Preparation of energy efficient paraffinic PCMs/expanded vermiculite and perlite composites for energy saving in buildings
Introduction
Buildings are one of the leading sectors of energy consumption in developed countries [1]. The increasing energy demands in buildings worldwide indicate the need for redistribution of energy to achieve good comfort conditions in the indoor environment, without compromise of energy efficiency. Thermal energy storage has become a key sector for energy savings in buildings. Thermal energy storage is generally classified into sensible and latent heat storage [2]. Phase change materials (PCMs) are a kind of latent heat energy storage material, and can be applied to store substantial thermal energy, which is collected from solar radiation [3]. PCMs can play an important role in an energy storage device, by utilizing their high storage density and latent heat property [4]. The latent heat property is the amount of heat released or stored by a substance during a change of state, without much change in temperature. This can be used to shift the cooling or heating load from the peak period, to the off-peak period [5]. PCMs have been studied in the field of buildings for saving energy. They can be expected to be integrated with various building materials, and gradually be developed into specialized materials. PCMs have been considered for TES in buildings since 1980. By implementing PCMs in gypsum boards, plaster, sandwich panel, concrete or other wall covering materials, TES becomes a part of the building structure, and is useful even for light-weight buildings. Thus, they are appropriate PCMs for the fabrication of building composite materials.
Vermiculite is a hydrous silicate mineral that is classified as a phyllosilicate, and when heated, it can greatly expand. Exfoliation occurs when the mineral is sufficiently heated, and the effect is routinely produced in commercial furnaces. Vermiculite is formed by the weathering or hydrothermal alteration of biotite or phlogopite [1]. Vermiculite generally has a melting point of 1330 °C, specific heat of 1.08 kJ/kg K, specific gravity of 2.5, and pH of 7–8. Vermiculite is a safe inert material, because when heated, it expands (exfoliates) up to 30 times its original volume. The exfoliation process converts the dense flakes of ore into light-weight porous granules, which contain innumerable minute air layers. The exfoliated (expanded) vermiculite is light, and clean to handle. Also, it has high insulation value, acoustic insulation properties, and is absorbent to a wide range of liquids. The expanded vermiculite is a lightweight material that is porous, inexpensive, ecologically harmless, and non-toxic. Therefore, vermiculite has been used in applications such as insulation materials in buildings. Perlite is a volcanic glass containing 65–75% of SiO2, and 2–5% of H2O. The characteristic property of perlite is that if it is exposed to rapid heating temperature of 900–1,200 °C, it increases its volume by 4–20 times. This happens because perlite includes bound water, which expands during heating, according to the well-known popcorn effect, and adopts the form of a porous swollen material. The size of swollen pellets varies between 1 and 10 mm, with an apparent density of 75–150 kg/m3. Thanks to these numerous tiny voids, the volcanic rock becomes light, and acquires unique thermal and sound proofing properties. Therefore, it is used in many commercial applications, such as construction, thermal, and acoustic insulation, agriculture, and horticulture. These properties make it one of the most suitable candidates to prepare composite PCMs for thermal energy storage in buildings.
In this paper, form-stable composites consisting of n-octadecane with eVMT and ePLT were prepared as novel potential PCMs for high thermal energy storage in building applications. Their thermal properties and reliability were analyzed by differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA). The thermal conductivity of composites was measured by TCi thermal conductivity analyzer. Their morphologies and structures were investigated by scanning electronic microscopy (SEM), and Fourier transformation infrared spectroscopy (FTIR).
Section snippets
Materials
For preparation of samples, n-octadecane purchased from Sigma-Aldrich company was used for the experiment as heat storage material. The n-octadecane has a latent heat capacity of 256.5 J/g and melting phase change range of 26.9–28.7 °C. The eVMT and ePLT were supplied from Misung Corporation in South Korea. The expanded vermiculite and perlite was ground in a grinder and sieved by 100–150 μm mesh. Table 1 shows the components of eVMT and ePLT that were used.
Preparation of the form-stable composite PCMs
To utilize PCMs in buildings, they need
Characterization of composite PCMs
Features of pore structure, such as the surface area, pore diameter distribution, and porosity, are important factors in the selection of porous materials for impregnating PCM. Fig. 2 shows the pore diameter distribution of the eVMT and ePLT. It can be seen that the eVMT and ePLT have a pore structure consisting of mesopores and macropores. Table 2 shows the physical properties of eVMT and ePLT. The eVMT and ePLT have macroporous structures, with pores of micrometer size. No pore of eVMT with
Conclusion
In this study, novel form-stable composite PCMs were prepared, by a vacuum impregnation method. We determined the characteristics of n-octadecane based composite PCMs that contained eVMT and ePLT, by using FT-IR, SEM, DSC, and TGA analysis techniques. Because of the effect of capillary force and surface tension force, n-octadecane was confined in mass fractions of 80% and 59% in the eVMT and ePLT, respectively, without any liquid n-octadecane leakage from the porous material of the composites.
Acknowledgment
This is research was supported by a grant (14CTAP-C078014-01) from Infrastructure and Transportation Technology Promotion Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.
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