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Abstract A series of paraffin/expanded graphite (EG) composites with various mass fraction were prepared. The microscopic geometry configuration, thermal conductivity and saturation sorption capacity of EG were obtained. In this paper, a method based on the experiments of EG pores was proposed for constructing geometric models. The geometric model constructed by this method has obvious anisotropy and has been employed in the calculation. Furthermore, the numerical results of thermal conductivity indicate anisotropy of a single-pore. Moreover, it had been observed by experiment that the anisotropy is weakened and transformed to isotropy in the process of combining geometric models with a large number of single pores. The final numerical results agree well with the experiments, which validates the precision and reliability of the proposed model. The increasement of thermal conductivity is also calculated and discussed during the phase change process of paraffin/EG. Highlights • A method of EG pores was proposed for constructing geometric models which has obvious anisotropy. • The SEM micrograph and saturation sorption capacity are employed to build the geometric models of EG pores. • Effective thermal conductivity during non-phase change process and the phase change process was calculated. • The numerical results agree well with the experiments. [ABSTRACT FROM AUTHOR]

Expanded graphite (EG) powder as a typical porous material has high thermal conductivity and large surface area that are often used to enhance the thermal conductivity of phase change materials (PCMs). Ten composites of paraffin/EG powder with various weight fractions were elaborated by high pressure compression. Their microstructure configuration and thermal properties were characterized with Scanning Electron Microscopy (SEM), Thermal Constant Analyzer. The saturation sorption capacity of EG under compression was experimentally determined. The thermal conductivity of the paraffin/EG composite is greatly enhanced by 41 times in maximum as compared to pure paraffin. Based on the experimental quantification, we propose a novel two-level scale model from micro-scale to macro-scale to compute the thermal conductivity of the composite. The numerical results agree well with the experimental data that validate the precision and reliability of the proposed model. Furthermore, the results reveal that both keeping porous EG structure at original morphology and increasing the saturated sorption capacity of paraffin in EG could eventually improve the heat transfer capacity of the composites. In addition, the proposed model would require more experimental validation based on the composites with different materials in future work. [ABSTRACT FROM AUTHOR]

In this study, expanded graphite (EG)/paraffin (PA)/silica (SiO 2) composite phase change materials (PCM), with good thermal conductivity and impermeability, were fabricated for potential applications in solar energy. The EG-PA was first prepared by vacuum adsorption and then encapsulated by SiO 2 through solid-phase wet grinding in the presence of sodium silicate. Our results show that the EG-PA-SiO 2 composite have optimal properties when the mass ratio of raw material is PA: sodium silicate: H 2 O = 1:9:2:8. The latent heat of melting of the EG-PA-SiO 2 composite prepared at this mass ratio is 106.2 J/g. The core material loss ratio, after washing with petroleum ether, is 4.79%. The thermal conductivity is 2.053 W/m·K, which is 8.5 times higher than that of pure paraffin wax, and the melt permeability is 9.35% after 120 min at 60 °C. Under simulated sunlight, the time required for the composite material to change from 30 °C to 65 °C is reduced by 70.78% compared to PA and the heat absorption and release times are nearly unchanged under multiple cycles. These results indicate that the material has a stable photothermal conversion efficiency. [Display omitted] • Expanded graphite, paraffin and silica composites were made by solid-phase wet grinding method. • Silica encapsulated the composite, reducing leakage. • Composites showed improved thermal conductivity and impermeability. • The composite has stable photothermal conversion, and there is no obvious leaking sign. [ABSTRACT FROM AUTHOR]

Xylitol with high latent heat of 243.28 kJ/kg and a significant supercooling degree has been considered to be a potential phase change material (PCM) for long-term thermal energy storage (TES). Xylitol can be kept in a supercooled or supersaturated metastable for a long time. In our study, it took 23 days and 30 min respectively that xylitol liquid and xylitol liquid adding 5 wt% ethanol crystallized completely at room temperature of 15 ± 5 °C. Unfortunately, the low thermal conductivity and leakage of xylitol restrict its thermal performance. In this paper, xylitol/expanded graphite (EG) composite PCMs with 5, 8, 10, 12 and 15 wt% EG content were prepared by an "impregnation and compression" two-step method. The results of SEM, XRD and FT-IR show that xylitol is well impregnated into the micropores of EG and has good chemical compatibility. The thermal performance test shows that the thermal conductivity of composite PCMs increases dramatically and the latent heat decreases with the increase of EG. The experimental results show that the latent heat of 10 wt% EG is 209.73 kJ/kg and 192.18 kJ/kg, respectively, during the charging and discharging process. The thermal conductivity of 10 wt% EG is 3.91 W/(m·K), as 9.29 times as xylitol. Besides, the crystallization of xylitol in EG and the mass loss of the composites have also been studied. The results show that 10 wt% EG composites crystallized completely after 16 days. These results prove that xylitol/EG composite PCMs have broad application prospects in long-term TES. ∙ A shape-stable xylitol/expanded graphite composite phase change material was prepared. ∙ The crystallization process of xylitol liquid and xylitol liquid adding 5 wt% ethanol at ambient temperature was studied. ∙ The thermal properties of shape-stable xylitol/expanded graphite composite phase change material were studied. ∙ The composite was characterized and the feasibility of its application in long-term thermal energy storage was preliminarily proved. [ABSTRACT FROM AUTHOR]

The latent heat thermal storage using Phase Change Materials (PCMs) is one of the essential aspects to be addressed for rapid expansion of utilisation of Renewable Energy sources and waste heat. The major disadvantage of PCMs, deployed in such thermal storage systems, is their low thermal conductivity and, in some cases, low latent heat. Paraffin wax/expanded graphite (PW/EG) composite PCMs with the EG mass fraction of 2%, 4%, and 6% were prepared by absorption of molten organic material into the EG structure. FireCarb TEG-315 and FireCarb TEG-160 were used as EG of A- and B-type, respectively, and PW 53–57 °C was deployed as the PCM in this study. Polarizing optical microscope, scanning electron microscopes and Fourier transform infrared spectroscopy were used to characterize the structure of composite PCMs. Thermal properties of pure Paraffin and produced composite materials were determined using a differential scanning calorimeter and thermal conductivity analyser. One of originalities of this work is that the statistical method of assessment of heterogeneity in composite PCMs was proposed for the first time with the use of Matlab software. This method was deployed for the quantitative assessment of uniformity of the EG distribution in composite PCMs structure. The novelty of obtained results is in detection of intermolecular interaction between B-type EG and PW. This interaction further enhances thermo-physical properties of the PW/EG composites. In contrast to pure PW and PW/A-type EG composite PCMs, in which the solid-solid transition and solid-liquid phase change are observed, PW/B-type EG compositions exhibit only the solid-liquid phase change. The heat storage capacity in both types of compositions was found to be almost identical. Despite of prolonged sample's ultrasonic treatment, the structure of PCM compositions was found to be far from being homogeneous. However, the thermal conductivity of compositions with 6 wt % of EG was determined to be 0.977 W/m°C for the PCM with A-type EG and 1.263 W/m°C for the PCM with B-type EG in comparison with the value of 0.258 W/m°C for pure PW. These values of the thermal conductivity correspond to the enhancement ratios of 3.79 and 4.9, respectively. If uniformity in the EG distribution in PW can be improved, then the thermal conductivity of composite PCMs would also considerably increase. Findings of this work are being used for designing a cost efficient solar thermal storage system with the reduced charging/discharging times as a part of an international project funded by EC. Image 1 • Paraffin wax/expanded graphite (EG) compositions with EG mass fractions of 2%, 4%, and 6% were analysed. • FTIR analysis indicates that B-type of two (A- and B-) types of tested expanded graphite interacts with paraffin. • The compositions with A-type EG exhibit solid-solid transition and solid-liquid phase change. • The compositions with B-type EG exhibit the solid-liquid only phase change. • Thermal conductivity enhancement ratio is 3.79 and 4.9 for PCM compositions with A- and B-type 6wt% EG, respectively. [ABSTRACT FROM AUTHOR]