Phase Change Material Heat Research Articles

Lattice Boltzmann simulation of flow and heat transfer evolution inside encapsulated phase change materials due to natural convection melting

A comprehensive study of the melting process inside a capsule can potentially take full advantages of latent heat of phase change materials (PCMs). The present study was devoted to the problem of complex interaction of natural convection and melting of PCMs inside a spherical capsule under different sizes. The numerical results, simulated by lattice Boltzmann method (LBM), were compared with experimental data and published simulations. The results showed that LBM presented desirable accuracy compared to traditional computational fluid dynamics (CFD) methods. Then, the effects of non-uniform PCM properties, expressed by the solid/liquid thermal diffusivity ratio, on the melting rate were found to be nonlinear in different melting stages. The non-dimensional fully melting time reduced with the increase of the surface temperature and the capsule size, and the former compared to the latter could have a greater influence on the melting rate. Moreover, the non-dimensional fully melting time reduced when increasing of the capsule diameter at the macro-scale; while there was a near-invariable non-dimensional fully melting time when the capsule size was changed at the micro-scale. The good understanding of the phase change process inside the capsule would provide essential information to develop a multi-scale model of microencapsulated PCM slurries.

Melting temperature and enthalpy variations of phase change materials (PCMs): a differential scanning calorimetry (DSC) analysis

ABSTRACTDifferential scanning calorimetry (DSC) analysis is a standard thermal analysis technique used to determine the phase transition temperature, enthalpy, heat of fusion, specific heat and activation energy of phase change materials (PCMs). To determine the appropriate heating rate and sample mass, various DSC measurements were carried out using two kinds of PCMs, namely N-octadecane paraffin and calcium chloride hexahydrate. The variations in phase transition temperature, enthalpy, heat of fusion, specific heat and activation energy were observed within applicable heating rates and sample masses. It was found that the phase transition temperature range increased with increasing heating rate and sample mass; while the heat of fusion varied without any established pattern. The specific heat decreased with the increase of heating rate and sample mass. For accuracy purpose, it is recommended that for PCMs with high thermal conductivity (e.g. hydrated salt) the focus will be on heating rate rather than sample mass.

Design of Phase Change Material Based Domestic Solar Cooking System for Both Indoor and Outdoor Cooking Applications

This paper mainly focuses on the design of solar concentric parabolic cooker with proper arrangement of phase change material (PCM) heat storage system. The receiver is a hollow concentric cylinder with inner and outer radii being 0.09 m and 0.1 m, respectively. The thickness or the gap between the two layers of the receiver is 0.01 m and is filled with heat transfer oil. The outer layer of the receiver is surrounded by the vertical cylindrical PCM tubes of diameter 0.025 m. The three modes of heat transfer, radiation, convection, and conduction, are explained and analyzed by heat transfer network. The schematic view of the receiver is shown with the help of sketchup software. The performance parameters, heat loss factor, optical efficiency factor, cooking power of the solar cooker, were calculated with and without PCM in the receiver. 7.74 W m−2 and 2.46 W m−2 are the heat loss factors, and 0.098 and 0.22 are the optical efficiency factors of the solar cooker without and with PCM presented in the receiver. The optical efficiency factor of the solar cooker with PCM receiver is two times more than that receiver without PCM. The cooking power of the solar cooker with PCM receiver is 125.3 W which is 65.6 W more than that of the cooking power without PCM receiver. From these results, it can be concluded that the design of PCM solar cooking system can expand the applicability of solar cookers as a compatible cooking solution for cooking applications instead of using fossil fuel based cooking systems.

A new ventilated window with PCM heat exchanger—Performance analysis and design optimization

This paper proposes a ventilated window with a phase change material (PCM) heat exchanger as a new window application. In summer, night ventilation mode is operated to discharge energy stored in PCM by the ambient cold air, which can be reloaded again, when ventilation pre-cooled air is provided.Numerical model is built and verified by full-scale experiment to evaluate the PCM ventilation system. The nonlinear properties and hysteresis of PCM are set in the model. The conclusion is that the configuration optimization should be based on different climates. In the case study in Copenhagen, the heat exchanger with 10 mm plate thickness is optimized. It can cool down the ventilated air 6.5 °C on average in 3.9 h pre-cooling effective time with 3.19 MJ/day energy saving. The material cost saving is 16.87% compared to 20 mm plate thickness which has similar discharged heat amount. Nevertheless, the heat exchanger with 5 mm plate thickness has a faster thermal response and a higher cost saving ability, which is good for the climate when the period of outdoor air temperature suitable for night ventilation in a day is short.

Loop-Mediated Isothermal Amplification Using a Lab-on-a-Disc Device with Thin-film Phase Change Material.

The design and fabrication of temperature measurement systems that facilitate successful realization of DNA amplification using a lab-on-a-disc (LOD) device are a highly challenging task. The major challenge lies in the fact that such a system must be directly attached to a heating chamber in a way that enables the accurate measurement of temperature of the chamber while allowing the LOD to rotate. This paper presents a temperature control system for implementing isothermal amplification of DNA samples using an LOD device. The proposed system utilizes a thin-film phase change material and non-contact heating system to remotely measure the actual temperature of the chamber and, if required, rapidly heat it to the desired temperature. The results of the experiments performed in this study demonstrate that the proposed system provides an automated platform for molecular amplification and exhibits an operational performance comparable to that of traditional microcentrifuge tube-based isothermal amplification systems.

Ca(NO3)2-NaNO3/expanded graphite composite as a novel shape-stable phase change material for mid- to high-temperature thermal energy storage

A novel shape-stable Ca(NO3)2-NaNO3/expanded graphite composite phase change material (PCM) for mid- to high-temperature TES was successfully synthesized by the developed impregnation and sintering two-step method. The low-cost binary salt Ca(NO3)2-NaNO3 was chosen as the PCM and expanded graphite (EG) was as the structural supporting material and thermal conductivity enhancer. The results showed that the binary eutectic nitrate and EG have good chemical compatibility, and the composite with 7 wt% EG can overcome the problems of cracks or liquid leakage during the phase change. Benefiting from the established better conductive passageway within the composites, the thermal conductivity of the composite with 7 wt% EG can be significantly increased by about 7.3 times in comparison with the binary eutectic nitrate. However, the measured phase change latent heats of the shape-stable phase change materials (SSPCMs) are markedly lower than the theoretical ones, meaning some losses of the latent heat during the material fabrication process. The developed SSPCM also showed little change of the phase change properties after 500 thermal cycles, indicating it has good thermal reliability for a long term period. Compared with other PCMs for mid- to high-temperature applications, this novel cost-effective composite SSPCM is very promising due to the wide operating temperature range and high thermal conductivity.

Geometry optimization of a heat storage system for concentrated solar power plants (CSP)

Abstract In the present study, geometry optimization of a phase change material (PCM) heat storage system is presented. The existing PCM-fins heat exchanger system works at the back side of a solar receiver in order to minimize the effect of the solar radiation fluctuations inside the cavity. As initially designed, the system does not accomplish the expected design purposes and thus optimization is needed. Optimization is usually time-consuming and some algorithms need a starting point, therefore one suitable method is geometrical optimization which aims to find the optimal shape of a system for a given criteria and providing a rough optimal geometry. Here, constructal theory, ‘point to volume’, is applied to find the optimum shape factor of the elemental volume of the presented PCM-heat exchanger. With this methodology, an optimum ratio of the PCM and fin width and length is found and beyond that the method is extended to ‘surface to volume’ problem. Results have been numerically validated using a CFD software and demonstrate that it gives a very good approximation of the real optimum which can be used as initial configuration for further optimization through CFD simulation or other optimization methods that require a starting point.

Cooling concentrator photovoltaic systems using various configurations of phase-change material heat sinks

A new hybrid concentrator photovoltaic-phase change material system is developed to attain rapid thermal dissipation by enhancing the typically low thermal conductivity of phase change materials. The developed system includes four different configurations of phase change material heat sinks: single cavity, three-parallel cavity, five-parallel cavity, and three-series cavity configuration. Furthermore, nine different pattern arrangements of phase change materials are studied. A comprehensive two-dimension model of photovoltaic layers integrated with phase change material heat sink is developed to predict the transient temperature variation at different concentration ratios of 10 and 20. The model is numerically simulated and validated with the available experimental data; the heat sink configurations with three and five parallel cavities are found to significantly reduce the solar cell temperature compared to the single cavity and three-series cavity heat sink configurations. Furthermore, the use of a five-parallel cavity heat sink is found to greatly enhance temperature uniformity of the solar cell. Substantial enhancement in temperature uniformity is also observed with different pattern arrangements of the phase change materials using the three-parallel cavity configuration. These findings can help identify the optimal configuration of heat sinks and pattern arrangements of phase change materials in order to achieve higher performance with concentrator photovoltaic systems.

Effect of scaling on thermal properties and switching operation of phase change memory devices

Phase change memory (PCM) is one of the most promising emerging non-volatile memory technologies. This paper simulates phase change memory devices (PCMDs) with careful attention to the scaling and its resulting impact on programming current during the switching operation, while Thomson heating within the phase change material and Peltier heating at the electrode interface are considered. The simulation results show that the device scaling has an influence on temperature distribution, volume of the molten region, heat diffusion and switching operation of PCMDs. The programming current decreases with smaller electrode size, greater thickness of phase change material and deeper isotropic scaling. The heat diffusion becomes more serious when the thickness of phase change materials decreases and the size of PCMD is isotropically scaled down. The scaling arguments also indicate that the impact of thermoelectric phenomena weakens with smaller dimensions due to the influence of programming current, heat diffusion and action area. This simulation provides useful insights to understand the switching operation of the PCMDs under the impact of thermoelectric effects. The process is instrumental for a complete understanding of device operation and hence provides valuable feedback for fine-tuning the device design so as to enhance its efficiency.

A hybrid energy storage concept for future application in industrial processes

The efficiency of many industrial processes, applying steam as heat transfer medium, can be increased by integrating Ruths steam accumulator. This component makes it possible to store surplus steam for consumption at later time, whereby high loading and unloading rates can be realized with this storage type. For improving this storage type in terms of storage capacity and application range a hybrid storage approach is presented. The concept combines a Ruths steam storage with phase change material and electrical heating elements. For a first analysis of the interaction between the Ruths steam accumulator and the phase change material, which surrounds the steam storage vessel, a dynamic model was created. Also, an example which consist of a charging, storing, and discharging phase is presented. The simulation results show a positive impact in terms of storage capacity. Therefore, the hybrid storage concept is a promising approach for integration in industrial processes.

INVESTIGATING THE USE OF PHASE-CHANGE MATERIALS FOR TEMPERATURE CONTROL DURING FAST FILLING OF HYDROGEN CYLINDERS

This paper explores the use of phase-change materials in the process of fast filling of hydrogen cylinders in order to limit the rise in the gas temperature by enhancing heat transfer from the gas. It is necessary to limit the temperature rise because the structural performance of the cylinder materials can be degraded at higher temperatures. Initially, two computational approaches for modeling the fast filling of hydrogen cylinders are presented and validated; the first is an axisymmetric computational fluid dynamics simulation and the second is a single-zone approach with one-dimensional conjugate heat transfer through the cylinder walls. The models are applied to study fast filling of a hydrogen-powered passenger car. The predictions show that the minimum safe fill time for Type III cylinders with aluminum liners is generally shorter than for Type IV cylinders with plastic liners, for given ambient and precooling temperatures. Alternatively, Type III cylinders require less precooling for a given fill time. Introduction of a phase-change material heat sink is assessed as a means of reducing the fill time for Type IV cylinders. Paraffin-based phase-change materials are considered. The predictions show that the use of pure paraffin wax does not help in reducing the gas temperature due to its low thermal conductivity, however materials with improved thermal conductivity, for example, mixtures of paraffin wax and graphite, can facilitate reduced fill times. Without use of phase-change material it is not possible to reduce the fill time of Type IV cylinders below three minutes unless the gas supply is precooled. While the fill time can be reduced by precooling the gas supply, the phase-change material reduces the degree of precooling required for a given fill time by 10–20 K, and reduces the minimum theoretical power consumption of the cooler by 50–100%, depending on the ambient temperature.

CFD modelling development and experimental validation of a phase change material (PCM) heat exchanger with spiral-wired tubes

Employing phase change materials (PCMs) for latent heat storage (LHS) application has a great potential to improve a solar thermal system performance. Despite this fact, the use of PCM in this area is quite limited due to the poor thermal conductivity of available PCMs. Therefore, heat transfer enhancement is one of the essential strategies that can overcome this obstacle. In this paper and related project, a PCM heat exchanger (HX) is purposely designed with spiral-wired tubes and integrated in an indirect solar assisted heat pump test system. Although the spiral-wired tube has not been applied in a PCM HX, it is expected to enhance significantly the PCM heat transfer and heat storage performance. To verify this and understand the PCM heat storage and releasement processes, a detailed 3D CFD model has been developed for the PCM HX and validated with measurements. The temperature variations and visualizations of the PCM during charging and discharging processes are therefore simulated and presented temporally. Furthermore, the effects of different inlet heat transfer fluid flow rates and temperatures on the PCM melting/solidification time are demonstrated in this study. Some significant simulation results have been obtained which can instruct efficiently the operation of the heat exchanger and its integration with the solar system.

Preparation and thermal performance of form-stable expanded graphite/stearic acid composite phase change materials with high thermal conductivity

Thermal energy storage plays an important role in improving the working reliability and utilization efficiency of renewable energy systems. Thermal energy storage includes sensible heat storage, latent heat storage using phase change material (PCM) and thermochemical heat storage. In comparison with sensible heat storage, latent heat storage has the distinct advantages of its high energy storage density and stable temperature during the energy storage and release processes. However, low thermal conductivity of PCM is the common drawback for various latent heat energy storage systems. The purpose of the research is to enhance the thermal conductivity of PCM by developing new form-stable composite phase change materials using expanded graphite matrix. Firstly, a preparation method “heating adsorption of liquid SA into expanded graphite (EG) matrix and compressing stable-shape composite” is employed to synthesis form- stable composite phase change material. Twenty samples with different EG contents and packed densities are prepared using stearic acid (SA) as phase change material and two kinds of EG with different dilatation as porous matrix. Secondly, the microstructures, thermal properties and thermal stabilities of different EG/SA samples are tested to analyze the influence of the dilatation of expanded graphite on the thermal performance of form-stable composite phase change materials. Moreover, EG/SA form-stable heat storage units with different EG contents and packed densities are fabricated to analyze the thermal storage/release performance through heating/cooling experiment. The microstructures of EG and EG/SA samples are observed using scanning electron microscopy (SEM) and the results show there exist obvious differences between two kinds of EG and SA distribution in the samples. The EG/SA samples with high dilatation EG are even more uniform because the high dilatation EG has larger pore size and pore volume. The data obtained from differential scanning calorimetry (DSC) show the addition of EG and compressing operation have negligible effect on the latent heat and slightly increase the phase transition temperature of pure SA. The form-stable EG/SA composite PCMs have lower supercooling degree than pure SA. LFA447 flash thermal diffusivity instrument is used to evaluate the effect of EG’s dilatation on the thermal conductivity of EG/SA samples. The results show that the use of high dilatation EG is more effective to improve the thermal conductivity of form-stable EG/SA composite PCMs when packed density is large. The radial thermal conductivity of form-stable 20 wt.%EG/SA phase change composite with packed density of 950 kg m−3 is as high as 19.6 W m−1 K−1 when high dilatation EG is utilized. This figure is more than 110 times higher than that of pure SA. Finally, the thermal stability of EG/SA samples is conducted by repeating the charging and discharging processes of SA. These samples prepared by high dilatation EG have better thermal stability than the samples prepared by low dilatation EG. The EG with high dilatation can assure the uniform distribution of SA inside EG/SA sample and thus prevent the leakage of liquid SA during the phase change transformation process due to its dilated microstructure. The heating/cooling test results show that the heat storage/release time of EG/SA composite units is about 1/8–1/4 of that of pure SA unit, and this means the heat transfer can be significantly enhanced. It suggests that the form-stable EG/SA composite phase change materials prepared by high dilatation EG has good comprehensive performance due to its high thermal conductivity, good thermal stability and no leakage problem.

Numerical analysis for maximizing effective energy storage capacity of thermal energy storage systems by enhancing heat transfer in PCM

Stratified water storage (SWS) is of high energy efficiency, but of low energy density as it relies on the sensible heat of water. Latent heat thermal energy storage (LHTES) system using phase change material (PCM) is of higher energy density, and might be an alternative to the bulky SWS system. However, the utilization of latent heat in PCM has been hindered by the low thermal conductivity of PCM. Previous research has demonstrated that high thermal conductivity components, such as metal foam, can be used to enhance the effective thermal conductivity of PCM, keff. In this work, we proposed an index of effective energy storage ratio, Est, to characterize the effective energy storage capacity of an LHTES system with reference to an ideal SWS system of the same volume. A conjugate numerical fluid flow and heat transfer method was first validated and then used to characterize the Est of a tube-in-tank design. The effect of keff on Est was demonstrated. It is shown that there is an optimum compactness factor (CF) for PCM at any finite keff, and that the optimum CF and the corresponding maximum Est are higher for PCM at a higher keff. It is also shown that, even at the optimal CF for PCM at any keff, quite a percentage of the theoretical energy storage capacity is not utilized, indicating that heat transfer enhancement in the fluid side may further improve the Est. This analysis provides implications on an optimal design of LHTES systems.

Optimizing LHS system using PCM in a tube-in-tank design for emergency cooling

Thermal Energy Storage (TES) systems are normally utilized to assist water chillers to cut down operational cost. With the development of heat-transfer-enhancement technique, there is potential for TES systems using the latent heat of Phase Change Materials (PCMs) for emergency cooling applications. The Latent Heat Storage (LHS) systems might be able to provide thermal energy around PCM melting temperature. This study numerically calculated the basic unit of the system in a tube-in-tank design and analyzed the results by the characteristic figure, which shows the relationship between the capacity effectiveness and the heat transfer effectiveness. The parameter of the equivalent thermal conductivity of the PCM is investigated to improve the characteristic capacity effectiveness of the LHS, and the parameter of inlet temperature is evaluated by testing the operational stability of the LHS when the thermal load is unsteady. According to the numerical results, most of the stored thermal energy in the LHS can be discharged effectively around the PCM melting temperature. The characteristic capacity effectiveness can reach approximate 3 at 80% heat transfer effectiveness, indicating a three times quantity of the thermal energy provided by an equivalent water tank.

Analytical Study on Rapid Accumulation Technology of Cold by Phase PCM Thermal Energy Storage System

Recently, uncertainty of demand and supply for energy has expanded as energy market change rapidly. Rapid accumulation storing of cold heat is a method for decreasing the gap between demand and supply of time, quantity and quality of heat energy. It is stored rapidly when electric power is on. And cold heat energy in heat is phase change material (PCM) heat storage is used when electric power is off. Porous media for cold heat is PCM. The present study has been conducted for improving performance of cold heat storage in refrigerator using characteristics on laten heat and phase change of PCM. This study provide basic data to improve performance of cold heat using characteristic of PCM. Also, this study predicts and measures specific points in PCM heat storage during process of phase change.

Experimental investigation of two-stage thermoelectric generator system integrated with phase change materials

Due to limitations in performance of thermoelectric materials, applying two-stage thermoelectric generator (TTEG) has been proposed to improve the performance of thermoelectric generator (TEG) system. In this paper, a novel prototype of a two-stage thermoelectric generator system is investigated experimentally. In the first stage, a TEG module installed between a phase change material (PCM) heat sink, as cooling system, and an electrical heater, as the heat source. Because of the inherent characteristics of PCMs to save the thermal energy as latent heat, the PCM heat sink is used as the heat source of the second stage TEGs. In the second stage, five smaller TEG modules are installed around the PCM with individual heat sinks for cooling with natural convection. In order to have a comparison between a common TEG system and the proposed two-stage TEG system, a one-stage thermoelectric generator with forced air cooling system has been tested. The results show the proposed TTEG system averagely generates 27% more electrical potential than the one-stage TEG system. Moreover, when the heater is off, the TTEG supplies 0.377 V open circuit voltage in average for about 7900 s, while the one-stage TEG generates this amount of voltage just for 2100 s. Therefore, the proposed design makes TEG systems more suitable for wireless sensor applications when the heat source does not provide steady thermal energy. In this study, four different patterns of thermal power applied to the TTEG system are considered. These patterns are used to simulate various transient thermal boundary conditions imposed to the system.

Thermodynamic comparison and dynamic simulation of direct and indirect solar organic Rankine cycle systems with PCM storage

A thermodynamic comparison between a novel direct solar ORC system (DSOS) and indirect solar ORC system (ISOS) is carried out in this study. A phase change material (PCM) heat storage unit is integrated with both systems to ensure the stability of power generation. Water and R245fa are selected as a heat transfer fluids (HTFs) for ISOS and DSOS respectively. However, R245fa is used as working fluid for both systems. Weekly, monthly and annual dynamic simulations are carried out to compare the performance of both systems using hourly weather data of Islamabad, Pakistan. ISOS has shown 1.71% system efficiency and able to provide 34.02 kW/day power while DSOS has shown 4.5 times higher system efficiency and 2.8 times higher power on annual basis. Numerical model for the PCM storage is developed and validated with the previous experimental data. Average annual amount of energy stored by PCM during charging phase for ISOS is 4.24 MW/day higher than DSOS. However, in comparison with ISOS, DSOS has delivered 33.80 kW/day more power to HTF during discharging phase of the PCM on annual basis. Maximum benefits of PCM storage are observed during the summer season compared to the winter season at selected operating conditions. Furthermore, average annual increment in capacity factor by using PCM storage are found to be 21.71% and 17% for DSOS and ISOS respectively.

Experimental thermal storage research of organic binary phase change materials in building environment

ABSTRACTThis paper aims to reveal the heat transfer mechanism of low-temperature phase change material (PCM) and design PCM heat storage device in building heating environment. Firstly, low-temperature binary PCMs of lauric acid and stearic acid are prepared, and their thermal properties are investigated by DSC. Then, shell and tube latent heat thermal energy storage units are conducted, and heat transfer experiments are carried out to analyze the heat transfer mechanism of PCM. The results demonstrate that natural convection plays an important role in heat transfer process, and the heat storage efficiency of PCMs can be significantly enhanced by increasing the fin width and improving the inlet heat transfer fluid (HTF) temperature. Furthermore, some proposals are put forward to guide the design of PCM storage device in building heating environment.

A review on phase change material application in building

In the past several decades, many literatures have emerged on the topic of phase change material and latent heat storage techniques used in building. Accordingly, it is essential to review previous...