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.     

Preparation and application of composite EG/Ba(OH)2·8H2O form‐stable phase change material for solar thermal storage

Composite expended graphite (EG)/Ba(OH)₂·8H₂O form‐stable phase change material (PCM) is prepared with porous adsorption method in this study to solve the problem of the leakage risk in the application process based on barium hydroxide octahydrate. In addition, the thermal properties and stability have been measured and verified. Thermal conductivity of each group composite material was enhanced by about two to four times, while the addition has little negative effect on the other properties. With microscopic features being characterized by scanning electron microscope (SEM), the experimental result demonstrates that the composite material with 7 wt% of EG is optimal to be a saturated state between two phases. After the thermal cyclings of 100, 300, and 500 times, thermal properties did not change dramatically, which can be used as an ideal material for solar thermal storage system within finite thermal cyclings. In order to simulate the operating condition of solar energy or waste heat storage, the composite material was encapsulated in the heat storage unit with pipe bundle, and heat storage/release experiences were performed to test the performance of the material and hot water supply. The results indicate that the composite material is qualified to storage and release sufficient heat. The experimental data can provide necessary technical reference for engineering design and effect prediction in practical application.     

The value of thermal radiation in assessing the charge/discharge rate of high‐grade thermal energy storage using encapsulated phase change materials (PCMs)

The charge/discharge rate of a spherical phase change material (PCM) capsule was assessed in consideration of phase change phenomenon and the combined effect of thermal radiation and heat convection in the charging/discharging processes. The heat transfer model was developed based on a single PCM capsule. The equivalent heat flux was evaluated by using the thermal resistance method. In consideration of the thermal radiation, the equivalent charge/discharge rate was improved, and the temperature rising of the PCM was actually much faster in the charging/discharging processes. It was indicated that the influence of the thermal radiation became more significant for PCM capsules under a small Re number (constant air velocity) and for high‐grade thermal energy storage. The analytical results showed that the highest heat flux contributed by cold thermal radiation occupied 30% and 62% of that by heat convection for PCM capsules with radius of 10 and 40 mm, respectively. This illustrated the crucial value of thermal radiation on the charge/discharge rate of PCM capsules with a large radius. However, for smaller size PCM capsules, the equivalent heat flux was larger under the same fluid flow velocity, and it decreased more promptly with time, because the heat convection that played the dominant role in charge/discharge processes was sensitively affected by the radius of the PCM capsules. Copyright © 2016 John Wiley & Sons, Ltd.     

The micro‐/nano‐PCMs for thermal energy storage systems: A state of art review

With advancement in technology—nanotechnology, various thermal energy storage (TES) materials have been invented and modified with promising thermal transport properties. Solid‐liquid phase change materials (PCMs) have been extensively used as TES materials for various energy applications due to their highly favourable thermal properties. The class of PCMs, organic phase change materials (OPCMs), has more potential and advantages over inorganic phase change materials (IPCMs), having high phase change enthalpy. However, OPCMs possess low thermal conductivity as well as density and suffer leakage during the melting phase. The encapsulation technologies (ie, micro and nano) of PCMs, with organic and inorganic materials, have a tendency to enhance the thermal conductivity, effective heat transfer, and leakage issues as TES materials. The encapsulation of PCMs involves several technologies to develop at both micro and nano levels, called micro‐encapsulated PCMs (micro‐PCM) and nano‐encapsulated PCMs (nano‐PCM), respectively. This study covers a wide range of preparation methods, thermal and morphological characteristics, stability, applications, and future perspective of micro‐/nano‐PCMs as TES materials. The potential applications, such as solar‐to‐thermal and electrical‐to‐thermal conversions, thermal management, building, textile, foam, medical industry of micro‐ and nano‐PCMs, are reviewed critically. Finally, this review paper highlights the emerging future research paths of micro‐/nano‐PCMs for thermal energy storage.     

Parametric analysis of space cooling systems based on night ceiling cooling with PCM‐embedded piping

A parametric analysis is conducted for space cooling systems based on cold water flowing, during the night, within regularly arranged pipes embedded in a layer of phase change material (PCM), located among the structural layers of the ceiling. The introduced PCM layer in conjunction with night cooling add to the usual ceiling cooling systems offers the advantages of low energy consumption, high cool storage capacity, operation under reduced night electricity price, smoothing of electricity consumption by eliminating daily peak loads, improved thermal comfort and elimination of ceiling dripping. Our parametric analysis is based on a transient three‐dimensional finite‐difference solution of the related heat‐transfer problem for various values of all the main system parameters. PCM phase change process is simulated by using the effective thermal capacity function, which is determined experimentally for PCM suitable for air‐conditioning applications. Our tests showed that the main parameters of the system are pipe spacing, PCM layer thickness, pipe depth within the ceiling, cooling water inlet temperature, night cooling duration and PCM properties (thermal conductivity, phase change heat and ends of phase change temperature range). The effect of all the above parameters is analysed and suggestions are made for selecting the proper combinations of their values in order to obtain the lowest energy consumption in conjunction with the highest level of thermal comfort. Copyright © 2010 John Wiley & Sons, Ltd.     

Investigation of heat transfer during melting of a PCM by a U‐shaped heat source

An experimental study was conducted to investigate the melting process of a phase change material (PCM) and the associated convection heat transfer due to a U‐shaped heat source embedded in the PCM. The experiments were conducted at four input heat fluxes that varied from 3450 to 5840 W/m². The results showed that the heat transfer behavior, interface movement, and the heat transfer coefficients differed both axially and vertically inside the chamber. The local convective heat transfer coefficient in the inner region, enclosed by the U‐tube, was found to be about 35% higher than that in the outer region over the input heat flux range, resulting in faster melting in the inner region than in the outer region. As melted domain grew vertically from 15% to 100%, it was observed that the overall h in the inner region increased by 40–55% from the lowest to highest heat flux. The melting rate was also found comparatively high up until 65–70% of the total PCM volume melted because of the higher contribution from the inner region. It was also observed that the Rayleigh number increased by approximately 23% in the inner region and 18% in the whole domain as the heat flux increased from 3450 to 5840 W/m². A new Nusselt–Rayleigh number correlation is proposed for the heat transfer during the melting process due to a U‐shaped heat source. Copyright © 2017 John Wiley & Sons, Ltd.     

Improved heating floor thermal performance by adding PCM microcapsules enhanced by single and hybrid nanoparticles

A heating floor is a low-temperature emitter consisting of pipelines in which a fluid circulates between 35°C and 45°C. To ensure energy efficiency, occupant comfort, and building material durability, proper heat management is crucial in buildings. By using phase change materials (PCMs) in building envelopes, the indoor temperature can be regulated through the storage and release of thermal energy, which reduces energy consumption and enhances occupant comfort. In this study, we evaluated numerically a heating floor that incorporates a PCM enhanced by nanoparticles (NePCM). The aim of the numerical analysis is to assess the impact of the addition of single and hybrid nanoparticles in different proportions to the PCM layer on the thermal performance of the PCM-based floor. Therefore, two main objectives are defined. The primary is to take advantage of the storage capacity of a PCM layer by integrating it into the ground; second, to evaluate the hot water temperature levels effect on the floor's performance. Additionally, we address the low thermal conductivity of PCM by enhancing PCM microcapsules with single and hybrid nanoparticles and comparing them to pure PCM. The numerical results obtained show that positioning the PCM microcapsules above the heating tubes (upper position) provides an optimum improvement in thermal performance. Moreover, the addition of hybrid nanoparticles within the base PCM, 1% of Cu mixed with 4% of Al₂O₃, allows an increase of 4°C, which relates to a reduction of 18% in the internal temperature amplitude and a phase shift of 6 h 30 min compared with the conventional heated floor in which there is no PCM.     

Lauric and myristic acids eutectic mixture as phase change material for low‐temperature heating applications

Lauric acid (m.p.: 42.6°C) and myristic acid (m.p.: 52.2°C) are phase change materials (PCM) having quite high melting points 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 lauric acid (LA) and myristic acid (MA). In the present study, the thermal analysis based on differential scanning calorimetry (DSC) technique shows that the mixture of 66.0 wt% LA forms a eutectic mixture having melting temperature of 34.2°C and the latent heat of fusion of 166.8 J g^?1. This study also considers the experimental establishment of thermal characteristics of the eutectic PCM in a vertical concentric pipe‐in‐pipe heat storage system. Thermal performance of the PCM was evaluated with respect to the effect of inlet temperature and mass flow rate of the heat transfer fluid on those characteristics during the heat charging and discharging processes. The DSC thermal analysis and the experimental results indicate that the LA–MA eutectic PCM can be potential material for low‐temperature solar energy storage applications in terms of its thermo‐physical and thermal characteristics. Copyright © 2005 John Wiley & Sons, Ltd.     

Performance analysis on industrial refrigeration system integrated with encapsulated PCM‐based cool thermal energy storage system

Cool thermal energy storage (CTES) is an advanced energy technology that has recently attracted increasing interest for industrial refrigeration applications such as process cooling, food preservation and building air conditioning systems. An experimental investigation on the performance of an industrial refrigeration system integrated with encapsulated phase change material (PCM)‐based CTES system is carried out in the present work. In the experimental set‐up a vertical storage tank is integrated with the evaporator of the vapour compression refrigeration system. Effect of the inlet temperature of heat transfer fluid (HTF) on the temperature variation of the PCM and the HTF in the storage tank and the performance parameters namely average rate of charging, energy stored, specific energy consumption (SEC) of the chiller with and without storage system are studied in detail. The effect of porosity variation in the storage tank is also studied. A 1°C decrease in evaporator temperature results in about 3–4% increase in SEC and 1°C decrease in condensing temperature leads to 2.25–3.25% decrease in SEC. The range of HTF inlet temperature and porosity values for optimum performance is reported. Copyright © 2007 John Wiley & Sons, Ltd.     

Heat pipe with PCM for electronic cooling

This article experimentally investigates the thermal performances of a heat pipe with phase change material for electronic cooling. The adiabatic section of heat pipe is covered by a storage container with phase change material (PCM), which can store and release thermal energy depending upon the heating powers of evaporator and fan speeds of condenser. Experimental investigations are conducted to obtain the system temperature distributions from the charge, discharge and simultaneous charge/discharge performance tests. The parameters in this study include three kinds of PCMs, different filling PCM volumes, fan speeds, and heating powers in the PCM cooling module. The cooling module with tricosane as PCM can save 46% of the fan power consumption compared with the traditional heat pipe.     

Thermal performance of PCM thermal storage unit for a roof integrated solar heating system

The thermal performance of a phase change thermal storage unit is analysed and discussed. The storage unit is a component of a roof integrated solar heating system being developed for space heating of a home. The unit consists of several layers of phase change material (PCM) slabs with a melting temperature of 29 °C. Warm air delivered by a roof integrated collector is passed through the spaces between the PCM layers to charge the storage unit. The stored heat is utilised to heat ambient air before being admitted to a living space. The study is based on both experimental results and a theoretical two dimensional mathematical model of the PCM employed to analyse the transient thermal behaviour of the storage unit during the charge and discharge periods. The analysis takes into account the effects of sensible heat which exists when the initial temperature of the PCM is well below or above the melting point during melting or freezing. The significance of natural convection occurring inside the PCM on the heat transfer rate during melting which was previously suspected as the cause of faster melting process in one of the experiments is discussed. The results are compared with a previous analysis based on a one dimensional model which neglected the effect of sensible heat. A comparison with experimental results for a specific geometry is also made.     

Thermal performance enhancement of melting and solidification process of phase‐change material in triplex tube heat exchanger using longitudinal fins

Efficient application of intermittent renewable energy sources, like solar, waste heat recovery, and so forth, depends on a large extent on the thermal energy storage methods. Latent heat energy storage with the use of phase‐change material (PCM) is the most promising one because it stores large energy in the form of latent heat at a constant temperature. The current study investigates melting and solidification of PCM in the triplex tube heat exchanger (TTHX) numerically. The two‐dimensional numerical model has been developed using Ansys Fluent 16.2, which considers the effects of conduction as well as natural convection. To overcome the limitation imposed by the poor thermal conductivity of PCM, use of fins is the better solution. In the current study, longitudinal fins are used for better performance of TTHX, which increases heat‐transfer area between PCM and heat‐transfer fluid. The effects of location of fins, that is, internal, external, and combined internal‐external fins, are observed. All three configurations improve melting as well as solidification process. During the melting process, internal and combined internal‐external fins are equally efficient, in which maximum 59% to 60% reduction in melting time is achieved. For solidification, internal‐external fins combination gives maximum 58% reduction in solidification time.     

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.     

Analysis of an encapsulated phase change material‐based energy storage system for high‐temperature applications

The capability of an encapsulated phase change material (EPCM)‐based thermal energy storage (TES) system to store a large fraction of latent energy at high temperatures was examined. A 3‐dimensional simulation of a prototype heat exchanger was conducted employing sodium nitrate as the phase change material (PCM). The k‐ω SST model was used to capture the turbulent flow of the HTF, while the melting front was tracked using the enthalpy‐porosity method. The results show that the use of metal deflectors yields a nearly constant heat transfer coefficient over the capsule's surface. Despite this, the presence of the void in the capsule and natural convection within the molten PCM influenced the storage characteristics of the system affecting the shape of the isotherms and melting front. Furthermore, the EPCM capsules consecutively undergo the same heat transfer starting from the capsule closest to the inlet. The EPCM capsules store 80% of the energy lost by the HTF. The 17.7 kg of sodium nitrate stores 14.5 MJ of energy where 20% of the energy stored is via latent heat. Of the energy released by the heat transfer fluid, 80% was absorbed by the EPCM capsules with the remaining energy going into the test section walls. A total of 14.5 MJ of energy was stored by the 17.7 kg of NaNO₃, of which 20% is attributed to the latent heat. The fraction of energy stored as latent heat would be larger if a smaller operating temperature range was used. Thus, an EPCM‐based latent heat TES system is capable of storing a large fraction of the supplied energy and presents efficient means of storing thermal energy for high‐temperature applications. Additionally, the strong agreement between the numerical and experimental works demonstrates that the numerical methods employed can predict the behavior of an EPCM capsule not only within a single capsule but on the system scale as well. Therefore, the applied numerical methods can be used for further design and optimization of EPCM‐based latent heat TES systems.     

Compatibility of plastic with phase change materials (PCM)

Even though paraffin interactions with plastics are known by industry, plastics are commonly proposed as container materials for encapsulating phase change materials (PCM) in many applications. In the literature, there are very few experimental studies of organic PCM migration in plastics and its effects on plastic properties. These interactions are a case study of environmental stress cracking, which is considered one of the most common causes of plastic failure. The aim of this work is an experimental study of interactions of some PCM typically used for thermal energy storage, and some plastic materials currently used as encapsulating materials. With the materials tested in this work, it can be concluded that, for encapsulating organic PCM, low‐density polyethylene and polypropylene showed worse behaviour than high‐density polyethylene. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical simulation of high‐temperature phase change heat storage system

In this paper, numerical results pertaining to cyclic melting and freezing of an encapsulated phase‐change material (PCM) have been reported. The cyclic nature of the present problem is relevant to latent heat thermal energy storage system used to power solar Brayton engines in space. In particular, a physical and numerical model of the single‐tube phase change heat storage system was developed. A high‐temperature eutectic mixture of LiF‐CaF₂ was used as the PCM and dry air was used as the working fluid. Numerical results were compared with available experimental data. The trends were in close agreement. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 32–41, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10132     

Corrosion of metal containers for use in PCM energy storage

In recent years, thermal energy storage (TES) systems using phase change materials (PCM) have been widely studied and developed to be applied as solar energy storage units for residential heating and cooling. These systems performance is based on the latent heat due to PCM phase change, a high energy density that can be stored or released depending on the needs. PCM are normally encapsulated in containers, hence the compatibility of the container material with the PCM has to be considered in order to design a resistant container. Therefore, the main aim of this paper is to study the corrosion effects when putting in contact five selected metals (aluminium, copper, carbon steel, stainless steel 304 and stainless steel 316) with four different PCM (one inorganic mixture, one ester and two fatty acid eutectics) to be used in comfort building applications. Results showed corrosion on aluminium specimens. Hence caution must be taken when selecting it as an inorganic salt container. Despite copper has a corrosion rate range of 6–10 mg/cm² yr in the two fatty acid formulations tested, it could be used as container. Stainless steel 316 and stainless steel 304 showed great corrosion resistance (0–1 mg/cm² yr) and its use would totally be recommended with any of the studied PCM.     

Analytical optimization of interior PCM for energy storage in a lightweight passive solar room

Lightweight envelopes are widely used in modern buildings but they lack sufficient thermal capacity for passive solar utilization. An attractive solution to increase the building thermal capacity is to incorporate phase change material (PCM) into the building envelope. In this paper, a simplified theoretical model is established to optimize an interior PCM for energy storage in a lightweight passive solar room. Analytical equations are presented to calculate the optimal phase change temperature and the total amount of latent heat capacity and to estimate the benefit of the interior PCM for energy storage. Further, as an example, the analytical optimization is applied to the interior PCM panels in a direct-gain room with realistic outdoor climatic conditions of Beijing. The analytical results agree well with the numerical results. The analytical results show that: (1) the optimal phase change temperature depends on the average indoor air temperature and the radiation absorbed by the PCM panels; (2) the interior PCM has little effect on average indoor air temperature; and (3) the amplitude of the indoor air temperature fluctuation depends on the product of surface heat transfer coefficient h_in and area A of the PCM panels in a lightweight passive solar room.     

Experimental and numerical study on charging processes of an ice‐on‐coil thermal energy storage system

In this study, an external melt ice‐on‐coil thermal storage was studied and tested over various inlet conditions of secondary fluid—glycol solution—flow rate and temperature in charging process. Experiments were conducted to investigate the effect of inlet conditions of secondary fluid and validate the numerical model predictions on ice‐on‐coil thermal energy storage system. The total thermal storage energy and the heat transfer rate in the system were investigated in the range of 10 l min ^?1?V??60 l min ^?1. A new numerical model based on temperature transforming method for phase change material (PCM) described by Faghri was developed to solve the problem of the system consisting of governing equations for the heat transfer fluid, pipe wall and PCM. Numerical simulations were performed to investigate the effect of working conditions of secondary fluid and these were compared with the experimental results. The numerical results verified with experimental investigation show that the stored energy rises with increasing flow rate a decreasing tendency. It is also observed that the inlet temperature of the fluid has more influence on energy storage quantity than flow rate. Copyright © 2006 John Wiley & Sons, Ltd.     

Experimental investigation on melting characteristics of ethanolamine–water binary mixture used as PCM

An experimental investigation of the melting process of ethanolamine–water binary mixture used as PCM (phase change material) in a rectangular enclosure with a heated vertical wall is reported in this work. The liquid–solid interfaces were captured and the instantaneous liquid fraction was presented. The effect of natural convection was studied in terms of the molten fraction and the shape of the solid–liquid interface. The correlations of molten fraction and time-averaged Nusselt number are obtained so that the time of the melting process can be predicted. The results indicate that natural convection enhances the rate of melting compared with the pure conduction model and that pure conduction mechanism only occurs at the initial stage of melting. Conduction–convection coupled model is necessary for predicting melting process exactly.