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).     

Preparation and properties of palmitic acid/SiO2 composites with flame retardant as thermal energy storage materials

Palmitic acid/silicon dioxide (SiO₂) composites with flame retardant as thermal energy storage materials were prepared using sol-gel methods. In the composites, palmitic acid was used as the phase change material (PCM) for thermal energy storage, and SiO₂ acted as the supporting material. In order to improve flame retardant property of the composites, melamine was added in the composites. Fourier transformation infrared spectroscope (FT-IR), X-ray diffractometer (XRD) and scanning electronic microscope (SEM) were used to determine chemical structure, crystalloid phase and microstructure of palmitic acid/SiO₂ composites, respectively. The thermal properties, such as phase change temperature and latent heat, were investigated by a differential scanning calorimeter (DSC). Thermal stability was determined by a thermogravimetric analyzer (TGA). The SEM results showed that palmitic acid was well dispersed in the porous network of SiO₂. The DSC results indicated that the composites solidify at 59.20 °C with a latent heat of 60.55 kJ/kg and melt at 59.76 °C with a latent heat of 85.11 kJ/kg when the mass percentage of palmitic acid in the composites was 41.1%. The TGA results showed that addition of melamine improved thermal stability. The microstructure of the charred residue after combustion indicated that the homogeneous and compact charred residue decreased the flammability of the composites.     

Polyethylene glycol (PEG)/diatomite composite as a novel form-stable phase change material for thermal energy storage

This paper deals with the preparation, characterization, and determination of thermal energy storage properties of polyethylene glycol (PEG)/diatomite composite as a novel form-stable composite phase change material (PCM). The composite PCM was prepared by incorporating PEG in the pores of diatomite. The PEG could be retained by 50 wt% into pores of the diatomite without the leakage of melted PEG from the composite. The composite PCM was characterized by using SEM and FT-IR analysis technique. Thermal properties of the composite PCM were determined by DSC analysis. DSC results showed that the melting temperature and latent heat of the composite PCM are 27.70 °C and 87.09 J/g, respectively. Thermal cycling test was conducted to determine the thermal reliability of the composite PCM and the results showed that the composite PCM had good thermal reliability and chemical stability. TG analysis showed that the impregnated PEG into the diatomite had good thermal stability. Thermal conductivity of the composite PCM was improved by adding expanded graphite in different mass fractions. Thermal energy storage performance of the composite PCM was also tested.     

Preparation of capric acid/halloysite nanotube composite as form-stable phase change material for thermal energy storage

A novel form-stable composite as phase change material (PCM) for thermal energy storage was prepared by absorbing capric acid (CA) into halloysite nanotube (HNT). The composite PCM was characterized by TEM, FT-IR and DSC analysis techniques. The composite can contain capric acid as high as 60 wt% and maintain its original shape perfectly without any CA leakage after subjected to 50 melt-freeze cycles. The melting temperature and latent heat of composite (CA/HNT: 60/40 wt%) were determined as 29.34 °C and 75.52 J/g by DSC. Graphite (G) was added into the composite to improve thermal storage performance and the thermal storage and release rates were increased by 1.8 times and 1.7 times compared with the composite without graphite, respectively. Due to its high adsorption capacity of CA, high heat storage capacity, good thermal stability, low cost and simple preparation method, the composite can be considered as cost-effective latent heat storage material for practical applications such as solar energy storage, building energy conservation and agricultural greenhouse in the near future.     

Preparation, thermal properties and thermal reliability of palmitic acid/expanded graphite composite as form-stable PCM for thermal energy storage

This study is focused on the preparation and characterization of thermal properties and thermal reliability of palmitic acid (PA)/expanded graphite (EG) composite as form-stable phase change material (PCM). The maximum mass fraction of PA retained in EG was found as 80 wt% without the leakage of PA in melted state even when it is heated over the melting point of PA. Therefore, the PA/EG (80/20 w/w%) composite was characterized as form-stable PCM. From differential scanning calorimetry (DSC) analysis, the melting and freezing temperatures and latent heats of the form-stable PCM were measured as 60.88 and 60.81 °C and 148.36 and 149.66 J/g, respectively. Thermal cycling test showed that the composite PCM has good thermal reliability although it was subjected to 3000 melting/freezing cycles. Fourier transformation infrared (FT-IR) spectroscopic investigation indicated that it has good chemical stability after thermal cycling. Thermal conductivities of PA/EG composites including different mass fractions of EG (5%, 10%, 15% and 20%) were also measured. Thermal conductivity of form-stable PA/EG (80/20 w/w%) composite (0.60 W/mK) was found to be 2.5 times higher than that of pure PA (0.17 W/mK). Moreover, the increase in thermal conductivity of PA was confirmed by comparison of the melting and freezing times of pure PA with that of form-stable composite. Based on all results, it was concluded that the form-stable PA/EG (80/20 w/w%) has considerable latent heat energy storage potential because of its good thermal properties, thermal and chemical reliability and thermal conductivity.     

Capric-myristic acid/vermiculite composite as form-stable phase change material for thermal energy storage

Phase change materials (PCMs) can be incorporated with building materials to obtain novel form-stable composite PCM which has effective energy storage performance in latent heat thermal energy storage (LHTES) systems. In this study, capric acid (CA)-myristic acid (MA) eutectic mixture/vermiculite (VMT) composite was prepared as a novel form-stable PCM using vacuum impregnation method. The composite PCM was characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis technique. Thermal properties and thermal reliability of the composite PCM were determined by differential scanning calorimetry (DSC) analysis. The CA-MA eutectic mixture could be retained by 20 wt% into pores of the VMT without melted PCM seepage from the composite and therefore, this mixture was described as form-stable composite PCM. Thermal cycling test showed that the form-stable composite PCM has good thermal reliability and chemical stability although it was subjected to 3000 melting/freezing cycling. Thermal conductivity of the form-stable CA-MA/VMT composite PCM was increased by about 85% by introducing 2 wt% expanded graphite (EG) into the composite. The increase in thermal conductivity was confirmed by comparison of the melting and freezing times of the CA-MA/VMT composite with that of CA-MA/VMT/EG composite. The form-stable PCM including EG can be used as energy absorbing building material such as lightweight aggregate for plaster, concrete compounds, fire stop mortar, and component of interior fill for wallboards or hollow bricks because of its good thermal properties, thermal and chemical reliability and thermal conductivity.     

Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage

As a phase change material (PCM), acetamide (AC) can be a potential candidate for energy storage application in the active solar systems. Its utilization is however hampered by poor thermal conductivity. In this work, AC/expanded graphite (EG) composite PCM with 10 wt% (mass fraction) EG as the effective heat transfer promoter was prepared; its thermal properties were studied and compared with those of pure AC. Transient hot-wire tests showed that the addition of 10 wt% EG led to about five-fold increase in thermal conductivity. Investigations using a differential scanning calorimeter revealed that the melting/freezing points shifted from 66.95/42.46 °C for pure AC to 65.91/65.52 °C for AC/EG composite, and the latent heat decreased from 194.92 to 163.71 kJ kg⁻¹. In addition, heat storage and retrieval tests in a latent thermal energy storage unit showed that the heat storage and retrieval durations were reduced by 45% and 78%, respectively. Further numerical investigations demonstrated that the less improvement in heat transfer rate during the storage process could be attributed to the weakened natural convection in liquid (melted) AC because of the presence of EG.     

Microencapsulated phase change slurries for thermal energy storage in a residential solar energy system

Phase change materials (PCMs) are attractive for use in thermal energy storage applications and thermal regulation/control due to their high-energy storage density over a small temperature range. The direct use of phase change materials for energy storage and/or heat transfer applications has been limited due to the low thermal conductivity of the PCM particularly when solidifying on the heat transfer surface. A Phase change slurry (PCS) consists of small micro-encapsulated PCM particles suspended in a carrier fluid which enhances the heat transfer to the PCM. The PCS can serve not only as the thermal storage media but also as the heat transfer fluid, and hence may have many potentially important applications including in the field of heating, ventilation and air-conditioning (HVAC), refrigeration, solar energy and heat exchangers. A test system to examine PCS performance in residential thermal energy storage applications has been developed to both observe and characterise the thermal processes that occur in a thermal store with a helical coil heat exchanger. These test results will be used to improve the system design and identify limitations when used for intermittent application.     

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.     

Numerical investigation of a PCM-based heat sink with internal fins

The present study explores numerically the process of melting of a phase-change material (PCM) in a heat storage unit with internal fins open to air at its top. Heat is transferred to the unit through its horizontal base, to which vertical fins made of aluminum are attached. The phase-change material is stored between the fins. Its properties used in the simulations, including the melting temperature of 23-25 °C, latent and sensible specific heat, thermal conductivity and density in solid and liquid states, are based on a commercially available paraffin wax.A detailed parametric investigation is performed for melting in a relatively small system, 5-10 mm high, where the fin thickness varies from 0.15 mm to 1.2 mm, and the thickness of the PCM layers between the fins varies from 0.5 mm to 4 mm. The ratio of the PCM layer to fin thickness is held constant. The temperature of the base varies from 6 °C to 24 °C above the mean melting temperature of the PCM.Transient three- and two-dimensional simulations are performed using the Fluent 6.0 software, yielding temperature evolution in the fins and the PCM. The computational results show how the transient phase-change process, expressed in terms of the volume melt fraction of the PCM, depends on the thermal and geometrical parameters of the system, which relate to the temperature difference between the base and the mean melting temperature, and to the thickness and height of the fins.In search for generalization, dimensional analysis of the results is performed and presented as the Nusselt numbers and melt fractions vs. the Fourier and Stefan numbers and fin parameters. In some cases, the effect of Rayleigh number is significant and demonstrated.     

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.     

Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems

Designing a cost-effective phase change thermal storage system involves two challenging aspects: one is to select a suitable storage material and the other is to increase the heat transfer between the storage material and the heat transfer fluid as the performance of the system is limited by the poor thermal conductivity of the latent heat storage material. When used for storing energy in concentrated solar thermal power plants, the solar field operation temperature will determine the PCM melting temperature selection. This paper reviews concentrated solar thermal power plants that are currently operating and under construction. It also reviews phase change materials with melting temperatures above 300 °C, which potentially can be used as energy storage media in these plants. In addition, various techniques employed to enhance the thermal performance of high temperature phase change thermal storage systems have been reviewed and discussed. This review aims to provide the necessary information for further research in the development of cost-effective high temperature phase change thermal storage systems.     

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.     

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.     

Evaluation of phase change materials for thermal regulation enhancement of building integrated photovoltaics

Regulating the temperature of building integrated photovoltaics (BIPV) using phase change materials (PCMs) reduces the loss of temperature dependent photovoltaic (PV) efficiency. Five PCMs were selected for evaluation all with melting temperatures ∼25 ± 4 °C and heat of fusion between 140 and 213 kJ/kg. Experiments were conducted at three insolation intensities to evaluate the performance of each PCM in four different PV/PCM systems. The effect on thermal regulation of PV was determined by changing the (i) mass of PCM and (ii) thermal conductivities of the PCM and PV/PCM system. A maximum temperature reduction of 18 °C was achieved for 30 min while 10 °C temperature reduction was maintained for 5 h at −1000 W/m² insolation.     

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.     

Direct contact PCM–water cold storage

Comfort cooling demand continues to increase throughout the world. Conventional cooling production results in high demand for electrical power during peak hours, leading to high emissions for producing cooling, and potential power shortages in electric grids. With a cold storage, the peak power demand is effectively managed and enables free-cooling. This paper examines one concept using phase change materials (PCM) for storing of cold, where the cold carrier (water) is in direct contact with the PCM. This is in order to enable high power for charging and discharging while providing a high storage capacity. A theoretical model highlights important design parameters for reaching large storage and power capacity. The capacity increases with the Packing Factor and temperature difference across the storage. For high power, the flow rate, temperature difference, and drop size is important parameters which is also verified in an experimental evaluation. The obtainable power is between 30 and 80 kW/m³ storage. Practical limitations of this concept are shown to be PCM–water bed expansion and non-uniform channeling due to asymmetric and unstable PCM shells.     

Thermal conductivity enhancement in a latent heat storage system

Latent heat storage systems especially those employing organic materials have been reported to exhibit a rather slow thermal response. This is mainly due to the relatively low thermal conductivity of organic latent heat materials. In this study, experiments were carried out to investigate a method of enhancing the thermal conductivity of paraffin wax by embedding aluminum powder in it. The size of the aluminum powder particles was 80 μm. The tested mass fractions in the PCM-aluminum composite material were 0.1, 0.3, 0.4, and 0.5 of aluminum. The used mass fraction in the experimental work was 0.5.The experiments were conducted by using a compact PCM solar collector. In this collector, the absorber-container unit performed the function of absorbing the solar energy and storing the phase change material (PCM). The solar energy was stored in the PCM and was discharged to cold water flowing in pipes located inside the PCM. Charging and discharging processes were carried out. The propagation of the melting and freezing fronts was studied during the charging and the discharging processes. The time wise temperatures of the PCM were recorded during the processes of charging and discharging. The solar intensity was recorded for the charging process. It was found that the charging time was reduced by approximately 60% by adding aluminum powder in the wax. In the discharging process, experiments were conducted for different water flow rates of 9-20.4 kg/h. It was found that the useful heat gained increased when adding aluminum powder in the wax as compared to the case of pure paraffin wax. The heat transfer characteristics were studied.     

Improvement of a thermal energy storage using plates with paraffin-graphite composite

This work aims at designing a thermal energy storage (TES) using air as heat transfer medium, efficient mainly for free-cooling but also for other applications, improving the low heat transfer rates dues to the thermal conductivity of the materials usually employed in these systems, phase change materials (PCM). In this paper, free-cooling means the storage of cold from the night to be used during the day to cool down a room. An experimental set-up has been constructed to simulate the application. The loading and unloading processes (melting and freezing of the PCM) have two disadvantages: a relative long duration, in the range from 3 to 8 h, and a very high power consumption of the fans. Using a porous matrix of graphite where the PCM is embedded, both handicaps can be noticeably overcome. The application is studied, both experimentally and numerically.     

Preparation and properties studies of halogen-free flame retardant form-stable phase change materials based on paraffin/high density polyethylene composites

The halogen-free flame retardant form-stable phase change materials (PCM) based on paraffin/high density polyethylene (HDPE) composites were prepared by using twin-screw extruder technique. The structures and properties of the form-stable PCM composites based on intumescent flame retardant system with expandable graphite (EG) and different synergistic additives, such as ammonium polyphosphate (APP) and zinc borate (ZB) were characterized by scanning electronic microscope (SEM), thermogravimetric analyses (TGA), dynamic Fourier-transform infrared (FTIR) spectra, differential scanning calorimeter (DSC) and Cone calorimeter test. The TGA results showed that the halogen-free flame retardant form-stable PCM composites produced a larger amount of charred residue at 700 °C, although the onset of weight loss of the halogen-free flame retardant form-stable PCM composites occurred at a lower temperature due to the thermal decomposition of flame retardant. The DSC measurements indicated that the additives of flame retardant had little effect on the thermal energy storage property, and the temperatures of phase change peaks and the latent heat of the paraffin showed better occurrence during the freezing process. The dynamic FTIR monitoring results revealed that the breakdowns of main chains (HDPE and paraffin) and formations of various residues increased with increasing thermo-oxidation temperature. It was also found from the Cone calorimeter tests that the peak of heat release rate (PHRR) decreased significantly. Both the decrease of the PHRR and the structure of charred residue after combustion indicated that there was a synergistic effect between the EG and APP, contributing to the improved flammability of the halogen-free flame retardant form-stable PCM composites.