Common Phase Change Materials Research Articles

Encapsulation of paraffin wax by rigid cross-linked poly (styrene divinylbenzene-acrylic acid) and its thermal characterization

The latent heat phase change materials (PCMs) have emerged as an important and effective thermal storage material. The massive change in volume and the requirement for specialized containment of PCMs limits the thermal storage application of the common PCM like paraffin wax. Herein, we report the encapsulation of paraffin wax with the cross-linked poly(styrene-divinylbenzene-acrylic acid) shell via suspension polymerization. The optical and SEM images of the prepared encapsulated phase change materials (EPCMs) were in the size range of 200–500 µm. The thermal charging-discharging rate of the EPCM was studied using a facile water bath technique. It was found that the optimized EPCM showed about 1.7 times faster-charging and 3.5 times faster discharging rate compared to reference PCM (without encapsulation). The latent heat for the EPCM was found to be about 3.3 times higher than the reference PCM. The thermal charging-discharging ability and latent heat values are higher for the encapsulated paraffin wax when compared with reference paraffin wax. The encapsulation of paraffin wax by polymeric materials via facial suspension polymerization using polymerizable vinyl monomers could find its application as heat storage materials.

Optimizing the Preparation of Semi-Crystalline Paraffin/Poly(Urea-Formaldehyde) Microcapsules for Thermal Energy Storage

Paraffin, the most common phase change material, has been widely utilized as the core component in thermal energy storage in the form of microcapsules. In this study, semi-crystalline paraffin is capsulated into a poly(urea-formaldehyde) (PUF) shell by a two-step polymerization process. To obtain the microcapsule with good morphology and high latent heat, sodium chloride and crosslinker (a mixture of ammonium chloride and resorcinol with a weight ratio of 1:1) are incorporated and their addition amounts were optimized through differential scanning calorimetry (DSC) and SEM. The optimized microcapsules were obtained by adding 4 wt% sodium chloride, and 0.25 wt% crosslinker exhibits a diameter of several microns and a melting enthalpy of 110 J/g. This detailed study shows that sodium chloride strongly affects the morphology of paraffin emulsion by enlarging droplets, widening the size distribution, and enhancing the stability, which should be attributed to the enhancement of electric double layer strength. In addition, sodium chloride can weaken the Zeta potential of prepolymer and provides more opportunity for prepolymer to deposit on the surface of emulsion droplets. The two components in crosslinker play different roles in the polymerization process. Ammonium chloride reacts with prepolymers and reduces the pH of system, which can accelerate the curing process, while resorcinol probably participates in polymerization as a comonomer.

Binary and Ternary Mixtures of Eicosane with Fatty Alcohols and Fatty Acids as Phase Change Material for Building Applications

Stable phase change materials (PCMS ) of binary and ternary mixtures of organic compounds were prepared in this work to be installed in buildings for purposes of heat energy storage. Binary mixtures of PCMS as salt hydrates, paraffins were heavily investigated but most of the mixing was within the same group very few articles deal with mixing PCMS from different organic groups or mixing more than two materials. A preparation and a differential scanning calorimetry ( DSC) investigation of binary and ternary mixtures of eicosane with fatty alcohols and fatty acids were performed. The eutectic mixtures of the binary systems were determined to be( 70% eicosane – 30% cetyl alcohol ) , (50% eicosane – 50% lauric acid) and (25% eicosane – 75% capric acid ) with melting temperature and latent heat of ( 33.77 ºC , 237.6 J/g ) , ( 30.63 ºC , 207.7 J/g ) and ( 24.96 ºC , 200.3 J/g ) respectively . The last one is very suitable for building applications. An eutectic mixture of eicosane with myristyl alcohol from previous work was considered as a new material and used to prepare two ternary mixtures( for the first time in literature) with both Cetyl Alcohol and Capric acid . An exciting results of temperatures were obtained : 26.86 ºC for the first ternary mixture and 16.23 ºC for the second one with moderate latent heats of 133.5 J/g and 157.7 J/g. The first one is very suitable for building applications and the second one is suitable for low thermal applications .These results will open the door to try ternary mixtures of common phase change materials to obtain various melting temperatures to suit different applications

Binary and Ternary Mixtures of Eicosane with Fatty Alcohols and Fatty Acids as Phase Change Material for Building Applications

Stable phase change materials (PCMS ) of binary and ternary mixtures of organic compounds were prepared in this work to be installed in buildings for purposes of heat energy storage. Binary mixtures of PCMS as salt hydrates, paraffins were heavily investigated but most of the mixing was within the same group very few articles deal with mixing PCMS from different organic groups or mixing more than two materials. A preparation and a differential scanning calorimetry ( DSC) investigation of binary and ternary mixtures of eicosane with fatty alcohols and fatty acids were performed. The eutectic mixtures of the binary systems were determined to be( 70% eicosane – 30% cetyl alcohol ) , (50% eicosane – 50% lauric acid) and (25% eicosane – 75% capric acid ) with melting temperature and latent heat of ( 33.77 ºC , 237.6 J/g ) , ( 30.63 ºC , 207.7 J/g ) and ( 24.96 ºC , 200.3 J/g ) respectively . The last one is very suitable for building applications. An eutectic mixture of eicosane with myristyl alcohol from previous work was considered as a new material and used to prepare two ternary mixtures( for the first time in literature) with both Cetyl Alcohol and Capric acid . An exciting results of temperatures were obtained : 26.86 ºC for the first ternary mixture and 16.23 ºC for the second one with moderate latent heats of 133.5 J/g and 157.7 J/g. The first one is very suitable for building applications and the second one is suitable for low thermal applications .These results will open the door to try ternary mixtures of common phase change materials to obtain various melting temperatures to suit different applications

Natural convection and solidification of phase-change materials in circular pipes: A SPH approach

Smoothed Particle Hydrodynamics is used to model natural convection and solidification of a liquid material inside a cylindrical pipe, whose wall temperature is kept below the freezing point. The model is validated with two benchmark cases and a parametric study targeting the properties of the most common phase-change materials is carried out. The results show three hydrodynamic regimes: the static regime, the dynamic regime, and the pseudo-static regime. In the static regime, viscous forces overcome buoyancy forces; liquid motion is minimal and the solidification front proceeds axisymmetrically from the wall to the centre of the pipe. In the dynamic regime, the flow shows the typical recirculation vortices of natural convection. Density differences drive the colder liquid in the lower (or higher, in the case of water) part of the pipe, where it quickly solidifies. Solidification does not proceed symmetrically and the solidified shell around the wall is thicker in the lower region of the pipe. The pseudo-static regime is the result of two competitive processes: on one hand a buoyancy-driven boundary layer tends to form around the solid-liquid interface, on the other hand the advancing solidification front tends to halt the motion by freezing the boundary. As a result, a small velocity profile around the solid-liquid interface is observed, but, as the solidification front moves, it has no time to transfer enough momentum to set the rest of the liquid in motion. A criterion for predicting the regime based on two dimensionless groups is given, and, since the regime affect the freezing time, a correlation for calculating the freezing time based on the hydrodynamic regime is provided.

Numerical simulation for solidification in a LHTESS by means of nano-enhanced PCM

Abstract In order to saving thermal energy, latent heat thermal energy storage systems (LHTESS) can be utilized. Common phase change material (PCM) has low thermal conductivity. In this paper, CuO nanoparticles have been used to enhance the performance of LHTESS. CuO–water nanofluid properties are estimated by means of KKL. This unsteady process has been simulated by Finite element method. Results prove that solidification process is accelerated by adding CuO nanoparticles in to pure PCM. As number of undulations increases average temperature and total energy profiles reduce while solid fraction profile increases. Also, it can be concluded that highest rate of solidification is obtained for dp = 40 nm.

Numerical study on the effects of fins and nanoparticles in a shell and tube phase change thermal energy storage unit

Energy storage is critically important for intermittent renewable sources such as solar or wind. This paper presents a numerical study on a shell and tube thermal energy storage unit using a common organic phase change material (PCM) – paraffin wax. To overcome the problem of slow charging due to low thermal conductivity of paraffin wax, this research applies a multiscale heat transfer enhancement technique, with circular plate fins on outer surface of the heat transfer fluid (HTF) tube and highly conductive nanoparticles (Al2O3) dispersed in the PCM on the shell side. The novelty of this research is that by simultaneous application of two enhancement methods, we are able to analyze the interactions between the two, which was not possible in previous studies on separate technique. A computational fluid dynamics (CFD) model is developed to simulate melting of the PCM with the following parameters: nanoparticle concentration ϕ from 0 to 4 vol%; fin angle α from −45° to 45°, and pitch p from 45 to 65 mm. The obtained numerical data was analyzed with a traditional method and a statistical response surface method (RSM). The latter represents another novelty of this research. The RSM analysis shows that fin angle and nanoparticle concentration are two significant parameters in affecting the PCM melting, but pitch of the fins does not show noticeable effect. Numerical results demonstrate that adding nanoparticles in the PCM does not accelerate the charging process; on the contrary it leads to longer charging time and lower overall heat transfer rate due to reduction of natural convection in the melted PCM. A strong interaction is also found between these two significant parameters, for example the charging time considerably increases when nanoparticles are added at α = −45°, but this effect is less pronounced when α = 45°. Positive fin angles are found to be favorable for PCM melting due to enhanced natural convection with strong local vorticities formed below the fins. A moderate fin angle of 35° leads to the shortest charging time among all studied cases. These new findings can be valuable in design of PCM units for renewable energy storage, waste heat recovery, or thermal management in engineering systems.

Optimizing fin design for a PCM-based thermal storage device using dynamic Kriging

A key challenge in the development of a practical thermal storage device (TSD) is the low thermal conductivity of common phase change materials (PCM). This low conductivity impedes both heat input and extraction. The most common solution is to use conductive metal fins to spread heat through the device. However, optimizing the effectiveness of the container and the fin arrangement is difficult due to the large number of potential design parameters. This paper develops a strategy to make simulation-based optimization process affordable and accurate. First, numerical techniques are designed to accurately and efficiently compute heat and mass transport in a variety of geometries without generating grids to conform to each geometry. This facilitates rapid prototyping and mitigates the expense of individual simulations. Second, a pre-screening process identifies the independent variables with the largest and most nonlinear effect on the objective function in the optimization process, thus narrowing the parameter space. Finally, a dynamic Kriging-based optimization approach constructs a multidimensional response surface using sparse input datasets; the response surface is then used to identify an optimal design. The combination of the above three strategies is shown to result in an approach that can aid in the design of optimal thermal storage devices that rely on a mixture of PCM and metal fins.

Empirical equation to estimate viscosity of paraffin

Thermal energy storage (TES) systems using phase change materials (PCM) are nowadays widely developed to be applied in solar power plants or cooling and domestic comfort services. The design of a TES system does not only rely on the energy density that a PCM can provide, but also on other important material properties such as its rheological behavior when the PCM is melted. Viscosity varies with temperature, but the lack of an empirical equation predicting its value has lead researchers to simulate the system performance taking constant viscosity values which, consequently, have led to errors on the designs. As paraffin are one of the most common PCM types used, the present paper evaluates the rheology of four commercial paraffin with different phase change temperatures in order to find out an empirical equation for the whole paraffin family. A polynomial 3 model type equation has been found as the best one to predict paraffin viscosity.

Investigation of unbranched, saturated, carboxylic esters as phase change materials

This study evaluates unbranched, saturated carboxylic esters with respect to their suitability to be used as storage media for latent heat storage applications. Therefore, important thermophysical properties are gathered both by means of literature research as well as by experimental measurements. Additionally esters are critically evaluated against other common phase change materials in terms of their environmental impact and their economic potential. The experimental investigations are performed for eleven selected ester samples with a focus on the determination of their melting temperature and their enthalpy of fusion using differential scanning calorimetry. Transient Hot Bridge was used to determine the thermal conductivity of the liquid samples while thermogravimetric analysis was employed for the evaluation of the 5% weight loss temperature as well as of the decomposition temperature of the non-volatile samples.Both experimental results and literature data reveal that the highest potential of esters against other phase change materials lies in low temperature applications where the main alternative is using salt hydrates.

Impact of the Relationship between Phase Change Temperature and Boundary Temperature on the Thermal Performance of a PCM Wall and the Presentation of PCM Thermal Performance Indexes

In the composite phase change material (PCM) building envelope, the matching relationship between the phase change temperature of the PCM and the wall's boundary temperature significantly affects the energy storage performance of the PCM building envelope. In this paper, a type of concrete hollow block with a typical structure and a common PCM were adopted to produce multiform composite PCM hollow blocks, and the temperature changing hot chamber method was performed to test the thermal performance of the hollow block walls under different temperature conditions. New indexes were proposed for the thermal performance evaluation of the PCM wall. Meanwhile, combined with experimental data, the effective heat capacity model and the enthalpy model were used to analyze the effect of correlations concerning how the relationship between phase change temperature and wall's boundary temperature influenced the thermal performance of PCM wall. Three main impact factors related to temperature were obtained through the analysis. In addition, approaches for improving the thermal performance of a composite PCM wall were put forward. This paper provides the theoretical basis, data reference and practical instruction for the proper use of a PCM wall and ways for improving the thermal performance of a composite PCM wall.

Experimental Research on the Thermal Performance of Composite PCM Hollow Block Walls and Validation of Phase Transition Heat Transfer Models

A type of concrete hollow block with typical structure and a common phase change material (PCM) were adopted. The PCM was filled into the hollow blocks by which the multiform composite PCM hollow blocks were made. The temperature-changing hot chamber method was used to test the thermal performance of block walls. The enthalpy method and the effective heat capacity method were used to calculate the heat transfer process. The results of the two methods can both reach the reasonable agreement with the experimental data. The unsteady-state thermal performance of the PCM hollow block walls is markedly higher than that of the wall without PCM. Furthermore, if the temperature of the PCM in the wall does not exceed its phase transition temperature range, the PCM wall can reach high thermal performance.

DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor

Reliable parameters of phase change material are essential for design and simulation of the PCM floor, which can be effectively used in heating system with non-continuous energy. This paper summarized latent heat, solidus temperature and liquids temperature of a typical PCM (capric acid) from reported tests based on differential scanning calorimeter (DSC). It is discovered that the results for the same PCM were significantly incongruent. Then, we arranged DSC tests with different procedures on capric acid and use the acquired parameters in simulations of a PCM floor, which has been reported with detailed experimental results in our former research. The aim is to present reliable latent heat, solidus temperature and liquids temperature of this common PCM and assess the impact of misinterpreted enthalpy-capacity function on the simulated thermal storage and releasing effect of the PCM floor. Errors with 33％–883％ deviation for phase transition range of PCM were discovered from the improperly arranged tests. In the cases of simulation, a maximum difference of 20% was observed for the floor surface temperature. It means it is worthwhile setting standard DSC tests and ascertaining right effective capacity or enthalpy function of PCM in simulations related to PCM system design.

相变材料在含翅片球形容器内的约束熔化传热过程

Spherical capsules are widely used as housing for phase change materials (PCMs) in heat exchangers for thermal energy storage applications. In view of the low thermal conductivity of common PCM candidates, extended surfaces, such as fins, are routinely added to PCM capsules to improve the thermal performance. In this paper, to quantitatively evaluate the influence of fins on the thermal performance of PCM-filled spherical capsules, constrained melting heat transfer of a PCM in a circumferentially finned spherical capsule with various fin heights was investigated both numerically and experimentally under constant-temperature boundary conditions. The enthalpy-based model was used in the numerical simulations to deal with phase change, while the control volume method was used to solve the governing equations for the melting problem with natural convection in the liquid phase. A spherical melting facility that allows for direct observation of the solid-liquid phase interface during melting was designed and constructed. The spherical capsule and fins were made of glass and aluminum, respectively. Octadecane with a nominal melting point at 28.2°C was used as the PCM. Qualitative agreement was obtained between the experimentally observed and numerically predicted results of solid-liquid interface evolution. The sources of the differences were identified to be the departure of the physical model from the real system, as well as simplifications of the numerical model. The evolution of the natural convective flow and temperature fields during melting are presented in the form of a series of snapshots of streamline and isotherm contours. The heat transfer mechanisms during melting were interpreted by these contour snapshots. The presence of circumferential fins not only enhances heat conduction, but also augments natural convection heat transfer in localized areas in the vicinity of the fins. The localized interactions between these two effects are significantly affected by the fin height. Under the specific conditions considered, fins with height-to-radius ratios of 0.25, 0.50, and 0.75 decrease the total melting time by 10.6%, 20.2%, and 28.7%, respectively, leading to a significant increase of the thermal energy storage rates in the spherical capsule. Although the presence of fins benefits heat transfer enhancement, a more comprehensive understanding of the effects of other important factors, such as the inclination angle of the spherical capsule, is required.

Improving thermal conductivity phase change materials—A study of paraffin nanomagnetite composites

Paraffin is a common organic phase change material (PCM) that finds many applications in thermal energy storage (TES) systems. One common characteristic of organic PCMs is their low thermal conductivities. This – in turn – causes a slow thermal response when paraffin is used in high power applications. In this study, paraffin–nanomagnetite (Fe3O4) composites (PNMC) were prepared by a dispersion technique to enhance their thermal properties. Nano magnetite prepared using the cost-effective sol–gel method was mixed into paraffin at two different mass fractions: 10% and 20%. Scanning electron microscope (SEM) was used to observe the morphologies of nanomagnetite and PNMCs during different stages of the composite production process. SEM analysis results showed that nanomagnetite particles of 40–75nm in size were homogeneously distributed in the paraffin structure. The latent heat storage capacity of PNMC, measured by differential scanning calorimetry (DSC) at 1°C/min, was found to be 8% higher than paraffin alone. Thermal conductivities and diffusivities of paraffin and PNMCs were determined by the “DICO” method. For 10% and 20% by weight PNMCs, the thermal conductivities were increased by 48% and 60%, respectively. These results clearly indicate that the addition of Fe3O4 nanoparticles is an efficient and cost effective method to enhance the heat transfer properties of paraffin, when they are incorporated into latent heat storage systems.

Quantitative measure of tetrahedral-sp3geometries in amorphous phase-change alloys

Phase change or Ovonic memory technology has gained much interest in the past decade as a viable solution for the rapid increase in the demand for memory storage. This unique technology, first proposed by S. Ovshinsky in 1968, is based on storing information on the crystalline and amorphous phases of a material. The most common phase-change materials (PCMs) use chalcogenide alloys such as the ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ (GST225). However, while the structure of its crystalline phase is relatively well characterized as consisting of a rhombohedrally distorted rock-salt lattice, the corresponding amorphous phase remains still poorly understood. Here, we show that $^{119}\mathrm{Sn}$ M\"ossbauer spectroscopy and angular constraint counting of simulated structures can provide a quantitative measure of the ${sp}^{3}$ tetrahedral fraction of Ge or Si cation in amorphous phase-change binary tellurides ${\mathrm{Ge}}_{x}{\mathrm{Te}}_{1\ensuremath{-}x}$ and ${\mathrm{Si}}_{x}{\mathrm{Te}}_{1\ensuremath{-}x}$. This represents the first quantitative estimate of such local structures, and reveals the fraction to be nearly 50%, while also revealing implications for the phase-change mechanism itself.

Bio-inspired phase change materials designed for high specific heat of solid phase

Abstract Two lipid blends with molar ratios of 2:1:1 and 1.13:1:1 of ceramide (type IV):cholesterol:palmitic acid, namely blends A and B, are prepared, respectively, based on stratum corneum of human. The phase behaviors and nanostructures of those blends have been fully characterized by low-temperature differential scanning calorimetry and small-angle X-ray scattering. The large-scale thermal properties of them have been measured by temperature-history method as well. The heat capacity of solid phase for blend A of 3.23 ± 0.14 J/g °C is higher than the value for common phase change materials (PCMs), such as lauric acid, stearic acid, polyethylene glycol 900, and paraffin wax. The lipid ratio in blend B is also changed to obtain an even higher heat capacity of solid phase of 3.69 ± 0.17 J/g °C. Therefore, we propose to concoct new lipid blends for the enhancement of heat capacity of solid phase values in the immediate future.

Characteristics of phase transition in boron‐implanted Ge2Sb2Te5 thin films for phase change memory applications

Phase change memory has been considered as one of the most promising alternatives for next‐generation nonvolatile semiconductor memory. Modification of phase change materials by impurity doping has been proved to be an effective way to improve phase change memory device performance. In this study, the most common phase change material, Ge2Sb2Te5 (GST), was employed, and its intrinsic properties were modified using boron–ion implantation. The GST films were implanted with 20 keV boron ions to fluences of 5 × 1014 and 5 × 1015 ions/cm2. The characteristics of the virgin and boron‐implanted GST films were investigated by SIMS, XRD, and in situ resistance measurement. The kinetics of the thermally activated phase transition was also analyzed on the basis of the Arrhenius relationship. The results revealed that both crystallization temperature and activation energy of crystallization increase with increasing the boron fluence. The data retention capability is also enhanced from 83 to 88 °C, corresponding to a criterion of 10‐year archival lifetime. In addition, the XRD spectra indicated that high‐fluence boron–ion implantation partially inhibits the transition of the GST films from face‐centered cubic phase to hexagonal close‐packed phase. In conclusion, the thermal stability of the GST films can be improved through boron–ion implantation. Copyright © 2014 John Wiley & Sons, Ltd.

Polarization readout analysis for multilevel phase change recording by crystallization degree modulation

Four different states of Si15Sb85 and Ge2Sb2Te5 phase change memory thin films are obtained by crystallization degree modulation through laser initialization at different powers or annealing at different temperatures. The polarization characteristics of these two four-level phase change recording media are analyzed systematically. A simple and effective readout scheme is then proposed, and the readout signal is numerically simulated. The results show that a high-contrast polarization readout can be obtained in an extensive wavelength range for the four-level phase change recording media using common phase change materials. This study will help in-depth understanding of the physical mechanisms and provide technical approaches to multilevel phase change recording.

Study on differential scanning calorimetry analysis with two operation modes and organic and inorganic phase change material (PCM)

Phase Change Materials (PCM) allow storing thermal energy as latent heat when phase change occurs. In order to characterize these materials the phase change enthalpy and temperature should be perfectly known. Differential Scanning Calorimetry (DSC) is the most used technique to determine PCM's thermophysical properties. There are mainly two DSC operation modes: dynamic and step mode. A selection of the two most common PCM materials (paraffin and salts hydrates) was performed to conduct measures with the two DSC operation modes. The main objective of this paper is to determine the most appropriate operation mode for each type of PCM. A slow dynamic mode is recommended when analyzing salt hydrates with DSC. No significant differences between the two DSC modes were observed for paraffin.