Thermal characteristics of a eutectic mixture of myristic and palmitic acids as phase change material for heating applications

The melting temperatures of the palmitic acid (PA) (61.2 °C) and the myristic acid (MA) (52.2 °C) are high for solar thermal energy storage under the climate conditions of the some regions in Turkey. However, their melting temperatures can be adjusted to a suitable value by addition of MA to PA. In this study, the thermal analysis by differential scanning calorimetry technique shows that the mixture of MA and PA with 58.0 wt% MA forms a eutectic mixture with melting temperature of 42.6 °C and the latent heat of fusion of 169.7 J g⁻¹. The melting temperature and latent heat of fusion of MA–PA eutectic mixture make it a suitable material for energy storage in passive solar space and greenhouse heating systems under the climate conditions of some residential and agricultural regions in Turkey. The present study also deals with the experimental establishment of thermal characteristics of the eutectic mixture as a phase change material in a vertical concentric two pipes––energy storage system. Reynolds and Stefan numbers were selected as experimental parameters and used for the evaluation of the results. The experimental results prove that the MA–PA eutectic mixture has attractive thermal characteristics during the melting and the solidification processes and it is a potential latent heat storage material for heating applications with respect to the climate requirements in Turkey.     

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.     

Microencapsulation of coco fatty acid mixture for thermal energy storage with phase change material

Thermal energy storage systems provide several alternatives for efficient energy use and energy conservation. Microcapsules of natural coco fatty acid mixture were prepared to be used as phase change materials for thermal energy storage. The coacervation technique was used for the microencapsulation process. Several alternatives for the capsule wall material were tried. The microcapsules were characterized according to their geometric profiles, phase transition temperatures, mean particle sizes, chemical stabilities, and their thermal cycling. The diameters of microcapsules prepared in this study were about 1 mm. Coco fatty acid mixtures have kept their geometrical profiles even after 50 thermal cycles for melting and freezing operations in temperature range from 22 to 34°C. It was found that gelatin+gum Arabic mixture was the best wall material for microencapsulating coco fatty acid mixtures. Copyright © 2005 John Wiley & Sons, Ltd.     

Novel inorganic binary mixture for low‐temperature heat storage applications

In this study, an inorganic mixture based on bischofite (industrial by‐product) was developed and characterized for its application as a phase change material for low‐temperature thermal energy storage. The most appropriate composition was established as 40 wt% bischofite and 60 wt% Mg(NO₃)₂ · 6H₂O. Thermophysical properties were defined and compared with those of the mixture with synthetic MgCl₂ · 6H₂O. The heat of fusion and melting temperature were measured as 62.0°C and 132.5 kJ kg^?1 for the mixture with MgCl₂ · 6H₂O and 58.2°C and 116.9 kJ kg^?1 for the mixture with bischofite. The specific heat capacity values, cycling, and thermal stability for both mixtures were also determined. For the mixture with MgCl₂ · 6H₂O, the densities of the solid and liquid states were 1517 kg m^?3 (ambient temperature) and 1515 kg m^?3 (at 60‐70°C), respectively. For the mixture with bischofite, the densities of the solid and liquid states were 1525 kg m^?3 (ambient temperature) and 1535 kg m^?3 (at 60‐70°C), respectively. Both mixtures show supercooling of about 23.4 and 34.1°C for the mixture with bischofite and MgCl₂ · 6H₂O, respectively. In addition, it was shown that supercooling may be reduced by increasing the quantity of material tested. Thereby, it was established that an inorganic mixture based on bischofite is a promising PCM for low‐temperature thermal energy storage applications.     

Fatty acid eutectic/polymethyl methacrylate composite as form-stable phase change material for thermal energy storage

This work is focused on the preparation and characterization of fatty acid eutectic/polymethyl methacrylate (PMMA) form-stable phase change material (PCM). Capric acid (CA), lauric acid (LA), myristic acid (MA) and stearic acid (SA) were selected to prepare binary fatty acid eutectic for the sake of decreasing the phase change temperature. Using the method of self-polymerization, CA–LA, CA–MA, CA–SA and LA–MA eutectics acting as the heat-absorbing materials and PMMA serving as the supporting material were compounded in the ratio of 50/50 wt.%. The relations between mass fraction of LA–MA eutectic and latent heat and compressive strength of LA–MA/PMMA composite were discussed, and the feasible maximum mass fraction of LA–MA eutectic was determined to be 70%. CA–LA/PMMA, CA–MA/PMMA, CA–SA/PMMA and LA–MA/PMMA composites were examined to investigate their potential application in building energy conservation. Scanning electron microscope and polarizing optical microscope observations showed that fatty acid eutectic was coated by PMMA thus the composite remained solid when the sample was heated above the melted point of the fatty acid. Fourier-transform infrared results indicated that fatty acid and PMMA had no chemical reaction and exhibited good compatibility with each other. According to the differential scanning calorimetry results, phase change temperatures of CA–LA/PMMA, CA–MA/PMMA, CA–SA/PMMA and LA–MA/PMMA composites were 21.11 °C, 25.16 °C, 26.38 °C and 34.81 °C and their latent heat values were determined to be 76.3 kJ/kg, 69.32 kJ/kg, 59.29 kJ/kg and 80.75 kJ/kg, respectively. Moreover, thermal stability and expansibility of the form-stable PCMs were characterized by thermogravimetric analysis and volume expansion coefficient respectively, and the results indicated that the composites were available for building energy conservation.     

Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system

The phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as phase change material (PCM) during the melting and solidification processes were determined experimentally in a vertical two concentric pipes energy storage system. This study deals with three important subjects. First is determination of the eutectic composition ratio of the palmitic acid (PA) and stearic acid (SA) binary system and measurement of its thermophysical properties by differential scanning calorimetry (DSC). Second is establishment of the phase transition characteristics of the mixture, such as the total melting and solidification temperatures and times, the heat transfer modes in the melted and solidified PCM and the effect of Reynolds and Stefan numbers as initial heat transfer fluid (HTF) conditions on the phase transition behaviors. Third is calculation of the heat transfer coefficients between the outside wall of the HTF pipe and the PCM, the heat recovery rates and heat fractions during the phase change processes of the mixture and also discussion of the effect of the inlet HTF parameters on these characteristics. The DSC results showed that the PA–SA binary system in the mixture ratio of 64.2:35.8 wt% forms a eutectic, which melts at 52.3 °C and has a latent heat of 181.7 J g⁻¹, and thus, these properties make it a suitable PCM for passive solar space heating and domestic water heating applications with respect to climate conditions. The experimental results also indicated that the eutectic mixture of PA–SA encapsulated in the annulus of concentric double pipes has good phase change and heat transfer characteristics during the melting and solidification processes, and it is an attractive candidate as a potential PCM for heat storage in latent heat thermal energy storage systems.     

A review of intercalation composite phase change material: Preparation, structure and properties

This paper reviews preparation, structure and properties of the intercalation composite phase change material (PCM). The layered structure of clay and graphite is utilized to prepare the intercalation composite PCM. It is concluded that the preparation methods include liquid phase intercalation and melting intercalation. The thermal conductivity and flame retardancy of organic PCM are improved by intercalating organic PCM into montmorillonite (MMT) or graphite. The phase change properties of the intercalation composite PCM can be measured by differential scanning calorimetry (DSC) and T-history method.     

Microencapsulation of phase change material with poly(ethylacrylate) shell for thermal energy storage

Microcapsules containing caprylic acid and polyethylacrylate shells were prepared using an emulsion polymerization technique for thermal energy storage applications. Ethylene glycol dimethacrylate was used as a crosslinking agent. The influence of the crosslinking agent concentration on the phase change properties of microcapsules was examined. The caprylic acid microcapsules (MicroPCMs) were analyzed by Fourier transform infrared spectroscopy, thermal gravimetric analysis, scanning electron microscopy, and differential scanning calorimetry. The results showed that microcapsules were synthesized successfully and that the best shell material:crosslinking agent concentration ratio was 1:0.2. The melting and freezing temperatures were measured through differential scanning calorimetry analysis and found to be 13.3 and 7.1°C, respectively. The melting and crystallization heats were determined to be 77.3 and ?77.0 kJ/kg, and the mean particle diameter was 0.64 μm. The thermal cycling tests of the microcapsules were performed for 400 heating/cooling cycles, and the results indicate that the synthesized microcapsules have good thermal reliabilities. Air stability test proved that the thermal properties and physical form of microcapsules were not affected by air. We recommend the prepared thermal, air, and chemically stable caprylic acid microcapsules for thermal energy storage applications as novel microPCM with latent heat storage capacities and properties. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermal buffering effect of a packaging design with microencapsulated phase change material

Temperature fluctuations during storage and transportation are the most important factors affecting quality and shelf life of food products. Phase change materials (PCM) with their isothermal characteristics are used to control temperature in various thermal operations. In this study, octanoic acid as PCM candidate was used in a packaging material design for thermal control of a food product. The PCM candidate was microencapsulated in different shell materials in our laboratory. Among the synthesized microcapsules, microencapsulated PCM (mPCM) (ΔHm = 42.9 J/g) with styrene polymer as the shell material was selected based on its properties of being cost effective and compatibility with human health. Thermal buffering effect of PCM in bulk and microencapsulated forms was tested in a packaging design with special PCM pockets. Results showed that packages with mPCM and bulk PCM provided 8.8 and 6 hours of thermal buffering effect for 160 g of chocolate compared with the package without PCM (reference package).     

Experimental study on performance of square tube absorber with phase change material

The intermittent nature of solar radiation has decreased the performance efficiency of solar heaters. Integrating the solar heater with thermal energy storage component could increase its performance effectively. In this article, an investigation on the effect of phase change material (PCM) as the thermal energy storage component on the performance of square aluminum tube was carried out experimentally. In the first phase, the temperature behavior of square aluminum tube with two types of PCM, namely, generic plant-based PCM (A2) and paraffin wax (A3), was compared with square aluminum tube without PCM (A1). In the second phase, the performance of square aluminum tube was investigated with different paraffin wax masses of 38 g (B1), 48 g (B2), and 58 g (B3). Based on the result, the A3 tube configuration performed better than A1 and A2 tube configurations with higher heat gain rate (0.08°C/s) and lower heat discharge rate (−0.04°C/s). The B2 tube configuration was found to have maximum heat gain of 3.73 kJ with higher heat discharge rate as compared with other square tube configurations. The average temperature difference between internal and external surface tube of B2 was lower (4.3°C) leading to higher average temperature difference at ambient temperature of 25.3°C. Instantaneous efficiency of the tube B2 is higher than the B1 and B3 tube configurations by 16% and 26%, respectively. The result suggests that the insertion of paraffin wax inside the square absorber tube improves the temperature response of the absorber in the situation of intermittent solar radiation.     

RETRACTED: Preparation and thermal properties of form-stable paraffin phase change material encapsulation

Form-stable paraffin phase change materials (PCMs), in which the paraffin as a latent heat storage material and the polyolefins as a supporting material, have to be encapsulated because of the paraffin leakage and lipophilicity. A novel microencapsulated PCM in which form-stable paraffin is encapsulated into inorganic silica gel polymer is prepared successfully by in situ polymerization. A differential scanning calorimeter (DSC) is used to measure the thermal properties of the PCM. Moreover, the Washburn equation associated with the wetting properties of powder materials, is used to test the hydrophilic–lipophilic properties of PCM. The results indicate that the optimum microencapsulated PCM is endowed with good hydrophilicity, and its specific enthalpy maintains 123.78 J g⁻¹.     

Development of a ventilation system utilizing thermal energy storage for granules containing phase change material

We have developed an experimental ventilation system that features direct heat exchange between ventilation air and granules containing a phase change material (PCM). Measurement of outlet air temperature when the inlet air temperature was periodically varied to simulate changes of outdoor ambient air temperature showed that the outlet air temperature was stabilized and remained within the phase change temperature range. This effect is expected to be useful in practical ventilation systems. The potential of such systems for reducing ventilation load was examined through computer simulation for eight representative cities of Japan. This revealed how different temperature conditions would affect required heat storage capacity.     

Performance analysis of a latent heat storage system with phase change material for new designed solar collectors in greenhouse heating

The continuous increase in the level of greenhouse gas emissions and the rise in fuel prices are the main driving forces behind the efforts for more effectively utilize various sources of renewable energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. In this study, the thermal performance of a phase change thermal storage unit is analyzed and discussed. The storage unit is a component of ten pieced solar air collectors heating system being developed for space heating of a greenhouse and charging of PCM. CaCl₂6H₂O was used as PCM in thermal energy storage with a melting temperature of 29 °C. Hot air delivered by ten pieced solar air collector is passed through the PCM to charge the storage unit. The stored heat is utilized to heat ambient air before being admitted to a greenhouse. This study is based on experimental results of the PCM employed to analyze the transient thermal behavior of the storage unit during the charge and discharge periods. The proposed size of collectors integrated PCM provided about 18–23% of total daily thermal energy requirements of the greenhouse for 3–4 h, in comparison with the conventional heating device.     

Experimental study on the phase change behavior of phase change material confined in pores

An experimental study on the phase change behavior of organic phase change materials (PCMs) in porous building materials is reported. Three kinds of porous materials and two kinds of PCMs were used. The phase change behavior of organic PCMs and phase change composites was measured by means of differential scanning calorimetry (DSC). The pore structure of the porous materials was characterized by means of mercury intrusion porosimetry (MIP). X-ray fluorescence spectrometry (XRF) and Fourier transformation infrared spectroscopy (FTIR) were used to characterize the chemical properties of porous materials and phase change materials. Quite different phase change behaviors were found for these two kinds of PCMs in porous materials. For capric acid with a functional group of –COOH, a remarkable elevation of melting temperature was found when confined in porous materials. But for paraffin with only inactive functional groups of –CH₂ and –CH₃, no elevation or depression of the melting temperature was found when confined in the porous materials. The interaction between functional groups of PCM molecules and alkaline spots on the inner pore surface of the porous materials and the Clapeyron equation were used to explain the different shift of the phase change temperature of capric acid and paraffin in porous materials.     

Thermal performance of myristic acid as a phase change material for energy storage application

Thermal performance and phase change stability of myristic acid as a latent heat energy storage material has been studied experimentally. In the experimental study, the thermal performance and heat transfer characteristics of the myristic acid were tested and compared with other studies given in the literature. In the present study is included some parameters such as transition times, temperature range, and propagation of the solid–liquid interface as well as heat flow rate effect on the phase change stability of myristic acid as a phase change material (PCM). The experimental results showed that the melting stability of the PCM is better in the radial direction than the axial direction. The variety of the melting and solidification parameters of the PCM with the change of inlet water temperature is also studied. The results show that the better stability of the myristic acid was accomplished at low inlet water temperature compared with the obtained results at high inlet water temperature. We also observed that while the heat exchanger tube is in the horizontal position, the PCM has more effective and steady phase change characteristics than in the vertical position. The heat storage capacity of the container (PCM tube) is not as good as we expected in this study and the average heat storage efficiency (or heat exchanger effectiveness) is 54%. It means that 46% of the heat acrually lost somewhere.     

Development and energy evaluation of phase change material composite for building energy‐saving

Phase change materials (PCMs) contributed to building energy‐saving and thermal comfort through increasing the thermal capacity of building envelopes. In this study, a phase change material composite was developed by using the PCMs mixture of capric acid (CA) and lauric acid (LA) as the primary phase change energy storage agent and using the solid waste fly ash as a carrier material. The results showed that for Guangdong, the ideal PCMs mixture should have a transition temperature of 25.5^oC, which could be obtained by using a mass ratio of CA/LA of 4:6. Then, experiment results also indicate that the optimum adsorption ratio of 2:1 (FA/PCMs) was detected for the synthesis of this FA/PCMs composite, which has the latent heat of 45.38 J/g and exists excellent thermal reliability. Moreover, simulation results by using EnergyPlus show that the proposed composite has a good building energy‐saving effect.     

强化导热相变材料对PV/PCM热控特性影响研究

文章建立了光伏/相变材料(PV/PCM)太阳能热控系统二维模型,并根据模拟结果研究了相变材料热导率对太阳电池热控特性的影响。模拟结果表明,当PCM热导率由0.3 W/(m·K)逐渐增加至1.1 W/(m·K)时,相变材料对太阳电池的热控效果越来越好。此外,文章设计了PCM热导率分别为0.8,1.1 W/(m·K)的PV/PCM太阳能热控系统实验装置,在模拟光源和自然光条件下,对太阳能热控系统实验装置的输出功率以及太阳电池的温度进行测试。实验结果表明:在模拟光源下,与无PCM太阳电池相比,PCM热导率分别为0.8,1.1 W/(m·K)的太阳电池的最高温度分别降低了4.6,10.8℃,平均输出功率分别提高了2.2%,4.1%;在自然光条件下,与无PCM太阳电池相比,PCM热导率分别为0.8,1.1 W/(m·K)的太阳电池的最高温度分别降低了9.7,12℃,平均输出功率分别提高了3.1%,5.98%。     

Preparation and thermal property of a tetradecane-octanoic acid eutectic phase change material

采用低共熔法研制了一种相变温度在0~3℃的二元有机相变蓄冷材料,该材料由十四烷和正辛酸按一定比例混合组成。首先通过理论计算预测二元最低共熔混合物的比例,确定其理论最低共熔点温度以及潜热值,然后围绕共晶点配制了5种不同比例的混合物。通过差示热量扫描仪、步冷曲线、Hot disk热常数分析仪测量其热物性,并利用高低温交变箱进行循环稳定性实验。当十四烷和正辛酸的摩尔质量比为51：49时,有最低共熔点温度为1.0℃,相变潜热为191.8 J/g,热导率为0.379 W/（m·K）。对其进行100次充放冷实验,循环后相变温度为0.9℃,相变潜热为191.5 J/g,相变蓄冷时间缩短了24.3%,热稳定性良好。实验结果表明,十四烷-正辛酸有机复合相变材料在低温储能中有可观的应用价值。     

Microencapsulated dimethyl terephthalate phase change material for heat transfer fluid performance enhancement

Micro‐phase change materials (micro‐PCMs) are proposed to increase the thermal conductivity and the thermal energy storage capacity of a heat transfer fluid (HTF). In this work, we have selected dimethyl terephthalate (DMT) to be used as a PCM for performance enhancement of a synthetic oil in the temperature range of approximately 100 to 170 °C. Silicon dioxide (SiO₂) was used as the microencapsulant, because of its desirable properties as containment material, including thermal stability. The SiO₂‐coated DMT micro‐PCM was characterized to determine relevant properties and its suitability for HTF performance enhancement. The SiO₂‐coated DMT was found to completely disperse in the synthetic oil, Therminol SP, silicone oil, at and above 100 °C. FTIR, thermal diffusivity and differential scanning calorimetry measurements were carried out on the materials, and these tests demonstrated that the coated particles can be used for HTF enhancement in the temperature range of 100–170 °C and potentially higher temperatures if pressurized pipes/vessels are utilized. Using the measured thermal diffusivity and known data for density and specific heat capacity, the thermal conductivity of the micro‐PCM was calculated. Our calculations indicate that both the thermal conductivity and the thermal energy storage heat capacity of the HTF would be enhanced by the addition of this micro‐PCM. It is expected that the thermal conductivity increase will enhance the heat transfer of the fluid when in use at temperatures above and below the melting temperature of the PCM. At the melting point, the latent heat of the PCM will increase the thermal energy storage capacity of the fluid. Copyright © 2016 John Wiley & Sons, Ltd.     

Feasibility study on a novel cooling technique using a phase change material in an automotive engine

The size of a cooling inventory is generally designed based on which size can endure the excessive heat load situations that occur sporadically. As a result, cooling systems are often too large for most normal driving modes. There have been numerous efforts to downsize the automotive engine cooling system using novel concepts and strategies. Efficient cooling in automobiles is beneficial in reducing harmful emissions as well as improving fuel economy. A simulation was conducted to validate the feasibility of using a novel cooling strategy that utilized the heat load averaging capabilities of a phase change material (PCM). Three prototypes were designed: full-size, down-sized, and a down-sized prototype with a heat accumulator containing the PCM inside. When the full-size of the cooling inventory was down-sized by 30%, this smaller design failed to dissipate the peak heat load and consequently led to a significant increase in the coolant temperature, around 25 °C greater than that in the full-size system. However, the peak heat load was successfully averaged out in the down-sized system with a heat accumulator. This novel cooling concept will contribute to a substantial reduction in the cooling system in terms of volume and hangover.