• 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

This study reports the results of experimental and numerical investigations on the thermophysical properties and the process of melting of a phase-change composite material. The proposed phase-change composite material based on epoxy resin with spherical shape paraffin wax (RT27) was used as a new thermal storage system. Thermal characterization was performed using a transient guarded hot plate technique. The results revealed the importance of thermal storage by latent heat. The numerical analysis is realized using numerical COMSOL® Multiphysics 4.3b. The effect of various parameters of the numerical solution on the results is examined: in particular, the term describing the mushy zone in the momentum equation and the influence of temperature melting range. The findings of the experimental investigation compare favorably with the numerical results.

NPRP grant # 4-465-2-173 from the Qatar National Research Fund (a member of Qatar Foundation)

The adobe is a widely used traditional material in popular constructions in rural areas of Peru and more generally in Andean countries. In order to increase comfort and energy efficiency of constructions, it is necessary to better know the thermal characteristics of the adobe, seen as a bio-composite material. Different adobes have been studied. Effective thermal conductivity and heat capacity were measured by means of a hot parallel-plate method. Density was estimated using a pycnometer and measuring physical dimensions and mass of each sample, which allowed the calculation of thermal effusivity and diffusivity. Some numerical simulation results displayed good agreement with experimental outcomes. The work presented here has implications for future studies of this traditional building material and might potentially help solving the problem of sustainable housing.

Abstract Phase change materials fabricated from high density polyethylene (HDPE) blended with 40 or 50 wt% commercial wax (melting point of 43.08 °C) and up to 15 wt% expanded graphite (EG) were studied. Techniques including scanning electron microscope (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and an experimental device to measure diffusivity and conductivity (DICO) were used to determine the microstructural, mechanical and thermal properties of the composites. The composites possessed good mechanical properties. Additionally, no leaching was observed during material processing or characterization. Although the Young’s modulus increased with the addition of EG, no significant changes in tensile strength were detected. The maximum Young’s modulus achieved was 650 MPa for the HDPE/40% wax composite with 15 wt% EG. The EG was well dispersed within the composites and did not affect the melting or crystallization of the HDPE matrix. The incorporation of EG increased the thermal stability of the composites by reducing chain mobility and inhibiting degradation. The intensification of thermal conductivity occurred with increasing fractions of EG, which was attributed to the high thermal conductivity of graphite. The maximum quantity of heat stored by latent heat was found for the HDPE/40% wax composite with EG. The addition of a relatively small quantity of EG enhances the heat conduction in the composite.

Abstract Phase change material (PCM) composites based on low-density polyethylene (LDPE) with paraffin waxes were investigated in this study. The composites were prepared using a meltmixing method with a Brabender-Plastograph. The LDPE as the supporting matrix kept the molten waxes in compact shape during its phase transition from solid to liquid. Immiscibility of the PCMs (waxes) and the supporting matrix (LDPE) is a necessary property for effective energy storage. Therefore, this type paraffin can be used in a latent heat storage system without encapsulation. The objective of this research is to use PCM composite as integrated components in a passive solar wall. The proposed composite TROMBE wall allows daily storage of the solar energy in a building envelope and restitution in the evening, with a possible control of the air flux in a ventilated air layer. An experimental set-up was built to determine the thermal response of these composites to thermal solicitations. In addition, a DSC analysis was carried out. The results have shown that most important thermal properties of these composites at the solid and liquid states, like the “apparent” thermal conductivity, the heat storage capacity and the latent heat of fusion. Results indicate the performance of the proposed system is affected by the thermal effectiveness of phase change material and significant amount of energy saving can be achieved.

Abstract This study aims at developing a new integrated component in a passive solar wall to achieve high thermal energy storage potential in building materials. This component was manufactured by conditioning the paraffin inside 24 copper tubes, which were regularly inserted and aligned in a gypsum matrix. Firstly, an investigation by means of a differential scanning calorimeter (DSC) was carried out to obtain the latent heat and the transition temperature of the selected phase change material (paraffin). Secondly, an additional study was conducted to gauge the improvement of energy storage performance in classical construction material (gypsum) when incorporating paraffin by the method of transient guarded hot plates. The results of this experiment revealed a clear improvement in different thermal properties when integrating paraffin in the composite. Besides, the results showed that the composite-PCM prevents the leakage of the molten paraffin during phase transition and proved a good thermal stability.