Bubble size and gas flow rate effects on bubbling-triggered crystallization of polyalcohol toward flexible thermal energy storage

The flexible thermal energy storage using supercooled phase change materials (PCMs) allows thermal energy storage lasting flexible periods up to months and release when heat is on demand. As an essential part of latent heat retrieval, the crystallization triggering techniques were not fully investigated. The present work deals with the bubbling-triggered crystallization. Special attention is paid to the gas flow rate and bubble size effects. Xylitol, as a familiar polyalcohol, is screened as an energy storage medium with persistent supercooling. It is found that the estimated diameters of primitive and detached bubbles show an overall increasing trend with gas flow rate and gas tube size. Bubbling-triggered crystallization behavior is concerned with the bubbling conditions. A full crystallization probability was obtained with a 5 mm gas tube among the parallel tests. The crystallization temperature seems independent of gas flow rate. Both the minimum average crystallization durations and bubbling induction durations were obtained with a gas flow rate of 500 ml/min. However, the performance of bubbling agitation is not acceptable due to the long induction duration and apparent standard deviations. Under the guidance of the secondary nucleation mechanism, a coupled technique of bubbling with seeding was introduced to sufficiently trigger crystallization. The required induction duration is only 21 % of that with bubbling alone. The triggering performance was also improved, indicated by the raised crystallization temperature and accelerated crystallization rate. The experiment reproduction was greatly improved as well. The obtained results help to understand the bubbling-triggered crystallization of supercooled PCMs and to guide the gas injector design of a flexible thermal energy storage system.