Processes Of Phase Change Material Research Articles

Thermal Energy Storage by the Encapsulation of Phase Change Materials in Building Elements-A Review.

The energy sector is one of the fields of interest for different nations around the world. Due to the current fossil fuel crisis, the scientific community develops new energy-saving experiences to address this concern. Buildings are one of the elements of higher energy consumption, so the generation of knowledge and technological development may offer solutions to this energy demand, which are more than welcome. Phase change materials (PCMs) included in building elements such as wall panels, blocks, panels or coatings, for heating and cooling applications have been shown, when heating, to increase the heat storage capacity by absorbing heat as latent heat. Therefore, the use of latent heat storage systems using phase change materials (PCMs) has been investigated within the last two decades. In the present review, the macro and micro encapsulation methods for construction materials are reviewed, the former being the most viable method of inclusion of PCMs in construction elements. In addition, based on the analysis of the existing papers on the encapsulation process of PCMs, the importance to pay more attention to the bio-based PCMs is shown, since more research is needed to process such PCMs. To determine its thermophysical and mechanical behavior at the micro and macro levels, in order to see the feasibility of substituting petroleum-based PCMs with a more environmentally friendly bio-based one, a section devoted to the excellent PCM with lightweight aggregate (PCM-LWA concrete) is presented due to the lack of description given in other reviews.

Acceleration of Heat Transfer and Melting Rate of a Phase Change Material by Nanoparticles Addition at Low Concentrations

The thermal analysis of melting process of phase change material in rectangular latent thermal energy storage unit is presented herein. To resolve the low heat transfer rate due to the low thermal conductivity of pure paraffin (the phase change material of this study), the nano-paraffin approach has been numerically investigated. Four nano-metal oxides (Al2O3, MgO, SiO2 and SnO2) with high thermal conductivity are dispersed into the pure paraffin at different concentrations (1 %, 3 % and 5 %, v/v). The adopted mathematical model that has been used to study the mass, momentum, and energy transport processes inside the nano-paraffin systems has been firstly validated by available literature data. The obtained results showed that both heat transfer and solid melting rate are significantly improved in the presence of nanomaterials, with an independent relation to the particles concentration for a long melting time, i.e. up to 10 000 s. However, above 10 000 s, high particles concentration slightly enhanced the liquid fraction. Besides, it is found that the type of particles did not affect appreciably the heat transfer and the solid melting rate. Nevertheless, the enhancing effect of the nano-paraffin system is strongly melting time dependence. For the four investigated metal oxides, the temperature system attained 351 K at 1000 s of operation, whereas that recorded for the pure paraffin at the same time is 345 K, which yields an improvement of 1.7 %. For the same conditions, the liquid fraction passes form 0.04 for pure paraffin to 0.1 for nano-paraffin system, showing 150 % of enhancement in the melting yield.

Characterization of size effect of natural convection in melting process of phase change material in square cavity* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51908197 and 12072107), the Tackle Key Problems in Science and Technology Project of Henan Province, China (Grant No. 202102310262), the Program for Innovative Research Team of Science & Technology of Henan

The accelerating effect of natural convection on the melting of phase change material (PCM) has been extensively demonstrated. However, such an influence is directly dependent on the size and shape of domain in which phase change happens, and how to quantitatively describe such an influence is still challenging. On the other hand, the simulation of natural convection process is considerably difficult, involving complex fluid flow in a region changing with time, and is typically not operable in practice. To overcome these obstacles, the present study aims to quantitatively investigate the size effect of natural convection in the melting process of PCM paraffin filled in a square latent heat storage system through experiment and simulation, and ultimately a correlation equation to represent its contribution is proposed. Firstly, the paraffin melting experiment is conducted to validate the two-dimensional finite element model based on the enthalpy method. Subsequently, a comprehensive investigation is performed numerically for various domain sizes. The results show that the melting behavior of paraffin is dominated by the thermal convection. When the melting time exceeds 50 s, a whirlpoor flow caused by natural convection appears in the upper liquid phase region close to the heating wall, and then its influencing range gradually increases to accelerate the melting of paraffin. However, its intensity gradually decreases as the distance between the melting front and the heating wall increases. Besides, it is found that the correlation between the total melting time and the domain size approximately exhibits a power law. When the domain size is less than 2 mm, the accelerating effect of natural convection becomes very weak and can be ignored in practice. Moreover, in order to simplify the complex calculation of natural convection, the equivalent thermal conductivity concept is proposed to include the contribution of natural convection to the total melting time, and an empirical correlation is given for engineering applications.

Simulation of solidification process of phase change materials in a heat exchanger using branch-shaped fins

Nowadays, due to the global need for energy and the limited amount of fossil fuels, it is required to utilize alternative forms of energy. Thermal energy storage systems together with phase change materials are considered potential options to help achieve renewable energy development goals. Special groups of materials are used for thermal management in thermal processes due to their specific properties such as high latent heat, appropriate phase change temperature, and slight volume change during phase change. In this study, the effects of branch-shaped fins with different thicknesses and lengths on the thermal conductivity of phase change materials are investigated. Furthermore, the different volume ratios is studied. The results show that using thinner branch-shaped fins and hybrid Al2O3 – Cu with volume ratio of 0.04 has greater influence on the speed of the solidification process. The results also illustrate that the use of fins significantly speeds up the solidification process. The comparison between thin and thick fins revealed that the thin fin is a more viable option in increasing heat transfer rate. In this simulation, the finite element method has been employed. Generally, simultaneous use of thin branch-shaped fins and nanoparticles with volume fraction 0.04 leads to maximum efficiency in the simulation.

Thermal Performance Evaluation of Circular-Stadium Double Pipe Thermal Energy Storage Systems

Thermal Energy Storage (TES) systems are employed in numerous practical applications, owing to their capability to solve the problems related to the reliability of thermal energy systems by using phase change materials (PCM). In this study, the charging and discharging processes of N-eicosane in a circular-stadium double pipe TES system for various aspect ratios (1, 0.35, 0.25, and 0.15) and angular orientations (0°, 30°, 60°, and 90°) of the inner stadium tube are numerically investigated using enthalpy-porosity formulation. The zero-heat flux is applied to the outer tube of TES systems while the inner tube wall is retained at constant temperatures. The numerical results are compared with the experimental results of the concentric double-pipe TES system which shows an accuracy with 2.34% error. The findings revealed that the best thermal performance is provided by the X/Y=0.15 case with 90° angular orientation which contributes the highest increase in the charging rate while the discharging rate is not substantially affected by aspect ratio and angular orientation. The melting time decreases by 59% and solidification time raises by 27%. The overall efficiency of the TES system is 75% which is 26% higher than a base case (X/Y=1). The results further conceived that gravitational acceleration plays an important role in the natural convection heat transfer during the charging process of PCM.

Effect of inclination angle on melting process of phase change materials in a square cavity under mechanical vibration

The effect of inclination angle on melting processes of solid–liquid phase change materials has been investigated numerically and experimentally. However, the available studies did not consider mechanical vibration which exists in most thermal systems. To bridge the knowledge gap on this important topic, in this work the effect of inclination angle on paraffin melting process under mechanical vibration condition is investigated for the first time. The results show that, under low vibration frequency condition, the inclination angle has a significant influence on the melting rate, melting heat transfer and melting interface. However, the effect of inclination angle on melting characteristics will be weakened by moderate/high frequency mechanical vibration. Mechanical vibration can accelerate the melting processes for all inclination angles, but the enhancement effect relies closely on the inclination angle. The present numerical results for the stationary condition are consistent to the published experimental exploration that did not consider mechanical vibration.

Numerical study of PCM melting processes in trapezoidal channel

In this paper, a numerical investigation had been performed to improve the melting process of phase change materials (PCM) by utilizing a trapezoidal channel. Different geometrical parameters of the trapezoidal channel were investigated. These parameters included: the height and pitch of the trapezoidal channel. Two-dimensional numerical models were developed with ANSYS (FLUENT) software. The unit is a horizontal channel that the heat transfer fluid (HTF) (hot water) passes through the channel to melt the PCM which contained at the top and bottom sides of the channel. The results showed that the trapezoidal channel with a longitudinal pitch of 6 mm and a height of 4 mm is the most suitable parameters for increasing the PCM melting process compared to other geometrical parameters. It was also concluded that the melting time is decreased by 27% at a height of 4 mm, and by 8% at a pitch of 2 mm.

Numerical investigation on latent heat thermal energy storage in a phase change material using a heat exchanger

AbstractThe use of a heat exchanger using phase change material (PCM) is an example of latent heat thermal energy storage (LHTES). In this study, the charging of PCM (RT50) is studied in a double pipe heat exchanger. The designing of the heat exchanger needs to be optimized for operating and boundary conditions to store latent heat efficiently. The size of the equipment and the amount of PCM are also important to calculate the latent heat storage capacity of the LHTES device. In this study, the amount of PCM taken is quite high to avoid sensible heat transfer and to maximize the heat content of PCM. The charging process of PCM is numerically simulated using an enthalpy‐porosity model. The study includes the effect of inlet temperature and flow rate of high‐temperature‐fluid (HTF) and concludes that both play an important role in determining the charging time. The continuous increase in inlet temperature of HTF can decrease the charging time of PCM in the heat exchanger. However, the continuous increase in the HTF flow rate cannot show the same effect. The charging time can only be minimized with a specified flow rate regime for a specific inlet temperature of HTF. These factors consequently affect the efficiency of the heat exchanger.

Numerical investigation of natural convection enhancement in latent heat energy storage units with punched-fin and slit-fin

We study the melting process of phase change materials (PCM) inside a shell-and-tube heat exchanger with punched-fin and slit-fin numerically based on the enthalpy-porosity model in the present paper. The phase change material used is paraffin wax. A comparison of the liquid fraction, the total melting time, and the average temperature was performed to assess the improvement of the melting processes for six fin designs. The numerical results reveal that the usage of punched-fins and slit-fins can improve the melting rate because of the decrease of the blockage effect. For slit-fin cases, the reduction percentage in the total melting time has nearly the same value at different Stephan numbers, and an almost 14.0% improvement is observed compared to the straight fin case, while for punched-fin cases, the reduction percentage varies for different Stephan numbers, and the maximum value of the reduction percentage is 10.1%.

The effect of inclination angle on the performance characteristic of a double-pipe latent heat storage unit

This paper presents a numerical investigation and experimental validation of the melting process of phase change material (PCM) in a double-pipe latent heat storage unit (LHSU) at different inclinations. The aim of this paper is determining the optimal inclination angle at different PCM thicknesses for the melting process in the double-pipe LHSU. The PCM is placed in the annulus while hot water flows through the inner tube. ANSYS-FLUENT 17 is used to create a transient multi-phase numerical model to simulate the thermal characteristic of the storage unit. Three masses of PCM have been numerically studied; namely 0.154, 0.308, and 0.46 kg which presents three PCM annulus thicknesses of 7, 11.28, and 14.5 mm. The length of the double-pipe LHSU is 500mm and the inclination angles are 0 ̊, 15 ̊, 30 ̊, 45 ̊, 60 ̊, 75 ̊, and 90 ̊. The numerical model has been verified through the comparison between its predicted results and those obtained from previous experimental published work. In addition, the comparison between the present experimental and numerical results shows good agreement. It has been found that the inclination of the storage unit has a significant effect on the melting behavior when the thickness of PCM is enough such that natural convection contributes to the melting process. For this condition, 45 degrees was the optimum inclination angle. Conversely, for thin PCM thickness, the horizontal orientation produced the least total melting time.

Numerical Study on Resistance Change Characteristics of Phase Change Materials

Phase change materials are a type of emerging materials whose states will change under certain conditions, which then lead to changes in resistance. To study the characteristics of the phase change materials, a numerical simulation model of the resistive change unit based on the finite element method and the classic nucleation/growth theory is established, while the partial differential equations of electricity and heat conduction and the discrete formula of the finite element are also derived. According to the phase transition process of phase change materials, a crystalline‐amorphous simulation model is also proposed in this paper to simulate the electrical and thermal properties and phase transition process of the resistive change unit. Simulations of the resistance change unit under single pulses with different amplitudes and widths as well as the simulations under continuous pulses are conducted in this paper. These results verify the characteristics of resistance change and can provide references for selecting the parameters of the resistance change units.

Heat transfer study of poly-c PV system integrated with phase change material under semi-arid area (Errachidia-Drâa Tafilalet)

The high operating temperature of the photovoltaic (PV) modules decreases significantly its efficiency. The integration of phase change material (PCM) is one of the feasible techniques for reducing the operating temperature of the PV module. A numerical simulation of the PV module with PCM and without PCM has been realized. The thermal behavior of the PV module was evaluated at the melting and solidification processes of PCM. The results show that the integration of RT35HC PCM with a thickness of 4 cm reduces the temperature of the PV module by 8 °C compared to the reference module. Compared the RT35 and RT35HC, we found that the latent heat has a significant effect on the PCM thermal comportment. Furthermore, it has been found that the thermal resistance of the layers plays an important role to dissipate the heat from the PV cells to the PCM layer, consequently improving the heat transfer inside the PV/PCM system.

CFD simulation for heat transfer enhancement in phase change materials

In this research work, a two dimensional CFD approach is presented to analyze the heat transfer phenomenon in Phase Change Materials (PCM). In this research study a rectangular shell type heat exchanger filled with PCM is selected and rectangular tubes are inserted to flow a heat transfer fluid. PCM is used as thermal energy storage material to store heat energy and a CFD simulation has been done to analyze the melting process of PCM and optimized the geometry of rectangular tubes inside the shell. Two geometries of rectangular tube, tubes without fins and tubes with four fins, are used to investigate the effect of geometry on heat transfer and melting of PCM. In this research work, CFD approach based on enthalpy porosity model is used for the enhancement of heat transfer and to reduce PCM’s melting time by inserting fins on circumference of the rectangular tubes in which a hot temperature fluid is flowing. The aim of this research is to optimize the geometry for increasing heat transfer in PCM and analyze the results in terms of liquid fraction at an applied temperature of 80 °C on the rectangular tubes. It can be seen from the analysis that fins incorporated to the tubes increases the heat transfer to the PCM and reduce the PCM’s melting time.

Performance evaluation of cascaded storage system with multiple phase change materials

The charging enhancement of latent heat thermal energy storage system is of great significance for the efficient transfer of non-uniform thermal sources. In order to solve the problem of long charging time caused by the low thermal conductivity of phase change materials (PCMs), multiple PCMs - uneven fins combination as a compound enhancement approach was proposed to improve thermal storage rate. The transient heat transfer and melting process of PCMs was numerically studied by the finite volume method and enthalpy-porosity method. Firstly, the filling ratio of high melting point PCM to high melting point PCM was optimized as 60%: 40%. Under the optimal ratio, the charging time was reduced by 29% and the thermal storage rate was increased by 9%. Then, the effect of natural convection on multiple PCMs melting characteristics was analyzed. Finally, the thermal storage performance of multiple PCMs - uneven fins combination was investigated. The results indicated that the reasonable fin arrangement (upper dense - lower sparse fins) strengthens the local convection and improves the melting uniformity, which reduces the melting time by 16% and increases the thermal storage rate by 7%. Therefore, multiple PCMs - uneven fins combination can obviously reduce the charging time and increase the thermal storage capacity, which contributes to promote efficient utilization of fluctuating thermal energy.

Enthalpy based mathematical modelling for thermal energy storage filled with paraffin encapsulated balls as storage material

Numerical modeling of latent heat thermal storage (LHTS) is always complex to solve because of the necessity to tediously track the moving boundary, which happens due to the continuous variation of solid–liquid interface, during the melting or freezing process of phase change materials (PCM). Moreover, when commercial grades of paraffin are employed as Phase change storage materials for thermal energy storage applications, the phase transition will not happen at a particular fixed temperature but in a narrow range. Also, such paraffin exhibits several zones of transitions such as solid–solid and solid–liquid. The enthalpy (or fixed domain) method is one of the convenient methods to model problems involving phase change (or phase change materials) with the above mentioned conditions. In this paper, the mathematical modeling by enthalpy method for a LHTS system filled with ball shaped PCM containers as storage materials and its numerical solution procedures were discussed. The numerical procedure that considers the solid to solid transition of PCM and enthalpy-temperature relationships for PCMs that exhibit unsymmetrical transition zones before and after the peak transition temperature during the solid–solid and solid–liquid transitions were introduced and analysed.

Melting and solidification processes of phase change material in evacuated tube solar collector with U-shaped spirally corrugated tube

Most important challenge in evacuated tube solar collector is lower performance in off-pick hours of sunlight which leads to utilizing thermal energy storage. In present study, thermal performance of evacuated tube solar collectors (because of its enormous application range) integrated with phase change material is evaluated, numerically. The corrugated tube is used instead of smooth tube. The proposed system which is an evacuated tube solar collector equipped with corrugated U-shaped tube and phase change material to store the additional thermal energy in pick hours of sunlight, is the innovation of present work. Numerical simulations were performed for both melting and solidification processes of phase change material. The influences of number of lobe, pitch and depth corrugated on thermal processes of proposed collector are analyzed. The results showed that although the collector outlet temperature in some cases were slight different, however, a significant enhancement in daily thermal efficiency is observed due to the longtime discharging process at night. The corrugated tube increases the collector efficiency by 21.55% compared to smooth tube. Furthermore, four-lobe corrugated tube retains the collector’s operation temperature up to 40 °C. Also, between investigated geometrical parameters, number of lobe has most impact on thermal performance.

Simulation and experimental investigation of a multi-temperature insulation box with phase change materials for cold storage

In terms of cold chain logistics, China is less developed with most refrigerated transport being mechanical compression refrigeration. To improve the economy of logistics and ensure the quality of fruits and vegetables in refrigerated transportation, a multi-temperature insulation box with different phase change materials (PCMs) was designed for cold storage in this work. The efficiency of PCMs was evaluated using a differential scanning calorimeter (DSC) and thermal conductivity tester. The experimental results reveal phase transition temperatures of 7.1 and −2.5 °C, latent heats of 146.1 and 256.2 J/g, and thermal conductivities of 0.2832 and 0.9427 W/(m·K) of PCM1 (n-octanoic acid-myristic acid composite) and PCM2 (potassium sorbate-water composite), respectively. Combined with vacuum insulation technology, a multi-temperature zone insulation box for cold storage was constructed. A three-dimensional unsteady model was established, and the temperature and velocity fields in different parts of the box were analyzed, especially the melting process of PCMs. An insulation box temperature test system was established using good supply practice (GSP). The experimental results reveal that temperature zones 2 (medium zone) and 3 (low zone) of the insulation box can be kept cold at 7–9 °C for about 13 h and at −2–0 °C for about 14 h, respectively. The results suggest that the designed multi-temperature storage insulation box is an effective transportation equipment for cold chain logistics.

Experimental and numerical investigation on enhancing heat transfer performance of a phase change thermal storage tank

There are extensive research and application prospects in the fields of solar energy utilization and waste heat recovery of low and medium temperature latent thermal energy storage (TES), but the low thermal conductivity of phase change material (PCM) seriously weakens the thermal response rate and the rate of heat storage and release of TES. In order to solve this problem, this paper adopts a multi-scale heat transfer enhancement technique. The following parameters by grouping experiments are investigated: expanded graphite (EG) particle content, the diameter and placement position of copper rods. Then the experimental results are analyzed by the statistical response surface method (RSM). At the same time, a 3D model of the phase change heat storage of a single tank is established considering the natural convection of liquid PCM. Meanwhile, the influence of the placement of copper fin on the charging process was analyzed numerically. RSM analysis and simulation results show that copper rod diameter and EG particle content are two important parameters affecting the melting of PCM. Meanwhile, it is predicted that in the range of D Cu = 4-6mm, if the EG particle content is 2 wt.%, it has a shorter melting time and higher economic performance. The placement of the copper rod has no obvious effect on the charging process, but the placement of the annular copper fin has an obvious effect on the melting process of PCM. This study will provide guidance for the heat transfer optimization design of a single tank TES.

Heat transfer enhancement and melting behavior of phase change material in a direct-contact thermal energy storage container

Abstract Experimental study was conducted to investigate the heat transfer performance and melting behavior of phase change material (PCM) in a direct-contact thermal energy storage (TES) container. The PCM was erythritol and the melting point was 119 ℃. The high temperature heat transfer oil (HTO), Thermia Heat Transfer Oil C, flowed into the PCM region from the bottom of the container. The results showed that the melting process of PCM could be divided into three stages, forming channel stage, melting stage and final stage. In the first stage, the HTO could not go through the solid PCM and the heat transfer was mainly thermal conduction. In the melting stage, the HTO flowed through the PCM region and convective heat transfer was dominated. The latent heat of phase change was stored in this stage. In the final stage, most of the solid PCM was liquefied and the temperature stratification in the PCM was almost disappeared under the strong disturbance generated by HTO. Sensible heat was stored in this stage. To make a quick flow channel in the first stage and accelerate the melting process, inlet tubes with plain fins was numerically discussed through computational fluid dynamic approach. The simulation utilized enthalpy-porosity formulation to solve the direct-contact melting process. The results indicate that plain fins significantly enhance the heat transfer performance and improved the phase change behavior of PCM due to the large surface contact area and natural convection. The flow channel was quickly built. The duration of forming channel stage was reduced from 71 min in the experiment to 28 min. However, the fins also impaired the buoyancy driven flow generated by the temperature difference. When the charging process entered the melting stage, the melting process was dominated by forced convection heat transfer and the solid PCM between two flow channels rapidly became liquid. The melted PCM removed from solid surface was brought into the liquid region and flowed along with the flow of HTO. The heat storage rate improved by up to around 270% compared to the experiment. When an auxiliary heat flux was added to the tube and fin, the heat transfer was further improved.

Analytical solution of the melting process of phase-change materials in thermal energy storage system

ABSTRACT The phase-change interface is a non-linear mathematical problem, and the superposition principle is not suitable for the solution. At present, only a few researches contribute to the analytical solution on the phase-change problem. The advantage of the analytical solution is that it costs a few computational sources and results can be used to predict another situation with different conditions. To understand the heat-transfer character of phase-change materials (PCM), a one-dimensional physical model of the phase transition is established and the corresponding experiments are developed. Phase-change process of pure paraffin and composite PCM is analyzed with the method of the Runge-Kutta and verified by experiments. The temperature curves of the analytical solution have a consistent tendency with that of the experimental results. The temperature difference of experimental data (10.04°C) is smaller in the melting process of pure paraffin, while that of analytical solution (5.18°C) is smaller in the melting process of composite PCM. By comparing the experimental data and analytical solutions, it is found that natural convection dominates the melting of pure paraffin, while heat conduction dominates the melting of composite PCM.