Phase change microcapsules can carry large amounts of heat and be dispersed into other mediums either as a solid composite or as slurry fluids without changes to their appearance or fluidity. These two standout features make phase change microcapsules ideal for use in thermal energy applications to enhance the efficiency of energy utilisation. This review paper includes methods used for the encapsulation of phase change materials, especially the method suitable for large scale productions, the trends of phase change microcapsule development and their use in thermal energy applications in static and dynamic conditions. The effect of phase change microcapsules on convective heat transfer through addition to thermal fluids as slurries is critically reviewed. The review highlighted that so far the phase change microcapsules used mainly have polymeric shells, which has very low thermal conductivities. Their enhancement in convective heat transfer was demonstrated in locations where the phase change material experiences phase change. The phase change results in the slurries having higher apparent local specific heat capacities and thus higher local heat transfer coefficients. Out of the phase change region, no enhancement is observed from the solid microcapsule particles due to the low specific heat capacity and thermal conductivity of the phase change microcapsules compared to that of water, which is normally used as slurry media in the test. To further the research in this area, phase change microcapsules with higher specific heat capacity, higher thermal conductivity and better shape stability need to be applied.   

Salt hydrates based phase change materials (PCMs) have a number of advantages such as high energy density, high thermal conductivity, moderate phase transition temperature and low price, making the materials promising candidates for numerous potential applications and hence a hot research topic over the past decade. This paper reviews and analyses the state-of-the-art development of the salt hydrate based PCMs and summarizes the progress in the thermophysical properties and applications of the materials. Main challenges in the industrial uptake of the salt hydrates based PCMs are supercooling, phase separation and corrosion. The progress and methods in resolving these challenges are discussed. Finally, recommendations are given for further research and applications of the salt hydrates based PCM.

 Nano-encapsulated phase change materials (NEPCM) have the potential to address the intermittent and less predictable nature of renewable energy resources. Other applications of the NEPCM include waste heat utilization, energy-saving buildings and thermal management of electronics. This paper introduces the latest research in NECPM preparation, characterization and heat transfer enhancement. The influence of preparation method on particle size and morphology of NEPCM are discussed. The characterization results show nanoencapsulation is able to effectively prevent the leakage of PCM and achieve an excellent thermal stability of PCM emulsions. In addition, the use of NEPCM could also effectively alleviate the rapid increase in the viscosity of PCM emulsions, and improve the thermal conductivity while reduce the subcooling degree of PCMs. Furthermore, both numerical and experimental studies have shown the benefit of NEPCM for heat transfer enhancement.

A erythritol-thiourea (TU) based binary phase change eutectic mixture was developed, which has a phase transition temperatures of 98.63 ℃. Using the phase change material (TU), PCM composite fibers were fabricated using electrospinning. In such composite fibers, erythritol (E), erythritol-thiourea (TU) and erythritol-xylitol (X) eutectic mixtures were the phase change materials, polyving akohol (PVA) acted as the supporting material and graphene was used to enhance the composite thermal conductivity. The effect of graphene on the morphology and thermal properties of composites was investigated. It was shown that there were no chemical reactions between the components and the composite were formed through physical interactions. The composites were cylindrical in shape with a smooth surface and had an average fiber diameters (AFDs) ranged from 0.95～1.12 μm. The AFDs of composites decreased with increasing graphene content as a result of the increase in the solution electric conductivity. The phase change enthalpies of the three composites were 241.6 J/g, 191.4 J/g and 192.7 J/g, respectively. The composites were shown to maintain the shape after 10 thermal cyclesand the changes to the latent heat were respectively 1.2%、0.5% and 0.5% after the thermal cycle. The addition of graphene (10%, weight ratio) markedly improved the thermal conductivity of composites, achieving 258%, 312% and 260%, respectively. These results demonstrated that the composites not only had a high stability both structurally and chemically, but also possessed excellent thermal properties.

An experimental study has been performed on the charging and discharging behaviour of composite phase change material (CPCM) modules. The CPCM modules were made of a eutectic carbonate salt based phase change material (PCM, NaLiCO3), a thermal conductivity enhancement material (TCEM, graphite flake) and a ceramic skeleton material (CSM, MgO). The CPCM modules were electrically heated from the lower surface with a constant heat flux. Analyses of the temperature differences between the heated surface and CPCM modules suggested that, compared to the use of pure PCM, heat transfer in the CPCM modules be significantly enhanced due to the addition of graphite. Further experiments were done to investigate the discharging behaviour of the material modules under both natural convection and forced convection conditions. The results showed that the solidification process of CPCM modules were faster than that of pure PCM samples, evidenced by the fact that the solidification time of the CPCM modules was one third shorter than that of pure PCM modules.

The influence of the addition of In on the microstructure and thermal properties of Sn-Bi-Zn eutectic alloy was investigated. The microstructure and phase compositions were investigated by electron probe micro-analysis (EPMA), X-ray diffusion (XRD) and X-ray fluorescence spectroscopy (XRF), and the thermal properties were measured with differential scanning calorimeter (DSC), thermogravimetry (TG/DTA), a pushrod type expansion meter (DIL 402C) and a laser flash analyzer (LFA 457). The results indicated that the microstructure of Sn-Bi-Zn eutectic alloy was mainly composed of Sn-rich phase, Bi-rich phase and Zn-rich phase, but (Sn48Bi50Zn2)100−xInx alloys showed InSn4 and BiIn intermetallic compounds and In-rich phases with increasing In content. The melting enthalpies increased with increasing In content, and the phase change temperature decreased with increasing In content in the (Sn48Bi50Zn2) 100−xInx alloys. The thermal expansion of (Sn48Bi50Zn2) 100−xInx alloys increased with increasing temperature and was tunable between 13×10−6 and 15×10−6/℃ by the In addition. The densities and thermal diffusion coefficient of all the alloys decreased with the addition of In, whereas the thermal conductivity increased with increasing In content.

Phase-change salt is an important heat transfer and storage material for concentrated solar power plants. This calls for a quantitative understanding of the relationship between the structure and properties of molten salts. We performed molecular dynamics simulations on NaNO3-KNO3-NaNO2 ternary molten salts using Buckingham potential with an aim to understand such relationship. The simulations gave structural information such as radical distribution function, coordination number and angular distribution function, and physical properties including density, shear viscosity, thermal conductivity and heat capacity. The results showed that all the properties calculated agree well with the literature data, suggesting the reliability of pair potential and simulation method used in the work.

Hybrid composites made of three-dimensional (3D) networked graphene and a phase change material (PCM) were synthesized by hydrothermal processing of graphene nanoplates (GNPs) and graphene oxide. Palmitic acid (PA) was used as the PCM. The composite PCM were prepared by vacuum impregnation. The effects of the type and content of GNPs on the thermal properties of the composite PCMs were investigated. The results showed that an increase in the content of GNPs increased the thermal conductivity of the composites PCMs. The type of GNPs had an influence on the thermal properties of the PCM composites The M-type of GNPs had the most significant effect on the thermal conductivity of the composite PCMs. No obvious changes to the latent heat and the phase change temperature were observed. When the content of GNPs was 8%, the thermal conductivity was 0.634 W/(m•K), which was almost a 3-fold increase compared with the PA. The heat transfer enhancement mechanisms were also discussed.

A series of glyceryltristearate (GT)-1,3:2,4-di-(3,4-dimethyl) benzylidene sorbitol (DMDBS) composite phase change materials (PCMs) were prepared with the DMDBS as a gelator and GT as the PCM. Through examining the gel-sol temperature and the leakage of the PCM composites, we found that the composite PCM can be form-stable. A 3%(weight ratio) of DMDBS addition to the formulation gave a gel-sol transformation temperature of 159 ℃ and the resulted leakage was only 6%. Scanning electron microscopy (SEM) analyses were performed to observe the surface microstructure of the composites and the xerogel. The thermal properties were studied by Differential scanning calorimetry (DSC). The results indicated that the melting and freezing enthalpy of GT-3%DMDBS composite were 126.4 J/g and 105 J/g, respectively. A laser flash diffusivity apparatus was used to measure the thermal conductivity of samples. The driving force of the formation of the three-dimensional network structure due to DMDBS was studied by UV absorption spectrum and IR absorption spectrum. The effect of DMDBS addition on the molecular structure of GT was characterized by X-ray diffraction (XRD).

This paper reports a study on ceramic based phase change materials (PCM). Sodium nitrate (NaNO3) was used as the PCM, magnesium oxide (MgO) as the ceramic matrix, and different types of carbon materials as the thermal conductivity enhancers (TCE). The composites were fabricated by mixing, compaction, and sintering. The effect of carbon material addition was studied and the morphological and thermophysical properties of the composites were characterized. The results showed that the addition of the TCE could either hinder or enhance the thermal diffusivity of the composites, depending on the type of TCE used. It was found that the thermal energy storage density increased with increasing PCM concentration in the composite. All the composites manufactured were found to be thermally stable up to 500 ℃.

Cold storage using phase change materials (PCMs) has wide applications on the transportation of food and medical products. This article presents a study on the performance of a cold storage device with two PCMs in a layered structure. Tridecane and dodecane were used as the PCMs, which have phase change temperatures and phase change enthalpies of −5.4 ℃ and 132 kJ/kg, and −9.6 ℃ and 173 kJ/kg, respectively. The main objective of the study was to find the best combination of the two PCMs giving the longest storage period. The results showed that the combination with tridecane placed outside and dodecane inside gave the longest time for the inner wall temperature to change from −15 ℃ to 0 ℃ although the combination only had the 3rd largest total latent heat. Same conclusions were obtained with pure dodecane, which was longer than the unit with pure tridecane and tridecane located outside layer and dodecane inside. Placing the PCM with a higher phase change temperature in the outside layer reduced the heat transfer rate from the ambient to the internal core of the cold storage unit, leading to an increase in the cold storage period and improvement in the cold storage performance.

Ice slurries have good thermal conductivity and fluidity, and hence are widely used in ice storage. In this work, a gas direct contact device was designed for rapid ice slurry preparation by using pre-cooling jacket, external vacuum insulation layer and multi-nozzle filling gas, and studied for the effects of these devices on the ice slurry production rate. The effects of the addition of molecular sieves, steel beads and glass beads as perturbation particles were then studied on the ice blockage. Finally, the effects of types and concentrations of additives on the quality of the ice slurry were discussed. By comparing the effect of each of the solutions, a method for improving the production efficiency of ice was put forward. It was showed that the use of alcohols as the additive was better than the use of salts, and, of all the alcohols studied, the use of 15% of glycerol gave the best performance.

We studied numerically a packed bed based cold energy storage device—A key component of liquid air energy storage (LAES) system and could play an important role in the system efficiency enhancement of LAES. Both the charging and discharging performance and associated efficiency were studied. Low cycle efficiency is found when the packed bed was fully charged/discharged in the first few cycles; suggesting that the outlet air temperature should be controlled with a cut-off point for each cycle. The packed bed based cold storage device approaches steady state after a number of cycles, with the charge/discharge processes repeatable, indicating the cold utilization efficiency of ~100% could be achieved. We also examined the effects of bed height, packed particle size and the cut-off point temperature on the performance. The results showed very low effective storage volume ratio. This should be considered when the storage device is scaled up.

Thermal energy storage (TES) is a key technology to resolve issues of dispatchability and load shift of renewable energy utilization. In this paper, we present an experimental study on a high-temperature combined sensible-latent TES, which uses air as heat transfer fluid, alumina rich ceramic balls as sensible storage material and salt as latent heat storage material. The results showed that the thermal stability of packed-bed based TES system was improved during both charging and discharging processes when PCM was used because the PCM released heat at an almost constant temperate at the phase point. The cyclic efficiency of the combined sensible-latent TES system was about 75%, which was higher than the sensible TES system.

A combined heating and refrigeration system (HRS) with off-peak thermal energy storage is proposed for load shift of electrical grids and efficiency enhancement of power generation. We show that the average electricity load rate (AELR) of power grid in Beijing area could be increased from 80.56% to 100% in summer, and from 83.10% up to 90% in winter, if 10% the residential central heating / cooling systems uses the HRS technology.

This paper reports a newly proposed phase change material based radiant floor system. The system consists of two phase change material layers with one for cold storage for space cooling and the other for heat storage for space heating. The phase change temperatures of the two materials are respectively 18 ℃ and 34 ℃. Two designs of the floor systems, A and B, were studied. The Design A had the cold storage layer placed on top of the heat storage layer; whereas the Design B placed the heat storage material on top of the cold. An experimental system was used to compare the two designs, which used a flow of water (120 L/h flowrate) with inlet temperatures of 45 ℃ and 15 ℃ for heating and cooling respectively. The results showed that the designs had a great influence on the indoor air temperature under the heating operations but exerted very little influence on the indoor air temperature under the cooling operations. This implied that the Design B had a better heat transfer performance than the Design A.

The work reported in this paper concerns the charging behaviour of an electrical storage heater using a high temperature composite phase change material (CPCM). A mathematical model was developed to study the transient heat transfer behavior of the composite PCM bricks. The model was validated experimentally. The results showed that the CPCM based electrical storage heater offered a better performance that was superior to MgO-based electrical storage heater. For a given volume, the same power rating, and the same amount of stored heat, the mass of the MgO based electrical storage heater exceeded 1.6 times that of the CPCM based storage heaters. For the same mass and the same power rating, the heat storage capacity of the CPCM based electrical storage heater was 68% higher than that of MgO-based unit. The results also indicated the importance of temperature control strategy. It was found that the average temperature and the maximum temperature inside the electrical storage heater could meet the requirements if the control temperature measurement point was selected to be 10 mm away from the heating elements. In such a case, heating elements only had two start-stops over the 8-hour charging period.

Thermal energy storage (TES) has been regarded as a key to the effective and efficient use of renewable energy and the recovery of waste heat, and hence attracted significant attention in the past few decades. Published research on TES includes materials, devices and systems. This paper presents a study for the optimization of thermal storage devices employing either sensible heat storage or latent heat storage. The process of thermal storage was modeled by the lumped parameter method and the design of the device was investigated by the effectiveness-NTU analysis. An exergy recovery ratio was defined for the device and the optimisation of the device design was achieved by maximizing the ratio by a search algorithm. A case study was carried out, which confirmed the feasibility and robustness of the optimization method. The results showed that there was an optimal combination of the material load and operation temperature range in sensible heat storage devices, while an optimal melting point is more important in latent heat storage devices. The thermal storage material amount required in a single-stage latent heat storage device could be reduced significantly compared with that for a sensible heat storage device. We also found that an optimized multi-stage latent heat storage device was shown to have a higher exergy recovery.

This paper concerns with liquid air energy storage particularly the use of different cycles and optimization. A large scale standalone liquid air energy storage system could achieve a round trip efficiency of ~60%. By using a combined Heylandt cycle and Rankine cycle, we show, through simulation with Aspen Plus software, a cycle efficiency of 64.5%. For further improvement of the performance of the combined cycle, thermal efficiency and exergy efficiency of the Rankine cycle and the Heylandt cycle were analyzed separately. We foundthat the maximum exergy loss was associated with the heat exchangers, and the exergy losses of the Rankine cycle and the Heylandt cycle accounted for 13.1% and 61.8%, respectively. As a consequence, the enhancement of the performance of heat exchangers holds the key to improving the whole combined cycle efficiency.

This bimonthly review paper highlights 100 recent published papers on lithium batteries. We searched the Web of Science and found 2564 papers online from Apr.1，2017 to May 31，2017. 100 of them were selected to be highlighted. Layered oxide and high voltage spinel cathode materials are still under extensive investigations for the structure evolution and modifications. Large efforts were devoted to Si and Si based anode material for the effects of size and composition and electrochemical mechanism. There are a few papers related to electrolyte additives, solid state electrolyte, Li/S battery, Li-air battery. Theoretical works are related to the structure of bulk and interface of materials and the tranportation properties. And more papers related to cell analyses, theoretical simulations, and modeling.

Driven by increasing demand for long range of electric vehicles, Ni-rich cathode materials have attracted lots of attention for the development of high energy density EV batteries. As the life span of EV batteries needs to be more than 10 years and the product development time is limited, an accelerated life span testing is often used to assess the long-term performance of the batteries. In this work, we prepared NCM811 cathode material through co-precipitation and high-temperature calcination and stored NCM811/Graphite pouch-type full cells at 60 ℃ in a fully charged state for the investigation of their storage performance. It was found that the storage capacity of the cells decreased to 80% after 180-day storage. XRD, SEM, ICP-AES, XPS and HRTEM techniques were used to investigate the differences between freshly made and the degraded electrodes. The results demonstrated the formation of by-products on the surface of cathode, and cathode materials exhibited layered-spinel-rock salt phase transformation after storage, both greatly increased the cell impedance. In addition, transition metal ions dissolved from the cathode were found to accumulate on the anode, which may have destroyed the SEI, leading to the consumption of the active lithium. Surface coating and bulk doping could resolve the problem through stabilizing the surface and bulk structure of the cathode materials.

Electrolytes play a crucial role in the storage and conversion of energy in a vanadium redox flow battery (VRFB). As an important transport property, the viscosity of electrolyte provides information of inter-particle interactions and solution microstructure. This is of significance to the optimization of the overall performance of the VRFB. We measured the viscosity and density of aqueous based VOSO4 solutions in a concentration range of 0.5～3.0 mol/kg at 283.15～323.15 K, calculated the activation energy associated with the viscous flow. Based on Eyring’s theory, a semi-empirical equation was proposed for the viscosity prediction. Such a relationship agreed well with the experimental data within 0.3%.

In this work, a computational fluid dynamics model is developed to analyze the cooling process of a molten salt tank during a standby period. The heat losses and temperature distribution under different conditions are obtained through the calculations. It is shown that the temperature distribution in the tank is mostly homogeneous, except for a temperature stratification closed to the bottom of the tank. Under the same conditions, the higher the level of molten salt, the longer time needed for the solidification of molten salt. For the cold tank at low molten salt level (290 ℃, 0.9 m), the estimated onset of solidification occurs in 3.8 days, and all the molten salt is solidified in 19.3 days.

This paper concerns the fundamental properties of quartz sand and brown alumina particles as natural gas hydrate media by using a laboratory gas hydrate and sediment instrument, SHW-III. We measured the electrical resistance, sound propagation speed, compressibility under two pore saturation conditions of 60% and 80%. The results showed little effects of the types of particles on the natural gas hydrate sediment resistance. Given particle size, the higher the porosity of the sediment, the faster the velocity of sound propagation; the velocity of sound propagation, however, increased with increasing particle size for the same type of sediment. In addition, cementation resistance of the gas hydrate deposits increased gradually with decreasing size of the sediment particles or increasing pore saturation. An empirical relationship was obtained through data fitting for the sound propagation of gas hydrate sediments as a function of particle size over a range of 0.42～1.8 mm for 60% and 80% pore saturations.

This paper concerns the internal resistance as a function of discharge rate of Lithium-ion batteries. The aim of the work is to improve the accuracy and adaptiveness of internal resistance model for battery management system (BMS), which is of significance to the accurate prediction of the status of batteries such as the state of charge (SOC). We used a second order RC equivalent circuit model to analyze the direct current internal resistance (DCIR) and pulse discharge internal resistance (PDIR) with 25 A•h LiFePO4 batteries under constant discharge and pulse discharge at various discharge rates. Data fittings were done on the DCIR, PDIR1, PDIR2 and PDIRtot at various states of charge as a function of discharge rate and a good agreement was obtained with a double-exponential relationship, whereas a linear relationship held for the PDIR0. The analyses also suggested independence of the internal resistance change to the temperature effect. Based on the chemical equilibrium between the formation and decomposition of the solid electrolyte interface (SEI), we concluded that large DCIR, PDIR1, PDIR2 and PDIRtot observed at low discharge rates was likely to be due to low decomposition rate than the formation rate of the SEI with a high resistance, similar to the standby situation. On the other hand, a lower resistance at a higher discharge rate could be attributed to higher decomposition rates than the formation rates of the SEI, leading to a thinner SEI until a new equilibrium status was reached, and hence a reduced internal resistance.

The influence of water-soluble binder, sodium carboxymethyl cellulose (CMC) with two different structures, was investigated on the stability of graphite anode slurry for lithium-ion batteries and the associated electrochemical performance. CMC with a higher degree of substitution (DS) of 0.85 and a higher molecular mass of 650000 was found to be adsorbed more on the graphite particle surface compared with a lower DS of 0.65 and a lower molecular mass of 250000. This was attributed to higher viscosity and better solubility of the higher DS and molecular mass. Based on repulsive interactions between the adsorbed CMC, the graphite particles were well dispersed in the aqueous medium. It was concluded that a high DS with a high molecular mass would improve the stability of graphite anode slurries and hence enhance the electrochemical performance of Lithium-ion batteries.

This article reviews different energy storage technologies. A statistical analysis is performed, on the basis of the information in the web of science, on the research papers and patents including publication year, originating countries/areas and institutions of the authors. The statistical analysis provides an indication of the development trend of the energy storage technologies. It is shown that, among all the energy storage technologies considered, fundamental studies on lithium ion batteries, supercapacitor, sodium ion batteries, flywheel, superconducting magnetic energy storage and solid state batteries are most actively studied, whereas the technology development of lithium ion batteries, flywheel, supercapacitors, compressed air energy storage, and lead-acid batteries are also very active.