Towards latent heat storage for days, or even extending to months in the context of solar energy utilization, the exploration of phase change materials (PCMs) with stable supercooling and the following crystallization triggering are carried out. The present work confirmed the persistent supercooling property of the xylitol/erythritol eutectic mixture. The coupled technique combining mechanical agitation with seeding was proposed to trigger crystallization, so as to facilitate the heat retrieval whenever heat is in demand. The xylitol/erythritol eutectic keeps supercooling state without crystallization at ambient temperature for at least 10 days, thus it can serve as applicable PCM for controllable heat retrieval. Such severe supercooling is mainly attributed to the particular molecular structure of polyalcohols and dramatic increase of dynamic viscosity . In particular, on the basis of secondary nucleation principle, the coupled technique combining mechanical agitation and seeding can be a preferable technique to efficiently trigger crystallization of the present eutectic PCM , so as to realize controllable heat retrieval. Such combined technique has main advantages of high efficiency and reproducible crystallization process . The agitation duration is shortened to 1.8 min with seeding, in comparison to a longer value of 43.3 min without seeding. Although the single technique of mechanical agitation can also trigger the crystallization, the agitation is found to be time-consuming and the triggered crystallization process is in poor consistency among parallel specimens. Furthermore, the crystallization initiating is greatly advanced with high agitation rate (300 r/min), and the agitation duration to sufficiently activate crystallization can be significantly shortened. The results suggest that appropriately high agitation rate favors the efficient heat retrieval. The obtained results are helpful to gain a better understanding on the supercooled PCMs based controllable heat retrieval.
Mengdi Yuan,Chao Xu,Tieying Wang,Tianying Zhang,Xinyu Pan,Feng Ye
The serious supercooling of erythritol (ET) limits its application in the field of mid-temperature thermal energy storage (TES), especially in the recently developed form-stable phase change materials (PCMs). The primary purpose of this paper is to find out the factors to mitigate the supercooling of ET/expanded graphite (EG) form-stable composite PCM and obtain the correlation among the morphology, kinetic parameters, and a supercooling degree in the non-isothermal crystallization process. Firstly, five types of supercooling relaxation agents (nano-Al 2 O 3 , nano-SiO 2 , CMC, urea, and citric acid) were added to prepare the ET/EG form-stable CPCMs. Among them, the ET/EG/nano-Al 2 O 3 exhibited excellent form-stable ability, a high heat release ratio of 78.9%, and the lowest supercooling degree (Δ T ) of 45.7 °C. Based on the chosen ET/EG/nano-Al 2 O 3 CPCM, the tablet pressures from 1 to 13 MPa were applied, obtaining lowest Δ T decreased by 59.0% in 1 MPa. Moreover, the non-isothermal crystallization processes were discussed on the ET/EG/nano-Al 2 O 3 with the Avrami equation modified by Jeziorny. ET tended to grow spherulites under the high cooling rate of 18 °C/min with triggering as the Johnson-Mehl-Avrami exponent n increased to 4.22. Meanwhile, the supercooling degree increased from 48.8 to 56 °C with the cooling rate growing from 2 to 18 °C/min. And the activity energy Δ E of the ET/EG/nano-Al 2 O 3 CPCM (7092.35 kJ/mol) was smaller than that of the ET/EG CPCM (8211.19 kJ/mol) because of the interaction between the heterogeneous nucleation effect and blocker effect of nano-Al 2 O 3 .
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