Assessing long-term performance of centralized thermal energy storage system

A validated computational fluid dynamics simulation tool is used to study the long-term performance of a centralized latent heat thermal energy storage system (LHTES). The LHTES system is integrated with a building mechanical ventilation system. Paraffin RT20 was used as a phase change material (PCM) and fins are used to enhance its performance.To reduce the computational time, artificial neural networks (ANN) was used to relate the relationships between the LHTES inputs and output parameters. Extensive CFD simulations were carried out to identify all the influential parameters for the development of ANN. They include phase change temperature range, air flow rate, the geometrical configuration of a LHTES system, fin size, and the unit's length. Further CFD simulations were carried out to provide sufficient data for proper training and testing of the ANN. The ANN model was used to predict the LHTES's outlet air-temperature. There was a good agreement between the ANN prediction and CFD model's prediction.The ANN model then was used to study the annual performance of a LHTES for application in Montreal. We found that the potential of use the centralized LHTES system to reduce the cooling load is high with a wider phase change temperature range. The centralized LHTES system contributes to reducing the cooling load from 21% to 36% when the length of the centralized LHTES system is increased from 500 to 650 mm at a flow rate of 1.5 m/s.     

Numerical analysis and parameters optimization of shell-and-tube heat storage unit using three phase change materials

In this paper, a mathematical model of shell-and-tube latent heat thermal energy storage (LHTES) unit of two-dimension of three phase change materials (PCMs) named PCM1, PCM2 and PCM3 with different high melting temperatures (983 K, 823 K and 670 K, respectively) and heat transfer fluid (HTF: air) with flowing resistance and viscous dissipation based on the enthalpy method has been developed. Instantaneous solid–liquid interface positions and liquid fractions of PCMs as well as the effects of inlet temperatures of the air and lengths of the shell-and-tube LHTES unit on melting times of PCMs were numerically analyzed. The results show that melting rates of PCM3 are the fastest and that of PCM1 are the slowest both x, r directions. It is also found that the melting times of PCM1, PCM2 and PCM3 decrease with increase in inlet temperatures of the air. Moreover, with increase in inlet temperatures of the air, decreasing degree of their melting times are different, decreasing degree of the melting time of PCM1 is the biggest and that of PCM3 is the smallest. Considering actual application of solar thermal power, we suggest that the optimum lengths are L₁ = 250 mm, L₂ = 400 mm, L₃ = 550 mm (L = 1200 mm) which corresponds to the same melting times of PCM1, PCM2 and PCM3 are about 3230 s and inlet temperature of the air is about 1200 K. The present analysis provides theoretical guidance for designing optimization of the shell-and-tube LHTES unit with three PCMs for solar thermal power.     

Heat transfer and exergy analysis of cascaded latent heat storage with gravity-assisted heat pipes for concentrating solar power applications

A thermal network model is developed to predict the performance of latent heat thermal energy storage (LHTES) systems including cascaded phase change materials (PCMs) and embedded heat pipes/thermosyphons. Because the design of LHTES systems involves a compromise between the amount of energy stored, the heat transfer rate, and the quality of the released thermal energy, an exergy analysis is also carried out to identify the preferred LHTES design. It is found that the LHTES with the lowest melting temperature PCM yields the highest exergy efficiency. However, a cascaded LHTES recovers the largest amount of exergy during a 24 h charging–discharging cycle. Quantitatively, the cascaded LHTES recovers about 10% more exergy during a 24 h charging–discharging cycle compared to the best non-cascaded LHTES considered in this work.     

Thermal Analysis for Package Cooling Technology Using Phase-Change Material by Using Thermal Network Analysis and CFD Analysis With Enthalpy Porosity Method

This paper describes a transient cooling technology for electronic equipments using phase-change material (PCM). The module is made of low-cost materials, yet it is designed to achieve a reasonably high level of heat transfer performance. Paraffin is used as the PCM. In previous our report, we can estimate the cooling performance of PCM by using a thermal network method, which cannot calculate melted PCM flow. In this paper, we consider the heat transfer phenomena of PCM module more deeply by using computational fluid dynamics (CFD) analysis with an enthalpy porosity method. By using this method, we can calculate phase-change phenomena and flow phenomena of melted PCM with CFD analysis. First, we briefly explain the results of the experiment and the thermal network analysis. Then we describe the details of CFD analysis with the enthalpy porosity method. In this calculation, melted PCM flow and heat absorption of latent heat can be analyzed. Therefore, we can discuss the reason why the thermal network analysis can estimate cooling performance of PCM module without dealing with melted PCM flow. The calculation results showed that natural convective flow of melted PCM affects the cooling performance of the PCM module. In the case where the PCM module is set vertically, high temperature and low temperature locations exist on the substrate. If several devices are cooled with the PCM module, device consuming the most power must be set in the lower part of the PCM module. From these results, we can conclude that no natural convective flow occurs in our experiment due to the shape of the PCM module.     

Numerical investigation on latent heat thermal energy storage in a phase change material using a heat exchanger

The use of a heat exchanger using phase change material (PCM) is an example of latent heat thermal energy storage (LHTES). In this study, the charging of PCM (RT50) is studied in a double pipe heat exchanger. The designing of the heat exchanger needs to be optimized for operating and boundary conditions to store latent heat efficiently. The size of the equipment and the amount of PCM are also important to calculate the latent heat storage capacity of the LHTES device. In this study, the amount of PCM taken is quite high to avoid sensible heat transfer and to maximize the heat content of PCM. The charging process of PCM is numerically simulated using an enthalpy-porosity model. The study includes the effect of inlet temperature and flow rate of high-temperature-fluid (HTF) and concludes that both play an important role in determining the charging time. The continuous increase in inlet temperature of HTF can decrease the charging time of PCM in the heat exchanger. However, the continuous increase in the HTF flow rate cannot show the same effect. The charging time can only be minimized with a specified flow rate regime for a specific inlet temperature of HTF. These factors consequently affect the efficiency of the heat exchanger.     

Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage

Fatty acids such as stearic acid (SA), palmitic acid (PA), myristic acid (MA), and lauric acid (LA) are promising phase change materials (PCMs) for latent heat thermal energy storage (LHTES) applications, but high cost is the most drawback which limits the utility area of them in thermal energy storage. The use of fatty acids as form-stable PCM will increase their feasibilities in practical LHTES applications due to reduced cost of the energy storage system. In this regard, a series of fatty acid/poly(methyl methacrylate) (PMMA) blends, SA/PMMA, PA/PMMA, MA/PMMA, and LA/PMMA were prepared as new kinds of form-stable PCMs by encapsulation of fatty acids into PMMA which acts as supporting material. The blends were prepared at different mass fractions of fatty acids (50, 60, 70, 80, and 90% w/w) to reach maximum encapsulation ratio. All blends were subjected to leakage test by heating the blends over the melting temperature of the PCM. The blends that do not allow leakage of melted PCM were identified as form-stable PCMs. The form-stable fatty acid/PMMA (80/20 wt.%) blends were characterized using optic microscopy (OM), viscosimetry, and Fourier transform infrared (FT-IR) spectroscopy methods, and the results showed that the PMMA was compatible with the fatty acids. In addition, thermal characteristics such as melting and freezing temperatures and latent heats of the form-stable PCMs were measured by using differential scanning calorimetry (DSC) technique and indicated that they had good thermal properties. On the basis of all results, it was concluded that form-stable fatty acid/PMMA blends had important potential for some practical LHTES applications such as under floor space heating of buildings and passive solar space heating of buildings by using wallboard, plasterboard or floor impregnated with a form-stable PCM due to their satisfying thermal properties, easily preparing in desired dimensions, direct usability without needing an add encapsulation and eliminating the thermal resistance caused by shell and thus reducing cost of LHTES system.     

Numerical assessing energy performance for building envelopes with phase change material

In hot climate, phase change material (PCM) can be incorporated into building envelopes to reduce heat gain through the building envelopes and therefore reduce its cooling demand. In this study, the energy performance of building envelopes integrated with PCM has been explored using a popular dynamic building performance simulation package, EnergyPlus, and the energy saving mechanism of PCM was investigated. The simulation results reflected that PCM could effectively help to reduce the building's annual energy consumption by 20.9% for Guangzhou, China. In addition, for the Guangzhou city, 27°C transition temperature, smaller thermal conductivity of roof, and higher amount of PCM can all help to improve the building's energy performance. Additionally, it is suggested that in real building development/retrofit projects, the selection of PCM needs to be based on both their thermal properties and the local climatic conditions of the building.     

Stearic acid/polymethylmethacrylate composite as form-stable phase change materials for latent heat thermal energy storage

The aim of this research is to prepare of a novel form-stable composite phase change material (PCM) for the latent heat thermal energy storage (LHTES) in buildings, passive solar space heating or functional fluid by entrapping of SA into PMMA cell through ultraviolet curing dispersion polymerization. The composite PCM was characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis technique. The results show that the form-stable microencapsulated PCM with core/shell structure was formed and the maximum encapsulated proportion of SA in the composite was 51.8 wt.% without melted PCM seepage from the composite. In the shape stabilized microencapsulated PCM, the polymer acts as supporting material to form the microcapsule cell preventing the leakage of PCM from the composite and the SA acts as a PCM encapsulated in the cell of PMMA resin. The oxygen atom of carbonyl group of skeleton is interacted with the hydrogen atom of hydroxyl group of SA. Thermal properties, thermal reliability and heat storage/release performance of the composite PCM were determined by differential scanning calorimetry (DSC), FT-IR and thermal cycling test analysis. The melting and freezing temperatures and the latent heats of the composite PCM were measured as 60.4 °C, 50.6 °C and 92.1 J/g, 95.9 J/g, respectively. The results of DSC, FT-IR and thermal cycling test are all show that the thermal reliability of the composite PCM has an imperceptible change. This conclusion indicates that the composite has a good thermal and chemical stability.     

Review on free cooling of buildings using phase change materials

The concept of Green building is gaining importance in the present energy scenario and related environmental issues. Free cooling or ventilation cooling is truly a green concept since even 1 g of carbon is not burnt for the purpose of cooling. Also it ensures that a good indoor air quality is maintained in the building. In this paper a detailed review of work carried out by various researchers on free cooling or ventilation cooling is presented. In addition the major challenges and facts posed in the use of phase change material for free cooling system design such as thermal resistance of air and phase change materials, geometry of encapsulation are discussed in detail. Also the method of energy efficient charging and discharging, effect of phase change temperature, insulation and geographical location are also discussed in this paper. This paper also provides lists the PCM candidates used for free cooling.     

基于火积理论的多级套管式相变蓄热系统优化

本文基于最小火积耗散热阻原理,在考虑相变材料导热热阻以及非稳态传热过程的基础上,对多级套管式相变蓄热系统的融化温度进行了数值优化,获得了最优融化温度分布。在此基础上,研究了相变材料导热系数和传热管长度对最优融化温度、火积耗散热阻和平均蓄热速率的影响。研究结果表明,与现有理论优化方法相比,本文提出的数值优化方法具有更好的适用性;优化后多级套管式相变蓄热系统可有效提高相变蓄热系统的平均蓄热速率,降低火积耗散热阻;随着相变材料导热系数增大和传热管长度增加,多级套管式相变蓄热系统最优融化温度的温差愈加明显,其强化传热性能呈上升趋势。     

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 dynamics of wallboard with latent heat storage

Wallboard impregnated with phase change material (PCM) will provide thermal storage that is distributed throughout a building, enabling passive solar design and off-peak cooling with frame construction. This paper examines the thermal dynamics of PCM wallboard that is subjected to the diurnal variation of room temperature, but is not directly illuminated by the sun. The purpose of this work is to provide guidelines useful in selecting an optimal PCM and in estimating the benefits of PCM architectural products. The energy stored during a daily cycle depends upon a) the melt temperature of the PCM; b) the temperature range over which melt occurs; and c) the latent capacity per unit area of wallboard. Situations with the wallboard on an interior partition or on the inside of the building envelope are investigated separately. The following findings are presented. The maximum diurnal energy storage occurs at a value of the PCM melt temperature that is close to the average room temperature in most circumstances. Diurnal energy storage decreases if the phase change transition occurs over a range of temperatures. The diurnal storage achieved in practice may be limited to the range 300–400 kJ/m², even if the wallboard has a greater latent capacity. The implications of these findings for test room experiments are discussed.     

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.     

A novel polynary fatty acid/sludge ceramsite composite phase change materials and its applications in building energy conservation

The preparation and characteristics of a composite phase change material (PCM) produced by incorporating polynary fatty acid eutectic mixture into sludge ceramsite were studied. According to Schröeder–Van Laar equation, five different kinds of polynary fatty acid eutectic mixture were prepared, and one of them, suitable for regulating room temperature, was absorbed into sludge ceramsite by vacuum impregnation method. The microstructures were observed by scanning electron microscope (SEM). The thermal properties and chemical structures were analyzed by differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectrometer, respectively. The durability or stability of the composite PCM was determined by heating–cooling cycles test, and the temperature regulation effect was tested by building models. The results indicated that polynary fatty acid eutectic mixture can be retained by 46 wt.% into the pores of sludge ceramsite without seepage. The melting temperature of composite PCM was 26.66 °C, and the corresponding melting enthalpy was 47.1 J/g, suitable to regulate building room temperature. The preparation of composite PCM was just a physical combination, and its chemical structures can remain stable in application process. In addition, model experiments showed that the prepared composite PCM can significantly reduce indoor temperature fluctuation, suitable for building energy conservation.     

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.     

PCM-facade-panel for daylighting and room heating

Double glazings combined with phase change materials (PCM) result in daylighting elements with promising properties. Light transmittances in the range of 0.4 can be achieved with such facade panels. Compared to a double glazing without PCM, a facade panel with PCM shows about 30% less heat losses in south oriented facades. Solar heat gains are also reduced by about 50%. This results in calculated U_eff-values of −0.3 to −0.5 W m⁻² K⁻¹, depending on PCM used. For an optimised panel, we calculated an U_eff-value of −0.6 W m⁻² K⁻¹. Although the U_eff-value of a double glazing is −0.8 W m⁻² K⁻¹, the PCM-systems may prove advantageous in lightweight constructed buildings due to their equalised energy balance during the course of day. Facade panels with PCM improve thermal comfort considerably in winter, especially during evenings. In summer, such systems show low heat gains, which reduces peak cooling loads during the day. Additional heat gains in the evening can be drawn off by night-time ventilation. If a PCM with a low melting temperature of up to 30 °C is used, thermal comfort in summer will also improve during the day, compared to a double glazing without or with inner sun protection. A homogeneous appearance of the PCM-systems is achievable by use of a concealment, like a screen-print glazing.     

Performance analysis of phase-change material storage unit for both heating and cooling of buildings

Utilisation of solar energy and the night ambient (cool) temperatures are the passive ways of heating and cooling of buildings. Intermittent and time-dependent nature of these sources makes thermal energy storage vital for efficient and continuous operation of these heating and cooling techniques. Latent heat thermal energy storage by phase-change materials (PCMs) is preferred over other storage techniques due to its high-energy storage density and isothermal storage process. The current study was aimed to evaluate the performance of the air-based PCM storage unit utilising solar energy and cool ambient night temperatures for comfort heating and cooling of a building in dry-cold and dry-hot climates. The performance of the studied PCM storage unit was maximised when the melting point of the PCM was ～29°C in summer and 21°C during winter season. The appropriate melting point was ～27.5°C for all-the-year-round performance. At lower melting points than 27.5°C, declination in the cooling capacity of the storage unit was more profound as compared to the improvement in the heating capacity. Also, it was concluded that the melting point of the PCM that provided maximum cooling during summer season could be used for winter heating also but not vice versa.     

Numerical and experimental study of melting in a spherical shell

The present study explores numerically and experimentally the process of melting of a phase-change material (PCM) in spherical geometry. Its properties used in the simulations, including the melting temperature, latent and sensible specific heat, thermal conductivity and density in solid and liquid states, are based on a commercially available paraffin wax, which is manufactured to be used mainly in latent-heat-based heat storage systems. A detailed parametric investigation is performed for melting in spherical shells of 40, 60, and 80 mm in diameter, when the wall-temperature is uniform and varies from 2 °C to 20 °C above the mean melting temperature of the PCM. Transient numerical simulations are performed using the Fluent 6.0 software. These simulations show the melting process from the beginning to the end, and incorporate such phenomena as convection in the liquid phase, volumetric expansion due to melting, sinking of the solid in the liquid, and close contact melting. The results of the experimental investigation, which included visualization, compare favorably with the numerical results and thus serve to validate the numerical approach. The computational results show how the transient phase-change process depends on the thermal and geometrical parameters of the system, including the temperature difference between the wall and the mean melting temperature, and the diameter of the shell. Dimensional analysis of the results is performed and presented as the mean Nusselt numbers and PCM melt fractions vs. an appropriate combination of the Fourier, Stefan, and Grashof numbers. This analysis leads to generalization which encompasses the cases considered herein.     

Numerical Analysis of Phase Change Material Characteristics Used in a Thermal Energy Storage Device

In this study, a numerical analysis is performed to investigate the freezing process of phase change materials (PCM) in a predesigned thermal energy storage (TES) device. This TES device is integrated with a milk storage cooling cycle operating under predefined practical conditions. Using this cooling unit, 100 litres of milk is kept cool at 4°C for 48 hours before it is collected. A 2-D model of the TES device is developed in COMSOL Multiphysics to analyze the phase change performance of water-based PCMs. The variations of thermal properties with temperature during the phase change are considered in the analysis. The model is used for exploring the solidification process of PCMs inside the TES device. Temperature variations with time, ice formation, and the impacts of boundary conditions are investigated in detail. Water PCM shows better characteristics in the solidification process in comparison to eutectic PCMs, which is mainly due to the differences between phase change temperatures of the PCMs.     

Thermal properties of building materials evaluated by a dynamic simulation of a test cell

A method for the measurement of thermal properties of building components under controllable conditions is presented. It is based on the use of a test cell designed to enable calculation of thermal properties solely from temperature readings, without the need for power measurements. The test cell’s low thermal inertia allows short testing times (about 3 h). Using an appropriate thermal network to simulate the dynamic behavior of the test cell, predicted results were found to have a 0.6°C maximum temperature deviation with measured test cell responses. The thermal transmittance (U-value) of an insulating block, a single glass sheet and a double glazing have been measured with an accuracy of about 5%. A simulation of a scaled-up test cell with dimensions 6.0 m×6.0 m×4.5 m has revealed that the test cell’s response to temperature changes depends strongly on the amount of wall insulation. The scaled-up test cell exhibits a relatively fast response to temperature changes (9 to 18 h) due to its low thermal mass.