• Energy Research

Thermal energy storage systems (TES) based on shape-stabilized phase change materials (SSPCM) designed from recycled Tetra Pak (TP) waste, paraffin wax (PW), and expanded graphite (EG) were investigated in this study. This work represents the first study to explore TP waste composed of low-density polyethylene (LDPE)/aluminum (Al) components for energy storage applications. The LDPE part serves as a matrix conserving a material in a compact, solid shape after PW melting; PW acts as an active phase change component contributing to heat absorption/release through a phase change (from a solid to a liquid state, and vice versa) of its crystalline phase. EG serves as a filler that enhances the thermal conductivity and mechanical properties of materials. The focus was put on the optimization of the composition of SSPCM including PW, and EG, to check thermal, mechanical, and rheological properties which influence the future processability of such systems through extrusion, as well as to investigate the synergic effect of graphite and residual Al component on thermal conductivity and leakage of PW. There are two main demands on polymer/PW blends, namely well-separated melting peaks for both components at significantly different temperatures, and good compatibility between polymer and PW. The best performance of SSPCM investigated in this study was found for a mixture having the composition TP/PW/EG = 50/40/10 w/w/w. This mixture shows well-balanced properties, including appropriate heat storage and release parameters, thermal conductivity, thermal diffusivity, toughness and strength, and low leakage of PW from the material. This system can store 116.2 J/g of heat energy and release 93.8 J/g of heat energy. The determination of the heat energy storage and release was performed by the transient guarded hot plate technique. Tensile testing revealed that Young's modulus of the TP/PW/EG = 50/40/10 w/w/w composition was 924 ± 71 MPa and the stress at break was 8.2 ± 1.2 MPa, which are sufficient values from the applicability point of view. The composition stability of the prepared system was confirmed by rotational rheometry. The environmental relevance of these materials lies in the utilization of the waste, which has minimal usage, and after the hydropulping of Tetra Pak packaging, it accumulates in large volumes. This is the first study indicating that LDPE/Al recyclate is a cheap alternative for preparing TES materials, fulfilling all the requirements for such materials. This study indicates the potential of TP waste for the preparation of SSPCM using PW as a phase change component. The selection of PW with a specific melting point determines potential applications, including the building industry, thermal management of electronics, solar vapor generators for desalination, solar water heaters, battery/computer heat protection, etc. This publication was made possible by Award NPRP13S-0127-200177 from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. The authors (M.M., M.I., and T.S.) gratefully acknowledge the Ministry of Education, Youth, and Sports of the Czech Republic - DKRVO (RP/CPS/2022/003). Author M.I. thanks the Slovak Research and Development Agency for the financial support provided through grant APVV-19-0338 . Scopus