In recent years, energy conservation and environmental protection have become most important issues for humanity. Phase change materials (PCMs) for thermal energy storage can solve the issues of ...energy and environment to a certain extent, as PCMs can increase the efficiency and sustainability of energy. PCMs possess large latent heat, and they store and release energy at a constant temperature during the phase change process. Thereby PCMs have gained a wide range of applications in various fields, such as buildings, solar energy systems, power systems and military industry. However, low thermal conductivity of PCMs leads to low heat transfer rate, thus, numerous studies have been carried out to improve thermal conductivity of PCMs. The main purpose of this paper is to review the methods for enhancing thermal conductivity of PCMs, which include adding additives with high thermal conductivity and encapsulating phase change materials. It is found that addition of thermal conductivity enhancement fillers is a more effective method to improve thermal conductivity of PCMs, where carbon-based material additives possess a more promising application prospect. Finally, the applications of PCMs in solar energy system, buildings, cooling system, textiles and heat recovery system are also analyzed.
•The synthesis methods of microencapsulated phase change materials were summarized.•The characterization of microencapsulated phase change materials were presented.•Thermal properties and reliability ...of microencapsulated phase change materials were discussed.•Applications of microencapsulated phase change materials were analyzed.
In recent years microencapsulation of phase change materials has become popular in thermal energy storage field. Commercially produced microencapsulated phase change material (MPCM) is also available in market today. Microencapsulation enhances thermal and mechanical properties of phase change materials used in thermal energy storage. Microencapsulation can be achieved through different techniques and using different shell materials. As the microencapsulation of PCM is gaining increased attention, more and more research works on MPCM are getting published. This review attempts to summarize the available research information on synthesis, characterization, properties and applications of microencapsulated phase change materials for thermal energy storage. The synthesis methods of microencapsulated phase change materials, such as physical synthesis methods like spray drying, physical chemical synthesis methods like complex coacervation and sol–gel process, and chemical synthesis methods like suspension polymerization, emulsion polymerization, interfacial polymerization, in-situ polymerization and condensation polymerization, are presented. The properties of microencapsulated phase change materials like physical properties, chemical properties and thermal properties are analyzed. The applications of microencapsulated phase change materials in buildings, textiles, MPCM slurry and composite foams are also expounded.
Microencapsulated palmitic acid (PA) with titanium dioxide (TiO2) shell as shape-stabilized thermal energy storage material was synthesized through a sol–gel process. Scanning electron microscope ...(SEM), Fourier transformation infrared spectroscope (FT-IR), X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) were used to determine the morphology, chemical structure, crystalloid phase and chemical state of the microcapsules, respectively. The thermal properties and thermal stability were investigated by a differential scanning calorimeter (DSC) and a thermogravimetric analyzer (TGA). The microcapsules have relatively spherical shape and average size of 200–400nm. The FT-IR, XRD and XPS results showed that the PA was well encapsulated in the TiO2 shell. The DSC results indicated that the typical microcapsules melt at 61.7°C with a latent heat of 63.3kJ/kg and solidify at 56.7°C with a latent heat of 47.1kJ/kg. The TGA results confirmed that the microcapsules have good thermal stability, resulting from the TiO2 shells. Based on all results, it can be concluded that the prepared microcapsules have good energy storage potential due to their non-inflammability, nontoxicity and good thermal stability.
•The microencapsulated palmitic acid with TiO2 shell was prepared through a sol–gel process.•Chemical structure, crystalloid phase and microstructure of the MPCMs were analyzed.•Thermal properties and stability of the MPCMs were investigated.
Phase change materials (PCMs) have received attention for various applications in solar heating systems, building energy conservation and air-conditioning systems. However, they need encapsulation in ...order to prevent leakage of the melted PCM during the phase change process. Now, these problems can be solved by using shape-stabilized PCMs. These shape-stabilized PCMs can be prepared by integrating the PCMs into the supporting material and microencapsulating the PCMs into the shell. This paper presents a review on preparation, thermal properties and applications of shape-stabilized thermal energy storage materials. The thermal properties of the composite phase change material and microencapsulated phase change material are analyzed and discussed. The applications of shape-stabilized thermal energy storage materials are summarized.
Microencapsulated phase change material (MPCM) is one of the most practical materials to enhance the energy efficiency for thermal energy storage. The microencapsulation technique is used to solve ...the leakage and volume change problems of pure phase change material (PCM). The MPCM slurry has become a novel heat transfer fluid in heat transfer and heat storage systems. In recent decades, the microencapsulation methods have been widely studied and proposed in many fields such as building, textile, food storage, solar and thermal energy storage, etc. The MPCM and its slurry prepared by these methods are widely investigated and put into practice. Based on these findings and applications, the different microencapsulation methods are characterized in this work. Next, the flow characteristics and basic thermal properties of this novel slurry are introduced in classification. At last the heat transfer problems under different situations are reviewed and analyzed. The future trend of the microencapsulation in thermal energy applications is presented.
In this work, bentonite–based composite phase change materials (CPCMs) were fabricated by the impregnation of fatty acid eutectics into bentonite clay. In the composites, the palmitic acid ...(PA)–stearic acid (SA) eutectics mixtures were undertook as phase change materials (PCMs) for thermal energy storage, and the bentonite were performed as the supporting material. Expanded graphite (EG) was employed for helping restrain the eutectic mixtures from leakage as well as improving thermal conductivity of the CPCMs. The differential scanning calorimetry (DSC) was adopted to assess the thermal properties of the composites, the results showed that the CPCMs have suitable melting temperature of around 54°C with latent heat capacity of 89.12–163.72kJ/kg. Fourier transformation infrared (FT–IR) and X–ray diffractometer (XRD) were utilized to test the chemical structure and crystalline phase of the CPCMs. The scanning electron microscope (SEM) images revealed that the organic PCMs homogenously spread to the surface and interior of the bentonite. The thermal gravimetric analyzer (TGA) detected that the CPCMs were provided with good thermal stability. As the content of the EG increased, the leakage of the PA–SA eutectics reduced considerably. The results from the thermal conductivity meter (TCM) showed that the thermal conductivity of the CPCM with content of 5% EG reached to 1.51W/(mK) in liquid state and 1.66W/(mK) in solid state, which was nearly 5.6 times and 4.9 times higher than that of the CPCM without the EG. Experiments displayed that the thermal storage and release rates were noticeably enhanced by combining the EG into original CPCMs. The CPCMs maintained thermal properties after 50 heating–cooling cycling. It is envisioned that the satisfactory CPCMs maintain considerable prospects in thermal energy storage.
The temperature curves of the CPCM3 and CPCM6 in melting and solidifying processes are shown. The heat storage time initiates from the same temperature (23°C) and ends at the same melting temperature (72.5°C). The interval starts from 70°C and finishes at the same solidifying temperature (26.5°C) in solidification period. The heat storage time of the CPCM3 and CPCM6 were 11min and 8.7min respectively. The heat release time of the CPCM3 and CPCM6 were respectively 68min and 42min Display omitted
•The fatty acid eutectics/bentonite/expanded graphite composites were synthesized for improving thermal properties.•Microstructure and chemical structure analysis of the CPCMs were displayed and analyzed.•Thermal properties and thermal reliability of the CPCMs were investigated and discussed.•Thermal conductivity of the CPCMs in liquid state by adding EG is 5.6 times higher than that of the pure CPCMs.
•The microencapsulated paraffin with TiO2 shells was prepared by a sol–gel process.•Chemical structure, crystalloid phase and microstructure of the MPCMs were analyzed.•Thermal properties and ...stability of the MPCMs were investigated.•The TiO2 shells can improve thermal stability of the MPCMs.
Microencapsulated paraffin with titanium dioxide (TiO2) shells as shape-stabilized thermal energy storage materials in buildings were prepared through a sol–gel process. In the core–shell structure, the paraffin was used as the phase change material (PCM), and the TiO2 prepared from tetra-n-butyl titanate (TNBT) acted as the shell material. Fourier transformation infrared spectroscope (FT-IR), X-ray diffractometer (XRD) and scanning electronic microscope (SEM) were used to determine the chemical structure, crystalloid phase and microstructure. The thermal properties and thermal stability were investigated by a differential scanning calorimeter (DSC) and a thermogravimetric analyzer (TGA). The FT-IR and XRD results presented that characteristic peaks of both paraffin and TiO2 can be observed in the microencapsulated paraffin with the TiO2 shells. The DSC results indicated that the microcapsules exhibited similar phase change characteristics as those of pure paraffin, and the typical ones melt at 58.8°C with a latent heat of 161.1kJ/kg and solidify at 56.5°C with a latent heat of 144.6kJ/kg when the microencapsulation ratio is 85.5%.
Shape-stabilized lauric acid/activated carbon composites as phase change materials were prepared by adsorbing liquid lauric acid into activated carbon. In the composites, the lauric acid was used as ...a phase change material for thermal energy storage, and the activated carbon was used as an adsorbent that acted as the supporting material. Fourier transformation infrared spectroscope, X-ray diffractometer, scanning electronic microscope and thermal conductivity apparatus were used to determine the chemical structure, crystalloid phase, microstructure and thermal conductivity, respectively, of the composites. The thermal properties and thermal stability were investigated by a differential scanning calorimeter and a thermogravimetry analyzer. The microstructural analysis results showed that the lauric acid was well adsorbed into the porous network of the activated carbon. The thermal conductivity measurement results presented that the thermal conductivity of the composites was enhanced. The differential scanning calorimetry analysis results indicated that the composites exhibited the same phase change characteristics as those of the lauric acid and their latent heats increased with increase of the lauric acid content in composites. The thermogravimetric analysis results presented that the activated carbon can improve the thermal stability of the composites.
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► LA/activated carbon composites were prepared by adsorbing LA into activated carbon. ► Chemical structure, crystalloid phase and microstructure of composites were determined. ► Thermal properties, thermal conductivity and stability of the composites were investigated. ► Activated carbon can improve thermal conductivity and stability of the composites.
Microencapsulated phase change materials (MPCM) have been recognized as effective materials to enhance heat transfer, and to improve heat storage performance in thermal energy system. ...Microencapsulated phase change material slurry (MPCS) can apply to heat transport and thermal energy storage systems. In order to fully develop the application of MPCS in thermal energy system, more researches on preparation and characteristics of MPCS have been done. This paper presents a review on microencapsulation methods and thermal characteristics of MPCS. It focuses on the thermal properties and heat transfer characteristics of MPCS flowing in horizontal circular pipe. Some phase change materials and microencapsulation methods are analyzed and discussed. Theoretical models for analyzing heat transfer characteristics of MPCS flowing in the pipe are presented. Several factors affecting the heat transfer characteristics of MPCS are also summarized.
