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 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.
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.
The commercial solar cells are currently less efficient in converting solar radiation into electricity. During electric power convention, most of the absorbed energy is dissipated to the ...surroundings. In order to improve energy efficiency, many efforts have been made to investigate and develop hybrid photovoltaic and thermal collector systems. A photovoltaic–thermal (PV/T) system does both the generation of electric power and collection of thermal energy at the same time. Thus, the overall efficiency of the photovoltaic–thermal (PV/T) system can increase accordingly. In this work, we attempt to summarize various research works on technologies like flat–plate PV/T systems and concentrator type PV/T systems, using different kinds of working fluids under a variety of environmental conditions. The purpose of this review is to define the appropriate environmental conditions and applications for different kinds of PV/T systems. Besides, it is also presented that the applications and developments of the PV/T systems. In order to develop novel PV/T systems, more effort is needed in accurate modeling, exploration of novel materials, enhancement of PV/T system stability and the design of a supporting energy storage system.
•The performances of flat–plate photovoltaic–thermal systems are analyzed and compared.•Developments of concentrator type photovoltaic–thermal systems are discussed.•Applications of photovoltaic–thermal systems are summarized in detail.•A view on the future of PV/T developments and the future work is presented.
Usage of renewable and clean solar energy is expanding at a rapid pace. Applications of thermal energy storage (TES) facility in solar energy field enable dispatchability in generation of electricity ...and home space heating requirements. It helps mitigate the intermittence issue with an energy source like solar energy. TES also helps in smoothing out fluctuations in energy demand during different time periods of the day. In this paper, a summary of various solar thermal energy storage materials and thermal energy storage systems that are currently in use is presented. The properties of solar thermal energy storage materials are discussed and analyzed. The dynamic performances of solar thermal energy storage systems in recent investigations are also presented and summarized.
Phase change material (PCM), stores and releases heat at a particular required temperature as it undergoes phase change at that temperature. Because of their large latent heat and constant ...temperature during the phase change process, the PCMs are extensively used in latent thermal energy storage system (LTES) and thermal management system (TM). Due to its virtue of competitive performance, PCM has been used extensively in heat recovery, solar energy, aerospace industry, buildings, textile industry etc. However, the PCM has limitations like low thermal conductivity and low heat transfer rate, which decrease the performance of LTES and TM systems. In this work, ways of improving thermal conductivity and heat transfer rate of the composite PCMs (CPCMs) are summarized from perspective of three kinds of CPCMs’ morphologies (fiber, porosity and sphere). This review paper presents morphological characterization of the CPCMs, and several fabrication methods of the CPCMs with enhanced thermal properties.
