Comparative study on bubbling and shearing techniques for the crystallization of xylitol in TES systems

Abstract

Xylitol is a promising phase change material for thermal energy storage at low and medium temperatures, but its supercooling and low crystallization rate can hinder its performance in actual systems. This problem can be overcome with the application of external stimuli that promotes the nucleation and subsequent crystallization of the supercooled xylitol. Seeding combined with mechanical stirring or air-bubbling, the techniques used in this study, are proven to achieve the crystallization (and the release of the stored latent heat) of xylitol, but the effect was not instantaneous. Rheological measurements were performed firstly to study the influence of shear and temperature in the crystallization of xylitol. Then, the use of different mechanisms to promote the crystallization of the material was evaluated by two different laboratory-scale prototypes; one of them included a mechanical stirring system while the other employed a gas-bubbling mechanism. Thermal power and discharged energy of xylitol were evaluated in the bubbling system prototype and the influence of supercooling in both discharged power and energy were also studied. A methodology to calculate induction time is proposed to evaluate crystallization systematically in both systems. This new systematic evaluation proposed in this work is different from that of the literature and aims to be used to compare different crystallization systems.

Keywords

• LHTES
• Crystallization
• Xylitol
• Bubbling
• Stirring
• Prototype