ENCAPSULATED PHASE CHANGE MATERIALS IN THERMAL BUFFERING APPLICATIONS: EXPERIMENT AND SIMULATION

Abstract

The energy demand of the world is steadily rising due to increasing population and standard of living, which demands higher production of energy and its effective storage. Phase change materials (PCMs) are found to be quite useful for storing large quantities of thermal energy from surrounding environment by virtue of their large latent heat capacities. The thermal energy stored should be recoverable later under thermal gradient. Thermal energy can be stored in various forms, e.g., sensible heat, latent heat, and thermo-chemical, of these, the latent heat involves the solid-liquid phase transformations, and the energy density (energy stored/mass) is significantly higher compared to other forms. Additionally, PCM temperature remains nearly unchanged during the complete phase transformation. PCMs are generally classified based on: (i) the phase changes involved (solid-solid, solid-liquid, liquid-gas, or solid-gas) and (ii) composition/origin (organic and inorganic). Organic PCMs are carbon-based fatty alcohols, fatty acids, fatty esters, paraffin, etc., while inorganic PCMs are non-carbonaceous compounds like inorganic salt hydrates, metals, etc. In spite of high energy storage density with PCMs, loss due to leakage in fluid (liquid/gas) state and contamination with the surrounding limit their wide range of applications. Enclosing PCMs inside leak-proof containers of different size ranges (macro/micro/nano) could substantially overcome leakage/contamination issues and broaden their application scopes. With minimization of container volume, the surface area of heat transfer increases, facilitating energy transport through increased surface area of containers. Material selection for the container wall and its thermal properties impart flexibility in tailoring the heat transfer rate leading to phase transformation/super-cooling phenomena. Methods of enclosing or loading PCM inside container may be classified into two types: (i) core-shell and (ii) shape stabilization, of which the later method is very simple, where PCM is mixed with/loaded in porous materials/objects of definite shape/dimension ensuring the pores get filled with the PCM. While synthesis of micro/nano-sized core-shell geometry with PCM as core involves multiple synthetic steps. The potential use of renewable materials is a crucial component of sustainable development. In this thesis, both organic (propyl palmitate, hexadecane, and isopropyl stearate) and inorganic PCMs (zinc nitrate hexahydrate) were encapsulated using direct impregnation and self-assembly methods. These encapsulated PCMs could be utilized in buildings for temperature maintenance, thermo-regulating textiles, and temperature control packaging applications. This report is prepared involving following main objectives: 1. Encapsulation of organic PCM hexadecane by inorganic calcium carbonate shell through a self-assembly process. 2. Encapsulation of organic PCM propyl palmitate within porous matrix expanded perlite (EP) through direct impregnation for cooling purposes in building application. 3. Encapsulation of inorganic PCM zinc nitrate hexahydrate within porous matrix EP through direct impregnation for electric radiant floor heating (ERFHS) in a building application. 4. Encapsulation of organic PCM isopropyl stearate using expanded graphite (EG) for cold chain applications. Hexadecane PCM with high enthalpy (ΔHm=222 J g-1) and the melting temperature (Tm= 19 °C) was microencapsulated using inorganic calcium carbonate shell through self-assembly technique. Mixed surfactants (SDS/PVP) was used in the encapsulation process. Hexadecane encapsulation inside calcium carbonate shell was achieved through self-assembly method, where two parameters (i) mass ratio of two surfactants (mixed surfactant template) and (ii) core/shell ratios were altered to understand encapsulation efficiency and morphology. Mass ratio of SDS/ PVP (1:1) resulted uniform spherical template for successive growth of CaCO3 shell. With various core: shell mass ratios, 1:1 was found to be optimum for stable microcapsules without leakage. DSC analysis showed encapsulation efficiencies as: 26.9±0.78%, 31.8±0.60%, 40.7±0.78%, and 43.1±0.78%, respectively with S1 (0.5: 1), S2 (0.75:1), S3 (1:1) and S4 (1.25:1). Encapsulation reduced supercooling of hexadecane by ~1.5 °C. Negligible change in thermal properties with S3 (1:1) sample while exposed to 100 thermal cycles using DSC. CaCO3 shell microcapsules showed rapid charging/discharging behavior compared to only hexadecane. CaCO3 supported hexadecane microcapsules might be a potential candidate for thermo-regulating textile applications. The lower encapsulation efficiency or core content limited the scope for commercial application. So, for the next exploration, shape stabilization of PCMs was considered for the encapsulation of PCMs as the preparation of shape stabilized PCM composites is economical and requires fewer processing steps than making core shell capsules. Shape stabilized composite of propyl palmitate (Tm: 18.99 °C, ΔHm: 149.94 J g-1) and expanded perlite (EP) was prepared by direct impregnation method. With the known weight of EP different amount of PCM was mixed to prepare PCM-EP composites containing 60 weight % (EP-60), 55 weight % (EP-55), 50 weight % (EP-50), 45 weight % (EP-45) and (EP-40) 40 weight %. The highest loading content PCM composite EP-55 (55 wt.% PCM) was chosen for thermal buffering experiments in building model. Two wooden-ply (1 cm thick) make boxes of different dimensions (the bigger one, B1, with L×W×H: 60× 40 × 42.5 cm, and the smaller box, B2, with L×W×H: 27.5×27.5×32 cm) were fabricated locally. The smaller box, B2 was termed as building and the space between boxes B1 and B2 was termed as ambience for the building. The room with 500g and 250g of EP-55 composite ceiling reduced the inner temperature by 7°C and 6°C respectively. The building model developed using COMSOL was validated using experimental data under the various amount of EP-55 composite in the building. The percentage error between simulated and experimental results was lesser than 4%. After encapsulating organic PCM with expanded perlite in the next work an inorganic PCM zinc nitrate hexahydrate (ZNHH) was chosen to explore electric radiant floor heating application in buildings. Form stable PCM composites with EP as the support material, and ZNHH (Tm: 36.49 °C, ΔHm: 138.24 J g-1) as the PCM was prepared by the impregnation method, with varied mass ratios of ZNHH/EP. Composites were prepared by keeping mass of EP constant and varying amount ZNHH i.e.70 mass % (EP-70), 65 mass % (EP-65), 60 mass % (EP-60), 55 mass % (EP-55), 50 mass % (EP-50). Finally, the EP-65 coated with paraffin was put onto filter paper to remove excess paraffin from the composite until no visible trace remained, and the CEP-65 was obtained. In the current work, an investigation was carried out to study the effect of variations in two crucial parameters, i.e., variation in the amount of PCM composite and the heat transfer area of the building model. The thermal-buffering property of CEP-65 composite (200 g) for variation in heat transfer area (keeping the amount of PCM composite constant) was investigated using three wooden boxes of different dimensions, i.e., wooden boxes B1 (15 × 15× 15 cm), B2 (20 × 20× 15 cm) and B3 (25 × 25× 25 cm). Further, experiments were conducted to understand the effect on thermal buffering capacity of floors containing various amounts of PCM composite (200 g, 300 g, and 400 g) in building B3. 200 g composite packets were kept at the bottom of three different size buildings, B1, B2, and B3, as floor provided thermal buffering time of 8364 s, 4611 s, and 4208 s, respectively. The thermal buffering time provided by arranging 200 g, 300 g, and 400 g PCM composite as floor inside building B3 was found to be 4208, 5737, 6832 s. Additionally, a three-dimensional building model based on an electric radiant floor heating system (ERFHS) developed using COMSOL software was authenticated by experimental data for various heat transfer areas and different amounts of CEP-65 in the building model. So far it was observed that the maximum encapsulation efficiency was 65 mass%; hence, a different porous support matrix of expanded graphite (EG) was explored for making shape stabilized PCM. For the preparation of EG-PCM composite, a constant mass of isopropyl stearate was considered to impregnate into varying mass of obtained expanded graphite (EG) matrix, and maximum encapsulation efficiency was found to be 88.9% with IPS:EG= 10:1.25. In case of IPS: EG=10:1.25 composite m.p. was 6.1±0.19 °C, enthalpy was 172.3±2.05 J g-1. Negligible change in the onset to melting/crystallization temperatures were recorded during 1st, 50th, 100th, 150th, 200th, 250th cycles. Thermal buffering experiments targeting cold chain application were carried out under two distinct conditions: the box was placed inside an oven (controlled condition) at a temperature of 30 ± 1°C, 40 ± 1°C, or 50 ± 1 °C, and the box was exposed to direct sunshine (uncontrolled condition). The total average insulation time for three different temperatures was found to be 21194 s (∼6h), 13776 s (∼4h), 10052 s (∼2.7 h), and 14942 s (∼4.15 h) by exposing the box to direct sunlight. Overall, the encapsulation of PCM using different supporting matrices successfully resolved the leakage issue of PCMs. It was observed that expanded graphite could be a better option as the supporting matrix based on the addressing of the leakage issue in PCMs and encapsulation efficiency. Based on applications of encapsulated PCMs mainly in building model and radiant floor heating, it was understood that for long-term applications, salt hydrate PCM (zinc nitrate hexahydrate) composite showed 18.8 % drop in enthalpy content, which could be reduced to 4.8 % by applying a thin coating of wax on the composite. Therefore, salt hydrate PCMs should be less preferred over other PCMs.