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.
In this work, polyethylene glycol (PEG) based polyurethane copolymer was synthesized using linear chain extension and branching methods. In linear chain extension, multiple PEG molecules were linked ...using urethane bonds. In branching, polyvinyl alcohol (PVOH) backbone was grafted with PEG molecules as branches and cross-linked to other PVOH molecules. 4,4′-Diphenylmethane di-isocyanate (MDI) was used as both chain extender and cross-linker. The resulting copolymers were characterized for chemical composition, microscopic morphology, crystalloid structure and thermal properties. Based on the results from characterizations, a detailed discussion on the phase change mechanism in polyurethanes is presented. 1H NMR and FT-IR results confirmed the chain extension and branching. 1H NMR spectra displayed characteristic signal of the urethane. DSC results revealed solid-solid phase change behavior of copolymers with high latent heat. Crystalline nature of spherulite lamellae structures was verified by observing birefringence using POM microscope and XRD peaks. Polyurethane copolymers had maximum latent heat absorption capacity of 144.14 kJ kg−1 at melting temperature of 51 ℃ and had maximum heat release capacity of 117.5 kJ kg−1 at freezing temperature of 33 ℃. Polyurethane copolymers were thermally stable up to 200 ℃. Polyurethane copolymers satisfied all criteria for use as form stable solid-solid phase change material (SSPCM) in solar thermal conversion storage.
•PCM with molecular chain-extension is synthesized as shape stabilized PCM.•Grafting PCM onto polymer molecule is prepared as shape stabilized PCM.•Characterization of shape stabilized PCM is performed for thermal energy storage.•Differences of urethane link in chain-extended and branched scenario are analyzed.
Due to humanity's huge scale of thermal energy consumption, any improvements in thermal energy management practices can significantly benefit the society. One key function in thermal energy ...management is thermal energy storage (TES). Following aspects of TES are presented in this review: (1) wide scope of thermal energy storage field is discussed. Role of TES in the contexts of different thermal energy sources and how TES unnecessitates fossil fuel burning are explained. Solar power generation, building thermal comfort and other niche applications of TES are presented. (2) Insight into classes of TES storage materials with details like their physical properties, cost, operational performance and suitability to application requirements is provided. (3) Insight into types of TES systems is presented. TES systems are classified using different types of criteria. Most common TES systems like seasonal TES systems, CSP plant TES systems, TES systems of domestic solar thermal applications, heat and cold storages of building HVAC systems etc are described. Active TES systems like thermocline, packed bed, fluidized bed, moving bed etc are analyzed. Passive TES systems implemented in buildings, textiles, automobiles etc are presented. TES systems operating in cold, low, medium and high temperature ranges are listed. Design parameters, operational issues and cost model of TES systems are discussed.
•Technology, material and research works in thermal energy storage were summarized.•Thermal properties of thermal energy storage materials were presented and analyzed.•Heat storage mechanism and applications based TES systems were shown in detail.•Performance parameters and operational issues based TES systems were discussed.
For solving the global problems of environmental pollution and energy shortages, thermal energy storage system that can improve the efficiency and utilization ratio of energy and solve the gap ...between energy demand and supply, has received more attention in recent years. More specifically, the latent thermal storage systems that use phase change materials (PCMs) as storage media, possessing high latent heat storage density and almost constant phase change temperature are the focus area in thermal energy storage. Previously, most of the researches on PCMs were organic, however in recent years, inorganic PCMs with large phase change temperature range have been paid more and more attention. In common inorganic PCMs, hydrated salts possess lower phase change temperature, applying in buildings, solar water heating systems, textiles, etc., and molten salts and metals have higher phase change temperature, applying in concentrated solar power (CSP) generation and industrial waste heat recovery etc. Each has its own outstanding merits, for example, inorganic salts possess a large latent heat storage capacity and metals possess an extremely high thermal conductivity. Therefore, this review focuses on the researches of inorganic PCMs in recent years and summaries their thermal properties, and introduces the integration of inorganic PCMs into heat exchangers and some applications of inorganic PCMs in main systems, seeking to give readers a relatively comprehensive awareness on them.
•Thermal properties of inorganic PCMs for thermal energy storage are analyzed.•Performances of heat exchangers integrated into inorganic PCMs are summarized.•Applications of inorganic PCMs in thermal energy storage systems are discussed.
•Preparation methods of microencapsulated phase change materials were summarized.•Thermal properties of microencapsulated phase change materials were presented and discussed.•Applications of ...microencapsulated phase change materials for thermal storage were analyzed.
Phase change materials (PCM) are characterized by storing a large amount of thermal energy while changing from one phase to another phase (normally solid–liquid states) at a certain temperature. The application of PCM devotes to elevate the global efforts to conserve energy with regard to fast depleting fossil fuels. The selection of supporting materials and encapsulation techniques has crucial effect on the use of the thermal energy released by PCM. The encapsulation method of PCM is an approach to alleviate leakage, phase separation and volume change problems. Microencapsulated phase change materials (MPCM) are one of the most popular techniques to enhance the efficiency in use of resources for thermal energy storage. This work attempts to summarize preparation methods and thermal properties of the MPCM, which helps to better understand the composition and working mechanism of the MPCM. Moreover, the MPCM provide enormous potential to meet the growing demands for applications of cooling and heating in buildings, textiles and MPCM slurry.
Phase change materials (PCM) have stable operation temperature and large storage capacity. However, PCM have problems of leakage and low thermal conductivity. For improving performances of PCM, ...stearic acid (SA) was encapsulated in silica shell by sol-gel method to form microencapsulated phase change materials (MPCM), and graphene oxide (GO) was attached to surface of silica by a self-assembly process to form GO@MPCM, so as to further improve performances of MPCM. The morphology of MPCM was observed through a scanning electronic microscope (SEM). The chemical structure and crystal phase of MPCM were measured by Fourier transformation infrared spectroscope (FT–IR) and X–ray diffractometer (XRD). Raman spectrometer was used to further verify that GO was attached to the MPCM. Thermogravimetric analyzer (TGA) analysis confirmed that MPCM have good thermal stability. Thermal properties of MPCM were measured by Differential scanning calorimeter (DSC), where melting temperature and latent heat of MPCM2 is 67.78 °C and 179.29 J/g. The melting temperature of GO@MPCM is similar to that of MPCM2, and the melting latent heat of GO1 @MPCM and GO2@MPCM is 146.72 J/g and 134.42 J/g, respectively. Besides, thermal conductivity of MPCM with GO is higher than that of pure SA.
•SA is microencapsulated with silica shell via sol–gel technique.•GO is attached to silica shell by a self-assembly process for improving thermal conductivity.•The melting temperature and latent heat of GO1@MCPM is 65.12 °C and 146.72 J/g.•The synthesized microcapsules have good thermal stability.•Thermal conductivity of GO1@MCPM is increased by 62.5% as compared with pure SA.
Microencapsulation of stearyl alcohol (SAL) with silica shell using sol-gel method has been attempted to obtain microencapsulated phase change materials (MPCM). Tetraethoxysilane (TEOS) was used as ...silica precursor to form silica shell. In order to inveatigate thermal properties of the MPCM, some tests were carried out. The results of Fourier transformation infrared spectroscope (FT–IR) indicated that the combination of the SAL with silica by sol-gel method is a physical action. X–ray diffractometer (XRD) was used to confirm PCM maintains stable crystal structure. Scanning electronic microscope (SEM) was employed to determine the MPCM has relatively stable physical structure. Thermal properties of MPCM were measured by Differential scanning calorimeter (DSC), where melting temperature and latent heat of MPCM1 (made from 10 g SAL, 10 g TEOS) is 55.89 °C and 229.73 J/g, respectively. Liquid–solid phase change has latent heat of 112.05 J/g at 56.74 °C while solid-solid phase change has latent heat of 72.01 J/g at 49.75 °C. Thermogravimetric analyzer (TGA) analysis confirms that MPCM has good thermal stability. After 100 thermal cycles of the MPCM1, there was no change in phase transition temperature and latent heat, which confirms that the MPCM has good reliability. The thermal conductivity of MPCM1 was measured to be 0.1508 W/m·K over melting point of MPCM1 and 0.1412 W/m·K at room temperature. Therefore, MPCM1 is found to be a promising candidate for thermal energy storage applications.
Myristyl alcohol (MA)/metal foam composite phase change materials (CPCMs) were fabricated by vacuum melting infiltration. MA was used as the phase change material (PCM) and the metal foam was used as ...skeleton. Nickel foam and copper foam were both employed for comparison. Furthermore, effects of pore size of metal foam on thermal performances were analyzed. Thermal properties, including differential scanning calorimeter (DSC) test results and thermogravimetry analyzer results (TGA) were investigated and recorded. Compared with latent heat of pure MA, the latent heat of CPCMs for the melting process decreased by 3-29%. TGA results showed that the composites have good thermal stability. Fourier transformation infrared spectroscope (FT-IR) and X-ray diffractometer (XRD) were used to determine chemical and crystalloid structure of the composites, respectively. Microstructures of the composites were analyzed by scanning electron microscope (SEM), and the results indicated that the PCM was well absorbed by the metal foam. Thermal conductivity of the composites was measured by thermal conductivity meter (TCM), and the outcome proved that MA/metal foam composites have an improved thermal conductivity.
•Form-stable palmitic acid/polyvinyl butyral/expanded graphite composites were fabricated by solution blending method.•Microstructure and chemical structure of composites were presented and ...analyzed.•The composites with 70 wt% palmitic acid have a higher latent heat of 128.1 J/g.•Thermal conductivity of palmitic acid/polyvinyl butyral composites can be remarkably improved by expanded graphite.
In order to improve the performances of phase change material (PCM) in applications, a novel form-stable phase change materials were fabricated via solution blending method in this work. Palmitic acid (PA) was used as phase change material to release and absorb large amounts of latent heat at operating temperature. Polyvinyl butyral (PVB) is the polymer that was used as supporting matrix to prevent the leakage of palmitic acid in melting state. Expanded graphite (EG) was used not only to enhance thermal conductivity of form-stable phase change materials, but also to help reduce leakage. A series of form-stable phase change materials were prepared, containing pure palmitic acid/polyvinyl butyral composites, and palmitic acid/polyvinyl butyral composites doped with expanded graphite of 3 wt%, 5 wt% and 7 wt%. Fourier transformation infrared spectroscope (FT-IR), X-ray diffractometer (XRD), scanning electronic microscope (SEM) and thermal gravimetric analyzer (TGA) were used to analyze the chemical structures, crystal structures, microstructure and thermal stability of the form-stable phase change materials. Thermal storage properties were determined by differential scanning calorimeter (DSC), the latent heat value of form-stable phase change material with the highest palmitic acid content (70 wt%) was 128.08 J/g, corresponding to melting point of 59.5 °C. The thermal conductivity of form-stable phase change materials which was measured by thermal conductivity meter (TCM) was greatly enhanced by expanded graphite, and the thermal conductivity of form-stable phase change materials can be increased 4.2 times by 7 wt% expanded graphite. Therefore, the novel form-stable phase change materials are promising in thermal energy storage systems, especially in low-temperature solar energy systems.
