination of the melting point and latent heat of fusion, a special attention was paid to the crystallization behaviors by undertaking consecutive melting-crystallization cyclic tests. It was found that the two candidates with the lowest melting points (both below 100 °C), i.e., xylitol and d-sorbitol, as well as the nine binary eutectic mixtures containing at least one of them, are unable to crystallize from the melt during cool-down at any cooling rates tested (down to 0.5 °C/min). Four other binary eutectic mixtures, i.e., erythritol (84 mol%) + d-mannitol, erythritol (95 mol%) + d-dulcitol, erythritol (96 mol%) + inositol and d-dulcitol (69 mol%) + inositol, were also shown to be unable to crystallize upon cooling, with the crystallization occurring during the reheating process instead, referred to as cold crystallization. The rest four pure sugar alcohols with relatively high melting points (110–230 °C), i.e., erythritol, d-mannitol, d-dulcitol, inositol, and two mixtures, i.e., d-mannitol (70 mol%) + d-dulcitol and d-mannitol (82 mol%) +inositol, were found to be able to crystallize upon cooling, although they all suffer from severe supercooling (e.g., up to over 100 °C for erythritol). The affordable pure and mixture sugar alcohols were deemed to have desirably high latent heat storage density, especially for those with higher melting points. However, they all face specific issues associated with crystallization, which must be addressed before they can really be utilized in real applications. In addition, it may not worth making eutectic mixtures, although this is deemed to be an effective way of manipulating the melting points of sugar alcohols.