Microencapsulated Phase Change Materials with Hybrid Latent Heat Storage and Oxygen Storage Functions

Oxygen plays an important role in reducing CO₂ emissions. Combustion with a high oxygen concentration reduces fossil fuel consumption due to its high reaction efficiency. Furthermore, the combustion support gas of a mixture of exhaust gas and pure oxygen increases the CO₂ concentration in the exhaust gas, reducing the cost of CO₂ separation and capture. However, the current method of producing oxygen using cryogenic distillation has the problem of consuming a large amount of energy.<br/>Oxygen production by the pressure swing adsorption (PSA) process using oxygen storage materials (OSMs) based on redox reactions is expected to be an alternative energy-saving oxygen production method. In this process, oxygen is stored in the OSM under ambient oxygen partial pressure and then released by decompression. During this process, the large reaction enthalpies of the redox reactions (exothermic oxidation and endothermic reduction reactions) change the temperature of the OSM during each reaction (temperature increase and decrease, respectively). These temperature changes occur in the direction of inhibiting each reaction, which greatly reduces the efficiency of PSA.<br/>The use of phase change materials (PCMs) is one excellent way to passively control the temperature change during a reaction without energy input. In this PSA, oxidation and reduction reactions are repeated within one packed bed, so it is expected that the PCM in the packed bed will store heat during the oxidation reaction and release heat during the reduction reaction, ideally maintaining a constant temperature inside the packed bed. We previously developed a microencapsulated PCM (MEPCM) with an aluminum alloy core covered with an alumina shell, which overcame the problems of leakage during melting and corrosion by liquid metal. Therefore, MEPCM is suitable as a material for temperature control in the packed bed.<br/>Preliminary experiments on a mixture of MEPCM prepared by a conventional method and an OSM revealed that alumina on the MEPCM surface diffuses to the OSM surface, reducing the oxygen storage/release reaction rate. To solve this problem, this study aimed to enhance the MEPCM-OSM performance by coating the surface of MEPCM with OSM precursor material and preparing the MEPCM-mixed OSM pellets from it. Since OSMs can also be used as a chemical heat storage material using redox reactions, MEPCM-mixed OSM pellets can be applied as a material that can store sensible heat, latent heat, and chemical heat in energy storage systems that use heat.<br/>The experiment was carried out using Y-doped Ca₂AlMnO_5+δ (CYAMO) as the OSM and Al-12mass%Si (Al-12Si) as the core alloy of the MEPCM. CYAMO was synthesized from the nitrates of each element as starting materials. CYAMO powder and Al-12Si powder (D &lt; 38 µm) were mixed by High-speed Impact Blending (HIB) to attach CYAMO to the Al-12Si powder. The alloy was then heat-treated in oxygen to encapsulate it. The melting and solidification characteristics of the products were analyzed by TG-DSC. The morphology after HIB and heat treatment was also observed by SEM and EDS.<br/>The trial on the variation of the peripheral speed during HIB at 40, 60, 80, and 100 m/s showed that a large amount of CYAMO fine particles successfully attached to the surface of Al-12Si at 40 and 100 m/s. Analysis of the melting and solidification characteristics after heat treatment showed that at speeds of 40, 60 and 80 m/s, a heat generation peak due to solidification was observed just below the melting point during temperature drop, while at 100 m/s, supercooling of approximately 35 °C occurred. Since this supercooling is a behavior unique to MEPCM that is not present in bulk Al-12Si, this result suggests successful encapsulation at a peripheral speed of 100 m/s.