SYNTHESIS AND CHARACTERIZATION OF MACRO/MICROCAPSULES CONTAINING PHASE CHANGE MATERIAL WITH -OH/-COOH FUNCTIONAL GROUP FOR THERMAL ENERGY STORAGE

Abstract

Thermal energy storage (TES) materials are capable of storing/releasing the latent heat of fusion/crystallization during their phase transformation, keeping the surrounding temperature mostly unaffected. Based on the melting/crystallization temperature, these materials are being used in the buildings, packaging, heat transfer fluid, etc. TES technology is dominated by latent heat storage than sensible heat due to high energy storage density and isothermal storage phenomena with the former. Materials capable of storing large latent heat during phase change are called phase change materials (PCMs) or latent heat thermal energy storage (LHTES). As there is no need for fossil fuel energy in this enthalpy exchange (phase transformation) with PCMs, this is considered a green technology without a carbon footprint. Out of various possible phase transformations (solid-liquid, solid-solid, liquid-gas, and solid-gas), the solid-liquid transformation is preferred due to ease in handling. Based on the sources, PCMs are derived; they are classified into two categories, i.e., inorganic (salts hydrates, metals from earth) and organic PCMs (mostly fossil fuel origin and plant-derived). Thermal cycle stability, easy availability, and congruent melting behavior with organic PCMs have made it explored more than inorganic PCMs. The existence of crystal defects in inorganic PCMs resulted in incongruent melting, supercooling, and loss of stability during thermal cycles, making these PCMs least explored [1]. Fossil fuel is being non-renewable in nature. Therefore, the current focus is on deriving PCMs (i.e., fatty acids, fatty alcohols, etc.) of similar performance but originating from a renewable plant. [2]–[4]. Fatty acids/fatty alcohol-based PCMs also exhibited large latent heat and consistent purity. Additionally, low or negligible supercooling, high thermal reliability, and stability of these renewable PCMs make them suitable alternatives to paraffin-based PCMs. Once PCM gets melted, handling it becomes a challenge; hence, it must be stored inside an appropriate enclosure. The process of enclosing PCMs inside a suitable container is known as encapsulation. This report focuses on encapsulation of selected fatty acids and fatty alcohol (renewable PCMs), characterization, and applications. Following important goals were set in executing the research: 1. Understanding phase transition and latent heat capacity with two categories of organic PCMs (fatty acids and fatty alcohol), i.e., molecules with different functional groups. 2. Microencapsulation of organic PCMs using a naturally derived polymer as shell and evaluating their thermal buffering performance 3. Microencapsulating organic PCMs using synthetic polymer(s) as shells and analyzing their thermal buffering ability 4. Development of PCM-loaded temperature control packaging (TCP) box for thermal buffering applications Gelatin-gum Arabic (GE-GA; natural polymers) and sodium alginate were naturally derived shell materials, while copolymers of (styrene, divinylbenzene, and n-butyl acrylate), and melamine-paraformaldehyde were of synthetic nature. Different physio-chemical and chemical methods like coacervation, sol-gel process, ionic gelation, interfacial polymerization, emulsion polymerization, and in-situ polymerization were adopted to encapsulate PCMs the selection of method was based on the nature of core and shell materials. Capric acid (Tm: 30.66 °C, ΔHm: 160.84 J/g) was encapsulated with glutaraldehyde cross-linked gelatin/gum Arabic, where a composite morphology was obtained with an enthalpy content of 85.38 J/g (core/shell: 2/3). Improvement in thermal stability and flow ability could be achieved by applying SiO2 coating over this microencapsulated PCM (MEPCM) composite, where ∼20 °C increase in thermal stability could be obtained with drop-in enthalpy content. Synthesized MEPCM composite showed poor encapsulation ratio (core: shell) and morphology, hence had limited scope for commercial applications. Subsequently, 1-Dodecanol (alcohol derived from lauric acid) with properties close to that of lauric acid was used as core PCM and was encapsulated using barium cross-linked alginate (natural shell material). The ionotropic gelation technique was adopted to encapsulate the core. The PCM capsules obtained were of poor mechanical stability with exudation issue; hence additional coating of polyurea layer was applied on these capsules. The latent heat of fusion (PCM@Ba-Alg beads ~149.15 J/g) got reduced to ~76.67 J/g with polyuria coating. Improved mechanical stability and with no exudation issue, these PCM@Ba-Alg@polyurea beads ( ̴ 1 to 2 mm) could be applied in a TCP (temperature-controlled packaging) box, where 60 g of PCM@Ba-Alg@polyurea capsules could delay 86.28 min for 240 g of chocolate to reach 25 oC, when exposed at 35±2 °C. 1-Dodecanol was subsequently encapsulated inside a synthetic copolymer shell made of divinylbenzene cross-linked copolymer of styrene and n-butyl acrylate prepared by emulsion polymerization technique. PCM encapsulation ratio of 69% could be achieved with the latent heat of fusion ~133 J/g. The size of the capsules was of ̴ 17μm, which were then embedded in a poly (vinyl chloride) (PVC) film to form a composite film targeting thermal buffering application. MEPCM-embedded PVC film showed enthalpy of ~94 J/g without any leakage issue while exposed to 50 °C, and it was suitable for the thermal buffering application. Surfactant free- encapsulation of 1-dodecanol was attempted using (i) TiO2 (Pickering Emulsion) and (ii) paraformaldehyde-based surfactant-free template to improve encapsulation efficiency and reduce the overall cost and process complexity. Prepolymer (methylolmelamine) acted as a self-dispersant and formed a sufficiently stable emulsion. The enthalpy of microcapsules made via Pickering emulsion was slightly lower than those prepared via the surfactant-free technique. PCM microcapsules ( ̴ 170 J/g) through the surfactant-free route, being cheaper and easier, are expected to become popular over the Pickering emulsion technique. Microencapsulated PCM capsules of fatty acids/fatty alcohols showed considerable loading efficiency, i.e., higher latent heat content, and are expected to be future-ready thermal energy storage materials for suitable applications.