Exergy analysis of two phase change materials storage system for solar thermal power with finite-time thermodynamics

A mathematical model for the overall exergetic efficiency of two phase change materials named PCM1 and PCM2 storage system with a concentrating collector for solar thermal power based on finite-time thermodynamics is developed. The model takes into consideration the effects of melting temperatures and number of heat transfer unit of PCM1 and PCM2 on the overall exergetic efficiency. The analysis is based on a lumped model for the PCMs which assumes that a PCM is a thermal reservoir with a constant temperature of its melting point and a distributed model for the air which assumes that the temperature of the air varies in its flow path. The results show that the overall exergetic efficiency can be improved by 19.0-53.8% using two PCMs compared with a single PCM. It is found that melting temperatures of PCM1 and PCM2 have different influences on the overall exergetic efficiency, and the overall exergetic efficiency decreases with increasing the melting temperature of PCM1, increases with increasing the melting temperature of PCM2. It is also found that for PCM1, increasing its number of heat transfer unit can increase the overall exergetic efficiency, however, for PCM2, only when the melting temperature of PCM1 is less than 1150 K and the melting temperature of PCM2 is more than 750 K, increasing the number of heat transfer unit of PCM2 can increase the overall exergetic efficiency. Considering actual application of solar thermal power, we suggest that the optimum melting temperature range of PCM1 is 1000-1150 K and that of PCM2 is 750-900 K. The present analysis provides theoretical guidance for applications of two PCMs storage system for solar thermal power.     

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.     

Micro-encapsulated paraffin/high-density polyethylene/wood flour composite as form-stable phase change material for thermal energy storage

Six novel polymer-based form-stable composite phase change materials (PCMs), which comprise micro-encapsulated paraffin (MEP) as latent heat storage medium and high-density polyethylene (HDPE)/wood flour compound as supporting material, were prepared by blending and compression molding method for potential latent heat thermal energy storage (LHTES) applications. Micro-mist graphite (MMG) was added to improve thermal conductivities. The scanning electron microscope (SEM) images revealed that the form-stable PCMs have homogeneous constitution and most of MEP particles in them were undamaged. Both the shell of MEP and the matrix prevent molten paraffin from leakage. Therefore, the composite PCMs are described as form-stable PCMs. The differential scanning calorimeter (DSC) results showed that the melting and freezing temperatures as well as latent heats of the prepared form-stable PCMs are suitable for potential LHTES applications. Thermal cycling test indicated the form-stable PCMs have good thermal stability although it was subjected to 100 melt–freeze cycles. The thermal conductivity of the form-stable PCM was increased by 17.7% by adding 8.8 wt% MMG. The results of mechanical property test indicated that the addition of MMG has no negative influence on the mechanical properties of form-stable composite PCMs. Taking one with another, these novel form-stable PCMs have the potential for LHTES applications in terms of their proper phase change temperatures, improved thermal conductivities, outstanding leak tightness of molten paraffin and good mechanical properties.     

基于火积理论的多级套管式相变蓄热系统优化

本文基于最小火积耗散热阻原理,在考虑相变材料导热热阻以及非稳态传热过程的基础上,对多级套管式相变蓄热系统的融化温度进行了数值优化,获得了最优融化温度分布。在此基础上,研究了相变材料导热系数和传热管长度对最优融化温度、火积耗散热阻和平均蓄热速率的影响。研究结果表明,与现有理论优化方法相比,本文提出的数值优化方法具有更好的适用性;优化后多级套管式相变蓄热系统可有效提高相变蓄热系统的平均蓄热速率,降低火积耗散热阻;随着相变材料导热系数增大和传热管长度增加,多级套管式相变蓄热系统最优融化温度的温差愈加明显,其强化传热性能呈上升趋势。     

Free cooling of a building using PCM heat storage integrated into the ventilation system

This article presents a study of the free cooling of a low-energy building using a latent-heat thermal energy storage (LHTES) device integrated into a mechanical ventilation system. The cylindrical LHTES device was filled with spheres of encapsulated RT20 paraffin, a phase-change material (PCM). A numerical model of the LHTES was developed to identify the parameters that have an influence on the LHTES’s thermal response, to determine the optimum phase-change temperature and to form the LHTES’s temperature-response function. The last of these defines the LHTES’s outlet-air temperature for a periodic variation of the inlet ambient-air temperature and the defined operating conditions. The temperature-response function was then integrated into the TRNSYS building thermal response model. Numerical simulations showed that a PCM with a melting temperature between 20 and 22 °C is the most suitable for free cooling in the case of a continental climate. The analyses of the temperatures in a low-energy building showed that free cooling with an LHTES is an effective cooling technique. Suitable thermal comfort conditions in the presented case-study building could be achieved using an LHTES with 6.4 kg of PCM per square metre of floor area.     

Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material

This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain form-stable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using transient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid–liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temperatures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values calculated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity.     

The effect of different inlet geometries on laminar flow combined convection heat transfer inside a horizontal circular pipe

The effect of different inlet geometries on laminar air flow combined convection heat transfer inside a horizontal circular pipe has been experimentally investigated for Reynolds number range of 400–1600, and the Grashof number range from 3.12 × 10⁵ to 1.72 × 10⁶. The experimental setup consists of an aluminum circular pipe as a heated section with 30 mm inside diameter and 900 mm heated length (L/D = 30) with different inlet geometries. A wall boundary heating condition of a uniform heat flux was imposed. The inlet configurations used in this paper are calming sections having the same inside diameter as the heated pipe but with variable lengths of L_calm. = 600 mm (L/D = 20), L_calm. = 1200 mm (L/D = 40), L_calm. = 1800 mm (L/D = 60), L_calm. = 2400 mm (L/D = 80), sharp-edged and bell-mouth. It was found that the surface temperature values for calming section length corresponding to (L/D = 80) were higher than other inlet geometries due to the lower mass flow rate and higher flow resistance. It was also observed that the Nusselt number values for bell-mouth inlet geometry were higher than other inlet geometries due to the differences in the average temperatures and densities of the air. The average heat transfer results were correlated with an empirical correlation in terms of dependent parameters of Grashof, Prandtl and Reynolds numbers. The proposed correlation was compared with available literature and it shows reasonable agreement.     

Heat transfer and exergy analysis of cascaded latent heat storage with gravity-assisted heat pipes for concentrating solar power applications

A thermal network model is developed to predict the performance of latent heat thermal energy storage (LHTES) systems including cascaded phase change materials (PCMs) and embedded heat pipes/thermosyphons. Because the design of LHTES systems involves a compromise between the amount of energy stored, the heat transfer rate, and the quality of the released thermal energy, an exergy analysis is also carried out to identify the preferred LHTES design. It is found that the LHTES with the lowest melting temperature PCM yields the highest exergy efficiency. However, a cascaded LHTES recovers the largest amount of exergy during a 24 h charging–discharging cycle. Quantitatively, the cascaded LHTES recovers about 10% more exergy during a 24 h charging–discharging cycle compared to the best non-cascaded LHTES considered in this work.     

Thermodynamics and kinetics analysis of hydrogen absorption in large-scale metal hydride reactor coupled to phase change material-metal foam-based latent heat storage system

Phase change materials (PCMs) have recently been coupled with metal hydride storage tanks (MHSTs) to store adsorption heat and subsequently deliver it for hydrogen desorption through melting and solidification cycles. This method might reduce process costs by eliminating the use of HTF (i.e. heat transfer fluid). However, thermodynamics and kinetic data are scarce for large-scale MH-PCM applications, particularly when PCM is loaded in metal foams (MFs) to promote heat transfer. The current work aimed to develop a 2D model for simulating H₂-absorption in a LaNi₅-metal bed integrating a PCM-MF unit in a large-scale tube-and-shell heat exchanger. The constructed model (via Fluent 15.0 CFD-platform) was first-validated using referenced experimental data. The resulting heat transfer was analyzed for different MFs [aluminum, copper, nickel and titanium] of different porosities (0.1–1.0). Excellent outcomes were retrieved. By trapping the H₂-absorption heat, the MF-PCM unit improved the LaNi₅ hydriding. The LaNi₅ charging was achieved after ～500 s, independently of the MF type and porosity. The PCM melting rate depends on tube position, porosity and the MF type. It increased with MFs incorporation (order of enhancement: Cu > Al > Ni > Ti) and MF-porosity decrease (from ε = 100% to 10%). Besides, the PCM tube above the H₂-feeding pipe melts more quickly than the other tubes, presumably to the gravitational-force effect. Longer times (i.e. 9 000 s to >16 000 s, depending on tube position) were recorded for complete melting of the PCM when ε_MF = 100%; however, when ε_MF is less than 80%, the required time for total melting was tremendously reduced to less than 500 s. Nevertheless, the MFs porosity could not be decreased considerably to avoid a huge loss of material storage (PCM), thereby diminishing the thermal storage performance of the PCM matrix.     

Experimental and numerical investigation of the melting process and heat transfer characteristics of multiple phase change materials

The melting and heat transfer characteristics of multiple phase change materials (PCMs) are investigated both experimentally and numerically. Multiple PCMs, which consist of three PCMs with different melting points, are filled into a rectangle-shaped cavity to serve as heat storage unit. One side of the cavity is set as heating wall. The melting rate of multiple PCMs was recorded experimentally and compared with that of single PCM for different heating temperatures. A two-dimensional mathematical model to describe the phase change heat transfer was developed and verified experimentally. The properties of multiple PCMs, including the effect of the melting point difference (combined type), thermal conductivity, and latent heat, on the heat transfer performance of the PCM were analyzed numerically. The results show that, the melting time decreases before it increases, with an increasing melting point difference for the multiple PCMs. In addition, the melting point decreases with increasing distance from the heating wall. Most of these types of multiple PCMs melt faster than the single PCM, and the multiple PCMs, with the melting point arranged as 322 K/313 K/304 K, has the shortest melting time in this study. The melting rate of the multiple PCMs, 322 K/313 K/304 K, accelerates faster than for the single PCM as the thermal conductivity, latent heat, and heating wall temperature increase. Finally, generalized results are obtained using a dimensionless analysis for both single and multiple PCMs.     

Characterization of melting and solidification in a real-scale PCM–air heat exchanger: Experimental results and empirical model

This paper describes the experimental studies carried out to test thermal cycling of a real-scale PCM–air heat exchanger at ambient temperatures. To achieve this goal an experimental setup previously designed and used for testing real-scale prototypes of PCM–air heat exchangers is modified. The PCM used is commercially available, organic, and paraffin based. The total energy exchanged during melting and solidification, as well as the time elapsed until total melting/solidification are determined from the power curves experimentally obtained. The influence of the inlet air temperature and air flow is studied, and results show that the continuous thermal cycling of the unit is a repetitive process: running experiments with similar conditions leads to the same thermal behavior, no degradation in the PCM properties is noticed. Pressure drop is measured for different air flows. Depending on the inlet air temperature, full solidification of the PCM could be achieved in less than 3 h for an 8 °C temperature difference between the inlet air and the average phase change of the PCM. Average thermal powers of up to 4.5 kW and 3.5 kW for 1 h are obtained for melting and solidification stages, respectively. An empirical model is developed from the experimental results, which could be a useful designing tool for applications that use such technology: green housing, curing and drying processes, plant production, HVAC, and free-cooling.     

Numerical study on thermal energy storage performance of phase change material under non-steady-state inlet boundary

Due to the solar radiation intensity variation over time, the outlet temperature or mass flow rate of heat transfer fluid (HTF) presents non-steady-state characteristics for solar collector. So, in the phase change thermal energy storage (PCTES) unit which is connected to solar collector, the phase change process occurs under the non-steady-state inlet boundary condition. In present paper, regarding the non-steady-state boundary, based on enthalpy method, a two dimensional physical and mathematical model for a shell-and-tube PCTES unit was established and the simulation code was self-developed. The effects of the non-steady-state inlet condition of HTF on the thermal performance of the PCTES unit were numerically analyzed. The results show that when the average HTF inlet temperature in an hour is fixed at a constant value, the melting time (time required for PCM completely melting) decreases with the increase of initial inlet temperature. When the initial inlet temperature increases from 30 °C to 90 °C, the melting time will decrease from 42.75 min to 20.58 min. However, the total TES capacity in an hour reduces from 338.9 kJ/kg to 211.5 kJ/kg. When the average inlet mass flow rate in an hour is fixed at a constant value, with the initial HTF inlet mass flow rate increasing, the melting time of PCM decreases. The initial inlet mass flow rate increasing from 2.0 × 10⁻⁴ kg/s to 8.0 × 10⁻⁴ kg/s will lead to the melting time decreasing from 37.42 min to 23.75 min and the TES capacity of PCM increasing from 265.8 kJ/kg to 273.8 kJ/kg. Under all the studied cases, the heat flux on the tube surface increases at first, until it reaches a maximum then it decreases over time. And the larger the initial inlet temperature or mass flow rate, the earlier the maximum value appearance and the larger the maximum value.     

Numerical investigation of the thermal mechanism of the solid-liquid phase changing process

由于相变换热储能技术可以协调能量在时间和空间尺度的分配,成为了目前研究的热点问题。本工作用焓值法分别对充填低温无机盐相变材料的二维和三维管壳式相变储能换热器模型的储/放热特性进行了模拟研究,采用Boussinesq近似研究了液相区密度变化引起的自然对流的影响。研究表明换热器的入口温度对相变换热效率影响显著;在储热过程中自然对流发挥了重要作用,换热效率与液相区的运动状态直接相关,而放热过程中的热交换主要依靠热传导完成;三维模拟的结果表明换热管出口温度与管壁的平均努赛尔数高度相关,且换热管水平放置的换热效率略低于竖直放置。     

Preparation and characterization of nano‐enhanced myristic acid using metal oxide nanoparticles for thermal energy storage

Fatty acids have been broadly used as phase change materials (PCMs) for thermal energy storage. However, low thermal conductivity limits their performances. This paper investigates the influence of metal oxide nanoparticle addition on myristic acid (MA) as nano‐enhanced PCM (NEPCM). Stability, chemical, and thermal properties were considered. Four types of nanoaprticles, TiO₂, CuO, Al₂O₃, and ZnO, were dispersed in MA at 0.1, 0.5, 1, and 2 wt%. Stability and dispersion were checked by sediment photograph capturing and scanning electron microscopy/energy‐dispersive spectroscopy. The Fourier‐transformed infrared (FTIR) and X‐ray diffraction analysis confirmed no chemical interaction between the nanoparticles and MA. The results revealed a ratio of thermal conductivity of 1.50, 1.49, 1.45, and 1.37, respectively, for 2 wt% of ZnO, Al₂O₃, CuO, and TiO₂. The T‐history method confirmed this enhancement. The latent heat thermal energy storage (LHTES) properties of the nano‐enhanced MA were evaluated using differential scanning calorimetry. The latent heat capacities of nano‐enhanced MA samples have dropped between 9.64 and 5.01 % compared with pure MA, and phase change temperature range was not affected significantly. The NEPCM was subjected to 500 thermal cycling, it showed a good thermal reliability as LHTES properties remained unchanged, while FTIR analysis showed similar characteristics compared with uncycled samples, indicating a good chemical stability. Based on the results regarding with the LHTES properties, cycling thermal reliability, and higher thermal conductivity improvement, it can be achieved that the MA/Al₂O₃ (2.0 wt%) and MA/ZnO (2.0 wt%) composites could be better PCMs for solar TES 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 characteristics of manganese (II) nitrate hexahydrate as a phase change material for cooling systems

The imbalance of electrical demand in summer due to cooling system demand is a big problem in many countries. One promising solution is shifting peak demand from early afternoon to night by utilizing natural cold energy resources such as cool outside air during night or running a refrigerator driven by midnight power. In these cases, using the thermal energy storage (TES) of phase change material (PCM) which has a melting point from 15 to 25 °C is one of the most effective ideas. However, few suitable PCMs for this temperature range are at present commercially available. This study aims to evaluate the potential of Mn(NO₃)₂ · 6H₂O (manganese (II) nitrate hexahydrate) as a new PCM for the TES of cooling systems. First, experiments on the modulation of the melting point of Mn(NO₃)₂ · 6H₂O and reduction of supercooling were made by dissolving small amounts of salts in the material. Consequently, MnCl₂ · 4H₂O was found to have good performance with regard to both modulation of the melting temperature and the heat of fusion. Next, a thermal response test was carried out by using a small cylindrical vessel. Results showed that the required temperature levels for charging and discharging the heat of this mixture were clarified. In addition, the price and safety of this material as a PCM are discussed.     

Experimental and numerical investigations of hydrogen–air premixed combustion in a converging–diverging micro tube

Experimental and numerical studies of hydrogen–air premixed combustion in a converging–diverging micro tube with inner diameters of the inlet, throat, and outlet of 2, 1, and 2 mm, respectively, have been performed to study the combustion and flame characteristics. The influences of the equivalence ratio (Φ) and inlet velocity (v_in) are investigated. The experiments reveal that the v_in range for stable combustion—between 3.4 and 41.4 m/s—was significantly expanded, particularly when Φ = 1.4. This effect can primarily be attributed to the converging–diverging structure. As Φ increased, both the wall and the flame temperatures exhibited an increasing–decreasing trend; the largest heat loss ratio occurred at Φ = 1.0. The ignition position initially moved upstream and then moved downstream. The flame thickness increased and then decreased, reaching its peak value at Φ = 1.2. The flame length decreased monotonously. As v_in increased, the wall temperature increased, the flame temperature decreased, and the flame moved downstream to grow thicker and longer.     

Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement

The present paper describes the analysis of the melting process in a single vertical shell‐and‐tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid‐dynamic (CFD) model that takes into account of the phase‐change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase‐change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase‐change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano‐enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano‐enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detected. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of inserted baffles on hydrogen heterogeneous reaction in planar catalytic micro combustor

Heterogeneous reaction characteristics of premixed H₂/Air mixture are numerically investigated in micro combustor with coating platinum (Pt) catalyst on the inner wall. The well-designed combustor with the inserted baffle is committed to improving the transport of bulk species on the catalytic surface, therefore enhancing the fuel conversion ratio. A two-dimensional numerical model with detailed heterogeneous reaction mechanism is developed and verified. In this work, the numerical results reveal that the heterogeneous reaction rate and fuel conversion ratio are significantly improved in the combustor with inserted baffle. The velocity of gaseous mixture swiftly increases between the baffle and catalytic surface, resulting in the increasing adsorption-desorption of bulk species on the catalytic surface. The main influencing factors of inserted baffle body comprise of slit width between two ribs in the same row (W), rib length (L), distance between adjacent baffles row (D), number of baffle row (n) which are studied to obtain a set of optimized parameters of baffle in the well-designed combustor. The optimized parameters of the baffle are W = 0 mm, L = 0.4 mm, D = 4 mm and n = 4, which are employed in the combustor to obtain a high hydrogen conversion ratio. Moreover, the effect of different inlet velocities and equivalent ratios of the mixture on the heterogeneous reaction characteristics are carried out in the designed combustor. As the designed combustor performs well, and the improvement effect of the optimized baffle gradually increases together with the inlet velocity, particularly for high inlet velocity (7 m/s). The promotion effects of baffle in the designed combustor are presented when the rich and lean fuel are adopted at inlet velocity of 7 m/s. Finally, the relative parameters of this work can serve as a reference and guidance for the design of micro scale combustor.     

Thermal behavior of paraffin-nano-Al2O3 stabilized by sodium stearoyl lactylate as a stable phase change material with high thermal conductivity

A new kind of nano composite phase change material (NCPCM) was prepared using sodium stearoyl lactylate (SSL) as a surfactant to improve the dispersion of the Al₂O₃ nano-particles (with 2.5, 5, 7.5, and 10 wt.%) in paraffin with a SSL/Al₂O₃ mass ratio of 1:3.5. To evaluate the efficiency of the prepared PCMs, the melting rate of them at a temperature range of 50–60 °C and the effective thermal conductivity values in the solid and liquid states at a temperature range of 25–75 °C were measured using the k-type thermocouple and the transient hot wire technique, respectively. The heat storage behavior of the samples was investigated and their melting temperature, latent heat, and thermal reliability were determined using differential scanning calorimetry (DSC). Results showed that effective thermal conductivity enhancement ratios for the sample containing 10.0 wt.% nano-Al₂O₃ were 31% and 13% in the solid and liquid states, respectively, which are higher than those reported in similar studies. In addition, melting rate increased by 27%. As expected, all the PCMs showed good thermal reliability after 120 melting/freezing cycles. Based on our results, it may be concluded that the prepared PCMs can be regarded as effective heat storage materials for application in energy storage systems.