Microencapsulated Phase Change Material Particles Research Articles

Heat-transfer characteristics of the two-phase-change process between a microencapsulated phase-change-material-slurry droplet and air: A numerical study

A concept is proposed for spray-heat exchangers with the aim of addressing the mismatch of heat supply-demand within and outside a water droplet. The concept involves using a microencapsulated phase-change-material (MPCM) slurry as the working medium, enabling two-phase-change heat transfer with air. The temperature of the water droplet rapidly decreases owing to continuous evaporation, leading to a rapid decrease in its heat-transfer capacity. The phase change of the MPCM particles not only provides sufficient heat for evaporation but also maintains a high surface temperature, thus sustaining effective evaporation. A numerical model was developed to theoretically investigate the effect of the MPCM phase change on external convection. The heat-mass transfer of a droplet with a radius of 0.25 mm composed of water and 20 vol% MPCM was simulated. The results indicate that the MPCM phase change effectively reduces the temperature decrease rate. However, when the overall surface temperature of the MPCM droplet exceeds that of water, the total heat-transfer coefficient is not necessarily higher. Further local analysis reveals that maintaining both an overall high surface temperature and a reasonable local temperature distribution is crucial to achieve a higher total heat-transfer coefficient. The MPCM phase change significantly influences the local surface-temperature distribution.

Performance analysis of groove-cut tube based double-pipe heat exchanger using microencapsulated phase change material slurry

It is commonly accepted practice to improve a thermal-fluid device by adopting surface enhancement techniques for better heat transfer. On the other hand, Microencapsulated phase change material (MPCM) slurry has been used as a heat transfer fluid (HTF) due to its improved heat capacity and heat transfer performance. The current paper attempted to study the combined effect of the two enhancement methods in a double-pipe heat exchanger. The results were first validated with experimental work taking into account the hydraulic and heat transfer characteristics of the MPCM slurry flow. Groove-cut geometrical models of MGGC, RGC, CGC, and TGC were proposed to modify heat transfer tube geometry. The effect of the number of helical turns (3, 4, 5, and 6-turns) and various Reynolds numbers were also considered for the investigation. The result showed that the proposed groove-cut models demonstrate enhanced flow mixing for MPCM slurry but with a higher friction factor, which increases with increasing helical turns. The impact was also manifested by improving the heat transfer by facilitating a uniform melting of MPCM particles, and the RGC model shows higher performance. The overall thermohydraulic performance assessment showed that the groove-cut tube models outperformed the corresponding plain tube. The highest performance factor value of 1.24 was observed for the RGC model, while the TGC model shows only 1.14 for the 6-turns. Similarly, the Q/Wp ratio indicates that the groove cut models transport more heat than the pumping power requirement for MPCM slurry.

Passive thermal regulation with 3D printed phase change material/cellulose nanofibrils composites

Application of phase change materials (PCM)-containing composite represents a promising passive thermal control strategy to provide indoor thermal comfort without using powered equipment. Yet, the trade-off between high thermal storage density and good thermal stability is the major obstacle that restricts the thermal regulation performance of the current PCM-containing products. This study proposed a facile approach to prepare PCM/cellulose nanofibrils (CNF) ink for 3D printed composite monolith, with very high PCM loading, good thermal stability, and robust mechanical properties. By incorporating a small amount of CNF gel as the stabilizer and viscosity modifier, the CNF-microencapsulated paraffin (model PCM) obtained essential printability to construct tailorable structure using Direct Ink Writing technology. After ice templating and freeze drying, the microencapsulated PCM particles were in-situ embedded in a CNF-assembled scaffold, forming a PCM/CNF composite. The lightweight nature of such scaffold (35.0 mg/cm3 for pure CNF matrix) endowed the composite monolith with high PCM loading (up to 82% with respect to the composite) and high thermal storage capacity (up to 153 J/g). Due to the 3D-interlinked CNF matrix and the coated CNF on PCM particles, the composite monolith not only exhibited high Young's modulus of up to 1.14 MPa but also showed exceptional thermal stability with negligible variation in microscopic morphology and thermal storage capacity, after high temperature conditioning at 80 °C for 30 h.

A study on energy transport performance of Microencapsulated Phase Change Materials (MPCM) slurry

Microencapsulated phase change materials (MPCM) slurry as a Heat Transfer Fluid (HTF) has been investigated due to the enhanced thermal energy storage capacity associated with the Phase Change Material (PCM). A numerical study employing the Eulerian-Eulerian flow model has been carried out to investigate the energy transport performance of MPCM slurry and water. Validation of the numerical model with the experimental work is first performed and further extended to identify the effects of temperature variations between the pipe inlet to outlet and volume concentrations of MPCM particles under a constant wall heat flux. Accordingly, it is found that water exhibits the highest pumping power demand over PCM-based HTFs to transport the corresponding thermal energy. The increase in temperature variation improves the energy capacity of both water and MPCM slurry; however, it affects the influence of the latent heat of the PCM. Pipe size and the latent heat of the phase change material significantly alter the ratio of energy transport by pumping power (Q/Wp) for MPCM slurry flow. Furthermore, it is observed that a flow of MPCM slurry with a low Reynolds number achieves a better value of Q/Wp under turbulent flow conditions.

Melting behavior and migration characteristics of microencapsulated phase change material particles in galinstan flow and application in minichannel heat sink

Mini/microchannel cooling performance can be improved with microencapsulated phase change material (MPCM) slurry as coolant by utilizing latent heat absorption capacity of MPCM. Compared with existing water, using liquid metal (LM) as basefluid can combine the advantages of high apparent heat capacity of MPCM particles and high thermal conductivity of LM. Thus, the prepared MPCM-LM slurry is proposed as coolant for minichannel heat sink. The melting of particles near the bottom enhances heat transfer performance of two-phase MPCM-LM slurry, but migration and aggregation cause the increase of pressure drop. Furthermore, the two-phase model is adopted to investigate the melting behavior and migration characteristics of MPCM particles in galinstan flow and the effect of particle volume percent on minichannel cooling performance. The results show that particles present boundary-to-center melting behavior in MPCM-galinstan slurry, resulting in the larger reduction degree in bulk mean temperature compared to MPCM-water slurry. Different from uniform distribution for small-diameter particles, the large-diameter particles migrate upward, leading to top-aggregation distribution and up-down asymmetry of velocity profile. Finally, the increase of particle volume percent significantly reduces heat capacity thermal resistance of heat sink, and maximum reduction is 59.02% for MPCM-galinstan slurry with φ s = 20% compared to pure galinstan.

Numerical investigation on heat transfer performance of a confined slot jet impingement with different MEPCM-water slurries using two-phase Eulerian–Eulerian model

An in-house Fortran-based two-phase Eulerian–Eulerian solver is used to study an impinging jet with microencapsulated phase change material (MEPCM)-water slurry in a minichannel. The heat transfer performance of water based n-eicosane and n-octadecane MEPCM slurries are reported here. The melting pattern of the particles within the minichannel are also reported at different parameters. Figure of merit (FOM) is used to quantify the heat transfer performance of the MEPCM-water slurry taking into account both the thermal and flow characteristics of the slurry. The heat transfer enhancement around 72% is found in the average Nusselt number for a 5%-n-eicosane water slurry, compared to that of pure water at a jet Reynolds number of 200. The melting region of MEPCM particles is found to come closer to the heated walls with an increase in Reynolds number. An increase in the Reynolds number from 100 to 300 has brought an increment of around 81% in the Nusselt number at the jet impinging region, while the increment in the average Nusselt number is around 145%. The figure of merit (FOM), increases from 1.34 to 1.64 with an increase in particle concentration from 1% to 10%, at Re = 300. By comparing the figure of merit values, the best heat transfer performance compared to pure water is obtained for water-based n-eicosane slurry at Re = 100, and εs=1%. A parametric study involving Reynolds number, particle concentration, and channel height to jet width ratio has been reported. Present results are validated with numerical and experimental results available in the literature.

Thermal performance of double-layer porous-microchannel with phase change slurry

Combining porous media paved on sidewall and microencapsulated phase change material (MPCM) suspension as coolant is presented in double-layered microchannel heat sink (MCHS), in which more surface area of porous copper matrix with high thermal conductivity as well as greater temperature difference between the heating wall and working fluid during phase transition of MPCM will enhance heat transfer. The Forchheimer-Brinkman-Darcy model together with energy equation in local thermal equilibrium are employed to deal with fluid flow and heat transfer in porous ribs, and the equivalent heat capacity method is used to describe the phase change process of microcapsules under laminar flow. The influences of coolants, heat sink designs (including the thickness, height, porosity and pore size of porous medium as well as channel number) and working conditions (involving inlet velocity, heat flux and suspension concentration) on thermal and hydraulic characteristics are numerically investigated. The performance evaluation factor (PEF) is introduced to evaluate the comprehensive thermal–hydraulic performance of double-layered MCHS, and the effective performance evaluation factor (PEFeff) is used to compare the overall performance of porous-wall MCHS between conventional arrangements, different channel aspect ratio and channel height ratio configurations. The thickness and height of porous media paved on sidewall should be adjusted reasonably, especially for the arrangement of non-equal porous media, to obtain better overall performance. Different from the variable porosity configuration, the double-layered MCHS cannot rely on losing part of flow performance to obtain better thermal performance in smaller pore size mode, so a larger pore size can be selected to obtain better thermal–hydraulic performance according to actual working conditions. The PEF at larger flow velocity decreases due to higher pressure drop exceeding the benefit of more convection on heat transfer enhancement, in which case the phase change effect in porous-wall mode with MPCM suspension as coolant is suppressed and higher viscosity amplifies the effect of frictional resistance, resulting in a greater drop ratio of PEF than water. The greater PEF in porous-wall MCHS with suspension concentration increasing from 3% to 20% arises than that in non-porous mode, due to the larger surface area of copper matrix with high thermal conductivity and more MPCM particles available for phase transition. The larger aspect ratio configuration can achieve greater PEFeff rise and better overall performance by optimizing fluid velocity in the current mode due to greater thermal resistance drop and less pressure drop rise at lower flow rates, but the variable height ratio structure makes the comprehensive performance of porous-wall MCHS inferior to conventional arrangement due to larger pressure drop rise outweighing the benefits of heat transfer enhancement at the same total volume flux.

The effect of micro-encapsulation on thermal characteristics of metallic phase change materials

Micro-encapsulated phase change material with metallic core can work under high temperature for energy storage purpose, making it an appealing candidate for renewable energy technologies, such as thermal energy storage for concentrating solar power plants. The objective of this work is to study the impact of encapsulation on the thermal performance of the micro-encapsulated phase change materials under various operating conditions. To evaluate the thermal performance of the micro-encapsulated phase change material particles, the governing equation of transient heat conduction with phase change in spheres with composite walls is solved numerically. The impact of encapsulation shell on the convective heat transfer rate, total energy absorption, latent heat ratio, energy density, and duration of phase change is presented and analyzed under different operating conditions. Three different core materials including tin, aluminum and copper with distinct Stephan numbers are investigated. In addition to the encapsulation shell effect, this model can be used to study micro-encapsulated phase change materials with different material compositions, core to shell ratios, and it can be used to analyze the impact of air gaps in between materials. Understanding the thermal performance at particle level is essential, and the results of this study could potentially be used as input for thermal energy storage system level analysis, such as the charging and discharging processes. • Encapsulation enhances convection due to the increased surface area • Thicker shell shortens core melting time • Heating time depends on the ratio of heat capacitance and convection rate • Optimal shell thickness exists for achieving high energy storage and fast absorption

Properties and performance of hybrid suspensions of MPCM/nanoparticles for LED thermal management

Mono suspensions of either nanoparticles or microencapsulated phase change material (MPCM) capsules (microcapsules) have shown superior thermophysical properties and heat transfer performance for thermal management. In this study, hybrid suspensions of nanoparticles and MPCM particles were prepared to integrate the advantages of both materials. The properties of hybrid MPCM/nanoparticle suspensions were investigated in terms of dispersion stability and thermal conductivity, and the cooling performance of the hybrid suspensions on thermal management of light-emitting diodes (LED) were studied. The experimental results show that the addition of graphene oxide, titanium dioxide (TiO2), and aluminum oxide (Al2O3) nanoparticles effectively improved the dispersion stability of the MPCM suspension (MPCMS). The thermal conductivity of the MPCMS was enhanced by the TiO2 and Al2O3 nanoparticles. The hybrid MPCM/Al2O3 suspensions, which exhibited remarkable improvement in both dispersion stability and thermal conductivity, were applied to an LED thermal management system. The results show that the cooling performance of the hybrid suspensions was enhanced compared with that of the mono suspensions of MPCM or nanoparticles. The optimal concentration of the hybrid suspension was obtained. The effects of inlet temperature of the hybrid suspensions on the cooling performance were studied.

Thermohydraulic performance analysis of graded porous media microchannel with microencapsulated phase change material suspension

The phase change process in microcapsules is described using equivalent heat capacity method, and the Forchheimer-Brinkman-Darcy equation based on the volume-averaging method is employed to account for the fluid flow in porous ribs. The feasibility in the novel design is validated by analyzing the effects of porosity (ε) arrangement, pore size (dp) arrangement as well as geometric parameters of porous-wall microchannel heat sink (MCHS) on the thermal and hydraulic characteristics under different inlet velocity, heating flux and mass fraction of microencapsulated phase change material (MPCM) suspension. The thermal resistance of the porosity gradient in porous media is less than that of linear variation, which is due to the more uneven velocity distribution in porous and fluid regions. The lower thermal resistance and more uniform temperature distribution occur in the graded porous media microchannel with higher height ratio, as well as the smaller and larger porosity in porous media respectively near the bottom and top walls. The size of microchannel limits the size of dp, making the thermo-hydraulic characteristics of gradient-dp mode worse than that of gradient-ε under the same conditions, and the thermal resistance decreases as well as pressure drop increases with the augment thickness of porous media, so it is necessary to select the appropriate structural parameters and coolants according to the specific working conditions to obtain better comprehensive performance. The performance evaluation factor (PEF) is introduced to evaluate the overall thermohydraulic performance of MCHS under different heat sink designs, and the stepwise increasing porosity mode with porosity varying along the y direction has larger velocity difference between the upper and lower layers and more reasonable MPCM particle distribution under the larger mass fraction of suspension, so as to obtain the larger rise ratio of PEF, up to 61%.

Investigation of Counterflow Microchannel Heat Exchanger with Hybrid Nanoparticles and PCM Suspension as a Coolant

The effect of the hybrid suspension on the intrinsic characteristics of microencapsulated phase change material (MEPCM) slurry used as a coolant in counterflow microchannel heat exchanger (CFMCHE) with different velocities is investigated numerically. The working fluid used in this paper is a hybrid suspension consisting of nanoparticles and MEPCM particles, in which the particles are suspended in pure water as a base fluid. Two types of hybrid suspension are used (Al2O3 + MEPCM and Cu + MEPCM), and the hydrodynamic and thermal characteristics of these suspensions flowing in a CFMCHE are numerically investigated. The results indicated that using hybrid suspension with high flow velocities improves the performance of the microchannel heat exchanger while resulting in a noticeable increase in pressure drop. Thereupon, it causes a decrease in the performance index. Moreover, it was found that the increment of the nanoparticles’ concentration can rise the low thermal conductivity of the MEPCM slurry, but it also leads to a noticeable increase in pressure drop. Furthermore, it was found that as the thermal conductivity of Cu is higher than that for Al2O3, the enhancement in heat transfer is higher in case of adding Cu particles compared with Al2O3 particles. Therefore, the effectiveness of these materials depends strongly on the application at which CFMCHE is employed.

Numerical flow characteristics of microencapsulated phase change slurry flowing in a helically coiled tube for thermal energy storage

As a new heat transfer medium, microencapsulated phase change slurry (MPCS) has better heat transfer performance than water. The MPCS consists of microencapsulated phase change materials (MPCM) particles and carrier fluid (water). Many experiments and numerical simulations have been carried out on its heat transfer characteristics, but few focused on its flow characteristics. In this work, a numerical model of the MPCS flowing in a two-turn helically coiled tube under constant wall heat flux was established. The numerical simulation results showed that inlet velocity was the most important factor affecting pressure drop and pumping power consumption of the MPCS. Except the inlet velocity, the mass fraction was the major factor affecting the flow characteristics of the MPCS. The latent heat and wall heat flux had little effects on the flow characteristics of the MPCS. In addition, the possible motion trajectories of MPCM particles were calculated by using Discrete Phase Model (DPM) model, and the motion state of MPCM particles was visually displayed.

Experimental study of jet impingement heat transfer with microencapsulated phase change material slurry

Based on the heat dissipation requirement of high heat flux for electronic chips, this paper establishes a closed-cycle experimental system and investigates jet impingement heat transfer characteristics with a microencapsulated phase change material (MEPCM) slurry as the working fluid. The phase change properties of the MEPCM particles and the thermal properties of the MEPCM slurry are analysed. The effects of several key parameters on the heat transfer and pressure drop of jet impingement are also discussed. Experimental results show that a 10% mass fraction of MEPCM slurry may enhance jet heat transfer by 32.8% compared with water because of the latent heat absorption of the PCM core. The heat transfer enhancement will be limited for higher mass fractions of slurry. The slurry exhibits a high heat transfer efficiency when the jet distance is ΔL/D = 4–9.3. The convective heat transfer coefficient of the slurry first increases and then decreases as the jet inlet temperature increases, and the optimum jet inlet temperature is approximately 12.1 °C lower than the melting peak temperature. Jet temperatures that are too high or too low will lead to deterioration of heat transfer for the slurry. These findings can provide guidance for the design of heat dissipation systems using MEPCM slurries.

Study the hydro and thermal performance of microchannel heat exchanger with hybrid suspension of nanoparticles and MEPCM particles

This paper aims to study the flow and heat transfer of hybrid suspension in counter flow micro channel heat exchanger (CFMCHE). In order to enhance the thermal properties of micro encapsulated phase change material (MEPCM) suspension. The hybrid suspension studied in this paper consists of nanoparticles and MEPCM particles, these particles are suspended in water as abase fluid, two types of hybrid suspension are used (Cu + MEPCM suspension) and (Al2O3+ MEPCM suspension) by n-octadecane as a PCM for both of them. The hydrodynamic and thermal characteristics of thes suspensions flow in micro channels of CFMCHE is numerically investigated. From obtained results, using of hybrid suspensions as a cooling fluid lead to modify thermal performance of a CFMCHE by increase its effectiveness but it also lead to high increasing in pressure drop. The results showed considerable enhancement in cooling effectiveness of (Al2O3 + MEPCM suspension) above of the pure PCM suspension, Nanofluids, and water. While (Cu + MEPCM suspension) showed enhancement above of the pure PCM suspension and water, but was not best effectiveness than Nanofluids. Extra increase in pressure drop in both types of hybrid suspension above are leads to reduce the overall performance compared with pure PCM suspension. Therefore its use depends on the application at which this heat exchanger is employed

Charging performance of latent thermal energy storage system with microencapsulated phase-change material for domestic hot water

A latent thermal energy storage system designed for domestic hot water is constructed using microencapsulated phase change material (MEPCM) as the energy storage medium. The temperature response and energy storage are experimentally investigated for different MEPCM particle fractions, PCM core fractions and carbon fiber fractions in the energy storage units (ESUs). A model for nominal energy storage is proposed. In the charging process, the ESU experiences three stages: the sensible heat stage before melting, the melting stage, and the sensible heat stage after melting. The ESU shows a high temperature uniformity in circumferential and axial directions. The temperature difference in the two directions is within 5 °C. A higher MEPCM particle fraction and higher PCM core fraction indicate a higher total PCM content, which results in higher energy storage capacity but slows down the energy storage rate. Carbon fiber fraction with a 10% variation shows a mild influence on temperature response and energy storage. In the consecutive alternate charging and discharging experiments, the latent heat of the PCM can be fully utilized and the system has a nearly constant energy storage/release rate. The stable operation and the high heat transfer rate of the system demonstrates that it is practical in the application for domestic hot water.

Paraffin core-polymer shell micro-encapsulated phase change materials and expanded graphite particles as an enhanced energy storage medium in heat exchangers

The Micro-Encapsulated Phase Change Materials (MEPCMs) with the melting point temperature of 28 °C was used as an energy storage medium to control the thermal behaviour of a heat exchanger. The NEPCM particles were mixed with Expanded Graphite (EG) microparticles to improve the heat transfer rate in the heat exchanger. The MEPCM particles are made of paraffin cores with a particle average size of 45 μm. The paraffin core of the particles can undergo a solid/liquid phase change and store a tremendous amount of thermal energy due to the latent heat of phase change. The heat exchanger is a copper pipe radiator with aluminium fins confined in either a bed of MEPCMs or a composite mixture of MEPCMs + Expanded Graphite (EG) microparticles. The thermophysical properties of MEPCMs and MEPCM + EG were measured. The outcomes show 132% enhancement in thermal conductivity and a 28% decrease in the sensible specific heat of the MEPCM + EG composite with the weight ratio of 70% MEPCM and 30% EG compared to pure MEPCM. The charging and discharging behaviour of heat exchanger for various flow rates and two cases of MEPCM and MEPCM + EG were studied experimentally. The results reveal that using EG notably enhances the heat transfer and capability of the heat exchanger during the charging and discharging process. Using MEPCM + EG reduces the temperature drop at the heat exchanger outlet about 15 °C compared to the case of pure MEPCM.

Investigation of thermal behavior of long-distance multilayer pipeline with MicroPCM particles

Long-distance transport of oil and gas in the harsh subsea environment needs reliable pipeline thermal management to prevent wax deposition and hydrate formation. Multilayer pipeline with the PCM-matrix composite material, made of microencapsulated phase change material particles (MicroPCM particles) and polymer matrix material, is proposed as an effective method to meet the flow assurance challenges owing to its thermal energy storage. Compared with the direct applications of PCM, the PCM-matrix composite material can avoid the possible leakage of its liquid phase and decrease the installation difficulty. Theoretical studies are carried out to investigate the proposed multilayer insulation system in a long horizontal pipeline, which embeds an intermediate insulation layer made of the PCM-matrix composite layer. Considering the storage or release of the latent heat and the changes of thermal properties during the phase change process, the global characteristics of the PCM-matrix composite layer is calculated based on the porous media theory. A pseudo-3D model coupled to the finite difference numerical method is developed to simulate the start-up and shut-in transient processes. To address the liquid and solid states of PCM and their interface, the model for the PCM-matrix composite layer is constituted in the form of enthalpy conservation. The influence of the thermal properties of different matrix and PCM materials on the cooldown time is examined. By analyzing the temperature distributions of produced fluid during start-up and shut-in, the current study shows that the use of MicroPCM particles in the composite layer can prolong the maximum cooldown time. In addition, the geometrical parameters, such as the volume fraction of MicroPCM particle and the thickness of the composite layer, are found to also play a role in affecting the extent of the prolongation of the cooldown time.

New hybrid suspension of MEPCM/GO particles with enhanced dispersion stability and thermo-physical properties

A new type of hybrid suspension comprising microencapsulated phase change material (MEPCM) spheres and graphene oxide (GO) nanoplatelets is proposed. The dispersion stability of the MEPCM/GO suspension was guaranteed by precisely adjusting the density of the MEPCM particles, which had n-heptadecane as the core and hexanediol diacrylate polymer as the shell. The MEPCM particles with excellent monodispersity were fabricated successfully using a co-flowing microfluidic method. The micromorphology of the hybrid MEPCM/GO suspension, measured using a transmission electron microscope, showed that the MEPCM particles and GO nanoplatelets were dispersed uniformly in the base fluid without aggregation. The high dispersity and stability of the hybrid MEPCM/GO suspension provide basic conditions for regulating its thermo-physical properties. Thermal conductivity, specific heat capacity, and viscosity were measured using a laser-flash analysis device, a differential scanning calorimeter, and a rotational rheometer, respectively. The effects of the concentrations of the MEPCM particles and GO nanoplates were investigated. Results showed that enhanced thermal conductivity and specific heat capacity were achieved for the hybrid MEPCM/GO suspension. This advantage of the hybrid suspension became increasingly obvious as the MEPCM and GO concentrations increased. The viscosity of the hybrid MEPCM/GO suspension changed only slightly with the GO concentration, whereas it increased with the MEPCM concentration, and thus showed a relatively good rheological property. The proposed hybrid MEPCM/GO suspension with enhanced thermo-physical properties possesses immense potential for use as a novel working fluid for high-efficiency heat transfer.

Microencapsulation of Paraffin with Poly (Urea Methacrylate) Shell for Solar Water Heater

Previous research has demonstred that microencapsulated phase change materials (MEPCMs) could significantly increase the energy storage density of solar thermal energy storage (TES) systems. Compared with traditional phase change materials (PCMs), MEPCMs have many advantages since they can limit their exposure to the surrounding environment, enlarge the heat transfer area, and maintain the volume as the phase change occurs. In this study, a new MEPCM for solar TES systems is developed by encapsulation of paraffin wax with poly (urea formaldehyde) (PUF). The experimental results revealed that agglomeration of MEPCM particles occurred during the encapsulation process which affected the uniformity of the particle size distribution profile when sodium dodecyl sulfate was used as an emulsifier. The differential scanning calorimetric (DSC) analysis results showed that the melting temperatures were slightly increased by 0.14–0.72 °C after encapsulation. A thermogravimetric (TG) test showed that the sample weight decreased while the weight loss starting temperature was slightly increased after encapsulation. Overall, the sample UF-2, fabricated with the binary emulsifiers of Brij 35 and Brij 30 and 5% nucleating agent, resulted in good particle dispersion and shell integrity, higher core material content and encapsulation efficiency, as well as improved thermal stability.

Modification of microencapsulated phase change materials(MPCMs) by synthesizing graphene quantum dots(GQDs) and nano-aluminum for energy storage and heat transfer applications

MPCMs and their suspensions, acting as the thermal storage, heat transfer or temperature constancy mediums, have gained concerns in various energy related sectors. However, problems involving high supercooling degree, low thermal conductivity and suspensions instability are barriers for their energy storage applications. The present study focuses on such properties by adding GQDs/nano-aluminum into MPCMs particles. Paraffin was selected as core material and urea–melamine–formaldehyde polymer as shell to prepare 10 MPCMs samples (no modifiers, GQDs, nano-aluminum, GQDs & nano-aluminum) via in situ polymerization. The morphology, thermal conductivity, thermal property and MPCM/suspensions stability were characterized. It was found, the selected modifiers didn't impact on the microcapsules morphology, but GQDs can make particle size smaller and distribution more uniform. Adversely, the mean particle size can be increased by nano-aluminum. GQDs are much more effective than nano-aluminum in improving thermal conductivity. GQDs can suppress supercooling effectively, however, nano-aluminum has no obvious effect on supercooling suppression. The MPCMs suspension modified by the selected amount of GQDs and nano-aluminum (Al-GQDs(4.5–2)) achieved a high physical stability. No structure instability of modified MPCM sample was observed. To sum up, the combined effort of GQDs and nano-aluminum enabled MPCMs to be more applicable in energy storage applications.