On the heat transfer rate reduction of structural insulated panels (SIPs) outfitted with phase change materials (PCMs)

This paper presents a study on the integration of two building technologies into one new unified form for application in residential and small commercial and industrial buildings. Structural insulated panel (SIP) technology was utilized as a structural vehicle, and also for thermal insulation, and phase change materials (PCMs) provided distributed thermal mass. This new type of wall panel was termed phase change material structural insulated panel (PCMSIP). The research conducted during this study provided the foundations for the development of this type of thermally enhanced wall panels and evaluated their thermal performance, based on heat transfer rate reduction, under full weather conditions. On average, the peak heat flux reductions produced by the PCMSIPs in combination with 10% and 20% PCM were 37% and 62%, respectively. The average reductions in daily heat transfer across the PCMSIPs were 33% and 38% for concentrations of 10% and 20% PCM, respectively. The percent PCM concentration was based on the weight of the interior wallboard.     

Application of a ground source heat pump system with PCM‐embedded radiant wall heating for buildings

This paper deals with the utilization of a renewable energy‐based integrated system with the latent heat storage option for building thermal management systems. Both energy and exergy‐based assessments of the current combined system are conducted. For this purpose, phase change material (PCM)‐embedded radiant wall heating system using solar heating and ground source heat pump (GSHP) is studied thermodynamically. Heat is essentially stored within the PCMs as used in the panels to increase the effectiveness. The stored heat is released when the solar energy is not available. In the thermal energy storage analyses, four different PCMs are considered. The present results show that the overall first ‐ law (energy) and second ‐ law (exergy) efficiencies of the PCM‐free radiant heating system are much lower than the case with the PCM‐embedded radiant heating system. Therefore, it is confirmed that the energy efficiency increases from 62% to 87% while the exergy efficiency rises from 14% to 56% with the option where SP26E PCM is employed accordingly.     

Assessment of PCM‐container interfacial heat transfer using a hot/cold probe technique

A novel technique for assessing heat transfer characteristics of salt‐based phase change materials (PCM) was proposed here. The method is based on solution to inverse heat conduction problem. Nanoparticles (Graphite, Graphene, and multi wall carbon nanotube [MWCNT]) were dispersed in the PCM (KNO₃) to assess their respective influence on heat transfer in the PCM. Graphite added PCM offered highest heat flow values and heating rates, while the pure salt‐PCM offered the least. The probe material had a significant influence on the heat transfer rates at the PCM‐probe interface.     

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.     

Eutectic compound (KNO3/NaNO3 : PCM) quasi‐encapsulated into SiC‐honeycomb for suppressing natural convection of melted PCM

The phase change eutectic compound, KNO₃/NaNO₃ (50/50 mol%) (phase change material (PCM)), which is used as the thermal energy storage material in the solar thermal power plant, was quasi‐encapsulated into the SiC‐honeycomb (SCH) for suppressing the natural convection occurring at the liquid state of PCM. The performance of the SCH as the material suppressing natural convection of PCM was investigated experimentally. PCM with three kinds of mixing ratios of SCH of 10%, 20%, and 30%, was prepared and packed in their respective stainless can with oil‐flowing pipe in the center, which is called thermal energy storage unit (TESU). Three units were linked together and stacked vertically by the connector at the inlet/outlet oil pipe. The time variation of temperature at the fixed positions inside the TESU in charging/discharging process and temperature gradient in the radial direction inside TESU when PCM was liquid state were investigated. It is concluded that the natural convection is suppressed by mixing the SCH with PCM up to around 30% in weight, because the PCM is quasi‐encapsulated into cell holes and porous structures of SCHs. And thus, the heat transfer of the PCM + 30%SCH composite is controlled mainly by its thermal conduction, which is also supported through comparison of simulation result with experimental one. And so, we conclude that SCH has a function as the quasi‐encapsulating material for suppressing the natural convection of PCM. Copyright © 2015 John Wiley & Sons, Ltd.     

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 on the thermal performance of phase change material/porous medium‐based battery thermal management in pore scale

With the latent heat, the phase change material (PCM) is widely used in battery thermal management (BTM) to control the temperature. In this paper, the porous medium is employed to enhance the heat transfer of PCM. The lattice Boltzmann model for PCM/porous medium in pore scale is considered, where the mesh system with porous medium (fixed point) is generated by quartet structure generation set (QSGS) method. The effects of the Rayleigh number and porosity on the heat transfer process in BTM are investigated. The results show that decreasing the porosity will accelerate the melting rate. When the porosities are 0.9, 0.8, 0.7, and 0.6, the total melting times are decreased by 23.7%, 43.3%, 58.0%, and 75.4%, compared with pure PCM. The heat is transferred through the high‐conductivity framework. The natural convection in the porous medium is weak, and the conduction is the dominated heat transfer. As a result, the area of solid–liquid interface will be increased, and the heat‐transferred rate is accelerated. However, when the Rayleigh number is raised to 10⁵, applying the porous medium with porosity of 0.9 will increase the total melting time, resulted from the stronger natural convection of PCM. The present study is helpful for design of PCM/porous medium‐based BTM.     

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.     

Modification on hydrated salt‐based phase change composites with carbon fillers for electronic thermal management

For electronic thermal management with hydrated salt phase change materials (PCM), supercooling and thermal stability usually inhibit its development. In view of this, novel of disodium hydrogen phosphate dodecahydrate (DSP)‐based composites PCM with miniaturized size is developed to solve these problems. Three kinds of carbon fillers employed as both nucleating agent and heat transfer promoter were added in DSP separately. The influences of carbon fillers' specific surface area, particle size, and adsorption mode on thermal properties of DSP‐based composites PCM were investigated experimentally. The thermal conductive chains of composites PCM were detected by energy dispersive spectroscopy. The supercooling degree was analyzed by melting‐freezing test. Temperature‐regulated property was captured by infrared imager. In the present work, the supercooling degree of GNS/DSP composites PCM is efficiently reduced to 97.24% compared with the pure DSP. Negligible change in phase change temperature of the CNTS/DSP composites PCM was confirmed after 200 times cycles. There was no obvious liquid leakage of DSP when 2 wt% of carbon fillers were added in composites PCM. The enhancement on thermal properties of DSP is a promising strategy for numerous thermal applications.     

Performance of coupled novel triple glass and phase change material wall in the heating season: An experimental study

A Trombe wall that uses a phase change material (PCM) as the heat storage medium is usually called a PCM Trombe wall or PCM wall. A research project was conducted to investigate the performance of coupled novel triple glass (NTG) and PCM wall, which is simply called PCM wall in this paper, as a solar space heater. The south façade of a test room was constructed using PCM walls. Rubitherm® GR35 and GR41 were incorporated into the plaster of the walls as PCM. The ratio of the solar energy gain provided by the PCM walls to the heat load of the test room per month varied from 4% to 70%; daily overall efficiency of the PCM walls varied from 20% to 36%; and solar transmittance of the NTG varied from 0.45 to 0.55 during the heating period. The overall efficiency of the PCM wall including GR35 was higher than that of the PCM wall including GR41.     

Preparation of stearic acid/halloysite nanotube composite as form‐stable PCM for thermal energy storage

A novel form‐stable composite as phase change material (PCM) for thermal energy storage was prepared by absorbing stearic acid (SA) into halloysite nanotube (HNT). The composite PCM was characterized by TEM, FT‐IR and DSC analysis techniques. The composite can contain SA as high as 60 wt% and maintain its original shape perfectly without any SA leakage after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (SA/HNT: 60/40 wt%) were determined as 53.46°C and 93.97 J g^?1 by DSC. Graphite was added into the SA/HNT composite to improve thermal storage performance, and the melting time and freezing time of the composite were reduced by 65.3 and 63.9%, respectively. Because of its high adsorption capacity of SA, 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 application. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Characterization of novel shape‐stabilized phase change material mortar: Portland cement containing Na2SO4·10H2O and fly ash for energy‐efficient building

In this work, a novel Na₂SO₄·10H₂O/fly ash shape‐stabilized phase change material mortar (PCM mortar) was prepared for building energy efficiency, in the context of energy conservation and environmental protection. The working, mechanical, and thermal properties of this proposed PCM mortar were investigated. The experiment results showed that the incorporation of PCMs greatly increased the thermal inertia of the mortar, while corresponding compressive strength was little affected. Specifically, when the blending amount of PCMs reached 15%, the thermal storage capacity of mortar sample (PCM‐15) was 6.18 × 10⁴ kJ/m³, which is 2.4 times of that for mortar sample without PCM (OPC) evaluated by theoretical calculations, while the corresponding compressive strength of mortar sample (PCM‐15) still remained above 31 MPa. Furthermore, the effects of PCM mortar on thermal comfort and energy use of buildings were studied by using experiment and simulation methods, respectively. The control experiments showed that PCM mortar can effectively alleviate the influence of outdoor temperature on indoor temperature compared with OPC. Temperature difference between PCM‐15 and OPC board can reach 4.6°C (inner surface) and 8.6°C (outer surface), respectively. Meanwhile, temperature difference of internal space reached 1.2°C. The simulation results showed that the energy consumption per unit building area was reduced by 4.4 and 18.7 kg/m² in Guangzhou and Harbin, respectively, with PCM mortar as the envelope structure. Hence, the proposed PCM mortar showed significant thermal and mechanical properties and had broad application prospects in regulating indoor temperature and constructing energy‐efficient buildings.     

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     

Accelerated melting of PCM in a multitube annulus‐type thermal storage unit using lattice Boltzmann simulation

Melting of the ice/water as the phase change material in a horizontal single‐tube annulus is sluggish when the stable stratification exists at the bottom of the configuration. To obviate this problem, three heat transfer enhancement techniques could be implemented using the enthalpy‐based lattice Boltzmann method with the double distribution function method to accelerate the process. The multifarious arrangements of the tubes in this horizontal annulus are investigated to expand the region affected by the natural convection. Also, the dispersion of the Cu nanoparticles in the base PCM could boost the thermal conductivity and melting rate. Finally, the metallic porous matrix made of nickel–steel alloys and saturated with the base PCM could be used to enhance the thermal conductivity of the base PCM. The solid–liquid phase change process is defined as the constrained melting of ice‐water in the tube heating mode. There is a thermal equilibrium between ice/water and the nickel–steel porous matrix and the Cu nanoparticles. The Prandtl number, Stefan number, Rayleigh number, and Darcy number are 6.2, 1, 10⁴–10⁵, and 10^?3, respectively. The volumetric concentric of the nanoparticles is between 0 and 0.02 and the porosity ranging from 1 to 0.9 in the representative elementary volume scale.     

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.     

Thermal behaviour of integrated solar collector/storage unit with 65 °C phase change material

A transient finite volume model was used to predict collection and retention of heat for rectangular cross section solar collector/storage systems when filled with water and various concentrations of phase change material (PCM) slurries with a 65 °C phase change temperature. It was found that the PCM slurry systems collected heat marginally less effectively than water filled stores. Retention of heat at higher temperatures in the PCM slurry systems may allow higher solar savings fractions to be realised depending on heat demand patterns.     

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.     

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.     

Fundamental heat transfer mechanism between bed‐to‐membrane water‐walls in circulating fluidized bed combustors

In the present work, the fundamental mechanism between bed‐to‐membrane water‐walls in the riser column of a circulating fluidized bed (CFB) combustor is presented. The bed‐to‐membrane water‐wall heat transfer depends on the contributions of particle heat transfer, dispersed phase heat transfer and radiation heat transfer. The fundamental mechanism of particle heat transfer and the effect of fraction of wall exposed to clusters and gas gap thickness between cluster and wall on particle heat transfer coefficient and bed‐to‐wall heat transfer coefficient are investigated. The influence of operating parameters like cross‐sectional average volumetric solids concentration and bed temperature on particle and bed‐to‐wall heat transfer are also reported. The present work contributes some fundamental information on particle heat transfer mechanism, which is responsible for increasing the bed‐to‐wall heat transfer coefficient (apart from dispersed phase convection and radiation heat transfer). The details on particle heat transfer mechanism will enable to understand the basic heat transfer phenomena between bed‐to‐membrane water‐walls in circulating fluidized bed combustors in a detailed way, which in turn will aid for better design of CFB combustor units. The particle heat transfer mechanism is significantly influenced by the fraction of wall exposed to clusters and gas gap thickness between clusters and wall. Copyright © 2003 John Wiley & Sons, Ltd.