Preparation of pentadecane/poly(melamine‐urea‐formaldehyde) microcapsules for thermal energy storage applications

Phase change materials (PCMs) with suitable melting ranges for thermal energy storage applications are alkanes, paraffins, fatty acids, eutectic mixtures, and inorganic PCMs. Paraffinic hydrocarbons and fatty acids with low solubility in water are usually the preferred candidates. Pentadecane, which is an alkane hydrocarbon with the chemical formula C₁₅H₃₂, was used as PCM in this study. The pentadecane was microencapsulated with a poly(melamine‐urea‐formaldehyde (MUF)) shell for thermal energy storage. Pentadecane/poly(MUF) microcapsules were prepared by in situ polymerization method. The morphological analysis of pentadecane microcapsules was analyzed with scanning electron microscopy (SEM). Thermal properties of microcapsulated pentadecane were determined by differential scanning calorimetry (DSC). The results demonstrated that pentadecane/PUF microcapsules were prepared successfully, and they offer proper phase transition temperature range (8.7°C and 8.1°C) and heat enthalpy values (84.5 and ?88.2 kJ/kg) for thermal energy storage applications. According to the results, it was determined that pentadecane/poly(MUF) microcapsules have good potential for thermal energy storage applications.     

Thermal Energy Storage Performance of Fatty Acids as a Phase Change Material

Thermal energy storage performance of fatty acids and a eutectic mixture as phase change materials (PCMs) has been investigated experimentally. The selected PCMs for this study were palmitic acid, myristic acid, stearic acid, and a mixture of stearic and myristic acids in eutectic combination ratio of 65.7 wt% myristic acid and 34.3 wt% stearic acid. The PCMs have a melting temperature range of 50.0°C to 61.20°C and a latent heat range of 162.0 J/g to 204.5 J/g. The inlet temperature and the mass flow rate of heat transfer fluid (HTF) were selected as experimental parameters to test the thermal energy storage performance of the PCMs. The transition times, temperature range, propagation of the solid-liquid interface, as well as heat flow rate characteristics of the employed cylindrical tube storage system were studied at varied experimental parameters. The experimental results show that the melting front moves to inward in the radial directions as well as in the axial directions from the top toward to the bottom of the PCM tube. It was observed that the convection heat transfer in the liquid phase plays an important role in the melting process. The changes in the studied HTF parameters have more effect on the melting processes than the solidification processes of the PCMs. The average heat storage efficiency calculated from data for all the PCMs is 51.5%, meaning that 48.5% of the heat actually was lost somewhere.     

Thermal properties of 3-hydroxy fatty acids and their binary mixtures as phase change energy storage materials

In the present work, we describe the chemical synthesis of 3-HFAs as prominent derivatives of fatty acids and assess if they could be applied as phase change materials (PCM). In addition, 3-HFAs were obtained by depolymerization of a bacterial biopolymeric material, polyhydroxyalkanoate. Thermal properties of 3-hydoxyoctanoic, decanoic, and dodecanoic acids are reported for the first time. These materials showed the potential to be applied as PCM in temperature range from 33°C to 66°C. In order to expand the temperature range for application of 3-HFAs as PCM, eutectic mass ratios of three kinds of binary mixtures of 3-HFAs were calculated, and their properties were predicted using the Schröder-van Laar equation. Thermal properties of these mixtures were validated by differential scanning calorimetry (DSC) analysis. These results showed that eutectics considerably expanded the scope of applications of 3-HFAs as PCMs. 3-HFAs originating from biotechnologically obtained polyhydroxyalkanoates also showed potential to be applied in development of PCMs.     

Thermal energy storage properties of paraffin and fatty acid binary systems

In this paper, 20 kinds of paraffin mixtures and fatty acid mixtures were prepared. Phase change temperatures of these mixtures are between 20°C and 30°C, and the phase change latent heats are big. The phase change temperatures and latent heats of these materials were tested by differential scanning calorimeter after multiple heat cycles. The testing results were compared in order to discuss the thermal stabilities of paraffin mixtures and fatty acid mixtures. The results showed that the thermal stabilities of phase change materials (PCMs) are good after 500 cycles, and the phase change temperatures and latent heats have small fluctuations. The thermal stability of fatty acids mixtures is better than that of paraffin materials. Paraffin mixtures and fatty acid mixtures are ideal PCMs used for the wall because of their good stabilities. The results can provide reference and basis for the application of paraffin and fatty acid in the energy-saving.     

High-chain fatty acid esters of myristyl alcohol with even carbon number: Novel organic phase change materials for thermal energy storage—1

High-chain fatty acid esters have not been investigated for their thermal properties as phase change materials (PCMs) in thermal energy storage. A series of high-chain fatty acid esters of myristyl alcohol (1-tetradecanol) were synthesized via esterification of lauric, myristic, palmitic, stearic and arachidic acids under vacuum and in the absence of any catalyst. The esterification reactions were studied by FT-IR spectroscopy. A differential scanning calorimeter (DSC) and a thermo-gravimetric analyzer (TGA) were intensively used to determine the thermal properties of the introduced thermal storage materials. The thermal properties were given in terms of phase change temperature, enthalpy, specific heat (C_p) and thermal decomposition temperature with related statistical data. The thermal reliability of the novel organic PCMs was investigated by thermal cycling with 1000 thermal cycles with respect to the thermal properties of the original synthesized PCMs. In addition to the synthesized esters, one commercial product was also investigated. The DSC analyses indicated that the melting points of the novel organic PCMs were between 38 and 53 °C with phase change enthalpy above 200 kJ/kg. The effect of chemical structure of the materials on thermal properties was also discussed. The results showed that these materials were favorable for low temperature heat transfer applications with superior thermal properties and reliability.     

Some fatty acids used for latent heat storage: thermal stability and corrosion of metals with respect to thermal cycling

The present study includes thermal stability of some fatty acids as phase change materials (PCMs). The selected fatty acids were stearic, palmitic, myristic and lauric acid with melting temperatures between 40–63°C and industrial-grade with 90–95 % purity. Latent heat storage capacity and phase transition temperature of the PCMs were determined by Differential Scanning Calorimetry (DSC) technique as a function of after repeated thermal cycles such as 40, 410, 700 and 910. The present work also comprises the investigation of corrosion resistance of some construction materials to the fatty acids over a long period. The containment materials tested were stainless steel (SS 304 L), carbon steel (steel C20), aluminium (Al) and copper (Cu). Gravimetric analysis as mass loss (mg/cm²), corrosion rate (mg/day) and a microscopic or matellographic investigation were performed for corrosion tests after 910 thermal cycles. DSC measurements showed that all fatty acids investigated as PCMs have a good thermal stability as a function of latent heat and phase transition temperature range for an actual middle-term thermal energy storage utility. However, in long-term solar thermal applications, the palmitic acid and myristic acid may be considered more suitable PCMs than the others. From the gravimetric and metallographic results, it can be concluded that stainless steel (SS 304L) with chromium oxide (Cr₂O₃) surface layer and Al with aluminium oxide (Al₂O₃) surface layer are essentially compatible with the investigated fatty acids. Carbon steel (Steel C20) and Cupper (Cu) are only preferantially compatible with PCMs.     

Developing microencapsulated 12‐hydroxystearic acid (HSA) for phase change material use

Phase change materials (PCM) have an increasingly more important role as a thermal energy storage (TES) media. However, leakage problem of PCM causes limitation during their integration in TES systems. Therefore, the encapsulation of PCMs is attracting research interest to extend usage of PCMs in real TES applications in recent years. In this study, hydroxystearic acid (HSA) was encapsulated with polymethyl methacrylate (PMMA) and different PMMA comonomer shells via emulsion polymerization method for the first time in literature. HSA with high melting temperature range (74–78°C) can widen the scope of using PCMs, and the encapsulated form can make it more versatile. The chemical structures, morphologies, and thermophysical properties of capsules were determined by FT‐IR, SEM, DSC, TGA, and thermal infrared camera. Among the produced HSA capsule candidates, PMMA‐HEMA is the most promising with latent heat of 48.5 J/g with melting range of 47 to 85°C. SEM analysis indicated that the capsules have spherical shape with compact surface at nano‐micro (100–440 nm) size range; however, some capsules exhibited agglomeration.     

Study on preparation and thermal properties of binary fatty acid/diatomite shape-stabilized phase change materials

The choice of fatty acids as shape-stabilized phase change materials (PCMs) will increase the feasibilities of PCMs in practical applications due to the low price of the fatty acids. Compounding different fatty acids for each other is an effective way to obtain a PCM with a suitable phase-transition temperature. In this study, a series of binary fatty acids composed by capric acid, lauric acid, palmitic acid and stearic acid for each other were prepared using the phase diagram thermal dynamics calculation method. Then these binary fatty acids are absorbed in four kinds of diatomites with different specific areas, which act as a supporting material, to prepare shape-stabilized PCMs. The prepared shape-stabilized PCMs are characterized by the Scanning electron microscope (SEM) and the differential scanning calorimetry (DSC) analysis method. The results show that there is an optimum absorption ratio between binary fatty acids and the diatomite. The latent heat of capric-lauric acid/diatomite decreases to 57% of that of capric-lauric acid, and the phase-transition temperature rises from 16.36 to 16.74 °C when the capric-lauric acid is absorbed in the diatomite. The prepared capric-lauric acid/diatomite composite PCM has proper melting temperatures and latent heat for thermal energy storage application in buildings.     

Binary mixtures of fatty alcohols and fatty acid esters as novel solid‐liquid phase change materials

The discovery of new eutectic phase change materials (PCMs) will overcome the current PCM challenges such as nonbiodegradability, super‐cooling, and limited thermal stability. This paper reports on the development of new bio‐based PCMs composed of binary mixtures of fatty acid esters and fatty alcohols at their eutectic compositions, which provide potential solid‐liquid PCMs for building applications. Six binary systems, namely 1‐dodecanol (DD) + methyl stearate (MES), DD + methyl palmitate (MEP), DD + methyl laurate (MEL), 1‐tetradecanol (TD) + MES, TD + MEP, and TD + MEL were prepared and their thermal behaviours were deliberated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), long‐term thermal stability test, and mass loss analysis. Amongst the studied systems, phase change transition temperature and latent heat of fusion of the eutectic mixtures of DD‐MES, DD‐MEP, TD‐MES, and TD‐MEP were found to be suitable for the building application with values of 22.46°C/201.91 J/g, 20.34°C/224.45 J/g, 32.05°C/209.38 J/g, and 26.72°C/210.15 J/g, respectively. The average degree of super‐cooling for all PCMs was below 2°C, and no significant changes in thermophysical properties of the developed PCMs were observed after 1000 thermal cycles.     

High-chain fatty acid esters of myristyl alcohol with odd carbon number: Novel organic phase change materials for thermal energy storage—2

A series of high-chain fatty acid esters of 1-tetradecanol (myristyl alcohol) were synthesized via esterification of 1-tridecanoic, 1-pentadecanoic, 1-heptadecanoic and 1-nonadecanoic acids under vacuum and in the absence of catalyst. The esterification reactions were controlled by FT-IR spectroscopy. Differential scanning calorimeter (DSC) and thermo-gravimetric analyzer (TGA) were intensively used to determine the thermal properties of the presented novel organic phase change materials (PCM). The thermal properties were given in terms of phase change temperature, enthalpy, specific heat (C_p) and thermal decomposition temperature with related statistical calculations. The thermal reliability of the synthesized PCMs, which is an important property for utilization, was determined via measuring the change in thermal properties after 1000 thermal cycles. The DSC analyses indicated that the melting points of the novel organic PCMs were between 40 and 50 °C with phase change enthalpy above 200 kJ/kg. The results showed that these thermal storage materials were favorable for low temperature heat transfer applications with superior thermal properties and reliability among the known PCMs.     

Lipid‐derived monoamide as phase change energy storage materials

One of the major shortcomings of current organic phase change materials (PCMs) is their relatively low melting points, typically below 80°C, which limits their integration into thermal energy storage (TES) systems. The present work was aimed at developing lipid‐derived PCMs with increased melting points which would be suitable for TES applications requiring higher melting points without compromising other key properties such as enthalpy. The introduction of an amide group into the structure of linear saturated fatty acids was used as a means to increase intermolecular interactions and therefore crystallization and melting points. A series of six linear monoamides with differing chain length and symmetry about the amide group were investigated for thermal stability, thermal transition, flow behavior, and crystal structure to establish the structure‐property relationships relevant to TES. The presence of the highly polar amide group in the aliphatic fatty acid–derived molecules resulted in notable improvement in performance compared with the analogous monofunctional molecules: Increases in melting points (79°C‐96°C) and high enthalpies of fusion (155‐201 J/g) were recorded. Fundamental relationships between structure, processing, and macroscopic physicochemical properties, never before elucidated, were revealed in the study. The study revealed a step‐like variation of macroscopic properties: a surprising outcome of the competition between intermolecular attractions, symmetry effects, and mass transfer limitations. The predictive structure‐function relationships established in this work will allow the straightforward engineering of monoamide architectures that can extend the range of organic PCMs and deliver thermal properties desirable for TES applications.     

Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage

Fatty acids such as stearic acid (SA), palmitic acid (PA), myristic acid (MA), and lauric acid (LA) are promising phase change materials (PCMs) for latent heat thermal energy storage (LHTES) applications, but high cost is the most drawback which limits the utility area of them in thermal energy storage. The use of fatty acids as form-stable PCM will increase their feasibilities in practical LHTES applications due to reduced cost of the energy storage system. In this regard, a series of fatty acid/poly(methyl methacrylate) (PMMA) blends, SA/PMMA, PA/PMMA, MA/PMMA, and LA/PMMA were prepared as new kinds of form-stable PCMs by encapsulation of fatty acids into PMMA which acts as supporting material. The blends were prepared at different mass fractions of fatty acids (50, 60, 70, 80, and 90% w/w) to reach maximum encapsulation ratio. All blends were subjected to leakage test by heating the blends over the melting temperature of the PCM. The blends that do not allow leakage of melted PCM were identified as form-stable PCMs. The form-stable fatty acid/PMMA (80/20 wt.%) blends were characterized using optic microscopy (OM), viscosimetry, and Fourier transform infrared (FT-IR) spectroscopy methods, and the results showed that the PMMA was compatible with the fatty acids. In addition, thermal characteristics such as melting and freezing temperatures and latent heats of the form-stable PCMs were measured by using differential scanning calorimetry (DSC) technique and indicated that they had good thermal properties. On the basis of all results, it was concluded that form-stable fatty acid/PMMA blends had important potential for some practical LHTES applications such as under floor space heating of buildings and passive solar space heating of buildings by using wallboard, plasterboard or floor impregnated with a form-stable PCM due to their satisfying thermal properties, easily preparing in desired dimensions, direct usability without needing an add encapsulation and eliminating the thermal resistance caused by shell and thus reducing cost of LHTES system.     

Ag-coated polyurethane fibers membranes absorbed with quinary fatty acid eutectics solid-liquid phase change materials for storage and retrieval of thermal energy

The novel quinary fatty acid eutectic (CA-LA-MA-PA-SA) of capric acid, lauric acid, myristic acid, palmitic acid and stearic acid was successfully prepared with the mass ratio of 61.09/24.61/8.13/4.01/2.16. Thereafter, the innovative Ag-coated polyurethane (PU) fibers membranes with different concentrations of Ag, which were selected as a supporting material to adsorb the CA-LA-MA-PA-SA eutectics, were successfully fabricated through electrospinning followed by magnetron sputter. The energy dispersive X-ray confirmed that Ag nanoclusters were successfully deposited on the surface of PU fibers as a result of sputter coating. The observations of atomic force microscope indicated that the surface roughness of the PU fibers significantly increased with increase in coating time. The scanning electron microscope images demonstrated that the CA-LA-MA-PA-SA eutectics were uniformly distributed into the three-dimensional porous structures of uncoated and Ag-coated PU fibers membranes. Furthermore, the differential scanning calorimeter curves suggested that the CA-LA-MA-PA-SA/PU/Ag composites phase change materials (PCMs) possessed melting enthalpies about 110 kJ/kg and melting temperature around 17 °C. The absorption ratios of the CA-LA-MA-PA-SA eutectic in composite PCMs was approximately at 73.74%–83.18%. The investigation on thermal performance indicated that we achieved higher melting and freezing rates of the CA-LA-MA-PA-SA/PU/Ag composites PCMs by increasing coating time. In addition to this, after depositing Ag nanoparticles the melting and freezing times of composites PCMs were shortened to about 21%–65%.     

A review of materials,heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS)

This paper reviews the development of latent heat thermal energy storage systems studied detailing various phase change materials (PCMs) investigated over the last three decades, the heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy and the formulation of the phase change problem. It also examines the geometry and configurations of PCM containers and a series of numerical and experimental tests undertaken to assess the effects of parameters such as the inlet temperature and the mass flow rate of the heat transfer fluid (HTF). It is concluded that most of the phase change problems have been carried out at temperature ranges between 0 °C and 60 °C suitable for domestic heating applications. In terms of problem formulation, the common approach has been the use of enthalpy formulation. Heat transfer in the phase change problem was previously formulated using pure conduction approach but the problem has moved to a different level of complexity with added convection in the melt being accounted for. There is no standard method (such as British Standards or EU standards) developed to test for PCMs, making it difficult for comparison to be made to assess the suitability of PCMs to particular applications. A unified platform such as British Standards, EU standards needs to be developed to ensure same or similar procedure and analysis (performance curves) to allow comparison and knowledge gained from one test to be applied to another.     

Fabrication and characterization of electrospun fatty acid form-stable phase change materials in the presence of copper nanoparticles

Latent heat storage system using phase change materials (PCMs) has been recognized as one of the most useful technologies for energy conservation. In this study, a novel type of fatty acid eutectic of methyl palmitate (MP) and lauric acid (LA)/polyacrylonitrile (PAN) composite phase change fiber is prepared by single electrospinning method. Additionally, copper nanoparticles (CNPs) with different mass ratio are combined for improving the thermal conductivity of the PCM. The structure and morphology of the fabricated composite PCMs are observed by scanning electron microscopy (SEM), and the thermal properties and performance are also characterized. SEM results show that the liquid fatty acid has been fully stabled by the three-dimensional structure of the fibers. Good compatibility among the components of the composites is also demonstrated. Besides, the addition of nanoparticles leads to an improved thermal conductivity by over 115.2% and a phase transition temperature 21.24 °C as well as a high latent heat of 85.07 J/g. Moreover, excellent thermal reliability of the phase change fiber is confirmed by multiple thermal cycles. Hence, the composite PCM prepared in this study shows a promising potential for thermal energy system such as building insulating and thermal mass regulating textiles.     

低温相变储能材料及其应用研究

利用相变储能材料（PCMs）潜热的热能存储（TES）是一种有效的热量利用方式。目前研究较多的储能材料包括无机体系（盐和水合盐）及有机化合物（石蜡、脂肪酸等）。本文对PCMs进行了归类并介绍了各类PCMs的基本特征;针对单纯的PCMs易泄漏的特点,介绍了多孔材料吸附PCMs形成复合PCMs及微胶囊封装技术;概括了PCMs在温度调控、热量储存等方面的应用;对目前PCMs的发展情况进行了总结,并对其未来的发展趋势进行了展望。     

The thermal storage performance of mixtures consisting of liquid paraffin and fatty acids

The thermal storage performance of binary mixtures consisting of fatty acids and liquid paraffin (LP) was studied experimentally. The study is to look for the material with suitable phase transition temperature and high phase change latent heat. The phase transition temperatures of binary mixtures consisting of capric acid and other four kinds of fatty acid are between 20°C and 30°C, and the phase change latent heat is high. They are ideal phase change materials used in the wall. The binary mixtures consisting of stearic acid, palmitic acid, lauric acid (LA) and myristic acid have high phase transition temperatures, and they are not suitable to use in the wall. The phase transition temperatures of mixtures consisting of LP and LA are between 20°C and 30°C, and the phase change latent heat is high. They can be used in the wall. The thermal stability of fatty binary mixtures is good.     

Medium‐ and high‐temperature latent heat thermal energy storage: Material database,system review,and corrosivity assessment

Latent heat thermal energy storage refers to the storage and recovery of the latent heat during the melting/solidification process of a phase change material (PCM). Among various PCMs, medium‐ and high‐temperature candidates are attractive due to their high energy storage densities and the potentials in achieving high round trip efficiency. Although a few review studies on high‐temperature PCMs have emerged in the past few years, the quantity, completeness, and accuracy of the presented data are relatively poor. Also, an efficient indexing methodology for retrieving useful PCM data is missing in the open literature. In this article, we created an up‐to‐date PCM database following a holistic review of the PCMs in medium‐ and high‐temperature applications over a temperature range of 100°C to 1680°C. Such effort then allows us to develop an accurate indexing tool for the fast selection of suitable PCM candidates and extraction of the related property data. More specifically, the created PCM database covers 496 entries of PCM materials, which are extracted from the scattered research works published during the year 1956 to 2017. The collected information includes both the basic thermo‐physical properties of PCMs (eg, melting temperature, heat of fusion, and thermal conductivity) and crucial design factors during construction and engineering phases (eg, energy storage density, volume expansion, liquid/solid densities, and cost). The reviewed PCMs comprise a wide variety of materials, including fluorides, chlorides, hydrates, nitrates, carbonates, metals and alloys, and other uncommon compounds and salts. In addition, the current work presents a brief review on high‐temperature latent heat thermal energy storage systems categorized into metallic and non‐metallic systems. The corrosivity and stability of PCMs, which are commonly ignored in previous studies, are also examined.     

Fatty acids and their mixtures as phase-change materials for thermal energy storage

An analysis of thermal properties of fatty acids and their binary mixtures has shown that they are attractive candidates for latent heat thermal storage in space heating applications. In this study, the method of differential scanning calorimetry was used to determine the transition temperatures and latent heat of transition of the fatty acids and their binary mixtures. These properties are of prime importance in the design of a latent heat thermal storage system.The melting range of the fatty acids (capric, lauric, palmitic and stearic) was observed to be approximately from 30°C to 65°C. Their heat of transition was observed to have a range from approximately 153 to 182 J/g. The eutectic points were determined for the binary mixtures of the fatty acids. The melting points of the eutectics for the binary systems of capric-lauric, lauric-palmitic, lauric-stearic and palmitic-stearic acids were found to be 18°C, 32.7°C, 34°C and 51°C respectively. The corresponding heats of melting were 120, 145, 150 and 160 J/g, respectively. The fatty acids and their eutectic mixtures were examined with an infrared spectrophotometer to ascertain the polymorphic forms and material purities.     

Development of pentadecane/diatomite and pentadecane/sepiolite nanocomposites fabricated by different compounding methods for thermal energy storage

Global warming is one of the most important consequences of excess energy consumption. Phase change materials (PCMs) have prominent advantages in thermal energy storage owing to their high latent heat capacities and small temperature variations during the phase change process. However, leakage is a major problem that limits the use of PCMs. Leakage may occur in encapsulated PCMs or in composites where the PCM is attached to the surface of a supporting material or within the pores of that material. In this study, pentadecane/diatomite and pentadecane/sepiolite nanocomposites were fabricated by using unmodified and microwave‐irradiated diatomite and sepiolite samples and by using different compounding processes, such as direct impregnation, vacuum impregnation, and ultrasonic‐assisted impregnation methods. The microstructures and the chemical and thermal properties of the composites were characterized by scanning electron microscopy, Fourier‐transform infrared spectroscopy, and differential scanning calorimetry. Subsequently, the thermal reliability and stability and the thermal conductivity of the PCM composites were also investigated. A melting temperature of 9.25°C and a latent heat capacity of 58.73 J/g were determined for the pentadecane/diatomite composite that was prepared with the direct impregnation method using a microwave‐treated diatomite sample. The pentadecane/sepiolite composite prepared in the melting temperature range 7.98°C to 8.53°C and latent heat capacity range 41.05 to 46.02 J/g. The results of the thermal analysis indicate that fabricated diatomite‐based or sepiolite‐based PCM composites have good potential as thermal energy storage materials.