Computational Analysis of Plastic Crystals as Solid-Solid Phase-Change Materials—Insight into the Molecular Mechanism of Thermal Energy Storage

Plastic crystals are promising candidates for solid-solid phase-change materials (PCMs) in thermal energy storage components and systems. The peculiar feature is the fact that they remain solid throughout the phase transition temperature range, thus reducing the need for containment and possible additivation [1]. Plastic crystals are organic molecular crystals that display solid-solid phase transitions at temperatures ranging from 310 K to 463 K with relatively large latent heat (over 100 J/g-K). Plastic crystals transform from a low-symmetry crystal structure to a high-symmetry one at a set temperature while absorbing a significant amount of heat (and vice versa).<br/>In this work, we perform theoretical investigations based on classical molecular dynamics (MD) simulations to obtain new physical insights into plastic crystals as PCMs and provide useful information for the rational design of new solid-solid PCMs [2]. We calculate the latent heat, density, number of hydrogen bonds, the energy of hydrogen bonds, radial distribution function, specific heat capacity, and thermal conductivity of three polyalcohols (Pentaerythritol, Pentaglycerine, and Neopentylglycol). MD simulations are also performed to elucidate the relationship between the thermophysical characteristics of polyalcohols and their crystalline structures at the atomic level. These plastic crystals are selected because relevant thermophysical properties have been reported being phase transition temperature and latent heat for pentaerythritol much higher than those for pentaglycerine and neopentylglycol. Our MD simulation results quantitively reproduce the properties of polyalcohols obtained in recent experiments for the first time [3]. Based on the analysis, we find that their latent heat originates mainly from the decrease in the number and strength of intermolecular hydrogen bonds upon solid-solid phase transition. Furthermore, we also examine the origin of the difference in latent heat between these three polyalcohols.<br/>Overall, this work reveals the molecular mechanism of thermal energy storage of solid-solid PCMs and provides valuable insights for the development of new materials with high latent heat without the need for encapsulation, with a prospected impact in the development of a new class of thermal energy storage systems based on solid-solid PCMs [4].<br/><br/><b><u>References</u></b><br/><br/>[1]. A. Ribezzo, G. Falciani, L. Bergamasco, M. Fasano, and E. Chiavazzo, “An overview on the use of additives and preparation procedure in phase change materials for thermal energy storage with a focus on long term applications,” <i>Journal of Energy Storage</i>, vol. 53. Elsevier Ltd, Sep. 01, 2022. doi: 10.1016/j.est.2022.105140.<br/>[2]. B. Feng, J. Tu, J. W. Sun, L. W. Fan, and Y. Zeng, “A molecular dynamics study of the effects of crystalline structure transition on the thermal conductivity of pentaerythritol as a solid-solid phase change material,” <i>Int J Heat Mass Transf</i>, vol. 141, pp. 789–798, Oct. 2019, doi: 10.1016/j.ijheatmasstransfer.2019.07.017.<br/>[3]. A. Mishra, A. Talekar, D. Chandra, and W. M. Chien, “Ternary phase diagram calculations of pentaerythritol-pentaglycerine- neopentylglycol system,” <i>Thermochim Acta</i>, vol. 535, pp. 17–26, May 2012, doi: 10.1016/j.tca.2012.02.009.<br/>[4]. M. Fasano, M. Bozorg Bigdeli, M. R. Vaziri Sereshk, E. Chiavazzo, and P. Asinari, “Thermal transmittance of carbon nanotube networks: Guidelines for novel thermal storage systems and polymeric material of thermal interest,” <i>Renewable and Sustainable Energy Reviews</i>, vol. 41. Elsevier Ltd, pp. 1028–1036, 2015. doi: 10.1016/j.rser.2014.08.087.