Multi-Scale Numerical Modelling for Predicting Thermo-Physical Properties of Phase- Change Nanocomposites for Cooling Energy Storage Alessandro Ribezzo 1 , Matteo Fasano 1 , Luca Bergamasco 1 , Luigi Mongibello 2 , Eliodoro Chiavazzo 1* 1 Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino 10129, Italy 2 ENEA Portici, P.le Enrico Fermi,1 – Località Granatello - Portici (Na) 8055, Italy Corresponding Author Email: eliodoro.chiavazzo@polito.it https://doi.org/10.18280/ti-ijes.652-409 ABSTRACT Received: 22 March 2021 Accepted: 27 May 2021 One major limitation of phase-change materials (PCM) for thermal energy storage comes from their poor thermal conductivity hindering heat transfer process and power density. Nanocomposites PCMs, where highly conductive nanofillers are dispersed into PCM matrices, have been exploited in the past decades as novel latent heat storage materials with enhanced thermal conductivity. A computational model based on continuum simulations capable to link microscopic characteristics of nanofillers and the bulk PCM with the macroscopic effective thermal conductivity of the resulting nanocomposite is the aim of this work. After preliminary mean-field simulations investigating the impact of the nanofiller aspect ratio on the thermal conductivity of the nanocomposite, finite element simulations at reduced aspect ratios have been performed with corrected thermal conductivity values of the filler, to take into account the thermal interface resistances between fillers and matrix. Finally, the thermal conductivity at the actual aspect ratios has been extrapolated by the results obtained at reduced aspect ratios thus saving computational time and meshing efforts. This method has been validated through comparison against previous literature evidence and new experimental characterizations of nanocomposite PCMs. Keywords: nanocomposites, phase change materials, thermal conductivity, composite materials, finite element 1. INTRODUCTION The growing relevance of renewable forms of energy has highlighted the need to store and manage thermal energy. Important renewables energy forms, such as solar, have the need to decouple the intermittent production and demand to exploit them in a more efficient way. Global thermal energy storage capacity for cooling applications needs to double to meet the expected demand in the next decade [1]. Phase-change materials (PCMs) are an efficient way to store thermal energy, as they assure a sufficiently higher energy density compared to traditional sensible heat storage. However, storage plants that exploit the use of PCMs present low power density and effective energy capacity due to the poor thermal conductivity of these materials, which hinders heat transfer processes [2, 3]. Nanocomposites, in which highly conductive nanofillers - e.g. graphite flakes and carbon nanotubes - are dispersed into PCM matrices, may alleviate the above issue [4, 5]. A synergistic approach between multi-scale numerical simulations and experiments to predict the thermophysical properties of PCM-based nanocomposites for cold storage is the aim of this work. The output from numerical simulations is the effective thermal conductivity of the nanocomposite at different volume fractions and for different filler sizes. Those properties are important to optimize the latent heat storage material (with or without microencapsulation) to be employed in thermal systems for domestic cooling, and to define its most suitable operating conditions as compared to state-of-art cold storage systems [2]. The numerical model described in this work is validated by comparing the effective thermal conductivity estimated numerically with previous literature results and new experimental evidence. The latter has been obtained by measuring the thermal conductivity of a commercial paraffin wax-based PCM for cold storage with the addition of graphite carbon flakes. 2. METHODS 2.1 Experimental methods The graphite flakes used in this work have been supplied by Sigma-Aldrich (mesh 100 – product number 332461). The paraffin wax-based phase change material comes from Rubitherm TM (PureTemp 15). It presents a melting temperature of 15°C, heat storage capacity of 182 J/g, a thermal conductivity of 0.15 W/mK in the liquid phase and 0.25W/mK in the solid phase. A TCi Thermal Conductivity Analyzer of C-THERM has been used to measure the thermal conductivity of both pristine PCM matrix and phase change nanocomposite. During experiments, the temperature control has been assured by a Tenney Tps Junior Test chamber. 2.2 Numerical methods Introducing highly conductive fillers in a poorly conductive phase change material has been widely demonstrated to be an effective way to enhance its overall heat conduction properties [6, 7]. Nevertheless, quantifying this thermal conductivity TECNICA ITALIANA-Italian Journal of Engineering Science Vol. 65, No. 2-4, July, 2021, pp. 201-204 Journal homepage: http://iieta.org/journals/ti-ijes 201