Triggering Mechanisms and Crystallisation Kinetics in Binary Supercooled Sugar-Alcohol Phase Change Materials

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Abstract
The transition to low-carbon heating in the UK requires large-scale electrification of space and water heating, supported by compact and flexible thermal storage to balance variable renewable electricity supply and heat demand. This work investigates a binary sugar-alcohol–based supercooled phase change material (PCM) as a controllable thermal battery for low-temperature heating systems. The PCM is designed to melt in the 50–90 °C range, providing useful discharge temperatures for radiators and domestic hot water while exploiting long-duration supercooling for deferred heat release. The study was conducted in three stages. First, material screening was performed using T-history analysis to identify suitable binary sugar-alcohol mixtures with high latent heat, stable supercooling, and peak crystallisation temperatures in the target range. A non-eutectic mixture of xylitol–erythritol (Xy–Er) was selected, delivering a cumulative enthalpy of ~287.78 kJ kg⁻¹ under triggered conditions while remaining stably supercooled after cooling to ambient. Second, four triggering strategies were systematically evaluated, namely seeding, localised cooling using a thermoelectric (TEC) heat sink, electrode-based activation, and mechanical agitation, including the effect of prior cold crystallisation (reheating to 60 °C). Third, in situ crystallisation image-processing analysis was combined with an Avrami-type (JMAK) model with fixed exponent n=0.5 to quantify effective crystallisation kinetics. The results show that mechanical agitation and electrode triggering at 60 °C outperform seeding and TEC-based cooling, with mechanical agitation yielding the highest effective rate parameters. Meanwhile, TEC-based cooling in the present configuration was insufficient to produce significant crystallisation. These findings demonstrate that appropriately screened sugar-alcohol mixtures, combined with practical triggering methods, can enable controllable supercooled PCMs to be used as on-demand thermal batteries for future low-carbon heating systems.

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