Temperature Thermal Energy Storage Applications Research Articles

Heat transfer of composite phase change material modules containing a eutectic carbonate salt for medium and high temperature thermal energy storage applications

This paper concerns the heat transfer behaviour of Composite Phase Change Material (CPCM) modules made of a eutectic carbonate salt of NaLiCO3 (phase change material, PCM), MgO (ceramic skeleton material, CSM) and graphite flakes (thermal conductivity enhancement material, TCEM). The CPCM has a melting point around 500 °C and is suitable for medium and high temperature thermal energy storage applications including peak shaving of power grids, effective use of curtailed wind energy, and concentrated solar power generation. Disk-like CPCM modules were fabricated for the work. The effects of TCEM loading and surface cooling conditions on the heat transfer were experimentally investigated and analysed. The results showed that the use of TCEM not only significantly enhanced heat transfer of the CPCM modules, but also reduced the temperature difference and hence the thermal resistance between heater surface and CPCM module surface, leading to a significant extent of enhancement of overall heat transfer. Temperature measurements of a flat surface of the CPCM modules as well as that within the modules showed a non-uniform temperature distribution perpendicular to heat transfer direction, suggesting the effect of CPCM microstructure on heat transfer. This microstructural effect was further investigated using a scanning electron microscope with energy dispersive spectrometry. The results indicated salt migration, particle breakage and particle redistribution in the interior of the CPCM modules during thermal cycling, leading to a more homogenous distribution of ingredients and more uniform heat transfer within CPCM modules.

A novel low-temperature fabrication approach of composite phase change materials for high temperature thermal energy storage

Phase change materials (PCMs) are generally integrated into matrix materials to form shape-stabilized composite heat storage materials (HSMs) used for high temperature thermal energy storage applications. The conventional fabrication of composite HSMs is prevalently implemented at quite high temperatures, which is energy-intensive and narrows down the range of applicable PCMs because of thermal decomposition. Therefore, this paper establishes a novel fabrication approach to accomplish highly dense matrix to encapsulate PCMs at extremely low temperatures, based on the recently developed cold sintering process. The feasibility of the proposed approach was demonstrated by a case study of NaNO3/Ca(OH)2 composite HSMs. It was observed that the Ca(OH)2 matrix formed dense microstructure with obvious sintered boundaries and successfully encapsulated NaNO3 as PCM. The HSMs maintained stable macroscopic shape after hundreds of thermal cycles, and exhibited an energy storage efficiency of 59.48%, little leakage of PCM, and good thermal stability. Mechanical tests indicated that the HSMs possessed excellent mechanical properties when the sintering pressure is over 220 MPa. The discharging time of stored heat was presented through infrared thermography, and the heat storage capacity measured for the composite HSMs was over four times as high as those of typical solid storage materials of sensible heat, which demonstrated their excellent heat storage performances. The HSMs can be used in the form of packed bed or parallel channel with multi-layered heat storage, which is beneficial for efficiently utilizing solar heat and improving the performance of current energy storage system. This study therefore provides a novel route for energy-saving and low-carbon fabrication of shape-stabilized composite HSMs.

Experimental study on microcapsules of Ag doped ZnO nanomaterials enhanced Oleic-Myristic acid eutectic PCM for thermal energy storage

Novel microcapsules of Ag doped ZnO nanomaterials enhanced Oleic-Myristic acid eutectic phase change materials (PCMs) were synthesized by in-situ polymerization process. X-ray diffraction analysis reveals that Ag doped ZnO nanomaterials has a hexagonal wurtzite structure. Nano enhanced composite PCMs are prepared by mixing different weight percentage of nanomaterials with Oleic-Myristic acid eutectic mixture act as core material. Microscopic analysis reveals the formation of uniform spherical structure of Melamine-Formaldehyde shell over the core material with a perfect core-shell structure. Differential scanning calorimetric analysis showed that microencapsulated PCMs (MEPCMs) melts and freezes in the range of 4.37–5.81 °C and 10.84–11.86 °C with the latent heat of 75.39–79.35 J/g and 74.9–77.95 J/g. MEPCMs have better thermal conductivity, stability and reliability determined by thermal conductivity, thermogravimetric and thermal cycling analysis. Based on the results, MEPCMs with Ag doped ZnO nanomaterials could be suggested as the potential core material for low temperature thermal energy storage applications.

Thermal conductivity enhancement in organic phase change material (phenol-water system) upon addition of Al2O3, SiO2 and TiO2 nano-inclusions

In spite of the immense industrial benefits of phase change materials (PCM) in energy conservation and management, the practical applicability of organic PCM is severely limited due to its low thermal conductivity. We report enhanced thermal conductivity during the first order liquid-solid phase transition in, phenol-water system, loaded with four different nano-inclusions, viz., α-Al2O3, SiO2, hydrophobic SiO2 and TiO2. Nano-inclusion loaded PCM showed a large thermal conductivity enhancement, which was attributed to the formation of nano-crystalline needle like microstructure in the PCM, during freezing, which squeezes the nano-inclusions towards the grain boundaries, thereby forming a quasi 2D network of high thermal conductivity percolation pathways. In the solid state, thermal conductivity enhancement is found to increase with concentration of the nano-inclusions due to the formation of larger well dispersed aggregates. Atomic force microscopy images, show an increased contact area for closely packed larger aggregates at higher concentration of the nano-inclusions. Our results show lower thermal conductivity enhancement for the PCM loaded with hydrophobic silica, which is attributed to the higher interfacial resistance of the hydrophobic interfaces. Addition of carbon black nano powder to the nano-inclusion loaded PCM showed a synergistic increase in thermal conductivity due to the fractal aggregates of carbon black that act as volume filling agents and reduce the inter-aggregate gaps, thereby enhancing the conductive pathways. Our studies show the nano-inclusion assisted thermal conductivity enhancement in phenol-water system as a PCM with tunable phase transition temperature for room temperature thermal energy storage applications.

Heat transfer behavior of elemental sulfur for low temperature thermal energy storage applications

Elemental sulfur provides a low-cost, high-performance thermal storage option for a wide range of applications and over an exceptionally wide range of temperatures (50 °C to over 600 °C). In previous efforts we have shown impressive performance for 200–600 °C, while in this study we examine the low-temperature (50–200 °C) thermal charge and discharge behavior of isochoric sulfur-based storage using a detailed computational model solving for the conjugate heat transfer and solid-liquid phase change dynamics. The model provides excellent agreement with experimental results. We show that sulfur exhibits lower viscosity because of reduction in the chain-length of polymeric sulfur caused by trace amounts of organic substances resulting in attractive charge and discharge performance. The results from the parametric analysis of pipe diameter on the charge and discharge heat transfer characteristics are used to develop a simple, generalized correlation that relates the transient sulfur temperature and liquid fraction evolution as a function of dynamically evolving buoyancy-Fourier number due to the solid-liquid phase change. This solid-liquid buoyancy-Fourier, BFs-l, correlation can be used for effectively designing sulfur-based thermal energy storage systems for transient operation in low temperature applications.

Preparation and investigation of multicomponent alkali nitrate/nitrite salts for low temperature thermal energy storage

A novel eutectic salt of alkali nitrate/nitrite mixture with low melting point was investigated using thermal analysis methods for thermal energy storage. The eutectic salt mixture system LiNO3-NaNO3-KNO3-NaNO2-KNO2 was prepared based on eutectic composition XLiNO3 = 33.5, XNaNO3 = 1.2, XKNO3 = 1.2, XNaNO2 = 17.4 and XKNO2 = 46.7 (in mole fraction). Using Differential Scanning Calormetry (DSC) apparatus, the melting point, enthalpy of fusion and specific heat capacity of the eutectic salt mixture were experimentally determined under an argon atmosphere. The density of eutectic salt mixture based on Archimedean principle was measured as a function of temperature. By means of the Thermogravimetric Analyzer (TGA) equipment, the decomposition temperature and the upper limit of operating temperature of eutectic salt mixture were determined. Viscosity of eutectic salt was also measured experimentally using a rotational coaxial cylinder viscometer constructed. Meanwhile, the empirical estimation method based on additive principle was used to predict thermal-physical properties (density and viscosity) of eutectic salt mixture. Results indicate that the predicted values were in good agreement with experiment values. Based on the thermal-physical properties of eutectic salt mixture, this novel five-component eutectic system can be used as excellent heat transfer and storage materials for low temperature thermal energy storage (TES) applications.

Sodium nitrate – Diatomite composite materials for thermal energy storage

The work reported in paper concerns the use of diatomite to form-stabilise sodium nitrate, a phase change material (PCM) for medium temperature thermal energy storage applications. The composite was found to be able to retain up to 70% of the nitrate salt. X-ray diffraction (XRD) analyses suggested an excellent chemical compatibility between diatomite and the salt. Scanning electron microscope (SEM) analyses demonstrated an even distribution of the salt within the diatomite structure. Differential scanning calorimetry (DSC) measurements showed that melting temperature of the material was approximately 307.8°C with a latent heat of 115.79kJ/kg. Mechanical characterization of the composite material showed a compressive strength of the composite materials as high as 22.17MPa. The composite materials were found to have a fairly low thermal conductivity of ∼0.5W/mK and an addition of graphite could give a substantial thermal conductivity enhancement (∼6-fold with an additional of 10wt% graphite).

Thermal properties and reliability of eutectic mixture of stearic acid-acetamide as phase change material for latent heat storage

The thermal properties and reliability of the stearic acid (SA) with acetamide (AC) binary mixture were characterized using differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), thermogravimetric analysis (TG) and Fourier transform infrared spectroscopy (FT-IR). The phase diagrams for the SA-AC binary mixture with AC in the metastable and the stable form were constructed. The eutectic system with stable AC is 0.604mol fraction SA, and displayed a melting temperature (Tm) of 64.55°C and latent heat of melting (ΔHm) of 193.87J·g−1. The eutectic systems with metastable AC are 0.397 and 0.604mol fraction SA. The melting temperatures are 62.23°C and 62.54°C, and latent heats of fusion are 222.10J·g−1 and 194.28J·g−1, respectively. Following accelerated thermal cycling tests, TG and FT-IR analysis indicate that the eutectic mixture (χSA=0.397) with the metastable AC has good cyclic and thermal stability. The results show that the SA-AC eutectic mixture use as phase change material (PCM) possess good prospect for low temperature thermal energy storage (TES) applications.

Phase diagrams, eutectic mass ratios and thermal energy storage properties of multiple fatty acid eutectics as novel solid-liquid phase change materials for storage and retrieval of thermal energy

A series of binary and multiple fatty acid eutectics such as ternary eutectics, quaternary eutectics and quinary eutectics were successfully prepared and developed as solid-liquid phase change materials (PCMs) by using five kinds of individual fatty acids such as capric acid (CA), lauric acid (LA), myristic acid (MA), palmitic acid (PA), and stearic acid (SA) through melt blending followed by ultrasonication method. The eutectic phase diagrams of binary/multiple fatty acid eutectics were plotted and analyzed on the basis of Schrader equation and MATLAB software, and then their eutectic mass ratios were precisely determined by eutectic phase diagrams and differential scanning calorimetry (DSC) analyses. The deviations between the theoretical calculated eutectic mass ratios and the experimental determined eutectic mass ratios of some fatty acid eutectics could be attributed to the experimental errors of data obtaining from the DSC analyses, impurities of fatty acids and cumulative errors of calculation. The results of DSC indicated that phase change temperatures and enthalpies of the synthesized fatty acid eutectics gradually decreased with the increase of the number of components in the mixtures. The melting peak temperatures of the prepared binary eutectics, ternary eutectics, quaternary eutectics and quinary eutectics were varied from about 22 to 56°C, from about 18 to 49°C, from about 19 to 32°C and about 17°C, respectively. Their minimum and maximum melting enthalpies were about 127.2kJ/kg and 204.7kJ/kg, respectively. The spectra obtained from Fourier transformation infrared spectroscope showed that the binary/multiple fatty acid eutectics exhibited the similar carboxyl group with the single fatty acids and had stable chemical properties. The developed fatty acid eutectic mixtures were suitable for low temperature thermal energy storage applications such as solar water heating systems, building energy storage systems, and thermal regulating textiles.

Ternary mixture of fatty acids as phase change materials for thermal energy storage applications

The present study deals with the development of ternary mixtures of fatty acids for low temperature thermal energy storage applications. The commercial grade fatty acids such as Capric Acid (CA), Lauric Acid (LA), Palmitic Acid (PA) and Stearic Acid (SA), have been used to prepare stable, solid–liquid phase change material (PCM) for the same. In this regard, a series of ternary mixture i.e. CA–LA–SA (CLS) and CA–PA–SA (CPS) have been developed with different weight percentages. Thermal characteristics of these developed ternary mixture i.e. melting temperature and latent heat of fusion have been measured by using Differential Scanning Calorimeter (DSC) technique. The synthesized materials are found to have melting temperature in the range of 14–21 °C (along with adequate amount of latent heat of fusion), which may be quite useful for several low temperature thermal energy storage applications.

Lithium manganese oxides as high-temperature thermal energy storage system

The reversible oxidation of LiMnO2 to LiMn2O4 and Li2MnO3 coexisting phases has been investigated in view of its possible application as high temperature energy storage system. By means of thermoanalytical techniques information regarding the heat exchanged during both oxidation and reduction reactions have been collected and the temperature range for cyclic reduction–oxidation has been explored. Initial tests regarding the cyclical operation highlight the optimal thermal stability of the mixture confirming the exploitability of the system for high temperature thermal energy storage applications. Preliminary information concerning the kinetic behavior of the powder during the reduction stage evidences a two-step mechanism. The reaction can be described through a first stage characterized by oxygen depletion from the spinel phase, followed by a major oxygen loss concomitant with the depleted LiMn2O4 and Li2MnO3 solid state transformation into LiMnO2.

Easy and industrially applicable impregnation process for preparation of diatomite-based phase change material nanocomposites for thermal energy storage

The high porosity, high oil and water absorption capacity and low density of diatomite make it ideal for industrial applications. The porous structure of diatomite protects phase change materials (PCMs) from environmental factors as a supporting matrix and phase changes occur in nanopores of diatomite. Previous research on diatomite/PCMs composites aimed optimal composite preparation but many methods were feasible only in laboratory scale. In large scale industrial fabrication, easy, continuous and steady state methods are need to be performed. The main purpose of this study was to prepare leakage-free, thermally stable nanocomposite PCMs (nanoCPCMs) by an easy, continuous and steady state method for high temperature thermal energy storage applications. A series of nanoCPCMs with different paraffin:diatomite mass ratios were prepared. The properties of nanoCPCMs have been characterized via scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The leak (exudation) test was performed on prepared composites at higher temperatures (95 °C) in comparison with literature. As the optimum composite for thermal energy storage applications, thermal reliability of nanoCPCM was evaluated after 400 cycles of melting and freezing. NanoCPCM melted at 36.55 °C with latent heat of 53.1 J/g.

Hydrated salts/expanded graphite composite with high thermal conductivity as a shape-stabilized phase change material for thermal energy storage

A novel shape-stabilized phase change material (PCM) was developed by impregnation of hydrated salts into expanded graphite (EG) and further coated with paraffin wax. It was displayed by scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FT-IR) measurements that the hydrated salts were filled into pores or adhered onto the flakes of EG by physical interactions, including capillary forces and surface tension. It was revealed from differential scanning calorimetry (DSC) analysis that phase segregation was inhibited and subcooling weakened in the coated composite PCM. The melting and freezing enthalpy of the coated composite PCM can reach 172.3kJ/kg at 32.05°C and 140.8kJ/kg at 17.11°C, respectively. Also, the results of thermal gravimetric analysis (TG) suggested that the coated composite PCM had good thermal stability in the working temperature range from 25 to 50°C. Furthermore, the enthalpy loss of melting and freezing was negligible after 100 cycles, indicating its good thermal reliabilities. The thermal conductivity of the coated composite PCM can be as high as 3.643W/(mK). According to the obtained results, the coated hydrated salts/EG composite PCM enjoys high latent heat, good thermal reliability and high thermal conductivity. Apart from its favorable thermal properties, the cost of the coated hydrated salts/EG composite PCM was quite low, making it promising for low temperature thermal energy storage applications.

Preparation of microencapsulated medium temperature phase change material of Tris(hydroxymethyl)methyl aminomethane@SiO2 with excellent cycling performance

Abstract Polyalcohol is one of the important medium temperature phase change materials (PCMs) for thermal energy storage, in which liquid-free solid–solid PCMs are more promising although sublimation can hardly be avoided when crystals transform to the “plastic phase”. We reported here the preparation of encapsulated Tris(hydroxymethyl)methyl aminomethane (Tris) with the SiO2 shell by sol–gel process. Through the hydrolysis and polycondensation of the bicomponent silicon precursors mixed with tetraethoxysilane (TEOS) and 3-aminopropyl triethoxysilane (APTS) of proper ratio, Tris@SiO2 microcapsules with excellent sealing property can be obtained, which prevent the leakage of Tris outside the capsules. Besides, silica encapsulation doubles the thermal conductivity of the PCMs to 0.478 W/(m K). The DSC measurement shows that the Tris@SiO2 capsule owns a reversible phase change in the range of 110–155 °C, which will stabilize at about 146 J/g during 70 times of thermal cycling. The Tris@SiO2 capsules have good potential for medium temperature thermal energy storage applications.

Sodium sulfate–diatomite composite materials for high temperature thermal energy storage

This work explores the use of sodium sulfate and diatomite to formulate composite materials for high temperature thermal energy storage applications. Sodium sulfate in the composite functions as a phase change material (PCM) and diatomite as a structural skeleton for shape stabilization. It is found that sodium sulfate and diatomite have an excellent chemical compatibility with the PCM melting temperature at around 880°C. It is shown that the composite containing 45% diatomite gives an optimal formulation in terms of energy density, salt leakage and mechanical strength. The results also suggest that the composite with the optimal formulation has an application window of 890–980°C. Failures occur to the composite materials at temperatures above 1000°C.

Preparation, thermal properties and thermal reliability of eutectic mixtures of fatty acids/expanded vermiculite as novel form-stable composites for energy storage

This paper deals with the preparation, characterization, thermal properties and thermal reliability of novel form-stable composite phase change materials (PCMs) composed of eutectic mixtures of fatty acids and expanded vermiculite for thermal energy storage. The form-stable composite PCMs were prepared by incorporation of eutectic mixtures of fatty acids (capric–lauric, capric–palmitic and capric–stearic acids) within the expanded vermiculite by vacuum impregnation method. The composite PCMs were characterized by SEM and FTIR techniques. Thermal properties of the composite PCMs were determined by differential scanning calorimeter (DSC) method. DSC results showed that the melting temperatures and latent heats of the prepared composite PCMs are in the range of 19.09–25.64 °C and 61.03–72.05 J/g, respectively. The thermal cycling test including 5000 heating and cooling process was conducted to determine the thermal reliability of the composite PCMs. The test results showed that the composite PCMs have good thermal reliability and chemical stability. Furthermore, thermal conductivities of the composite PCMs were increased by adding 10 wt% expanded graphite. Based on all results, the prepared form-stable composites can be considered as promising PCMs for low temperature thermal energy storage applications due to their satisfactory thermal properties, good thermal reliability, chemical stability and thermal conductivities.

Thermal Energy Storage Characteristics of Myristic and Stearic Acids Eutectic Mixture for Low Temperature Heating Applications

Stearic acid (67.83°C) and myristic acid (52.32°C) have high melting temperatures that can limit their use as phase change material (PCM) in low temperature solar heating applications such as solar space and greenhouse heating in regard to climatic requirements. However, their melting temperatures can be adjusted to a suitable value by preparing a eutectic mixture of the myristic acid (MA) and the stearic acid (SA). In the present study, the thermal analysis based on differential scanning calorimetry (DSC) technique shows that the mixture of myristic acid (MA) and stearic acid (SA) in the respective composition (by mass) of 64% and 36% forms a eutectic mixture having melting temperature of 44.13°C and the latent heat of fusion of 182.4J.g −1. The thermal energy storage characteristics of the MA-SA eutectic mixture filled in the annulus of two concentric pipes were also experimentally established. The heat recovery rate and heat charging/discharging fractions were determined with respect to the change in the mass flow rate and the inlet temperature of heat transfer fluid. Based on the results obtained by DSC analysis and by the heat charging/discharging processes of the PCM, it can be concluded that the MA-SA eutectic mixture is a potential material for low temperature thermal energy storage applications in terms of its thermo-physical and thermal characteristics.

Lauric and palmitic acids eutectic mixture as latent heat storage material for low temperature heating applications

Palmitic acid (PA, 59.8 °C) and lauric acid (LA, 42.6 °C) are phase change materials (PCM) having quite high melting temperatures which can limit their use in low temperature solar applications such as solar space heating and greenhouse heating. However, their melting temperatures can be tailored to appropriate value by preparing a eutectic mixture of the lauric and the palmitic acids. In the present study, the thermal analysis based on differential scanning calorimetry (DSC) technique shows that the mixture of 69.0 wt% LA and 31 wt% PA forms a eutectic mixture having melting temperature of 35.2 °C and the latent heat of fusion of 166.3 J g −1. This study also considers the experimental determination of the thermal characteristics of the eutectic mixture during the heat charging and discharging processes. Radial and axial temperature distribution, heat transfer coefficient between the heat transfer fluid (HTF) pipe and the PCM, heat recovery rate and heat charging and discharging fractions were experimentally established employing a vertical concentric pipe-in-pipe energy storage system. The changes of these characteristics were evaluated with respect to the effect of inlet HTF temperature and mass flow rate. The DSC thermal analysis and the experimental results indicate that the LA–PA eutectic mixture can be a potential material for low temperature thermal energy storage applications in terms of its thermo-physical and thermal characteristics.