Xylitol used as phase change material: Nucleation mechanisms of the supercooling rupture by stirring

Abstract

With a very high energy density, Xylitol is a promising phase change material for low temperature heat storage (< 100 °C). However, it has a high and persistent supercooling, which inhibits the latent heat restitution during thermal discharges. Bubbling in supercooled Xylitol has already been identified as an efficient crystallization triggering technique. This paper provides a detailed analysis of the nucleation mechanisms involved. A first conclusion is that primary nucleation has shown to be impossible to activate by stirring or shearing in reasonable timescales. This leads to a description of the crystallization based on secondary nucleation. A particular attention was paid to unintentional seeding, which showed to be likely the starting point of most of the Xylitol crystallizations observed in literature. Coupling seeding and bubbling was then identified as an improvement of the nucleation triggering technique. Adding a seeding step to the discharge protocol indeed leads to faster and reproducible crystallizations. In a second part, a parametric study, conducted for a mechanical and a bubbling agitation, showed that secondary nucleation in Xylitol is a thermally activated mechanism. Therefore, surface nucleation was proposed as the main source of secondary nuclei. Finally, a first model for the surface nucleation in supercooled Xylitol, emphasizing on the influence of Xylitol viscosity, was proposed.