Polyalcohols for flexible thermal energy storage : Understanding persistent supercooling from Gibbs free energy

Toward a low-carbon future, flexible thermal energy storage using supercooled phase change materials (PCMs) allows renewable energy utilization at scheduled stages. In other words, renewable sources are preserved in thermal energy for flexible periods subject to days and even months, then liberated by triggering crystallization when heat is in demand. Polyalcohols are suggested to serve in flexible thermal energy storage due to their inherent supercooling. Unfortunately, the reported supercooling of xylitol, one of the typical polyalcohols appears inconsistent, particularly, the responsible mechanism remains unclear. The present work examines the supercooling of xylitol under specific conditions and analyzes the basic reasons based on the fundamental Gibbs free energy theory. It was found that the closed xylitol sample remained supercooled for over four months when preserved from unintentional impurities. It confirms that xylitol can be a promising candidate for flexible thermal energy storage. From a thermodynamic viewpoint, xylitol has a high free energy barrier for nucleation, particularly the surface free energy and activation energy, making homogeneous nucleation extremely difficult. The corresponding free energy barrier was determined to be 20.8 × 10−18 J at a subcooling of 10 °C, which equals 10 times of erythritol, another polyalcohol with a high crystallization tendency. In the kinetic aspect, xylitol shows large dynamic viscosity (4.47 Pas at 30 °C subcooling) and low diffusivity (0.03 × 10−12 m2/s at 50 °C subcooling), which significantly limits the crystal growth. Consequently, xylitol keeps persistent supercooling and has a low crystallization tendency. Furthermore, mechanical stirring is introduced to sufficiently trigger xylitol crystallization, thus retrieving latent heat in the heat demand period. The investigation contributes to a better understanding of the persistent supercooling of polyalcohol and further suggests an approach to stabilize the supercooling of PCMs for various flexible thermal energy storage scenarios.