• Energy Research

Abstract As covering layers, barrier envelopes of VIPs are always exposed to severe environmental conditions, including high temperatures, alkaline environments and local stress concentration when used as thermal insulation material in concrete structures. This study investigated the time-dependent degradations of three types of commonly used envelopes (aluminium film, metallized film and metallized film coated with alkali-resistant (AR) fibreglass mesh) and VIPs covered by these three envelopes by simulating environmental conditions with four types of alkaline solutions (NaOH solution with pH = 7, 11, 13 and saturated Ca(OH)2), two different temperatures (20 °C and 60 °C) and local stress concentration. The results showed that stress and high temperature accelerated the degradation of envelopes and that such degradation became more serious with increased pH value and temperature. It was also observed that the thermal conductivity of VIPs increases quickly when they are exposed to higher alkalinity combined with high temperatures. After 6-week submerged in saturated Ca(OH)2 solution at 60 °C, the thermal conductivity of VIPs increased from 4.413 mW/m K to 13.049 mW/m K for aluminium foil, from 5.375 mW/m K to 10.982 mW/m K for metallized film, and from 5.786 mW/m K to 8.110 mW/m K for AR fibreglass mesh-reinforced film, respectively.

Abstract In this study, granular phase change material (PCM) composites were developed by absorbing paraffin into the pores of expanded perlite particles with two grades of particle size. Because of the different particle sizes and pore structures, the absorption mechanisms of the expanded perlite particles were found to differ. A significant amount of paraffin leakage was found when the PCM composites were applied directly into the cement mixture. To prevent such leakage, a new method by using hydrophobic silane was investigated as surface modification for the PCM composites. The method was then compared with nanosilica deposition method. Although both methods prevented leakage effectively, cement composites incorporating silane-modified PCM composites had lower compressive strength than those incorporating nanosilica modified PCM composites. Thermal performances of expanded perlite/paraffin composites were compared with that of microencapsulated PCM and the results showed expanded perlite/paraffin composites were superior under certain conditions.

Abstract Thermal energy storage composites were developed by incorporating granular phase change materials (PX25 from Rubitherm®) into cement paste. Without prevention, however, a significant amount of phase change material (paraffin) leaked from the PX25 during the fabrication process. To prevent paraffin leakage during fabrication, three types of nano-SiO 2 powder (hydrophilic fumed silica (fs1), hydrophobic fumed silica (fs2), and precipitated silica (ps2)) were chosen to modify PX25. The influence of dosage of these modifiers was investigated with reference to PCM leakage during fabrication. Paraffin leakage was eliminated when hydrophobic fs2 or ps2 was used in a 2.3% and 9.0% mass fraction (relative to PX25), respectively. However, hydrophilic fs1 did not prevent leakage of paraffin. Microstructural and mechanical analyses of the thermal energy storage composites were used to analyze the prevention mechanism of hydrophobic nano-powder against paraffin leakage.

Abstract The influence of particle characteristics, such as shape, size, and volume fraction, on the permeability of porous media was investigated by combining the randomly packed beds of Platonic particles with the lattice Boltzmann method. Quantitative solutions of the permeability as a function of these characteristic parameters in mono-sized particle packing structures were obtained. The D3Q19 model is presented here, which was tested by three simple benchmark tests. A series of packed beds of Platonic particles as well as spherical particles were generated in a random manner. Numerical studies on factors influencing the permeability of materials were carried out to comprehensively study their impacts. The results revealed that the permeability significantly increased with increasing equivalent diameter of the particles (or decreasing volume fraction). At a fixed size and volume fraction of particles, the permeability of the Platonic particle packing structures was also influenced by particle morphology: permeability significantly reduced as the particle sphericity decreased. The permeability of tetrahedral particle packing structures dropped by more than 40% compared with that of corresponding spherical particle systems.

Abstract We investigated the integration of form-stable paraffin/nanosilica composites into vacuum insulation panels (VIPs) to improve the thermal mass. In total, six form-stable paraffin/nanosilica composites were produced by absorbing paraffin in different types of nanosilica with different surface characters, hydrophobic and hydrophilic. In view of cost, a phase change material (PCM) composite prepared by hydrophilic precipitated silica was chosen to produce the PCM-integrated VIPs. The microstructure and phase change behaviour of the PCM composites were investigated. The thermal mass of the PCM-integrated VIPs was found to be improved. With integration of PCM the thermal conductivity of the VIPs was increased, although it was still very low compared to normal foam insulations.